Inorganic Particles Alter Competition in Grazing Plankton: The Role of Selective Feeding

Abstract

Suspended sediments (clay and silt particles) differentially inhibit cladoceran populations, but not rotifer populations, and can change the outcome of competition between rotifers and cladocerans in favor of rotifers (Kirk and Gilbert 1990). This paper provides a mechanistic explanation for the population and community effects of suspended clay. Feeding experiments with radioactively labeled phytoplankton cells (Cryptomonas) showed that the presence of suspended clay (<2—@mm particle size) significantly decreased the phytoplankton ingestion rates of five cladoceran species (Ceriodaphnia dubia, Daphnia ambigua, D. galeata mendotae, D. magna, and D. pulex) by 13—83% but not those of three rotifer species (Keratella cochlearis, K. crassa, and Synchaeta pectinata). When radioactively labeled phytoplankton cells and clay particles were offered simultaneously, cladocerans were less selective for phytoplankton over clay than were rotifers. Thus, cladocerans ingested more suspended clay particles than did rotifers, and this caused the reductions in phytoplankton ingestion rates. Smaller cladocerans were less selective for phytoplankton over clay than were larger cladocerans, and their phytoplankton ingestion rates were thus reduced more by clay. Rotifers fed more selectively than either large or small cladocerans, were able to avoid ingesting clay particles, and therefore were not inhibited by suspended clay. Thus, differences in the feeding modes and selectivities of rotifers and cladocerans form the mechanistic basis for the change in competitive outcome in favor of rotifers observed in the presence of suspended clay. This mechanism may form the basis for changes in the relative abundance of rotifers and cladocerans in turbid lakes and reservoirs.