P. M. Detwiler, Marie F. Coe & Deborah M. Dexter. Department of Biology, San Diego State ...... Watling, 1975. Virginia estuary. 140 000. Virnstein, 1979.
Hydrobiologia 473: 139–160, 2002. D.A. Barnum, J.F. Elder, D. Stephens & M. Friend (eds), The Salton Sea. © 2002 Kluwer Academic Publishers. Printed in the Netherlands.
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The benthic invertebrates of the Salton Sea: distribution and seasonal dynamics P. M. Detwiler, Marie F. Coe & Deborah M. Dexter Department of Biology, San Diego State University, San Diego, CA 92182, U.S.A. Key words: saline lakes, Neanthes succinea, Balanus amphitrite, Gammarus mucronatus, Corophium louisianum, Streblospio benedicti, Thalassodrilides
Abstract The Salton Sea, California’s largest inland water body, is an athalassic saline lake with an invertebrate fauna dominated by marine species. The distribution and seasonal dynamics of the benthic macroinvertebrate populations of the Salton Sea were investigated during 1999 in the first survey of the benthos since 1956. Invertebrates were sampled from sediments at depths of 2–12 m, shallow water rocky substrates, and littoral barnacle shell substrates. The macroinvertebrates of the Salton Sea consist of a few invasive, euryhaline species, several of which thrive on different substrates. The principal infaunal organisms are the polychaetes Neanthes succinea Frey & Leuckart and Streblospio benedicti Webster, and the oligochaetes Thalassodrilides gurwitschi Cook, T. belli Hrabe, and an enchytraeid. All but Neanthes are new records for the Sea. Benthic crustacean species are the amphipods Gammarus mucronatus Say, Corophium louisianum Shoemaker, and the barnacle Balanus amphitrite Darwin. Neanthes succinea is the dominant infaunal species on the Sea bottom at depths of 2–12 m. Area-weighted estimates of N. succinea standing stock in September and November 1999 were two orders of magnitude lower than biomass estimated in the same months in 1956. During 1999, population density varied spatially and temporally. Abundance declined greatly in offshore sediments at depths >2 m during spring and summer due to decreasing oxygen levels at the sediment surface, eventually resulting in the absence of Neanthes from all offshore sites >2 m between July and November. In contrast, on shoreline rocky substrate, Neanthes persisted year round, and biomass density increased nearly one order of magnitude between January and November. The rocky shoreline had the highest numbers of invertebrates per unit area, exceeding those reported by other published sources for Neanthes, Gammarus mucronatus, Corophium louisianum, and Balanus amphitrite in marine coastal habitats. The rocky shoreline habitat is highly productive, and is an important refuge during periods of seasonal anoxia for Neanthes and for the other invertebrates that also serve as prey for fish and birds. Introduction The Salton Sea has great ecological importance within the Pacific Flyway due in part to the abundant invertebrate populations that serve as a food base for migratory and resident bird species. Restoration objectives of the Salton Sea Reclamation Act of 1998 recognize this importance, and mandate the maintenance of habitat components for waterfowl as well as the present fishery. The infaunal polychaete Neanthes succinea is the key benthic link between detritus accumulating on the sediments and higher trophic level organisms, including predaceous birds and fish. Neanthes constitutes a major portion of the diet of
adult bairdiella (Bairdiella icistia Jordan & Gilbert) and juvenile corvina Cynoscion xanthulus Jordan & Gilbert (Quast, 1961; Whitney, 1961) and is heavily preyed upon by eared grebes (Podiceps nigricollis Brehm) staging at the Sea (J. Jehl, pers.com.). Despite the importance of Neanthes within the Sea, few in situ studies on the species have been attempted. The abundance of Neanthes in offshore sediments was last estimated in 1956 by Carpelan & Linsley (1961a), and monthly abundance of Neanthes at one shoreline location was recorded throughout 1972 when salinity was 38 g l−1 (Reilly, 1974). More recent surveys of the Salton Sea biota examined contaminants and potential toxins associated with invertebrate fauna, but did
