Coral Reefs (2001) 19: 318±329 DOI 10.1007/s003380000122
R EP O RT S
R. W. Thacker á D. W. Ginsburg á V. J. Paul
Effects of herbivore exclusion and nutrient enrichment on coral reef macroalgae and cyanobacteria
Accepted: 25 September 2000 / Published online: 10 February 2001 Ó Springer-Verlag 2001
Abstract Although phase shifts on coral reefs from coral-dominated to algal-dominated communities have been attributed to the eects of increased nutrient availability due to eutrophication and reduced herbivore abundance due to over®shing and disease, these factors have rarely been manipulated simultaneously. In addition, few studies have considered the eects of these factors on benthic, ®lamentous cyanobacteria (bluegreen algae) as well as macroalgae. We used a combination of herbivore-exclusion cages and nutrient enrichment to manipulate herbivore abundance and nutrient availability, and measured the impacts of these treatments on macroalgal and cyanobacterial community structure. In the absence of cages, surface cover of the cyanobacterium Tolypothrix sp. decreased, while surface cover of the cyanobacteria Oscillatoria spp. increased. Cyanobacterial cover decreased in partial cages, and Tolypothrix sp. cover decreased further in full cages. Lower cyanobacterial cover and biomass were correlated with higher macroalgal cover and biomass. Dictyota bartayresiana dominated the partial cages, while Padina tenuis and Tolypiocladia glomerulata recruited into the full cages. Palatability assays demonstrated that herbivore-exclusion shifted macroalgal species composition from relatively unpalatable to relatively palatable species. Nutrient enrichment interacted
R. W. Thacker (&)1 á D. W. Ginsburg2 á V. J. Paul Marine Laboratory, University of Guam, Mangilao, Guam 96923, USA E-mail:
[email protected] Tel.: +1-205-934-4006; Fax: +1-205-975-6097 Present addresses: Department of Biology, University of Alabama at Birmingham, CH 267, 1530 3rd Avenue South, Birmingham, Alabama 35294±1170, USA 1
2
Department of Biological Sciences, University of Southern California, Los Angeles, California 90089, USA
with herbivore exclusion to increase the change in cover of D. bartayresiana in the uncaged and fully caged plots, but did not aect the ®nal biomass of D. bartayresiana among treatments. Nutrient enrichment did not signi®cantly aect the cover or biomass of any other taxa. These results stress the critical role of herbivory in determining coral reef community structure and suggest that the relative palatabilities of dominant algae, as well as algal growth responses to nutrient enrichment, will determine the potential for phase shifts to algal-dominated communities. Key words Eutrophication á Herbivory á Algae á Community ecology
Introduction Phase shifts on coral reefs from coral-dominated to algal-dominated communities have been attributed to the eects of increased nutrient availability due to eutrophication and reduced herbivore abundance due to over®shing and disease (Littler and Littler 1984; Done 1992; Hughes 1994; Lapointe 1997). Although the relative importance of these factors has been debated (e.g., Hughes et al. 1999; Lapointe 1999; McCook 1999; Miller et al. 1999; Aronson and Precht 2000), the role of herbivores in determining the abundance and species composition of macroalgae on coral reefs is well-documented (Carpenter 1986, 1997; Lewis 1986; Hay 1991, 1997; Hixon 1997). Herbivory can increase community diversity by removing dominant competitors (Lubchenco 1978; Menge and Farrell 1989), by clearing substrates for new individuals (Menge and Lubchenco 1981), and by maintaining equilibria between competing species (Gleeson and Wilson 1986). Herbivory can also decrease community diversity by selectively removing preferred prey (Lubchenco and Gaines 1981) and altering rates of succession (Hixon and Brosto 1996; Kim 1997; McClanahan 1997).
