Latitudinal gradients in benthic community ... - Wiley Online Library

Journal of Biogeography, 26, 1007–1023

Latitudinal gradients in benthic community composition in Western Atlantic estuaries V. D. Engle and J. K. Summers U. S. Environmental Protection Agency, National Health and Environmental Effects Research Laboratory, Gulf Ecology Division, 1 Sabine Island Drive, Gulf Breeze, Florida 32561 USA

Abstract Aim The community composition of benthic macroinvertebrates from 295 estuarine sites was examined in order to either confirm or challenge established boundaries of zoogeographical provinces. We also investigated the postulate that, while local distributions were determined by natural habitat characteristics such as salinity, sediment type and depth, distributions on a large geographical scale would be correlated with temperature. Location The Atlantic coast of the United States (on a latitudinal gradient from 42° to 25°N). Methods Using the descriptive techniques of cluster analysis and nonmetric multidimensional scaling, we determined the similarities in benthic community composition between sites within 1° latitudinal bands and compared the biotic ordinations to natural habitat characteristics such as salinity, sediment type and depth. We then evaluated the overall community composition within each 1° latitudinal band and established whether or not similarities existed between adjacent 1° latitudinal bands. In this manner, we were able to confirm that a latitudinal gradient existed in estuarine benthic community composition along the western Atlantic coast. This latitudinal gradient was demarcated by biogeographical boundaries at (1) Biscayne Bay, Florida, (2) the southern end of the Indian River Lagoon, Florida, (3) St John’s River, Florida, (4) Cape Island, South Carolina, (5) the mouth of the Cape Fear River, North Carolina and (6) Cape Cod, Massachusetts, with a subprovince boundary just north of Cape May at Wildwood, New Jersey. Results The major divisions approximated widely recognized biogeographical boundaries. Average summer water temperatures correlated better than sediment type, depth, or salinity with the latitudinal groupings of sites determined by cluster analysis. Conclusions Based on this relationship, we speculate on the potential impacts of current global climate change scenarios on the distribution of benthic macroinvertebrates along the western Atlantic coast. Keywords Benthic, estuarine, latitude, climate, province.

INTRODUCTION Latitudinal gradients in the distribution, community composition and diversity of marine organisms are well known along both the Atlantic and Pacific coasts of North America Contribution no. 1052 of the U.S. Environmental Protection Agency, National Health and Environmental Effects Laboratory, Gulf Ecology Division, Gulf Breeze, Florida, 32561, U. S. A. Disclaimer: The mention of commercial products or trademarks does not constitute endorsement or recommendation by the U.S. Environmental Protection Agency.  1999 Blackwell Science Ltd

(Hedgpeth, 1957; Valentine, 1966; Hayden & Dolan, 1976; Calder, 1992; Kendall & Aschan, 1993). Compared with terrestrial fauna, marine organisms tend to have larger ranges and exhibit distinct latitudinal zonation more frequently, especially in the near-shore environment where physical barriers prevent the migration of littoral species (Pielou, 1979; Rapoport, 1994). Biogeographers have delineated boundaries along both coasts that correspond to zoogeographical provinces and recognized climatic zones. On the Atlantic coast of the United States, four provinces have been recognized with boundaries occurring at Cape Cod, Massachusetts, Cape Hatteras, North

1008 V. D. Engle and J. K. Summers

Figure 1 Map of the Atlantic coast of the United States with provinces delineated by U. S. EPA’s Environmental Monitoring and Assessment Program.

Carolina and Cape Canaveral, Florida, or Palm Beach, Florida (see Fig. 1; Hutchins, 1947; Gosner, 1971). These boundaries or changepoints are associated with different seasonal temperature regimes where the regions north of Cape Cod and south of Palm Beach experience stable temperatures in winter and summer, respectively, but the region between Cape Cod and Palm Beach endures large annual temperature changes (Hutchins, 1947). This region is called the American Atlantic Temperate Region (Gosner, 1971), and the resident fauna are limited by summer conditions in the north and by winter conditions in the south (Hutchins, 1947). Biogeographical provinces are defined by the observed distribution of organisms (Hedgpeth, 1957; Hall, 1964). Although western Atlantic biogeographical provinces may coincide with oceanographic or climatic zones, they depend on the existence of well-defined breaks in the distribution of organisms (Hutchins, 1947). In the western Atlantic, controversy exists concerning what types of benthic fauna (i.e.

temperate, transitional or tropical) are represented in each province. The Virginian province (Cape Cod to Cape Hatteras [42° to 35°N]; Fig. 1), for example, may represent a unique biogeographical region (Briggs, 1974) or may simply contain fauna that are transitional between the boreal province to the north and the warm-water fauna to the south (Gosner, 1971). Another controversy exists as to whether the Carolinian province (Cape Hatteras to Cape Canaveral; Fig. 1) represents the southern portion of the American Atlantic Temperate Region (Gosner, 1971), the northern border of a warmtemperate province (Briggs, 1974), a transitional region between temperate and tropical provinces (Cerame-Vivas & Gray, 1966) or an outer tropical region with a fauna similar to the subtropical and tropical fauna from the south (Hall, 1964). A third controversy involves the ill-defined boundary between the Carolinian and West Indian (or Caribbean) provinces. Depending on the group of organisms studied, the changepoint may occur at various places along the eastern Florida coast,  Blackwell Science Ltd 1999, Journal of Biogeography, 26, 1007–1023

Latitudinal gradients in benthic community composition 1009

between the Florida–Georgia border and Palm Beach, Florida (or between 31° and 26°N; Fig. 1), but the actual location is the subject of some debate (Briggs, 1974). Vermeij (1978: 4) summarizes the controversies surrounding geographical limits of temperate-zone provinces by claiming them to be ‘arbitrary and subject to varying interpretations’. While conceding that provincial boundaries usually occur along the coast at points where many species reach their range limits, Vermeij (1978) emphasizes that species range limits do occur at many locations along the coast and can be variable over time. Regardless of the controversies, however, Hedgpeth (1957: 367) notes that ‘. . . the substantial agreement in the extent of major biogeographical divisions of the shore provided by diverse and unrelated studies does indicate that elaborate statistical treatment may do little more than refine certain details’. The causes of the observed differences in community composition that occur along latitudinal gradients are also the subject of some debate. While temperature and global climate change have been cited as the most important factors governing marine biogeographical provinces (Hutchins, 1947; Hall, 1964; Cerame-Vivas & Gray, 1966; Valentine, 1966; Gosner, 1971; Vermeij, 1978; Angel, 1991), other hypotheses also have some merit. Species may be confined to biogeographical regions by barriers that are physical, geomorphological, chemical, spatial or even biological (Hayden & Dolan, 1976; Golikov et al., 1990; Vermeij, 1991) or by latitudinal gradients in other oceanic factors such as upwelling intensity (Connolly & Roughgarden, 1998). Our main purpose in this paper was to examine the distribution of estuarine benthic macroinvertebrates in order to either confirm or challenge the boundaries of the zoogeographical provinces that are widely accepted in the literature. Our secondary objective was to confirm the tenet that natural habitat characteristics such as salinity, sediment type and depth determined the distribution of estuarine benthic genera on a local scale but that, on a large geographical scale (spanning 17° latitude), temperature would be the overriding factor determining the distribution of benthic genera. To accomplish these objectives we analysed benthic macroinvertebrate community composition data that were collected by the U. S. Environmental Protection Agency’s (U. S. EPA) Environmental Monitoring and Assessment Program for Estuaries (EMAP-E). One of the primary objectives of EMAP-E was to determine the condition or status, extent and geographical distribution of ecological resources within and among biogeographical provinces (Summers et al., 1995). Although EMAP-E was neither designed nor intended specifically for biogeography, both the sample design and the data collected lend themselves well to answering traditional biogeographical questions. Most previous biogeographical studies relied on gathering data from many different researchers on species range limits or distributions, without the benefit of large amounts of data collected in a standardized manner (Peters, 1971; Hayden & Dolan, 1976). Standardization of methods, however, is required in order to attribute gradients in diversity or community composition to latitude and to discount any confounding factors (Abele & Walters, 1979; Kendall & Aschan, 1993). Our study uses data collected across  Blackwell Science Ltd 1999, Journal of Biogeography, 26, 1007–1023

