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Reservoirs built on rivers draining watersheds are often turbid with suspended sediments. Filter—feeding zooplankton, e.g., members of the genus Daphnia, seem to be as abundant in such reservoirs as in natural lakes. Using controlled laboratory conditions, we have investigated the potential role of suspended sediments in the nutrition of Daphnia from a turbid reservoir. In three sets of experimental procedures we measured: (1) the physical effect of sediments on the ingestion and incorporation rates of algae by daphnids, (2) the ingestion rates of two sizes of clay mineral sediments particles by daphnids, and (3) the growth and survival of daphnids fed yeast and sediments with and without organic matter adsorbed onto the particle surfaces. Increases in suspended sediment concentration from 0.0 to 245 l mg/L decreased ingestion rates of 14C—labeled Chlorella vulgaris by Daphnia parvula and D. pulex by 895% and decreased incorporation rates by 99%. Sediment concentrations of 50—100 mg/L reduced the algal carbon ingested by daphnids to potential starvation levels. 65Zn—labeled fine and coarse (1.88 and 4.65 μ mean diameter) clay mineral sediment particles were fed to D. parvula, D. pulex, and D. similis. Ingestion rates of sediments by daphnids were limited at particle concentrations of 85.0 ° 106 particles/mL and were dependent on particle size, daphnid size, daphnid species, and body size. Fine particles were ingested at greater rates by D. pulex and D. parvula than were coarse particles, while large D. similis ingested coarse and fine particles at similar rates. In addition, D. pulex had higher ingestion rates in each particle size suspension than did D. parvula or D. similis. The differential ability to ingest these particles suggest a mechanism of regulating daphnid species composition if the organic matter adsorbed to these sediment particles is useful as food. To evaluate this mechanism, we adsorbed dissolved organic protein to sediment particles. When D. pulex were fed the amended sediment suspension, they grew larger than when fed control sediments, but not as large as when fed yeast. D. parvula grew equally well when fed either yeast or amended sediments. Thus, dissolved organic matter can be made available for daphnid growth by the mechanism of adsorption.

F i l t e r i n g comb a r e a s were g r e a t e s t f o r Daphnia p u l e x and g. s i m i l i s , and l e a s t f o r 2. p a r v u l a , r e l a t i v e t o body l e n g t h . The s l o p e s of t h e r e g r e s s i o n s of comb a r e a body l e n g t h were d i f f e r e n t from expected (based on s i m p l e geometry) f o r g. p u l e x and g. s i m i l i s . F i l t e r i n g r a t e s of t h e s e daphnids i n suspensions of i n e r t sediment p a r t i c l e s were l e s s r e l a t e d t o comb a r e a s t h a n they were t o t h e g u t f i l l i n g c a p a c i t y of t h e s e p a r t i c l e s .

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