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The eects of eutrophication on macroalgal abundance and community structure have also been demonstrated. Increases in nutrient availability can dramatically increase macroalgal growth rates and biomass (Fong et al. 1993; Lapointe 1999) and remove the competitive advantage of zooxanthellate corals over seaweeds (Dubinsky and Stambler 1996). Nutrient enrichment can also change the species composition of macroalgal communities (Lukatelich and McComb 1986; Lavery et al. 1991). Increased nutrient availability due to upwelling has resulted in overgrowth of corals by the green alga Enteromorpha sp. in the Red Sea (Genin et al. 1995), while eutrophication from sewage was correlated with the dominance of the macroalga Dictyosphaeria cavernosa in Kaneohe Bay, Hawaii (Smith et al. 1981; Hunter and Evans 1995). Studies of nutrient uptake rates and nutrient-limited growth have demonstrated the physiological mechanisms by which nutrient enrichment alters macroalgal growth (Littler et al. 1988; Littler and Littler 1990; Peckol et al. 1994; Larned 1998; Schaelke and Klumpp 1998). Investigations of phase shifts on coral reefs have often overlooked benthic, ®lamentous cyanobacteria (bluegreen algae), which generally have been grouped with turf algae in ecological research (e.g., Steneck and Dethier 1994). Benthic cyanobacteria may play an important role in these phase shifts, as they can be early colonizers of dead coral and disturbed substrates (Tsuda and Kami 1973; Borowitzka et al. 1978; Larkum 1988). Large mats of the cyanobacterium Lyngbya wollei can dominate benthic fresh-water communities following disturbance (Cowell and Botts 1994; Doyle and Smart 1998). Once these mats are established, their modi®cation of pH and carbonate concentrations can prevent recolonization by other macrophytes (Cowell and Botts 1994; Doyle and Smart 1998). Miller et al. (1999) found that marine cyanobacterial abundance increased with a combination of reduced herbivory and nutrient enrichment. Although rapid increases in the abundance of benthic cyanobacteria may be caused by factors similar to those that in¯uence macroalgal blooms, the eects of eutrophication and reduced herbivory on benthic cyanobacteria are not well understood. Despite experiments that demonstrate enhanced cyanobacterial growth rates after nutrient enrichment (Fong et al. 1993), cyanobacterial abundance often is not positively correlated with either nitrogen or phosphorus availability (Cowell and Botts 1994; Thacker and Paul 2001). The role of herbivory in determining the diversity and abundance of benthic, ®lamentous cyanobacteria may be limited by the wide variety of secondary metabolites that they produce, many of which are toxic or pharmacologically active (Moore 1981, 1982; Nagle and Paul 1999). Compounds produced by marine cyanobacteria deter feeding by several species of herbivorous ®shes (Paul and Pennings 1991; Pennings et al. 1996, 1997; Thacker et al. 1997; Nagle and Paul 1998, 1999), sea urchins (Pennings et al. 1997; Nagle and Paul 1998) and crabs (Pennings et al. 1997). Although defensive compounds may reduce
the direct eects of herbivory on cyanobacteria, indirect eects of herbivory on macroalgae may strongly aect cyanobacterial abundance. Selective browsing by herbivorous ®shes on relatively palatable macroalgae may remove these potential competitors and favor the establishment of relatively unpalatable cyanobacteria (Tsuda and Kami 1973). A high abundance of macroalgae and cyanobacteria and a low abundance and diversity of herbivorous ®shes characterize Guam's reef ¯ats and patch reefs. However, large recruitment pulses of juvenile rabbit®shes (Siganus spinus and S. argenteus) can occur in April and May each year (Kami and Ikehara 1976). These rabbit®shes can dramatically reduce the abundance of macroalgae and seagrasses, profoundly aecting community structure (Tsuda and Bryan 1973; Amesbury et al. 1993). To better understand the combined eects of nutrient enrichment and reduced herbivory on cyanobacteria and macroalgae, we conducted a ®eld experiment that simultaneously excluded herbivorous ®shes and enriched local nitrogen and phosphorus availability. We hypothesized that reduced herbivory would shift the species composition of the community from unpalatable macroalgae and cyanobacteria to palatable macroalgae, and that nutrient enrichment would increase the biomass of these taxa.