a large geographical area within the framework of a single program that relies on standardized methods for collection, identification and quality control of samples. We examined the similarities of benthic communities within 1° latitudinal bands and compared the observed similarities to habitat characteristics (i.e. salinity, depth and sediment type). We then investigated the similarities between benthic communities in adjacent 1° latitudinal bands. Using the results from cluster analysis and nonmetric multi-dimensional scaling, we determined breakpoints in benthic macroinvertebrate community composition in estuaries along the Atlantic coast. We compared our findings with established latitudinal boundaries and speculated on the potential impact of recent global climate change scenarios on the distribution of benthic macroinvertebrates in western Atlantic coast estuaries. METHODS EMAP-E collected benthic samples in western Atlantic coast estuaries from Nantucket Sound, Massachusetts to Biscayne Bay, Florida over a 5-year period (Fig. 1). Based on established biogeographical boundaries, EMAP-E divided the Atlantic coast into four provinces (Acadian, Virginian, Carolinian and West Indian; Fig. 1; Holland, 1990) and this study incorporated data from all provinces except the Acadian. As Fig. 1 shows, the province boundaries recognized by EMAP-E were slightly different from the biogeographical changepoints referenced in the literature. EMAP-E moved the Virginian–Carolinian border north of Cape Hatteras to Cape Henry, Virginia, so as not to split the Albermarle–Pamlico Sound system between two separate provinces. For the same reasoning the Carolinian–West Indian boundary was moved south of Cape Canaveral to St Lucie Inlet, so as not to split the Indian River Lagoon system between two provinces. The changepoints at the Capes indicated in Fig. 1 have generally referred to climatic zones, oceanic currents and fauna of the continental shelf, not estuarine fauna. In addition, previous efforts that delineated biogeographical provinces at the capes have often focused on individual faunal groups (e.g. molluscs) rather than the complete assemblage of benthic macroinvertebrates. EMAP-E employed a probabilistic design to select 527 sampling stations within a triangular grid framework. Stations were located in the subtidal zone of estuarine systems along the western Atlantic coast. Each province was responsible for implementing EMAP-E but all provinces adhered to all aspects of the EMAP-E sampling design from the probabilistic station matrix to the collection and quality control of benthic invertebrate samples (Holland, 1990; Strobel et al., 1995; Hyland et al., 1996). EMAP-E chose to sample during a summer index period to represent the time frame during which the most severe impacts on the biota would be expected to occur (Summers et al., 1995). Each station was sampled once during the summer months of 1990–93 in the Virginian province, 1994–95 in the Carolinian province and 1995 in the West Indian province. Additional data from 1996 and 1997 for the Carolinian province were not yet available for inclusion in this analysis. No station was sampled more than once during the same year and a different, random selection of stations was sampled each year. All provinces


also used standardized methods for collecting samples in the field and processing samples in the laboratory, thus ensuring standardization of procedures for the entire data set. This standardization allowed us to combine the data from all provinces. At each site, at least three replicate benthic samples were collected using a Young-modified Van Veen grab that sampled a surface area of 440 cm2. Because stations were located in the subtidal zone of estuaries, primarily soft-bottom habitats were sampled. Samples were sieved in the field and the fauna, retained on a 0.5-mm screen, were preserved in rose-Bengal buffered formalin. At the laboratory, samples were sieved again on a 0.5mm screen, sorted into major groups and then individuals were identified to lowest practical taxonomic level and counted. Due to difficulties encountered during sample processing at the laboratory, only one grab per station was evaluated for the West Indian province. Because the enumeration and identification of benthic invertebrate taxa were carried out at several different laboratories, considerable effort was expended to reconcile potential benthic taxa identification discrepancies from the different provinces. Using cross-province taxonomic identification quality control checks, taxa identification issues were resolved with relatively little difficulty and a master taxa list was created. Although both macroinvertebrate infauna and epifauna were included in the master list, any chordates (i.e. fish, cephalochordates [Amphioxus], urochordates [tunicates] and hemichordates [acorn worms]) and meiofauna (e.g. copepods, nematodes) that were identified were omitted. Salinity, temperature, pH and dissolved oxygen were also recorded at each station at 1-m intervals from the surface to the bottom. The three provinces used different instruments to record water quality profiles: the Virginian province used a SeaBird Sealogger CTD, the Carolinian province used a Hydrolab DataSonde 3 and the West Indian province used a Hydrolab Surveyor II. Depth at the station was recorded from the boat’s fathometer. Sediment samples for grain size analysis were homogenized from the top 2 cm from several grabs and sent to a laboratory for determination of silt–clay and sand fractions. Because 69% of the stations came from 4 years of sampling in the Virginian province, we were concerned that these data would bias any comparisons. The EMAP-E design called for sampling the same set of large estuaries (> 280 km2) and large tidal rivers each year (although not the same station each year), providing four replicates for the estuaries in the province. Only one-fourth of the small estuaries (< 280 km2) were sampled each year, so that all small estuaries would have been sampled after 4 years. Comparison of the cumulative distribution function and associated 95% confidence intervals for Virginian province large estuaries from 1990 to 1993 revealed that there was no significant year-to-year variation in the assessment of benthic indicators. We therefore reduced the number of Virginian province stations by omitting the large estuary sites that were sampled in 1990, 1991 and 1992, but retaining the 1993 large estuary sites. Ecological indicators for determining whether or not a station had degraded sediment quality were developed by all EMAPE provinces. Because our interest was in benthic community composition based on natural gradients, it was desirable to omit all stations with degraded sediment quality that could be

attributed to an anthropogenic origin. By so doing, we removed the potential for the benthic community composition at a station to be determined by confounding anthropogenic influences rather than natural habitat characteristics. Each province employed slightly different guidelines for determining degraded sediment quality. We decided to take a conservative approach and defer to the best judgement of personnel from each province rather than apply a single set of guidelines across all provinces. For the Virginian province, we omitted all stations that had a negative benthic index, and where at least one of the following conditions was true: (1) at least three contaminants had concentrations greater than ER-L guidelines or at least one contaminant was greater than ER-M guidelines (Long et al., 1995), (2) control-corrected survival of the amphipod, Ampelisca abdita, from sediment toxicity tests was less than 80%, or (3) bottom-dissolved oxygen concentrations were less than 2 mg/L. The ER-L and ER-M guidelines were developed by Long et al. (1995) from a biological effects data base (BEDS) that contained

