Reduction of resuspended sediments by the pelagic herbivore,Daphnia pulex

Abstract

We examined the removal of resuspended sediments from ambient lake water by the pelagic herbivore, Daphnia pulex, in a short-term field-enclosure experiment during the spring clear-water phase. We assessed Daphnia’s ability to clear the water of suspended sediments by evaluating removal of tripton (detritus and inorganic particles) by Daphnia when ambient water conditions were turbid (>100 NTU) or clear (<20 NTU) in 2 trials, conducted 2 wk apart. Treatments consisted of 6 (0, 100, 200, 400, 700, and 1000 ind/L) or 7 Daphnia densities (0, 50, 100, 200, 400, 700, and 1000 ind/L) in the 1st (turbid) and 2nd (clear) trials, respectively. We used scanning electron microscopy to examine Daphnia gut contents and measured fecal-pellet settling in settling columns. In general, Daphnia presence significantly reduced both the detrital and inorganic components of tripton, but increasing Daphnia densities did not lead to increased reductions of tripton. Ambient turbidity did not alter this effect. These results indicate that Daphnia are capable of clearing resuspended sediments during both low- and high-turbidity conditions, signifying that Daphnia can affect springtime water clarity in lakes with high suspended sediment concentrations. This clearing mechanism is particularly relevant in shallow lakes during spring when rapid transport of aggregates to the lake bottom could substantially increase water transparency at a time when macrophytes are regrowing from overwintering structures.