Methods Site description Our manipulations were conducted at Marc's Reef, a patch reef in Cocos Lagoon, Guam. Nutrient concentrations at Val's Reef, another patch reef in Cocos Lagoon approximately 100 m from Marc's Reef, have been monitored as part of the US EPA ECOHAB program for the past 18 months. During that time, phosphate concentrations have generally been low, ranging from below detectable limits to a maximum of 0.19 lM and only exceeding 0.1 lM on 2 of 12 sampling days. Nitrate concentrations have also been low, ranging from below detectable limits to a maximum of 0.79 lM and only exceeding 0.5 lM on 1 of 12 sampling days (Thacker and Paul 2001). Ammonium concentrations were not monitored regularly because a pilot study found that 70% of total nitrogen was represented by nitrate and nitrite, while only 8% was represented by ammonium (Thacker and Paul 2001). Common ®lamentous cyanobacteria at this site include Tolypothrix sp. and Oscillatoria spp., while common macroalgae include Dictyota bartayresiana, Padina tenuis, and several species of Halimeda and Caulerpa (hereafter, these species will be referred to by genus). We timed our study to begin shortly before the predicted rabbit®sh recruitment in April 1999 and continued to monitor our manipulations for 4 months. Initial cover measurements were made on 13 April 1999 and ®nal cover measurements were made on 16 August 1999. Experimental approach A two-way factorial experiment was used to test for the eects of herbivory and nutrient addition on cyanobacterial and macroalgal community composition. We manipulated herbivory with three levels of herbivore-exclusion: fully caged plots, partially caged plots, and uncaged plots. The partially caged plots were used to control for shading and reduced water ¯ow in the fully caged plots (Steele 1996). We used two levels of nutrient enrichment: with and without bags of fertilizer placed in the center of each plot. The reef
320 area available limited our design to ten replicates for each of the six treatments. To control for dierences in macroalgal and cyanobacterial cover across the reef, a randomized block design was used. Treatments within blocks were placed a maximum of 1 m apart. Blocks were a minimum of 2 m apart. Uncaged experimental plots were marked on the reef by placing nails at two opposite corners of a 50 ´ 50 cm square, which was measured by a metal frame. Cages were constructed from 6-mm mesh hardware cloth and measured 50 ´ 50 ´ 25 cm. Small doors, held closed with cable ties, allowed fertilizer to be placed inside the full cages. Two opposite sides of the partial cages contained large holes measuring 20 ´ 10 cm, while the remaining two sides contained two smaller holes measuring 10 ´ 10 cm. Holes of these sizes allowed ®shes to feed inside the cages, but left enough structural rigidity to support the top of the cage. Cages were cleaned each week using scrubbing brushes and pads. We assessed the eectiveness of the herbivore-exclusion by observing whether ®shes were inside the partial and full cages each week. Nutrient enrichment and sampling A small bag constructed from ®berglass window screening and containing 10 g of slow-release fertilizer was placed into the nutrient-enriched plots. These bags were changed every 2 weeks. During the ®rst 5 weeks, we used Osmocote 14±14±14 (N-P-K) fertilizer, while during the remaining 14 weeks we used Osmocote 18±6±12 fertilizer. We estimated the rate of nutrient release by drying the bags after 1 and 2 weeks of exposure, weighing the bags again, and calculating the mass lost. These estimates are a minimum estimate, as the addition of salts from the seawater exposure biased against loss. We measured the concentration of water column nitrogen and phosphorus in three uncaged plots without fertilizer and three uncaged plots with fertilizer on 15 June 1999. Water samples were collected in the center of the plots using 50-ml Nalgene bottles that were rinsed three times with seawater immediately before sampling. Empty bottles were opened approximately 2 cm above the substrate surface and within 2 cm of the fertilizer bags. The bags had been in place for 1 week before sampling. Nutrient analyses were performed by the Guam Water and Environment Research Institute, blind to sample identity. Nitrate and nitrite were measured together in Jones' (1984) modi®cation of the cadmium reduction method described in Parsons et al. (1984). Standards were run at 0, 3.57, 7.14, 17.9, and 35.7 lM. Ortho (reactive) phosphate was measured by the method described in Parsons et al. (1984). Standards were run at 0, 0.81, 1.61, and 3.23 lM. Since the observed variances were not homogeneous, we compared nutrient concentrations using onetailed Mann-Whitney U-tests.