the concentrations of contaminants at which adverse biological effects occurred (i.e. altered benthic communities, sediment toxicity and histopathological disorders in demersal fish). The guidelines are referred to as effects range – low (ER-L) and effects range – median (ER-M) which delineate concentrations of contaminants at which adverse biological effects occur rarely (< ER-L), occasionally (between ER-L and ER-M) or frequently (> ER-M). The Virginian province benthic index represented a combination of diversity and abundances of indicator species that was used to discriminate stations with degraded benthic communities (Strobel et al., 1995). The Carolinian province statistical summaries listed sites that exhibited evidence of degraded biological conditions accompanied by significant pollution exposure (Hyland et al., 1996; 1998a). They used indicators similar to those in the Virginian province to determine degraded sediment quality but evaluated Microtox assays and benthic diversity, species richness and abundance in addition to a benthic index (Hyland et al., 1998b). Only nine sites from the West Indian province were located on the Atlantic Coast of Florida (latitude 25°N). Because these sites, which were in Biscayne Bay, exhibited neither evidence of degraded benthic conditions nor poor sediment quality, all were included in the analyses. All of these omissions reduced the number of stations that were used in the analyses to 295 (from a total of 527). In the first phase of the analysis, stations were grouped arbitrarily into 1° latitudinal increments, so that all stations located between adjacent degrees of latitude were combined and labelled with the lesser degree of latitude. This produced eighteen 1° latitudinal increments from 25 to 42°N. An arbitrary division of coastline into 1° latitudinal increments was also used by Valentine (1966), who noted that this subjective partitioning of the data resulted in variation in sample size, especially in the in-shore and shallow-water habitats. The number of stations in our groups ranged from two in latitude 29°N to forty-nine in latitude 38°N. There were no stations sampled in latitude 26°N and only one station was located at 42°N. These two latitudinal groups were dropped from all further analyses, which decreased the  Blackwell Science Ltd 1999, Journal of Biogeography, 26, 1007–1023


number of latitudinal increments being compared to sixteen. In order to minimize the potential for redundancy in the benthic taxa, the abundance data were constrained to genus as the lowest taxonomic level of identification. All taxa that were listed as ‘unidentified’ for a taxonomic group higher than genus (except for ‘unidentified oligochaeta’) were omitted (e.g. ‘unidentified polychaeta’ was omitted from the data). Although a few oligochaete genera were actually identified, we classified all oligochaetes as unidentified oligochaeta’ to avoid redundancy. All genera used in the analyses are listed by major taxonomic group in Appendix I. The reason for our use of the genus level was primarily to avoid redundancy in the data (i.e. to avoid the pitfalls of Genus A sp. and Genus A sp. 1 and Genus A species B included in one sample) but the reduction in complexity of the data also streamlined the cluster analysis. The use of genera but not higher order taxonomic distinctions in determining latitudinal zones is supported by some researchers (e.g. Peters, 1971; Pielou, 1979) but disputed by others (e.g. Thorson, 1955; Golikov et al., 1990), who claim that species distributions rather than genera are usually associated with climatic zones, whereas genera are more likely to be limited by physical barriers such as substrate type and depth. Transformation of abundance values using loge(1+x) enabled us to evaluate patterns effectively in the entire community by properly weighting both common and rarer species (Clarke & Warwick, 1994). All statistical analyses were accomplished using SAS/STAT procedures (SAS Institute, 1990) and complementary procedures from Plymouth Marine Laboratory’s PRIMER package (Clarke & Warwick, 1994). Within each 1° latitude, a matrix of all stations as rows and all genera as columns was created. The Bray–Curtis (dis)similarity coefficient, which does not count mutual absences, was calculated for every pair of stations. Classification of sites using SAS/STAT was accomplished by performing average-linkage cluster analysis on the matrix of Bray–Curtis dissimilarities, resulting in a dendrogram that plotted clusters by distance. Confirmatory classification analysis was performed using PRIMER, which performed hierarchical agglomerative cluster analysis with groupaverage linkage and produced a dendrogram of Bray–Curtis similarities. Using the results from SAS/STAT, station clusters were identified by drawing a line on the dendrogram at distance=1.0. Cluster analysis using PRIMER showed that this subjective demarcation of clusters corresponded to a Bray–Curtis similarity of 10–30%. Multi-dimensional scaling (MDS) was performed on the (dis)similarity matrix. The station groups identified by cluster analysis were transferred to the MDS plot and then overlaid with labels representing the classification of salinity, sediment or depth values or with estuary names (see below, Fig. 4). For classification purposes, stations were grouped by bottom salinity into tidal freshwater (0–0.5 p.p.t.), oligohaline (0.5–5 p.p.t.), low mesohaline (5–12 p.p.t.), high mesohaline (12–18 p.p.t.), polyhaline (18–35 p.p.t.) and marine (> 35 p.p.t.). Similarly, stations were also classified by sediment type as sand (< 20% silt–clay content), mixed (20–80% silt–clay), or mud (> 80% silt–clay). Overlaying classifications of salinity, sediment or depth  Blackwell Science Ltd 1999, Journal of Biogeography, 26, 1007–1023

values can be used to visualize which environmental factors are best associated with the grouping of stations determined by the dissimilarity coefficients (McRae et al., 1998). The BIO-ENV procedure in PRIMER was used to confirm the relationships between the benthic ordination and the environmental variables by comparing the ranked similarities between the benthic ordination and the abiotic ordination (Clarke & Ainsworth, 1993). BIO-ENV produces a Spearman’s q which, when ≥0.8, indicates that an ordination of stations using the combination of environmental variables closely matches the ordination of the benthic communities (Clarke & Warwick, 1994). The results from the first phase of the analysis were used to identify stations within each 1° latitudinal band that had similar benthic community composition. The MDS plot suggested which combination of environmental factors, such as salinity, sediment type or depth, provided the ‘best’ match to the similarities among benthic communities. From this analysis we determined which, if any, environmental factors (other than temperature) were highly correlated with the benthic community composition. These factors would have to be taken into account during the analysis to detect latitudinal differences. In the second phase of analysis, the entire benthic community was grouped by 1° latitudinal increments and any important environmental factors, and similarities and dissimilarities between latitudes were identified using the same clustering and MDS techniques. In order to perform these comparisons, the mean abundance of each taxa over all stations within each 1° latitudinal band was calculated. Then a matrix of all latitudes as rows and all taxa as columns was created and all missing abundances were converted to zeros. The Bray–Curtis coefficient was calculated for all pairs of latitudes. Cluster analysis and MDS were applied as before and the results were plotted as dendrograms and MDS ordinations. Because we accounted for other important environmental factors, we assumed that temperature would be the overriding factor that would distinguish between latitudes as one moves from north (42°N) to south (25°N). Average bottom water temperatures for the stations in each 1° latitude were superimposed on the MDS plots, along with the groupings determined by cluster analysis. Duncan’s means comparison test was used to determine if the average temperatures (as well as salinity, silt–clay and depth averages) were significantly different among latitudinal clusters. A modified Bray–Curtis index (Clarke & Warwick, 1994) was also calculated for each genus to identify its contribution to the dissimilarity between adjacent clusters of latitudes. RESULTS A total of 535 genera and Oligochaeta were identified from the EMAP-E Atlantic coast samples (Appendix I). At stations within each 1° latitude, the benthic communities clustered mainly along salinity gradients, either alone or in combination with sediment and, occasionally, depth gradients. Results from the BIO-ENV procedure indicated that good correlations (q≥0.7) between the benthic ordinations and salinity occurred


