Preferential dreissenid mussel grazing on small plankton can favor cyanobacteria: Experimental evidence from western Lake Erie

Abstract

Herein we provide experimental evidence for the effects invasive mussels (Dreissena) grazing can exert on a natural assemblage of plankton that included both hetero- and photo-trophic components in western Lake Erie. Five mussel feeding experiments were performed seasonally in 2018 (May, June, July August, October). Pre- and post-grazing samples were collected from seven 20-L mesocosms (3 control, 4 mussel), and analyzed using microscope cell counts. Results from our experiments showed that Dreissena were active grazers of plankton on all dates, with significant correspondence between mussel clearance rates measured using microscopy versus size-specific chlorophyll and FluoroProbe based estimates (Spearman rank correlation, r=0.45, and r=0.48, respectively, p<0.05). Clearance rates (ml mg-1 h-1) were variable among taxonomic groups and seemed to track the abundance of ambient plankton assemblage (range 1.70 to 25.00, mean ± SE 11.70 ± 1.33). Dreissena grazed consistently on nano-sized hetero- and phototrophic plankton that constituted key trophic linkages in the Lake Erie foodweb. The most actively grazed plankton were phototrophic cryptophytes (Rhodomonas minuta, Cryptomonas spp.), centric diatoms (Cyclotella sp. and Discotella spp.), and non-pigmented chrysomonads (Chromulina sp., Ochromonas sp.). Conversely, clearance rates were low for cyanobacteria (e.g. Microcystis), dinoflagellates (Gymnodinium varians), and some colonial chlorophytes (Desmodesmus, Pediastrum); these plankton groups occurred during specific temporal windows (one-way ANOVA, p<0.05). Our results indicate the potential mussel grazing can suppress typical, non-harmful plankton species in the nanoplankton size range, thereby favoring the occurrence of less-edible, larger cyanobacteria and chlorophyte species in western Lake Erie.