Sediment with different densities of Corophium volutator (Pallas), ranging from 0 to 19 800 ind. m-', were incubated in laboratory microcosms, and rates of oxygen uptake, denitnfication and nitrate ammonification were determined from sediment-water fluxes. The measured processes were stimulated differently by C. volutator; oxygen uptake, denitrification of NO, from nitnfication within the sediment, and denitrification of NOT from the overlying water were enhanced 2-, 3-and 5-fold respectively in the presence of 19 800 ind. m-?. This differential stimulation was explained by the different characteristics of diffusional solute transport at the sediment-water interface and mass transfer of water into the burrows where O 2 and NO? was depleted. Denitrification rates were calculated by using the lSN isotope pairing technique. The applicability of the ' 5~ isotope pairing technique for measunng coupled nitrification-denitrification in bioturbated sedlment was confirmed in a test incubation with different levels of 1 5~0 < added to microcosms with 12 000 C. volutatorind. m-'

Ingestion of picoplankton-size particles was studied in two marine ciliates, a typical grazer of pica-and nanoplankton, the oligotrich Strombidium sulcatum, and a bacteriovorous scuticociliate, Uronema sp. In laboratory experiments, both logarithmic-(food-unlimited) and stationary-phase (food-limited) populations were presented with particles of different sizes and surface properties: plain microspheres, protein-adsorbing carboxylate microspheres, fluorescently labelled heterotrophic bacteria, and cyanobacteria. In both log-and stationary-phase populations, Strombidium clearance rates varied linearly as function of prey size based on the ingestion of plain microspheres. In contrast, Uronema clearance rates were invariant with particle size for log-phase populations but increased with particle size in stationary-phase cells. Discrimination among prey with different surface properties was also dissimilar in the two ciliates. For Strombidium, log-phase populations cleared l-pm-diameter plain microspheres at higher rates than those with carboxylate surfaces. The pattern was repeated by stationary-phase cells for both -0.5. and l-pm-diameter microspheres. In Uronema, there was no significant difference in clearance rates with surface property in log-phase populations, similar to the findings with regard to particle size. Only in stationary-phase populations of Uronema was some selection evident, as 0.97-pm carboxylate microspheres were cleared at rates higher than were 0.95.pm plain microspheres. Overall, selection was higher in stationary-phase ciliates, and Strombidium seemed to feed more size-selectively among picoplankton particles than did Uronemu.

No abstract

No abstract