Effect of grazing and water column nutrient supply on biomass and nutrient content of sediment microalgae

Field experiments were conducted to investigate the effects of grazing and nutrient supply on sediment microflora in a freshwater habitat (Lake Erken, Sweden) and at the brackish Baltic Sea coast (Vaddo, Sweden). The two sites were of similar productivity, but had contrasting herbivore composition. In a full-factorial experiment design, closed cages excluded macrozoobenthos (>1 mm) from sediment patches, whereas open cages allowed grazer access. The cage design applied here proved to successfully prevent in- and epifauna to access the sediment in closed cages. In half of the treatments, nutrients were added to the water-column by a slow-release fertilizer. The experiments were seasonally replicated four times at Vaddo and two times in Lake Erken. After 4-5 weeks, sediment cores were sampled and analyzed for chlorophyll, carbon, nitrogen and phosphorus. The benthic microalgae showed strong seasonal variation in biomass and internal nutrient content. At Vaddo, neither grazing nor nutrients affected the algal biomass significantly, but significant grazer effects were detected on C:N:P ratios. In Lake Erken, grazer presence reduced algal biomass by ca. 50%, whereas nutrients were without effect on biomass or on nutrient content. Compared to results from hard substrata at the same sites, sediment microflora was less affected by nutrients and grazing. This may be due to the harsh physico-chemical environment on sediments, to low grazer density at the coastal site and to low availability of water column nutrients to sediment microalgae. In our experiments, sand-dwelling microphytobenthic communities represented a highly dynamic assemblage, which, however, is less structured by biotic interactions than epilithic periphyton. (C) 2002 Elsevier Science B.V. All rights reserved.