initial exposure of zooplankton to radioactive food and their removal 

 from the water for counting, must be less than the time for gut passage 

 as sizable errors are introduced at longer intervals due to excretion 

 and respiration of carbon-14 (Rigler, 1971). We chose the 5- and 10- 

 minute intervals for the two copepods to coincide with Heinle's (1973, 

 unpublished) observations of pel let formation in these two species. 



b. Effects of Suspended Solids on Feeding Activity at Maximum 

 Ingestion Time . Considerable quantities of inorganic material mixed 

 with particulate food interferes with food ingestion by the copepods 

 E. affinis and A, tonsa. Additions of two different particle-size 

 distributions of fine sand. Fuller's earth, and Patuxent River silt to 

 cell suspensions of heavily labeled (NaHCl'+Os) M. lutheri (50,000 cells 

 ml"-^) significantly reduced the maximum ingestion rate of these cells 

 by E. affinis at solids concentrations in excess of 250 milligrams 1"-^ 

 (Fig. 11, Table 1). At 100 milligrams 1"-^ concentrations of Fuller's 

 earth a significant increase occurred. Some nonsignificant reduction at 

 this concentration occurred for other particle types. 



The Patuxent River silt caused a greater reduction in ingestion 

 of radiocarbon-labeled cells than other particle types over all concen- 

 trations tested except at the low concentration of 50 milligrams 1"^ 

 (Fig. 11, Table 1). The apparent enhancement effect of the silt at 100 

 milligrams 1"-^ was smaller than for all other particle types. 



Aoartia tonsa exhibited biologically significant reductions in 

 ingestion at all concentrations of all particle types tested (Fig. 12, 

 Table 2). The effects increased with increasing concentration. How- 

 ever, with this species the effect of Patuxent River silt was lower 

 than Fuller's earth or Si02 particles ( < 15 micrometers) except at the 

 highest concentration (1,000 milligrams l"^)* 



The differences in the shapes of the uptake curves may be related 

 to the habitats of the two species. Eurytemora affinis typically 

 occurs in upper, more turbid estuarine areas (Herman, Mihursky, and 

 McErlean, 1968), where normally low concentrations of suspended solids 

 may stimulate feeding. Low concentrations of suspended solids may 

 indicate the presence of food to this species, and in turn stimulate 

 the organism to begin feeding or to increase its feeding rate. 



Stimulation of pumping activity in the American oyster at low 

 particle concentrations (Loosanoff , 1961) , increased activity of 

 Doliotida and Satpida by the presence of suspended particles (Jorgensen, 

 1966) , and tripling of the ingestion rate of algal cells by Artemia sp. 

 upon the addition of fine sand to the cell suspension (Reeve, 1963) 

 lend support to our observations of the response of E. affinis. This 

 type of feeding response will increase the organism's chances for 

 survival. 



26 