140 not include quantitative information on distribution, abundance, or seasonal trends (Setmire et al., 1990, 1993; Schroeder et al., 1993). Furthermore, there is no information on the seasonality of the invertebrate communities associated with the rocky substrates and barnacle shell substrates found along the perimeter of the Sea. From the 1930s to 1961, California Department of Fish and Game introduced at least 29 species of marine invertebrates into the Sea to develop potential commercial fisheries and to create a food base for emerging sportfisheries (Carpelan & Linsley, 1961a). Of those species, only Neanthes succinea remains, persisting under highly eutrophic conditions, a 19– 21 ◦ C range in seasonal mean water temperature, and sulfate concentration nearly 3 times greater than in seawater (S. Hurlbert, unpubl. data; Watts et al., 2001). The present species composition of benthic macroinvertebrates represents inocula from the Gulf of California, Pacific Ocean, and the Gulf of Mexico. As adults, Neanthes and the other invertebrate species provide food for fish and birds that forage on the bottom; their juveniles and reproductive stages provide food for fish feeding in the water column and at the water surface. Because there are so few species in the food chain, changes in the physical and chemical environment in the Salton Sea have the potential for destabilizing the food base, adversely affecting fish and birds. The purpose of this study is to document the species composition, abundance, and seasonality of macroinvertebrates associated with specific benthic habitats. We sampled the sublittoral sediments of the Sea at depths of 2–12 m, and the shoreline habitats comprising barnacle shell substrates, barnacle-covered rocks, and algae-covered rocks. Also, we determined the seasonal abundance of swarming heteronereids of Neanthes succinea to quantify their availability to fish and birds foraging at night.
Description of major species Neanthes succinea is a euryhaline nereid polychaete first reported in the Sea by Hartman (1936) and was probably introduced in 1930 from Mission Bay, San Diego (Carpelan & Linsley, 1961a). At sexual maturity, benthic adults metamorphose into nektonic heteronereids, which are distinguished from nonreproductive adults by the presence of natatory setae and an enlarged medial region for gamete storage.
Reproduction occurs when heteronereids swim to the surface and release gametes through ruptures in the body wall. After spawning, individuals die. Fertilized eggs develop within 24 h into pelagic trochophore larvae which settle on the bottom after approximately 2 weeks (Carpelan & Linsley, 1961b). Balanus amphitrite Darwin is a common midlittoral barnacle found throughout temperate and tropical waters, and tolerates eutrophic and polluted marine environments and salinity levels of up to 78 g l−1 (Simmons, 1957; Calcagno et al., 1998). Introduction of this species into the Salton Sea probably occurred in 1944 via military seaplanes or buoys brought in from the Pacific coast (Cockerell, 1945; Rogers, 1949). The amphipod Gammarus mucronatus Say is found throughout the Atlantic Seaboard (Quebec to the Gulf of Mexico) inhabiting estuaries, other brackish intertidal zones, and hypersaline lagoons. This crustacean is often associated with seagrasses and/or chlorophytes (Barnard & Gray, 1968; Bousfield, 1973). In the Salton Sea it is epibenthic in the littoral zone, and also neritic. It most likely became established in the Salton Sea after 1956, as no amphipod was recorded by Carpelan & Linsley (1961a) during their study. In the laboratory, Gammarus reproduces over increasing salinities of Salton Sea water up to 65 g l−1 (D. Dexter & P. McNair, unpubl. data). This study confirms the tentative identification by Barnard & Gray (1968) of Corophium louisianum Shoemaker in the Salton Sea. Corophium is a euryhaline amphipod distributed along the entire Gulf coast, and is found in brackish and marine waters up to 75 g l−1 (Hedgpeth, 1959). In those locations, C. louisianum lives in self-constructed tubes attached to roots of marsh grass or other firm substrates, and forms an important component in the diet of estuarine fish (Thomas, 1976; Heard, 1982; Rozas & LaSalle, 1990). Both C. louisianum and G. mucronatus may have been introduced into the Salton Sea in 1957 when California Dept. Fish and Game planted stands of shoal grass (Diplantha wighti) from Texas for wildfowl food (Barnard & Gray, 1968). The spionid polychaete Streblospio benedicti Webster inhabits the upper 2–3 cm of muddy to slightly sandy sediments in marine environments throughout the world and can recruit to disturbed and polluted environments (McCall, 1977; Pearson & Rosenberg, 1978; Levin, 1986; Chandler et al., 1997). This species can exploit both suspension and surface-deposit feeding (Levin, 1986). The presence of Streblospio represents a new report for the Salton Sea, and pos-