within latitudes 25° and 37–40°N (e.g. see results for latitude 37°N in Fig. 4). Moderate correlations (0.5≤ r < 0.7) linked the benthic ordinations with salinity and either depth or sediment characteristics within latitudes 28°, 33–36° and 41°N. Tidal freshwater and oligohaline communities, regardless of sediment type, usually clustered separately, sometimes mixed with a few low mesohaline sites. Latitudes 27–32° and 34°N contained no undegraded oligohaline stations. Low mesohaline communities were usually clustered with oligohaline stations while high mesohaline and polyhaline stations clustered together. Polyhaline stations usually formed several clusters based on sediment type or depth. Marine salinities occurred only at stations in North Carolina and those communities clustered separately. This is not surprising as, in coastal waters, salinity is considered an ‘ecological master factor’ that determines the distributions of invertebrates (Kinne, 1971). Sediment type and depth, although occasionally important, are relatively minor factors when compared with salinity. The second phase of analysis divided the Atlantic coast estuaries by latitude and salinity zone. Salinity zones, however, were not evenly distributed along the Atlantic coast. When degraded stations were removed from the data set there were, in fact, few stations south of latitude 35°N that had salinities in the mesohaline or oligohaline range. That is not to say that only polyhaline conditions existed in the southern estuaries, but that fewer oligohaline and mesohaline areas exist south of latitude 35°N when compared with the Virginian province which is dominated by Chesapeake Bay. Although, initially, we performed separate cluster analyses on five different salinity zones, the resulting latitudinal divisions within the tidal freshwater, oligohaline and low mesohaline zones were similar enough to group these zones together; similarly, we grouped the high mesohaline, polyhaline and marine zones together. The results from cluster analyses across latitudes, separated by salinity (≤12 p.p.t. v. > 12 p.p.t.) are presented here. For the high salinity zone (> 12 p.p.t.) the latitudinal bands clustered in groups from north to south (Fig. 3). The southernmost latitude (25°N) clustered entirely separately from the rest of the region and represented Biscayne Bay in the West Indian province. Four (possibly five) additional clusters were identified at a Bray–Curtis similarity of 35%. The first included Indian River Lagoon in Florida (27–29°N; Fig. 3), with average summer temperatures ranging from 29° to 30°C. The second cluster included stations from the Carolinian province that ranged from the St John’s River, Florida to Cape Island, South Carolina (30–32°N; Fig. 3), with average summer temperatures from 27° to 29°C. Latitude 33°N clustered separately and represented stations from the Carolinian province that ranged from Cape Island, South Carolina, to the mouth of the Cape Fear River in North Carolina (Fig. 3). The last and largest cluster included the remainder of the Carolinian province and the entire Virginian province from 34° to 41°N with a possible subdivision between 38° and 39°N latitude located near Cape May, New Jersey and intersecting the upper Chesapeake Bay (Fig. 3). Average summer temperatures in the entire 34–41°N cluster ranged from 21° to 29°C. The cluster analysis for the low salinity zone (≤12 p.p.t.) produced identical latitudinal groups. Several latitudes,

however, were omitted from this analysis. There were no low salinity stations in latitudes 27–31°N, nor in 34°N. Regardless of the reduction in the number of latitudinal bands, they clustered from north to south into three groups: (1) latitude 25°, (2) latitudes 32° and 33°N, and (3) latitudes 35–41°N. Even though salinity was a major determining factor in the distribution of benthic genera and salinity zones were not evenly distributed along the Atlantic coast, the latitudinal cluster analyses for different salinity groups produced the same results. Based on these results, we recombined the data and performed the cluster analyses on all of the data without regard to salinity. Again the latitudinal bands clustered from north to south in the groups described previously for the high salinity zone (Fig. 4). The following section details the results of this cluster analysis using all the data. The mean summer temperature for stations within each latitudinal cluster was a significant factor in determining differences between latitudinal groups (P =0.0001; R2=0.52). Average summer temperatures in Biscayne Bay and Indian River Lagoon (25–29°N; 30°C) were significantly different from the average summer temperatures in the lower Virginian (34–38°N) and Georgia–South Carolina (30–32°N) groups. The upper Virginian (39–41°N) had the lowest average summer temperature (23°C) and was significantly cooler than all other latitudinal groups (Table 1). Average salinity, percentage of silt–clay and depth, while significant (P < 0.001), did not explain as much of the variability among latitudinal groups as did temperature (Table 1). The southernmost cluster, Biscayne Bay (25°N; Fig. 3), was characterized by predominantly polyhaline, shallow (< 4 m), sand or muddy–sand conditions. One station, in Long Sound, was oligohaline. Salinity (Spearman’s q=0.847) best explained the ordination of benthic taxa similarities within this latitude. The top ten genera that contributed most to the average dissimilarity between this cluster and the adjacent one to the north (Indian River Lagoon, 27–29°N) included two molluscs, two crustaceans and six polychaetes. The bivalve, Mysella, occurred only in Biscayne Bay, while Mulinia occurred in Indian River Lagoon but not in Biscayne Bay. Similarly, the amphipod, Leptochelia, and the tanaid crustacean, Halmyrapseudes, were common in Biscayne Bay, while the amphipod, Ampelisca, was common in Indian River Lagoon. Biscayne Bay had high abundances of the polychaetes, Exogone, Potamilla and Caulleriella, while Paraprionospio, Streblospio and Mediomastus were common in Indian River Lagoon. The second latitudinal cluster, Indian River Lagoon (27–29°N; Fig. 3), was characterized by salinities that were greater than 12 p.p.t. (high mesohaline to polyhaline), predominantly sandy sediments and shallow depths (< 4 m). No environmental factors correlated strongly with the benthic ordination within these latitudes suggesting that the benthic community was fairly homogeneous throughout this cluster. Clusters within latitudes were identified by combinations of salinity (high mesohaline or polyhaline), sediment type (sand, mud or mixed) and depth (> or < 2 m). McRae et al. (1998) performed a similar analysis of the benthic community composition of Indian River Lagoon (using 1994 stations from EMAP-E’s Carolinian province) and concluded that a  Blackwell Science Ltd 1999, Journal of Biogeography, 26, 1007–1023


Figure 2 Multi-dimensional scaling (MDS) of Bray–Curtis dissimilarities from loge(1+x)-transformed genera from EMAP-E stations in latitude 37°N. Salinity classes are superimposed where POLY=polyhaline (.> 18 p.p.t.), HMSEO=high mesohaline (12–18 p.p.t.), OLIGO=oligohaline (0.5–5 p.p.t.) and TFRSH=tidal freshwater (0–0.5 p.p.t.). Three groupings from the cluster analysis are indicated by circles.

combination of natural habitat characteristics such as salinity and sediment type and sediment contaminants were associated with differences among stations. Of the top ten genera from our analysis that contributed most to the average dissimilarity between Indian River Lagoon and the adjacent cluster to the north (North-east Florida, Georgia and southern South Carolina; 30–32°N) two polychaetes, Sabellaria and Tharyx, occurred only in the higher latitudes and a bivalve, Erycina, and an amphipod, Grandidierella, occurred only in Indian River Lagoon. Six additional genera all occurred in greater abundances in Indian River Lagoon than in the 30–32°N cluster (Mulinia, Caecum, Exogone, Diopatra, Phoronis and Acteocina). Latitudes 30–32°N clustered together and included stations from the St John’s River in north-east Florida to Cape Island, South Carolina (Fig. 3). All but one station was characterized by high salinities (high mesohaline to polyhaline) and predominantly sandy sediments. Depths ranged to 13 m, but averaged 4–6 m. The depth range of the stations in these latitudes distinguished them from stations to the south that were much shallower (< 4 m). The benthic communities in latitudes 30–32°N differed from those in latitude 33°N primarily because of an abundance of Protohaustorius, Brachycercus (an insect larva) and the polychaete, Aricidea, in latitude 33°N and an abundance of Ampelisca and the polychaetes, Scoletoma, Neanthes and Monticellina in latitudes 30–32°N that were absent from latitude 33°N. Latitude 33°N extends from Cape Island, South Carolina to the mouth of the Cape Fear River at Wilmington Beach, North Carolina (Fig. 3). Only four sites were clustered in this latitude, one oligohaline site and three polyhaline sites. Salinity  Blackwell Science Ltd 1999, Journal of Biogeography, 26, 1007–1023

