No abstract

The aims of this work were to analyse the feeding selectivity of L. fortunei in a natural assemblage of phytoplankton in a short-term microcosm experiment and to assess whether this selectivity is affected by the presence of Rotifera as a secondary, palatable feeding resource. This bivalve preferred Desmidiales, Chlorococcales, Euglenophyceae and Chrysophyceae algae with a maximum linear dimension from 20 to 100 lm. Organisms between 500 and 40 9 10 3 lm 3 belonging to Desmidiales, Chrysophyceae and Euglenophyceae were also positively selected. Volvocales, Cryptophyceae and one group of medium-size Euglenophyceae (Trachelomonas sp.) had a high, negative selectivity index independent of their cell shape or size (Ivlev's index of feeding selectivity \-0.7). The mussel positively selected Rotifera, and this only had a measurable effect on large Euglenophyceae, which increase their selectivity value in the absence of Rotifera. The nonparametric multiplicative regression showed that selectivity is largely explained by a combination of cell shape, biovolume and the phytoplankton taxa offered (R 2 [ 0.8).We concluded that the impact on phytoplankton community structure could be severe, considering that the presence of zooplankton does not have an effect on the majority of phytoplankton groups and that the mussel tends to feed on both items to improve its diet. The negative selection of some phytoplankton taxa is possibly related to the morphophysiological characteristics of their cell shells.

The effect of different concentrations of the bivalve Limnoperna fortunei on the algal density and structure and its filtration rate was estimated in a mesocosm experiment. The experiment was carried out using containers of 200 L and three treatments: Control (without bivalves), C1 (with 128 individual bivalves), and C2 (with 256 individual bivalves). Decrease in phytoplankton density was detected in bivalve treatments after 6 h. A decrease of 52% and 86% of the initial density was measured in C1 and C2, respectively, after 48 h. All algae groups showed substantial decrease in densities (460%) and the composition of the algae assemblages changed in the presence of the bivalve. In particular, there was an increase in the density of Flagellates. The maximum estimated bivalve filtration rate was 357 mL ind À1 h À1 at 28 C. Our results suggest that the invading bivalves could significantly affect both the plankton density and assemblage structure in natural systems.