141 sibly a new salinity record for this species. Based on its lecithotrophic mode of development (L. Levin, pers. com.), the origin of the Salton Sea population appears to be the Pacific Coast of North America. The tubificid oligochaetes Thalassodrilides belli Cook and T. gurwitschi Hrabe are commonly found in marine and brackish sediments both intertidally and subtidally throughout the Caribbean and the Gulf of Mexico, while T. gurwitschi also has a circumtropical distribution (Milligan, 1986; Erséus, 1990). The enchytraeid could not be identified from preserved specimens, but is most likely a species of Marionina, a genus with a cosmopolitan distribution commonly found in intertidal and supralittoral habitats (Giere & Pfannkuche, 1982). We did not quantitatively sample for other marine taxa present in the Sea (the turbellarian Macrostomum pusillum Ax, the ostracod Cyprideis beaconensis LeRoy, and the harpacticoid copepod Cletocamptus deitersi Richard). Thirteen putative species of nematodes were distinguished in benthic samples collected from a variety of habitats, and are discussed in Warwick et al. (2002). Two non-marine insects, the corixid Trichocorixa reticulata Guérin–Menéville and larvae of the brine fly Ephydra cinerea Jones, can be abundant within hypersaline pools in shoreline regions around the Sea, but are rarely found in the Sea proper, and thus were not included in our study. Scanning electron micrographs of all these species are presented in Kuperman et al. (2002). Methods Benthic fauna of the soft offshore sediments Benthic grab samples were taken at stations along 3 transects extending out from shore. These transects were selected in relation to the three stations (S1, S2, S3) used for plankton sampling and water quality monitoring during 1997–1999 (Fig. 1). Transect 1 began at the State Recreation Area and extended along a course of 165◦ toward S1. Transect 2 began offshore of Bombay Beach and extended on a course of 261◦ toward S2. Transect 3 began off of the shoreline approximately 8 km south of Bombay Beach, and extended on a course of 244◦ toward S3. Stations S1, S2 and S3 are located at 115◦ 55 W, 33◦ 25 N (9.6 km south of State Recreation Area); 115◦ 5l W, 33◦ 21 N (8.0 km northeast of Salton City); and 115◦ 48 W, 33◦ 18 N (9.0 km east of Salton City), respectively. The angle of Transect 3, the southernmost
transect, was selected with the expectation that laminated sediments reported by Walker (1996) would be encountered along that trajectory. The infaunal benthos was sampled on January 18– 19, March 27, April 16–17, May 19, July 13, September 27–28, and November 20, 1999. Triplicate grab samples were taken along each transect at each of 6 stations corresponding to depths of 2, 4, 6, 8, 10, and 12 m. We used a petite Ponar grab, which samples an area of 15 × 15 cm to a depth of 15 cm. The grab always filled during our sampling. GPS coordinates for each station were recorded on each sampling date. Because the sediment was impenetrable at Station 2 on Transect 3 in January, the replicates for that station were obtained at a slightly deeper (2.3 m) location nearby. The contents of each grab were rinsed on a 1000 µ sieve and material remaining on the screen was preserved in a solution of buffered 3.4% formaldehyde with rose bengal added. On March 27, stations 4 to 12 m on Transect 3 were sampled but the Ponar grab was lost at station 2 m as a result of a cable break. Thus, T3-2 m and Transects 1 and 2 were sampled on April 16–17. The counts of benthic invertebrates for the March and April sampling dates were not treated separately during data analysis. Sediment analysis Sediment characteristics can be important factors affecting the distribution of infaunal invertebrates. Replicate samples of the top 15 cm of sediment were collected at each station in March and September. Total carbon, total nitrogen, and organic carbon content were determined using a CE Instruments