and depth or sediment type and depth (both with Spearman’s q=0.643) provided the best explanations of the biotic ordination within latitude 33°N. The sites in this latitude had very few genera in common with the latitudes to the north (North Carolina and the Virginian province). Of the top ten genera that contributed to the difference in benthic communities between latitude 33°N and all locations to the north, Sabellaria, Aricidea, Protohaustorius, Brachycercus and Balanus were frequent in latitude 33°N, while Mediomastus, Ampelisca and three molluscs, Mulinia, Macoma and Acteocina, were common in the northern latitudes, but were virtually absent from latitude 33°N. The remaining latitudinal cluster included the rest of North Carolina and the Virginian province exclusive of the upper tidal portion of the Hudson River (Fig. 3). If we extended the division of clusters to a Bray–Curtis similarity of 40%, this cluster would split into 34–38°N and 39–41°N (Fig. 4). Out of the entire region sampled, this cluster contained the only tidal freshwater sites and the majority of oligohaline and low mesohaline sites. Average bottom temperatures ranged from 21° to 29°C, with the lowest temperatures occurring in latitudes 40° and 41°. The coldest, deepest and lowest salinity sites were located throughout this region. The genera that contributed most to the difference in benthic communities between this region and the rest of the clusters (excluding latitude 25°N) included three genera found only in this region (Marenzelleria, Limnodrilus and Leptocheirus). Three additional genera are found in much greater abundance in this region than in more southern latitudes (Gammarus, Macoma and Ampelisca). The polychaete, Sabellaria, was more common to the south of latitude 34°N than to the north of this latitude.


Figure 3 Map of the Atlantic coast of the United States showing latitudinal groups determined by clustered analysis for the high salinity zone and for all of the data combined. Landmarks discussed in the text are also indicated.

DISCUSSION Comparisons of benthic invertebrate community composition along latitudinal gradients would ideally include Arctic, temperate and tropical sites (Kendall & Aschan, 1993). The EMAP-E sites along the Atlantic coast are mainly in the temperate zone with the exception of latitudes 25–30°N, which are considered tropical. The entire region, from Cape Cod to Cape Canaveral in Florida, is generally considered to be transitional from the Arctic or boreal to the subtropical and tropical waters (Cerame-Vivas & Gray, 1966; Gosner, 1971). Within this temperate region, however, there is a widely recognized biogeographical change point occurring around Cape Hatteras, North Carolina (Hutchins, 1947; Cerame-Vivas & Gray, 1966; Gosner, 1971; Calder, 1992). Using the duration of critical temperatures that limit the ranges of shallow-water marine molluscs, Hall (1964) defined marine climates along the

Atlantic coast of North America to be mild temperate from 35° to 41°N corresponding to the Virginian province, outer tropical from 30° to 35°N corresponding to the Carolinian province and inner tropical from the equator to 30°N corresponding to the Caribbean (or West Indian) province. The Virginian province is transitional between two regions of relative thermal stability (the cold Arctic and the warm Carolinian) and, as a temperate province, experiences a wide range of annual temperature fluctuations (Gosner, 1971). This characteristic allows the fauna from each of the adjoining provinces to migrate into the Virginian province during the appropriate seasons. Therefore, in the summer, for example, species may be found that are endemic to the Carolinian province occurring in the lower Virginian province. The benthic invertebrate fauna discussed here, however, are not highly motile and would not be expected to migrate long distances within a season.  Blackwell Science Ltd 1999, Journal of Biogeography, 26, 1007–1023


Figure 4 Dendrogram illustrating clustering of benthic macroinvertibrates from estuaries of each latitude (i.e. L25, L27, L41) with those from other latitudes. Similarity ceofficient of cluster represented by vertical crossbars using the Bray–Curtis Similarity Index and group-average clustering methods.

Our purpose in this paper was to examine the distribution of estuarine benthic macroinvertebrates in order to either confirm or challenge whether the zoogeographical provinces that are widely accepted in the literature apply to estuarine fauna. We also intended to confirm the belief that, on a small geographical scale (1° latitude), natural habitat characteristics such as salinity, sediment type and depth determined the Table 1 Duncan’s means comparison of temperature, salinity, % silt–clay and depth among latitude groups identified by normal cluster analysis. Groups connected by a solid line are not significantly different at a=0.05.

Latitude cluster

distribution of estuarine benthic genera but, on a large geographical scale (spanning 17° latitude), temperature would be the overriding factor determining distribution. Biogeographers widely agree that, in establishing boundaries or zones based on the distribution of organisms, one must be sure that homogeneity exists within each unit and that adjacent units are distinctly different from one another (Hutchins, 1947; Peters, 1971; Pielou, 1979; Diaz, 1995). There is also some conjecture that provincial boundaries may be drawn based on the number of endemic organisms within a given zone and on the extent to which organisms have their northernmost or southernmost range limits within a zone (Valentine, 1966; Briggs, 1974; Vermeij, 1978). We examined our data for evidence to support both of these concepts. Cluster analysis indicated that a latitudinal gradient exists in the composition of benthic communities in estuaries along the western Atlantic coast. In general, cluster analysis divided the communities into relatively distinct groups from north to south. With the exception of latitudinal bands that had no low salinity zones, the divisions were almost identical for high salinity and low salinity zones. The resultant clusters approximated the established Virginian, Carolinian and West Indian biogeographical provinces although the boundaries did not match exactly. Cape Hatteras has been widely recognized as a major breakpoint for the distribution of marine organisms (Hutchins, 1947; Cerame-Vivas & Gray, 1966; Briggs, 1974; Vermeij, 1978) because of the convergence at this point of a number of oceanographic conditions. Cape Hatteras represents the land point where (1) the Florida Current becomes the Gulf Stream which veers north east away from land at this juncture, (2) the Virginian Coastal Current changes direction, curving away from land (3), the Carolinian Coastal Current drifts southwest along the coast and (4) a significant temperature gradient occurs as a result of water-current patterns, particularly in winter (Calder, 1992). The first major breakpoint for benthic communities from our data, however, did not occur at Cape Hatteras, but occurred further south at Wilmington Beach, North Carolina (34°N). This breakpoint occurred at a Bray–Curtis similarity of 40%. The similarity between latitudes

Mean

Duncan grouping

Ο 27–29°N 30.2 25°N 29.6 33°N 28.7 Ο Ο 30–32°N 27.9 34–38°N 27.0 Ο Ο 39–41°N 22.9 Ο Temperature, P=0.0001, R2=0.52

30–32°N 33°N 25°N 27–29°N 39–41°N 34–38°N

Ο Ο

Ο

34–38°N 41.6 Ο 39–41°N 39.8 25°N 32.8 27–29°N 14.3 Ο 30–32°N 12.1 Ο 33°N 6.3 % Silt–clay, P=0.0001, R2=0.11  Blackwell Science Ltd 1999, Journal of Biogeography, 26, 1007–1023

Latitude cluster

Ο

Ο

39–41°N 30–32°N 33°N 34–38°N 25°N 27–29°N

Mean

Duncan grouping

Ο 27.5 25.2 25.1 23.5 22.4 Ο 15.4 Salinity, P=0.0001. R2=0.16 Ο 8.9 6.5 5.2 Ο 3.8 2.5 1.9 Depth, P=0.0001, R2=0.19

