The effect of dense patches of Stratiotes aloides L. on the spatial structure of microcrustacean assemblages in an oxbow lake

Abstract

The role of an emergent form of Stratiotes aloides (water soldier) in creating a habitat for microcrustaceans (Cladocera and Copepoda) in an oxbow lake of the Bug River (Poland) was studied from June to September in years 2003 and 2004. Water samples were collected monthly, both in open water and from Stratiotes beds in the littoral zone at half of the water depth. Plants were sampled in the littoral zone from the area of one square meter. Densities and biomass of microcrustaceans, concentrations of oxygen, chlorophyll a, seston, temperature, pH, conductivity and Stratiotes densities and biomass were determined. Microcrustacean assemblages within macrophyte beds were taxonomically richer, more diverse and abundant than in open water (on average 10 versus 2 taxa, 460 versus 128 ind. dm−3, 0.54 versus 0.03 mg DW dm−3, respectively). Cyclopoida dominated in the densities and biomass in both habitats with copepodites and adults being the most numerous in the Stratiotes stands. Cladocera were represented almost exclusively by Bosmina longirostris in open water and mainly by chydorids, Ceriodaphnia spp. and Simocephalus vetulus among plants. Daphnia longispina complex was recorded only in the littoral zone. Microcrustacean diversity in the oxbow lake was positively correlated with Stratiotes abundance. The observed microcrustacean composition and spatial distribution suggest fish predation as the main factor shaping the microcrustacean community in the oxbow lake, while Stratiotes stands acted mainly as a habitat for benthic and plant-associated species as well as a likely refuge for small-bodied free-swimming Ceriodaphnia spp., adults and copepodites of Cyclopoida.