NCS 2500 elemental analyzer calibrated against Montana Pine standard. Particle size analysis was performed on sediments collected in March utilizing a combination of wet sieving through a US standard sieve series and the hydrometer technique (Cox, 1996). A Mettler electronic balance was used to weigh dried sediment fractions to the nearest mg to determine size distributions. Macrozooplankton tows On each sample date beginning at sunset, we conducted 3 replicate tows for macrozooplankton to determine the abundance and biomass of nektonic heteronereids of Neanthes succinea. A 1000 µ mesh net (0.8 m in diameter, 2.5 m long) was towed along a course of 280◦ from the mouth of Varner Harbor at
142
Figure 1. Location of sites sampled bimonthly throughout 1999. Benthic transects are numbered 1–3. Barnacle sand sampling occurred at North Shore, Salt Creek, and Bombay Beach. Rocky substrata were sampled at Red Hill Marina
0.8 km/h for 10 min, sampling ∼67 m3 if we assume 100% filtration efficiency. Water depth along this trajectory ranged from 2.7 m to 6.2 m. Organisms were preserved on site in a solution of buffered 3.4% formaldehyde with rose bengal added. The lengths of heteronereids from each collection were measured to the nearest mm. In collections where there were >50 heteronereids, 50 individuals were randomly selected for measurement using a plankton splitter. Benthic flora and fauna associated with shoreline littoral substrates Rocky substrates Hard substrates available in the Salton Sea for epifaunal colonization consist of rocks, submerged structures, construction materials, imported fill (e.g. con-
crete riprap) and precipitated calcium carbonate deposits. Quadrat sampling of invertebrates on hard substrates was carried out on the southern jetty at Red Hill Marina on two substrate types: rocky substrate lacking macroscopic algae (and dominated by Balanus amphitrite) and rocky substrate dominated by the chlorophytes Chaetomorpha linum Müller and Enteromorpha intestinalis (L.) Link. The locations of five quadrats on barnacle-covered rock were haphazardly selected along the exposed side of the jetty, while five quadrats on algae-covered rock were selected along both the exposed and protected sides of the jetty. Quadrat sites were restricted to smooth surfaces between 0 and 90◦ slope. All samples were collected within 15 cm of the water surface, and sites were selected at least 3 m apart from each other.
143 Each quadrat was sampled using a scraping device constructed specifically for this purpose. The device is a stainless steel box measuring 15 × 15 × 5 cm. The bottom surface of the box has a sharpened flange directed 45◦ downward, creating a 10 × 1.5 cm opening into which the sample is scraped. The top face of the box is screened with 1000 µ mesh to prevent loss of organisms, and is hinged to allow removal of collected material. On both sides of the box are handles 10 cm long, which were used to delimit a 10 × 10 cm area for scraping. Samples were preserved in a solution of buffered 3.4% formaldehyde with rose bengal added. Barnacle shell substrates A major shoreline habitat all around the Salton Sea, especially at very shallow depths (less than 0.3 m), consists of intact and broken barnacle shells and associated unconsolidated sediments, ranging in particle size from coarse sand to gravel. Invertebrates present in barnacle shell substrates were sampled on open shoreline sites at Bombay Beach, Salt Creek, and State Recreation Area Headquarters (Fig. 1). Three replicate samples at each site were collected using a stainless steel corer (area 0.01 m2 ) which penetrated the sediment 10 cm. Samples were collected at 0.3 m depth, within 0.5 m of the shoreline. Each sample was rinsed on a 1000 µ sieve, and all of the remaining material on the sieve was preserved in the field with a solution of buffered 3.4% formaldehyde with rose bengal added. In the laboratory, each sample was divided using a Folsom plankton splitter to obtain a subsample with 1/8th the original volume of the field-collected sample. This sample was transferred into 70% ethyl alcohol for later enumeration of organisms. Sample processing In the laboratory, organisms were separated from the associated debris, identified under a dissecting microscope, and transferred to 70% ethyl alcohol. Count data of polychaetes and amphipods represented intact individuals and head segments. Specimens were identified to the lowest taxonomic level possible. In the case of unknown species, representative specimens were sent to taxonomic specialists for further identification. Complete collections of preserved Neanthes from grabs and from all quadrats collected on algae-covered rocks were blotted on filter paper and weighed on an analytical balance to determine weight per indi-
