Effect of grazing and water column nutrient supply on biomass and nutrient content of sediment microalgae
Introduction
Sediment microflora is an important contributor to primary productivity in aquatic habitats. In unvegetated areas like intertidal mudflats, the community consisting mainly of diatoms and cyanobacteria is the main source of organic carbon fixation (Cadee, 1980), but even in seagrass meadows, the benthic algae contribute substantially to the autotrophic production (Daehnick et al., 1992, Pinckney and Zingmark, 1993).
However, sediments represent harsh conditions for algal photosynthesis and growth. The substratum is unstable and characterized by strong chemical and physical gradients on small spatial scales. Algal biomass and production on sediments was shown to be highly affected by sand movements (Delgado et al., 1991), grain size (Watermann et al., 1999), current velocity (Berninger and Huettel, 1997) and various physico-chemical gradients such as sulfide, oxygen, temperature (Admiraal, 1984). These factors vary on different temporal and spatial scales and thus, result in distinct temporal and spatial patterns of algal biomass and production (Admiraal, 1984, Underwood and Paterson, 1993, Pinckney et al., 1995, Saburova et al., 1995, Light and Beardall, 1998).
There is less agreement on the importance of competition and grazing on sediments compared to other littoral habitats. Besides the abiotic environment, biotic interactions (competition, grazing, nutrient regeneration) have been found to be important for microalgae colonizing hard substrata in freshwater (Feminella and Hawkins, 1995, McCormick, 1996) and marine habitats (Hillebrand and Sommer, 1997, Hillebrand et al., 2000). For sediments, however, the role of competition was proposed to be minor due to the harsh physical environment and high nutrient concentrations in sediment pore-water (Admiraal, 1984, McIntire and Amspoker, 1986). Still, there has also been experimental evidence of enhanced algal growth in sediments following nutrient addition (Sundback and Snoeijs, 1991, Flothmann and Werner, 1992, Pinckney et al., 1995). Grazing has been investigated for both meiofauna and macrozoobenthos. Although, grazing rates by meiofauna and protozoa on sediment microalgae can be high (Blanchard, 1991, Bott and Borchardt, 1999), it is less clear how effective grazing by these small consumers is on a longer time scale allowing numerical responses of prey and grazers (Reise, 1992; Epstein, 1997a, Epstein, 1997b). Grazing by macroconsumers such as snails (McClatchie et al., 1982), crustaceans (Hargrave, 1970, Gerdol and Hughes, 1994) and annelids (Smith et al., 1996) was shown to have important impact on biomass and community structure of sediment microflora. However, other studies indicated only a minor importance of grazing in the carbon flux of sediment communities (Barranguet et al., 1997). Here, we analyze the impact of nutrient enrichment and macrograzer presence on sediment flora integrated over a period of 4–5 weeks and compare the results to experiments involving epilithic periphyton. We conducted full-factorial field experiments with exclusion of macroconsumers and addition of nutrients in two habitats, a freshwater lake (Lake Erken) and a brackish coastal site (Väddö) in Sweden. The sites were chosen to represent similar habitat productivity but contrasting herbivore fauna. Moreover, at both sites the experiments were placed in shallow bays reducing the impact of physical forces on the algae, since the bays were protected from strong waves or currents and tides are virtually absent from the Baltic coast. We tested the following specific hypotheses:
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ambient grazer densities are able to significantly reduce algal biomass;
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nutrient enrichment increases algal biomass, especially in the absence of grazing;
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algal nutrient stoichiometry reflects the increase of nutrient availability by nutrient enrichment and grazer excretion.
Section snippets
Experimental sites
Lake Erken (59°25′N, 18°15′E) covers 23.7 km2 within a catchment area of 141 km2. Mean depth is 9.0 m, pH around 8.0 and mean conductivity 280 μS cm−1. Mean total phosphorus (TP) in the pelagic was 0.94 μmol l−1, mean total nitrogen (TN) 46.1 μmol l−1. The TN:TP ratio, calculated on a molar basis, was 49. Mean phytoplankton biomass was 5.6 μg Chl. a l−1. All background data are means of biweekly to monthly measurements from the Lake Erken monitoring program during the experiment period (autumn 1999 to
Results
Benthic invertebrate composition differed strongly between the two sites. The fauna in Lake Erken was dominated by insect larvae (Diptera, Trichoptera, Ephemeroptera), whereas at Väddö, annelid and crustacean grazers were important together with benthic filtration feeders (Bivalvia) (Table 1). Only the dominance of hydrobiid snails among the gastropods was similar between both sites, but with higher abundances at Väddö. The seasonal difference between the experiments at Väddö was marginal, with
Discussion
Our results showed that macrograzers on sediments were able to reduce algal biomass, but to a lesser extent compared to removal rates known from experiments on hard substrata. At Väddö, the grazing impact was marginal, whereas it was significant at Lake Erken. Grazer presence changed also the C:N:P ratios of benthic samples at Väddö. Nutrient enrichment of the water column had no significant effects on Chl. a or nutrient content and only a weak effect on organic C in Lake Erken. This was
Acknowledgements
We would like to thank Ulla Holm at Väddö Skjutfält for giving us the opportunity to conduct our experiment in this military area. We are gratefully indebted to Karl Hillebrand and Christa Hillebrand, who built the cages and mesh nets. Lars Peters helped with the macrozoobenthos analysis. Karin Larsson, Erik Törnblom, Ann-Louise Haglund, Sonja Stendera, Jan Johansson and Monika Feiling helped with the conduction of the experiments and the analysis of the samples. HH received financial support
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