The importance of freshly produced phytoplankton detritus in the diet of marine meiobenthos in a soft-bottom community was examined in a 14m3 microcosm. Radiolabelled phytodetritus was continuously produced in the water column from January to June by the addition of ^C-bicarbonate. The specific activity of organic carbon in plankton, sediment and meiobenthos was measured during this time. All meiobenthos reached specific activities higher than that of the sediment; recently produced, labelled detritus was more readily assimilated than older sedimentary carbon. However, the proportion of assimilated carbon that was apparently derived from labelled detritus differed greatly between 2 distinct faunal groups found in the top 5mm of sediment. One group, dominated by harpacticoid copepods, reached specific activities identical to that of the phytoplankton. These fauna thus almost exclusively assimilated labelled detritus, and for harpacticoids there was a lag of less than 2mo between detrital deposition and assimilation. The second group included most other meiobenthic taxa and, by late May, still had remarkably low specific activities -from 10 to 30 Oh of phytoplankton values. It appears that most of the organic carbon in the diet of these fauna had been produced prior to January. I suggest that these 2 meiobenthic groupings were segregated by depth in the surface millimeters and that the low specific activity fauna utilized a large, buried reservoir of older detritus.

Three experiments were conducted to investigate the responses of benthic rneiofauna to long-term, low-level additions of No. 2 fuel oil in large (13 m3), outdoor tanks (mesocosms) containing sediment and sea water from Narragansett Bay, Rhode Island, USA. In the first experiment, an average water column oil concentration of 190 ppb was maintained for 168 d followed by a 64 d period of no oil additions. During the second experiment, an average oil concentration of 90 ppb was maintained for 122 d. A 386 d period of no oil additions (Experiment 111) followed Experiment 11. The abundances of metazoan meiofauna decreased during oil addition periods in Experiments I and I1 with the oil more extensively affecting the meiofauna in the first experiment. In both experiments, ostracods and harpacticoid copepods were the most sensitive metazoan groups. In contrast, abundances of protozoan meiofauna (foraminiferans and ciliates) were higher in the oiled mesocosms. Abundances of most meiofaunal groups returned to levels similar to the controls within 2 to 7 mo following the termination of oil additions. However, the abundances of kinorhynchs and halacarids remained depressed for more than 1 yr after the last oil addition, presumably due to residual oil in the sediments.