vidual and biomass per m2 . Individual Neanthes were weighed live and after preservation to determine a factor to convert alcohol wet weight biomass to wet weight: Wet weight = alcohol wet wt. × 1.0649. Wet weights were averaged over transects, and the overall mean was multiplied by geometric mean density of Neanthes along each transect per date, then those numbers were averaged into the means plotted in Figure 3B. The proportion of gravid amphipods collected from algae covered rock quadrats was determined on each sampling date by examining individuals under the dissecting microscope for the presence of eggs or young in the brood pouch. Data analysis For the various sorts of abundance data, geometric means were calculated. The first step was to add a constant (c) to each raw datum, corresponding to the smallest non-zero value possible based upon the sampling and counting methods used. When the entirety of each sample is examined and results are reported as number per sample, the constant is 1, as in the case of grab and quadrat sampling. For macrozooplankton tows, the constant is 0.015 and for sand cores, the constant is 8. For weight data, the constant corresponds to the weight of the smallest individual encountered (0.0015 g). Standard errors were determined from log transformed data. Backtransformation of the standard error (SE) from log units yields the standard error factor (SEF). The value of SEF includes the constant. Thus for geometric means, the upper and lower boundaries (B) of the ‘mean ± standard error’ are obtained as follows: B upper = [(Geom. Mean) × SEF] – c B lower = [(Geom. Mean) ÷ SEF] – c Values for Neanthes density in offshore sediments represent the mean of 9 replicates taken on each sampling date at each depth. Standard errors are based on 3 means, one from each transect. To estimate the area-weighted standing stock of Neanthes on each sample date, we used lake surface area as a proxy for bottom area. First, we computed the area of lake surface that would be exposed if the lake elevation dropped 2, 4, 6, 8, 10 and 12 m in depth, using recent bathymetric data (Ferrari & Weghorst,
144 1997). We assumed an initial lake elevation of −69 m sea level. Next, we subtracted those areas from the present lake area to obtain areas of lake bottom lying at 0–2, 2–4, 4–6, 6–8, 8–10,10–12, and 12– 14 m depth. Each final area was multiplied by the mean density of Neanthes at each depth, and this product was then multiplied by the average mass per individual at each station. We assumed that numerical and biomass density of Neanthes at ≥12 m were equivalent to values at 12 m, and that 14 m represented the bottom of the Sea. Estimated standing stock of Neanthes in 1956 was recomputed using more accurate measurements of area as described above. September and November 1956 biomass density data were separately analyzed. Data from 2 m depth intervals were converted to geometric means for comparison to 1999 data. When there was no data for a given depth, biomass density at the next lower depth was used. Mean values at a given depth were multiplied by the area of lake bottom at that depth, and all depth values summed to determine standing stock. Water column and sediment properties Oxygen concentration and temperature values reported in Figure 3 represent the means of all temperature and oxygen measurements recorded from the mid-lake vertical profiles along S1, S2, and S3 for each month when benthic sampling occurred (Watts et al., 2001, see their ‘Methods’). We assumed that the oxygen concentration at the mid lake stations at a particular depth represents the mean oxygen concentration on the sediment surface at the depth we sampled. We compared differences in carbon, nitrogen, and C:N ratios of sediments collected in March and September by multiple t tests on means of log-transformed data, and compared the relationship between sediment elemental content and depth using Spearmann rank correlation analysis.
