Macrofaunal processing of phytodetritus at two sites on the Carolina margin: in situ experiments using C-13-labeled diatoms

Tracer experiments using C-13-labeled diatoms Thalassiosira pseudonana were carried out at two 850 m sites (I off Cape Fear and III off Cape Hatteras) on the North Carolina, USA, slope to examine patterns of macrofaunal consu mption of fresh phytodetritus. Experiments examined the influence of taxon, feeding mode, body size and vertical position within the sediment column o n access to surficial organic matter. delta(13)C measurements were made on macrofaunal metazoans and agglutinating protozoans from background sediment s and from sediment plots in which C-13-labeled diatoms were deposited and then sampled 0.3 h, 1 to 1.5 d, 3 mo and 14 mo later. Significant between-s ite differences were observed in background delta(13)C signatures of sedime nts, metazoans, and large, agglutinating protozoans, with values 2 to 3 par ts per thousand lower at Site III than at Site I. Background delta(13)C sig natures also varied as a function of taxon and of vertical position in the sediment column at Site III. The background delta(13)C value of carnivores was higher than that of surface-deposit feeders among Site I annelids, but no annelid feeding-group differences were observed at Site III. delta(13)C data from short-term (1 to 1.5 d) experiments revealed rapid diatom ingesti on, primarily by agglutinated protozoans and annelids at Site I and mainly by annelids at Site III. Selective feeding on diatoms was exhibited by para onid polychaetes, especially Aricidea spp. Exceptionally high uptake and re tention of diatom C also was observed in the maldanid Praxillella sp., the nereid Ceratocephale sp. and several other surface-deposit feeding polychae tes. After 14 mo, little of the diatom C-13 remained at Site III, but high concentrations of the tracer were present in annelids and agglutinating pro tozoans at Site I. At both sites, nonannelid metazoans and subsurface-depos it feeding annelids exhibited the least uptake and retention of diatom C. O ur hypotheses that large-bodied taxa and shallow-dwelling infauna should ha ve greatest access to freshly deposited organic matter were not borne out. Some small, deep-dwelling taxa acquired label more readily than large or ne ar-surface forms. Differences in tracer fates between sites reflected great er vertical mixing at Site III. These results indicate heterogeneity in ben thic processes along the Carolina margin. but suggest that labile organic m atter is consumed quickly at both sites. Because most of the taxa found to consume freshly deposited diatoms in these experiments are typical of bathy al settings, we infer that phytodetritus reaching the seabed in margin envi ronments is rapidly processed by protozoan and metazoan components of the b enthic fauna.