Ο Ο

Ο Ο

Ο Ο


34°N and 35°N was 55%, and this cluster joined the cluster of 36–38°N at a similarity of approximately 50%. This would suggest that the estuarine benthic communities directly to the north or south of Cape Hatteras are not distinctly different. A point should be made here that, although Cape Hatteras is a major changepoint for marine fauna of the continental shelf, the changepoint for estuarine fauna may occur further south and would not be expected to occur in the middle of the Albermarle–Pamlico Sound. Excluding latitude 25°N, 460 genera were identified from the region between Cape Cod, Massachusetts and Palm Beach, Florida. With a biogeographical breakpoint at 34°N, 37% of the genera were common between the northern (34–41°N) and southern (27–33°N) latitudes, while 37% of the genera were exclusive to the north and 25% occurred only in the southern latitudes. These numbers exceeded the number of endemics that have been cited as desirable for defining a biogeographical province (e.g. 19% for fish, 18% for echinoderms, 28% for decapods; see Briggs, 1974) for review). Clusters of adjacent latitudinal groups were added at sequentially lower similarities. Many researchers have indicated that the region from Cape Hatteras to Cape Canaveral represents a biogeographical province with similar marine communities. EMAP-E delimits the Carolinian province between Cape Henry, Virginia and St Lucie Inlet, Florida (just north of Jupiter, Florida; Fig. 3). Our data indicate that the temperate fauna common to the Virginian province extends south to Wilmington Beach, North Carolina (34°N; Fig. 3). The region from 34°N south to 27°N separated into three location groups at a Bray–Curtis similarity of 30% (Fig. 4). Breakpoints occurred at latitude 33°N (north of Charleston, South Carolina) and at latitude 30°N (south of Jacksonville, Florida, or at the head of tide of the St John’s River; Fig. 3). The benthic fauna of southern South Carolina and Georgia are distinctly different from the benthic fauna of Indian River Lagoon. Within this region (27–32°N), 283 genera were identified, 33% of which were common to both the 27–29°N and 30–32°N latitudinal groups. The South Carolina and Georgia group (30–32°N) contained 38% of the genera that were not found to the south of latitude 30°N, while 29% were found only in the Indian River Lagoon group (27–29°N). A distinct breakpoint for benthic fauna occurred between latitudes 25°N and 27°N. Only 22% of the 535 genera identified overall occurred in both the West Indian province and the regions to the north of 27°N. The benthic communities at latitude 25°N were distinguished from communities to the north more by the genera that were absent from this latitude than by the number of genera endemic to this latitude. Only 14% occurred exclusively at latitude 25°N, leaving 64% of the genera that did not occur at latitude 25°N. The separation of benthic communities at this breakpoint corresponds to a wellrecognized but ill-defined boundary between Carolinian and West Indian biogeographical provinces. EMAP-E delimits the northern limit of the West Indian province on the Atlantic coast of Florida to be St Lucie Inlet. Evidence has been presented to suggest that the boundary between the Carolinian and West Indian provinces was located somewhere along the south-east coast of Florida between the Florida–Georgia border and Palm

Beach (Hutchins, 1947; Hall, 1964; Briggs, 1974; Vermeij, 1978; Calder, 1992) but that the border was ill-defined. Cape Canaveral has been cited most often as a biogeographic change point, but our study indicated that the change point for estuarine fauna occurs further south near Palm Beach. A possible reason for the change in fauna at this location is that the Florida Current swings away from the coast at Palm Beach redirecting the flow of warm, tropical water east into the Atlantic. Because of the arbitrariness of our divisions at 1° latitudinal increments and our lack of data between 26° and 27°N, we can only suggest that the boundary for the West Indian province is located no further north than Jupiter, Florida (27°N) and no further south than Biscayne Bay (25°N). The communities in Indian River Lagoon were more closely related to the communities to the north than to those in Biscayne Bay. Biscayne Bay most probably represents the northern boundary for the tropical fauna of the West Indian province. Up to this point we have used cluster analysis to determine similarities among latitudinal groups and to suggest provincial boundaries or biogeographical changepoints. Valentine (1966) and Briggs (1974), however, indicated that defining the boundaries of zoogeographical provinces also required the identification of latitudes where the most range endpoints occurred and the areas where the highest degree of endemism occurs. We re-examined our data and, in the manner of Valentine (1966), plotted the latitudinal distribution of range endpoints for all species that were identified by EMAP-E along the Atlantic coast. This type of analysis did not rely on the community composition at a specific location but, rather, focused on the northernmost and southernmost occurrences of a given species. We were therefore able to use all our data at the species, not genus, level but limited the inclusion of species to only those that were actually identified to species. We defined a boundary latitude to be one that has approximately 100 species with range endpoints (total of northernmost and southernmost) within that latitude. Using this rather arbitrary guideline, we drew latitudinal boundaries at 25°, 27°, 30°, 32°, 37° and 41°N (Fig. 5). These boundaries coincide closely with the boundaries that were indicated by the cluster analysis for benthic genera (25°, 27°, 30°, 33°, 34° and 39°N). However, a surprisingly high number of species (344) were endemic to 1° latitude (i.e. the species distribution was found entirely within 1° latitude; see Fig. 6). These included rare species and species that were collected in a single sample. Valentine (1966) also found a disproportionately high number of 1° endemic species on the Pacific coast and postulated that this reflected a lack of knowledge about true ranges rather than real endemism. Similar to Valentine (1966), we combined the information about range endpoints and 1° endemic species with the idea that the overlap of the two would provide more concise indication of latitudinal boundaries. The overlap of the number of species that span at least 2° latitudes with range endpoints at each latitude (bars) and the number of 1° endemic species at each latitude is shown in Fig. 7. The latitudinal boundaries were drawn at the latitudes with the highest number of 1° endemic species and the highest number of range endpoints and were the same as before (25°, 27°, 30°, 32°, 37° and 41°N). A disproportionately high number of species had either their  Blackwell Science Ltd 1999, Journal of Biogeography, 26, 1007–1023


species. Whether we examined the similarities between benthic communities from adjacent latitudes using cluster analysis of genera or the degree of endemism and the number of range endpoints of species that occurred within 1° latitudinal bands, the outcome was similar. Three major provinces were identified by both analyses with at least two subprovinces. POTENTIAL RESPONSES OF BENTHIC DISTRIBUTIONS TO GLOBAL CLIMATE CHANGE

Figure 5 Latitudinal distribution of range endpoints for benthic macroinvertibrates from the western Atlantic coast (estuaries from Cape Cod, MA to Biscayne Bay, FL. Grey bars represent the number of species for which each latitude is an endpoint at the northern limit. Black bars represent the number of species for which each latitude is an endpoint at the southern limit. Ranges are as indicated by EMAP-E data and may actually extend beyond 25°N or 42°N for individual species.