similarities between pairs of samples to calculate the statistic R, which re¯ects the observed dierences between treatments, contrasted with dierences among replicates within treatments. The magnitude of R is proportional to the magnitude of the dierences between treatments. The signi®cance of R is calculated using a permutation test (Clarke and Warwick 1994). For post-hoc pairwise comparisons of caging treatments, additional ANOSIMs were conducted between pairs, adjusting the level of signi®cance for the number of comparisons made. We also compared each of the six treatments over time by ANOSIM of the initial and ®nal communities, again adjusting the level of signi®cance for the number of comparisons made. Data for multiple species of Caulerpa, Halimeda, and Oscillatoria were pooled for each genus. We compared the responses of major macroalgal and cyanobacterial species to the six treatments over time by calculating change in surface cover as the dierence in cover between the end and the start of the experiment. We also compared all macroalgae and all cyanobacteria by pooling the appropriate genera. We used blocked two-way analyses of variance (ANOVA) to examine the eects of caging, nutrient enrichment, and their interaction on change in cover and ®nal wet biomass for each taxa, blocking by replicate. To meet the assumptions of ANOVA, cover data were rank-transformed for Dictyota, Halimeda, Tolypothrix, total macroalgae, and total cyanobacteria. Biomass data were log-transformed for Dictyota, Halimeda, Padina, and total macroalgae. We used Bonferroni's test to conduct post-hoc means comparisons for signi®cant eects. Since Padina and Tolypiocladia glomerulata (hereafter, Tolypiocladia) were virtually restricted to the full cages, the eect of caging and its interaction with nutrient enrichment were not included in the ANOVAs for Padina cover and Tolypiocladia cover and biomass. Palatability tests The impact of herbivory on cyanobacteria and macroalgae depends on the relative palatabilities of these taxa to herbivores. We measured the relative palatability of the major macroalgal and cyanobacterial species in our plots by exposing clumps of algae to herbivorous ®shes. Since Padina, Rosenvingea, and Tolypiocladia were not found outside the full cages, we used algae from one replicate of the six treatments to conduct this assay, consequently reducing the number of replicates for wet biomass measurements. For each species, twelve small pieces, approximately 2 cm long, were placed in weighted clothespins on the reef ¯at at Val's Reef, approximately 100 m from the experimental plots at Marc's Reef. Herbivorous ®shes, including parrot®shes, surgeon®shes, and rabbit®shes, immediately targeted the novel algae. After 1 and 2 h, we counted the number of clumps of each species remaining. For each time period, we used a G-test to compare the number of clumps eaten among species.