Results Sediment characteristics The sediment characteristics of all stations sampled by grab are shown in Figure 2. Most of the sediments at depths below 2 m consist of coarse silt (mean particle size 59 µ). While there was little evidence of difference in mean silt content of sediments at T2 and T3 stations between 4 and 12 m (P = 0.11, paired t test), mean silt content of 4–12 m sediments along T1 was
Figure 2. Sediment composition of samples from six stations along three transects in the Salton Sea, California in March 1999.
lower than along T2 and T3. Sediment characteristics at 2 m differed markedly among transects. At T1-2 m, a surprising finding was that much of the substrate was composed of loose clusters of live barnacles at high densities (up to 18 026 m−2 ), and this precluded the determination of C and N content of those sediments. At T2-2 m, the bottom was sandy with moderately-sorted sediments, while the sediments at T3-2m consisted of moderately-sorted clay. Mean total carbon content, organic carbon content, nitrogen content, and C:N ratio all varied between locations and sampling dates (Table 1). Along T3, both mean total carbon and nitrogen content were higher in March than in September, but distinctive seasonal differences were not apparent along the other two
145 Table 1. Seasonal changes in carbon and nitrogen content of sediments sampled along three transects in the Salton Sea, California. Nitrogen and carbon content was not determined for T1–2 sediments. Values presented are means of 4 replicates. Correlation coefficients relating elemental content to depth are compared below. 2 m stations were not included in the calculation of mean values Station
Total Carbon Content (mg C g dry wt−12 ± SD) March Sept.
P
Organic Carbon Content (mg C g dry wt−12 ± SD) March Sept.
P
Nitrogen Content (mg C g dry wt−12 ± SD) March Sept.
P
C:N molar ratio ± SD March Sept.
P
1-4 1-6 1-8 1-10 1-12
5.98±0.13 6.36±0.41 6.78±0.11 7.46±0.29 7.21±0.60 Mean 6.76±0.60 r 0.80 P 0.000
7.22±0.15 7.11±0.12 7.09±0.47 8.87±0.34 8.41±0.86 7.74±0.84 0.70 0.001
2.82±0.51 5.43±0.69 5.42±0.85 6.37±0.19 6.66±0.28 0.06 5.34±1.51 0.73 0.000
5.97±0.14 5.48±0.19 5.26±0.41 7.58±0.26 7.02±0.34 6.26±1.00 0.55 0.013
0.46±0.04 0.50±0.06 0.45±0.01 0.56±0.10 0.52±0.02 0.29 0.50±0.05 0.39 0.088
0.41±0.08 0.35±0.04 0.25±0.06 0.52±0.07 0.48±0.06 0.40±0.11 0.43 0.057
15.2 ± 1.32 14.9 ± 2.16 17.4 ± 0.70 15.9 ± 3.53 16.1 ± 0.72 0.12 15.9 ± 0.98 0.18 0.45
21.22± 23.83± 34.16± 20.15± 20.60± 23.99± −0.23 0.32
2-2 2-4 2-6 2-8 2-10 2-12
0.60±0.07 3.21±0.20 4.13±0.06 5.69±0.16 5.51±0.22 6.04±0.36 Mean 4.92±1.19 r 0.91 P 0.000
1.13±0.03 4.17±0.24 5.16±0.56 4.15±0.28 4.16±0.13 7.81±0.66 5.09±1.58 0.63 0.001
0.24±0.04 1.78±0.14 2.35±0.05 3.17±0.27 3.37±0.14 4.44±0.77 0.29 3.02±1.02 0.97 0.000
0.57±0.05 2.10±0.22 2.53±0.73 2.93±1.04 3.58±0.53 7.64±0.33 3.76±2.24 0.90 0.000
no data 0.29±0.06 0.32±0.02 0.26±0.02 0.29±0.01 0.36±0.01 0.18 0.30±0.04 0.39 0.09
0±0A 0.08±0.03 0.11±0.03 0.10±0.03 0.15±0.02 0.51±0.07 0.19±0.18 0.62 0.003
13.5 ± 2.97 14.9 ± 1.12 26.0 ± 1.16 22.1 ± 1.33 19.5 ± 0.84 0.21 19.2 ± 5.12 0.56 0.01
69.3 ± 28.06 55.15± 11.73 51.83± 14.77 31.98± 3.76 18.12± 1.50 45.3 ± 20.2 0.02 −0.75 0.000
3-2 3-4 3-6 3-8 3-10 3-12
3.32±0.31 5.40±0.21 5.62±0.40 5.53±0.46 5.81±0.73 6.31±0.54 Mean 5.73±0.35 r 0.72 P 0.000
2.78±0.08 4.54±0.33 4.52±0.06 3.77±0.10 3.84±0.26 4.40±0.61 4.21±0.38 0.21 0.32
0.69±0.17 2.03±0.25 2.95±0.44 2.98±0.62 3.23±0.23 5.17±0.26 0.00 3.27±1.15 0.91 0.000
1.14±0.10 2.05±0.08 2.20±0.06 2.15±0.14 2.52±0.14 3.97±0.69 2.58±0.08 0.92 0.000