Figure 6 Number of species with latitudinal ranges from 0° (endemic to 1° latitude) to 16° (ubiquitous across the full range of latitudes from 25°N to 42°N).

northernmost range endpoint at latitude 41°N or were endemic to this latitude (Figs 5 and 7). We suspected that this high number was due to the number of stations sampled from areas that were not technically estuarine but that resembled true oceanic habitat (i.e. the areas adjacent to and including Nantucket Sound; Fig. 3). Sites in these sounds had average depths of 6 m, bottom salinities of 32 p.p.t. and bottom temperatures less than 20 °C. When the thirty-seven sites from these areas were removed and the analysis repeated, the number of species with their northernmost range endpoints at latitude 41°N decreased from 387 to 248 and the number of 1° endemics decreased from 123 to sixty-six. These numbers, although reduced, are still high enough to support latitude 41°N as a boundary. Our findings (Figs 5, 6 and 7) support Valentine’s (1966) conclusion of a high correlation between the range endpoints of nonendemic species and the number of 1° endemic forms, indicating that the factors that dictate the boundaries for longranging species are associated with the presence of 1° endemic  Blackwell Science Ltd 1999, Journal of Biogeography, 26, 1007–1023

The data and conclusions that we have presented here should serve as a good baseline of the latitudinal distributions of benthic macroinvertebrates in western Atlantic estuaries. The results from future monitoring efforts could be used to compare distributions and attempt to correlate changes in global climate with any observed alterations in species’ distributions. This analysis indicated that temperature is the overriding factor in distinguishing estuarine benthic community composition on a latitudinal gradient. Based on this observed relationship, we now consider how this type of analysis could be used to evaluate the potential impacts of current global climate change scenarios on the distribution of benthic macroinvertebrates along the western Atlantic coast. In order to determine if global climate change will alter the distribution of estuarine benthos, we need to identify first what forces in the estuarine environment may have led to the observed latitudinal distribution of invertebrates. Although Gaston (1996) argued that the latitudinal diversity gradient from the poles to the equator is only a general pattern and does not hold for all groups of organisms, especially those in shallowwater coastal systems, the tenet that latitudinal gradients in species composition exist is well-accepted. Two primary views prevail on the driving forces for latitudinal gradients in the distribution of species. One view holds that temperature or climate is the primary force and that marine communities tend to coincide with oceanic provinces which are distinguished by prevailing currents and temperature (Hutchins, 1947; Hall, 1964; Cerame-Vivas & Gray, 1966; Vermeij, 1978; Angel, 1991). The mechanisms of temperature change, however, seem to be more important than the magnitude of temperature in determining province boundaries because some invertebrates may have large ranges of thermal tolerances but find sharp changes in temperature limiting (Hall, 1964; Valentine, 1966; Gosner, 1971; Fields et al., 1993). Optimal temperatures for adult survival are not the limiting factor either; rather, it is the temperature variations that are necessary for reproductive success and/or survival of larvae that dictate species’ geographical distribution (Bhaud et al., 1995). The second view is that geographical or physical barriers (i.e. capes, rivers, land bridges) provide major breakpoints in the latitudinal distribution of species that are coincidentally associated with climatic factors (Hayden & Dolan, 1976; Golikov et al., 1990; Vermeij, 1991). Some researchers have suggested that biogeographical provinces are defined by a combination of physical or geographical barriers and current global climate scenarios (Pielou, 1979) and that subprovince divisions may have arisen from local temperature regimes influenced by


Figure 7 Number of species that exhibit their northernmost and southernmost range limits at each latitude. Bars represent the number of species whose range spans at least 2° latitude. Line represents the number of species that occurred within only 1° latitude (1° endemics).

topography (Valentine, 1966). Vermeij (1991) and Angel (1991) also suggested that the range limits of species may have been determined not only by geography or temperature but also by their ecological history or by their interaction with other species (i.e. biological barriers). Because present-day latitudinal zonation in marine biota coincides with current abiotic conditions, one can assume that there has been no real lag in the response of marine communities to global climate changes (Pielou, 1979). Common climate change scenarios predict that a doubling of CO2 from preindustrial levels will result in an increase in temperature of 2–4°C at the equator and 6–9°C at higher latitudes (50–70°N) (Manabe et al., 1991; Viner et al., 1995). Such changes in atmospheric temperature will influence latitudinal and vertical shifts in oceanic water temperature (Manabe et al., 1991). Given the influence of temperature on the survival, reproductive success, dispersal patterns, behaviour and competitive advantage of marine species, a 2°C rise in temperature is sure to lead to considerable changes in marine communities (Southward et al., 1995) and, subsequently, on sedimentary processes, benthic–pelagic coupling and energy flow (Justic et al., 1996). Global warming may result in a shift of tropical water masses and their resident biota toward the poles. One climate equability model predicts that northern taxa will move south if the summers become cooler but will be restricted in range if the summers become warmer, with the opposite being predicted for southern taxa and winter temperatures (Graham & Grimm, 1990). Species ranges will be limited by their ability to survive changing temperatures. Some species will be restricted by geographical barriers or physical limitations to ranges that are less than optimal in the face of climate change (Fields et al., 1993). Other species may respond to warming temperatures by moving into cooler, deeper waters rather than shifting their distributions to the north.

Comparing historical records of changes in distribution patterns and relative abundances of different species to measured environmental changes would allow us to predict the effects of global warming on marine communities. However, we would need at least 150 years in order to detect subtle biological effects associated with small changes in climate (Southward et al., 1995). Then, to conclude that the observed changes represent a marine biological response to global warming, we would have to see increases in abundance and ranges of warm-water species that are much greater than any that have been observed in recent history (since the 1950s) (Southward et al., 1995). CONCLUDING REMARKS We have demonstrated that latitudinal gradients in benthic community composition exist in estuaries along the Atlantic coast using cluster analysis of genera and an examination of endemism and range endpoints of species. Although we do not dispute the well-established biogeographical provinces, we suggest that the boundaries or changepoints for estuarine benthic macroinvertebrate communities may differ from those for other organisms or specific taxonomic groups. We found that the boundary between the Virginian and Carolinian provinces should most probably be located south of Cape Hatteras, in that the fauna of Albermarle–Pamlico Sound were more similar to the Virginian province fauna to the north than to the estuarine fauna of South Carolina and Georgia. Even though we did identify that salinity zones and sediment types were not evenly distributed along the Atlantic coast, we support the postulate that temperature or climate is the primary factor determining the distributions of estuarine benthos along a latitudinal gradient. We speculate that if the current global climate change scenarios (of an increase in global temperature  Blackwell Science Ltd 1999, Journal of Biogeography, 26, 1007–1023


of approximately 2°C) are realized, major distributional shifts of benthic communities are likely to occur along the Atlantic coast.

ACKNOWLEDGMENTS The authors would like to thank M. Hughes at the Virginian province office in Narragansett, Rhode Island and T. Snoots at the Carolinian province office in Charleston, South Carolina for providing their benthic data and for being available to answer questions. We also thank D. Flemer, U. S. Environmental Protection Agency, and J. Hyland, National Oceanic and Atmospheric Administration for their insightful reviews of this manuscript. Finally, we thank R. Conner and L. Haseltine, Johnson Controls World Services for their valuable assistance in the final preparation of this manuscript.

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BIOSKETCHES Virginia Engle is an ecologist and Kevin Summers is a research ecologist, both with the U.S. Environmental Protection Agency’s Office of Research and Development at the Gulf Ecology Division. They currently work with the Environmental Monitoring and Assessment Program. Ms Engle has expertise in estuarine ecology and the analysis of environmental data. Her primary accomplishments include the development of an indicator of benthic condition for estuaries and a report on the ecological condition of estuaries in the Gulf of Mexico. Dr Summers has been conducting research in systems ecology and monitoring of estuarine ecosystems for the past 20 years. His primary contributions include the development of analytical methods and tools for environmental data, particularly indices of coastal eutrophication, estuarine integrity, and environmental sustainability.

