A BIOASSAY APPROACH TO SEASONAL-VARIATION IN THE NUTRITIONAL-VALUE OF SEDIMENT

Smaller surface deposit feeding macroinvertebrates probably rely on both microbial and non-living sources of organic matter, but only scant evidence supports the hypothesis that food is limiting to natural populations. Summer declines in deposit feeding populations have been ascribed to food limitation and to the seasonal influx of predators. We used a bioassay approach to test the following hypotheses: (1) Sediment supports more population growth in the spring and early summer, relative to the late summer and fall. (2) The nutritive value of sediment declines from spring to early fall, and absorption therefore should decrease. Laboratory tests using the common deposit feeding oligochaete Paranais litoralis partially support both hypotheses. Population growth studies of spring and summer sediment show a relatively nutritionally enriched surface sediment layer, suggesting that the sediment is recharged in the late fall and winter, either by particulate organic matter deposition or microbial transformation of previously unabsorbable material. High summer temperature contributes to the negative effect of poor sediment quality. In Flax Pond, Long Island, New York, field studies show summer declines of several species of deposit feeding invertebrates, but caging studies demonstrate that predation is not a likely cause of the crashes, which proceed at similar rates inside and outside of cages.