0.10±0.01 0.18±0.01 0.20±0.01 0.23±0.01 0.27±0.02 0.38±0.02 0.48 0.25±0.08 0.98 0.000
0.01±0.01 0.05±0.04 0.09±0.02 0.10±0.04 0.15±0.03 0.26±0.07 0.13±0.08 0.93 0.000
40.4 ± 0.82 141.8 ±195.7 34.8 ± 1.33 64.43± 46.92 32.6 ± 1.38 58.35± 11.75 28.6 ± 0.68 47.49± 17.05 24.7 ± 1.73 30.58± 3.83 19.1 ± 0.77 20.69± 2.75 0.04 27.96± 6.2 44.3 ± 18.4 0.16 −0.97 −0.95 0.000 0.000
4.07 2.41 6.03 1.79 0.76 5.86 0.01
a Below detection limits.
transects. Sediment organic carbon content remained similar between seasons along all transects. Mean nitrogen and total carbon content at T1 at any given depth on both dates (n = 10) was up to 2.5 times higher than at the same depth along T2 and T3. Total carbon and organic carbon content were lowest in the sandy sediments at T2-2 m. Total carbon increased with depth along all 3 transects in March, but this relationship decoupled along T3 in September. Organic carbon increased with depth along all transects in both March and September, while nitrogen content increased with depth at each transect in September. C:N ratios were consistently greater in September than in March, and showed clear decreases with depth along T3 on both dates, but less obvious patterns at the other transects.
Benthos of soft sediments Neanthes succinea was the dominant benthic species, present at all depths along the three transects during late winter and early spring (Fig. 3). Density of Neanthes was consistently highest at the 2 m stations, particularly in July at 2 m (8325 ind m−2 ), and biomass density at that depth was also greatest (32.19 g m−2 ). However, density declined over the summer throughout the lake, as oxygen content decreased and mean water temperature remained elevated. In July, worms were present only at 2 and 4 m stations. By September, pileworm density was reduced 95% relative to July, and worms were present only at the 2 m stations. By November, Neanthes had begun to recolonize sediments at 12 m. During January and March, mean biomass of individual Neanthes from 4 to 12 m stations was 3 times greater than for specimens collected at 2 m
146
Figure 3. Variation of Neanthes density, oxygen concentration, and temperature with depth on six sampling dates, 1999. Each density value at each depth represents the geometric mean ± SE of 3 transects, and includes the constant (c=1). Oxygen concentrations and temperature at sediment surface are estimated from the mid-lake vertical profiles of Watts et al. (2001) (see ‘Methods’).
depth (Table 2). Mean size and mean biomass per m2 were greatest in March, and lowest in September (Fig. 4A,B). The standing stock of Neanthes was highest in March, and about 6.5% of the total standing stock was found between 0 and 4 m depth, increasing to 41% in May. From July to November, 99% of the standing stock was restricted to sediments 0–4 m depth (Table 2). During September, there was little correlation between density of Neanthes and sediment organic content (Spearmann r = −0.40, P = 0.12). The other infaunal macroinvertebrates present in offshore sediments were the oligochaete Thalassodrilides belli and the spionid Streblospio benedicti. Thalassodrilides belli was observed only once during 1999 (within barnacle clusters at T1–2 m in July) at
high density (13 542 m−2 ). Streblospio was found in sporadically and in low numbers at shallow (2 & 4 m) stations along T1 and T2, but was always present at station T3-2 m, where clays composed nearly 40% of the sediments. Maximum density of Streblospio reached 2797 m−2 in January 1999. On rare occasions, Gammarus was present at 2 m stations but at low density (