 Blackwell Science Ltd 1999, Journal of Biogeography, 26, 1007–1023


Appendix I List of genera used in latitudinal cluster analyses. Phylum: Arthropoda Amphipods Acanthohaustorius Aeginina Americhelidium Ameroculodes Ampelisca Ampithoe Anamixis Argissa Batea Bathyporeia Bemlos Byblis Calliopius Caprella Caribboecetes Cerapus Chevalia Colomastix Corophium Cymadusa Deutella Dulichiella Dumosus Dyopedos Dyspanopeus Elasmopus Eobrolgus Ericthonius Eudevenopus Gammarus Gitanopsis Globosolembos Grandidierella Harpinia Ischyrocerus Jassa Lembos Lepidactylus Leptocheirus Leucothoe Listriella Luconacia Lysianopsis Phylum: Mollusca Bivalves Abra Agriopoma Aligena Amygdalum Anadara Anomalocardia Anomia Arctica Astarte Asthenothaerus Barbatia Barnea Brachidontes

Maera Melita Metharpinia Microdeutopus Microprotopus Monoculodes Mucrogammarus Neomegamphopus Orchomenella Paracaprella Parahaustorius Parametopella Paraphoxus Perioculodes Photis Phoxocephalus Plesiolembos Protohadzia Protohaustorius Pseudohaustorius Pseudunciola Rhepoxynius Rudilemboides Shoemakerella Stenopleustes Stenothoe Tethygenia Unciola Varohios Decapods Alpheus Biffarius Callianassa Callichirus Callinectes Cancer Crangon Euceramus Hexapanopeus Hippolyte Leptochela Libinia

Ogyrides Ovalipes Paguristes Pagurus Palaemonetes Panopeus Penaeus Pinnixa Polyonyx Portunus Rhithropanopeus Synalpheus Upogebia Isopods Amakusanthura Ancinus Carpias Chiridotea Cyathura Dynamenella Edotea Erichsonella Harrieta Idotea Kupellonura Limnoria Mesanthura Paracerceis Pleurogonium Ptilanthura Xenanthura

Other Arthropoda Achelia Almyracuma Americamysis Anoplodactylus Apseudes Axarus Balanus Bezzia Bodotria Bowmaniella Brachycercus Caenis Callipallene Chaoborus Chironomus Cladotanytarsus Coelotanypus Cryptochironomus Cubanocuma Cumella Cyclaspis Demicryptochironomus Diastylis Dicrotendipes Dubiraphia Endochironomus Eudorella Glyptotendipes Halmyrapseudes Hargeria Harnischia Heteromysis Hexagenia Hutchinsoniella Hydroptila Iungentitanais Kalliapseudes Leptochelia Leucon Lightiella Mancocuma Microchironomus Mysidopsis

Nanocladius Neomysis Oecetis Oxyurostylis Pagurapseudes Palpomyia Paranebalia Polypedilum Pontomyia Probezzia Procladius Pseudochironomus Pseudoleptochelia Pseudoleptocuma Pseudotanais Rheotanytarsus Sphaeromias Squilla Stictochironomus Tanaissus Tanypus Tanytarsus Vaunthompsonia

Other Phyla Mulinia Musculium Musculus Mya Mysella Mytilopsis Mytilus Noetia Nucula Ostrea Pandora Paramya Parastarte


Gastropods Acteocina Alvania Amnicola Amphithalamus Anachis Ascobulla Astraea Barleeia Bittiolum Bittium Boonea Bulla Busycotypus

Melongena Microeulima Mitrella Nassarius Odostomia Olivella Parvanachis Patelloida Physella Pilsbryspira Pleurocera Polinices Prunum

Acanthochitona Amphiodia Amphioplus Amphipholis Aspidosiphon Asterias Axiognathus Caudina Ceriantheopsis Dentalium Glottidia Havelockia Hemipholis


Appendix I continued Phylum: Mollusca—continued Bivalves—continued Bushia Cerastoderma Cf.raeta Chione Cooperella Corbicula Corbula Crassinella Crassostrea Crenella Cryptopleura Cyclinella Cyclocardia Cylichnella Diplodonta Donax Ensis Erycina Gemma Geukensia Glycymeris Hiatella Ischadium Laevicardium Lioberus Lucina Lyonsia Macoma Mercenaria Modiolus Phylum: Annelida Polychaeta Aglaophamus Amaeana Amastigos Ampharete Amphicteis Amphitrite Ancistrosyllis Anobothrus Antinoella Aphelochaeta Apoprionospio Arabella Aricidea Armandia Asabellides Autolytus Axiothella Bhawania Boccardiella Boguea Branchioasychis Branchiomma Branchiosyllis Brania Cabira

Other Phyla—continued Parvilucina Petricola Pisidium Pitar Pleuromeris Polymesoda Rangia Semele Solemya Solen Sphenia Spisula Tagelus Tellina Yoldia

Enoplobranchus Euchone Eumida Eunice Euphrosine Eupolymnia Exogone Fabricinae Fabricinuda Fimbriosthenelais Galathowenia Glycera Glycinde Goniada Goniadella Goniadides Grubeosyllis Gyptis Haplosyllis Harmothoe Hartmania Hemipodus Heteromastus Hobsonia Hydroides

Gastropods—continued Caecum Cerithiopsis Cerithium Conus Costoanachis Cratena Crepidula Diodora Doridella Epitonium Eubranchus Eulimastoma Eupleura Fargoa Ferrissia Finella Goniobasis Granulina Haminoea Henrya Hydrobia Ilyanassa Jaspidella Kurtziella Lacuna Laevapex Littoridinops Longchaeus Marginella Melanella

Malmgreniella Manayunkia Marenzelleria Marphysa Mastobranchus Mediomastus Melinna Mesochaetopterus Microphthalmus Microspio Monticellina Mooreonuphis Myxicola Naineris Neanthes Nematonereis Nephtys Nereiphylla Nereis Ninoe Notocirrus Notomastus Odontosyllis Onuphis Ophelina

Pyrgocythara Rictaxis Rissoina Sayella Schwengelia Seila Sinum Smaragdia Tectonatica Teinostoma Terebra Tricola Turbonilla Valvata Vermicularia Vitrinella Zebina

Ischnochiton Leptogorgia Leptosynapta Microphiopholis Ophiactis Ophiolepis Ophionereis Ophiophragmus Ophiopsila Ophiostigma Pentemera Phascolion Phoronis Saccoglossus Sclerodactyla Thalassema Thyonella

Pista Plakosyllis Platynereis Podarke Podarkeopsis Poecilochaetus Polycirrus Polydora Polygordius Potamilla Prionospio Proceraea Protodriloides Protodrilus Pseudeurythoe Pseudobranchiomma Pseudopolydora Pseudopotamilla Pseudovermilia Pygospio Questa Rullierinereis Sabaco Sabella Sabellaria

Syllis Taylorpholoe Terebellides Tharyx Thelepus Therochaeta Travisia Trochochaeta Vermiliopsis Oligochaeta



Appendix I continued Phylum: Annelida—continued Polychaeta—continued Capitella Carazziella Caulleriella Ceratonereis Chaetopterus Chaetozone Chone Cirriformia Cirrophorus Clymenella Cossura Dasybranchus Decamastus Demonax Diopatra Dispio Dorvillea Drilonereis

Hypereteone Inermonephtys Isolda Kinbergonuphis Laeonereis Laonome Leiocapitella Leitoscoloplos Lepidametria Lepidasthenia Lepidonotus Levinsenia Loimia Lysidice Lysilla Macrochaeta Macroclymene Magelona


Ophryotrocha Orbinia Owenia Paleanotus Parahesione Paranaitis Paraonis Parapionosyllis Paraprionospio Parougia Pectinaria Pettiboneia Pherusa Pholoe Phyllodoce Pionosyllis Piromis

Scalibregma Scolelepis Scoletoma Scoloplos Scyphoproctus Sigalion Sigambra Sphaerosyllis Spio Spiochaetopterus Spiophanes Spirorbis Sternaspis Sthenelais Streblosoma Streblospio Streptosyllis