Algal community sampling Every 2 weeks, we estimated macroalgal and cyanobacterial abundance using a 0.5 ´ 0.5-m quadrat divided by strings into 100 5 ´ 5-cm cells. For each species, we recorded the total number of cells occupied as a measure of spatial cover (Pennings and Callaway 1992; Sutherland 1996). At the end of the experiment, we harvested the biomass of each macroalgal species and combined all cyanobacterial species for each experimental plot. For nine of the ten sets of treatments, we measured the wet weight of these samples after drying them for 30 spins in a salad spinner. The tenth set was used in palatability assays (see below). Statistical analysis We evaluated dierences in community structure among experimental plots due to the main eects of caging and nutrient enrichment at the start and end of the experiment using analysis of similarity (ANOSIM) and the PRIMER software package (Clarke and Warwick 1994). ANOSIM uses a matrix of Bray-Curtis
Results Eectiveness of treatments Throughout the experiment, we observed ®shes feeding in the uncaged and partially caged plots. Juvenile rabbit®shes (Siganus argenteus and S. spinus) recruited in large numbers beginning approximately 9 May. These ®shes were abundant at the study site throughout the months of June and July and were not observed in the fully caged plots. A large harvest of juvenile rabbit®shes was reported island-wide between May and August, with over 1,300 kg caught and over 400 kg sold at local stores (Guam Department of Agriculture, Division of Aquatic and Wildlife Resources). By August, maturing rabbit-
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®shes had moved o the reef ¯ats and ®sh abundance had declined. Osmocote bags placed in the ®eld for 1 week lost 167.48.0 mg/day (meanSE, n=30), while bags placed in the ®eld for 2 weeks lost 172.810.4 mg/day (n=114), yielding a minimum release of 20 mg N/day and 9 mg P/day. The similarity of loss rates after 1 and 2 weeks suggests a relatively constant rate of nutrient release. Water-column concentrations of N and P just above the substrate were signi®cantly higher in the nutrient-enriched plots than in the control plots (N control=0.670.09 lM, N enriched=6.810.47 lM, U=0.0, P=0.043; P control=0.090.01 lM, P enriched=0.470.14 lM, U=0.0, P=0.043). Changes in community structure At the beginning of the experiment, all plots were dominated by the cyanobacteria Tolypothrix and Oscillatoria and the macroalgae Dictyota and Halimeda (Fig. 1A±F). Other taxa present in minor amounts (less
than 1% mean cover) included the cyanobacterium Lyngbya majuscula, and the macroalgae Amphiroa spp., Caulerpa spp., and Turbinaria ornata. Analysis of similarity (ANOSIM) revealed small, but signi®cant, dierences in community structure among the cage treatments at the start of the experiment (Table 1). The fully caged plots (Fig. 1E, F) contained more Tolypothrix and less Oscillatoria and Dictyota than the partially caged and uncaged plots (Fig. 1A±D). At the end of the experiment, the uncaged plots were dominated by Oscillatoria, with less Tolypothrix, Dictyota, and Halimeda, and minor amounts of Amphiroa spp., Hormothamnion enteromorphoides, Padina, T. ornata, and L. majuscula (Fig. 1G, H). The partially caged plots were dominated by Dictyota, with less Tolypothrix, Oscillatoria, and Halimeda, and minor amounts of H. enteromorphoides and L. majuscula (Fig. 1I, J). The fully caged plots were dominated by Padina and Tolypiocladia, with less Dictyota and Halimeda, and minor amounts of Tolypothrix, Oscillatoria, Caulerpa spp., T. ornata, Rosenvingea intricata (hereafter, Rosenvingea), Acanthophora spicifera, and Hydroclathrus clathratus (Fig. 1K, L). The last three species were found only in the fully caged plots. ANOSIM revealed no signi®cant Table 1 Results of ANOSIMs to compare community structure among herbivore-exclusion and nutrient enrichment treatments at beginning of experiment. Global tests use a=0.05 as level of signi®cance, while level of signi®cance for pairwise comparisons among the three herbivore-exclusion treatments is adjusted for the number of comparisons to a=0.017
Global tests Source Caging Nutrient Pairwise comparisons Caging treatments None, partial None, full Partial, full
R
P
0.094 ±0.049
0.007 0.911
0.034 0.125 0.129
0.214 0.018 0.016
Table 2 Results of ANOSIMs to compare community structure among herbivore-exclusion and nutrient enrichment treatments at end of experiment. Global tests use a=0.05 as level of signi®cance, while level of signi®cance for pairwise comparisons among the three herbivore-exclusion treatments is adjusted for the number of comparisons to a=0.017
Fig. 1 Mean percent cover (SE) of major taxa at beginning and end of ®eld manipulations. Left (A±F) Initial distributions; right (G±L) ®nal distributions. Each row corresponds to a treatment: uncaged, unfertilized (A, G); uncaged, fertilized (B, H); partial cage, unfertilized (C, I); partial cage, fertilized (D, J); full cage, unfertilized (E, K); full cage, fertilized (F, L)
Global tests Source Caging Nutrient Pairwise comparisons Caging treatments None, partial None, full Partial, full
R
P
0.598 ±0.011
