Uptake of sedimentary organic matter by the deposit-feeding Baltic amphipods Monoporeia affinis and Pontoporeia femorata

Some benthic deposit-feeders mainly eat freshly deposited phytodetritus, while others feed more on older material that has been mixed with the sediment and modified by diagenetic processes before being ingested. We studied the uptake of sedimentary foods of different ages by the Baltic amphipods Monoporeia affinis (Lindstrom) and Pontoporeia femorata Kroyer in laboratory experiments using 3 isotopic tracers. The amphipods were offered fresh 14 C-labelled diatoms spread on top of a thin unlabelled sediment layer, underlain by 1 yr old sediment to which 13 C- and 15 N-labelled diatoms had been added. Thus, 14 C uptake represented surface feeding on fresh organic material, and 13 C: 15 N uptake subsurface feeding on aged phytodetritus. Experiments using a single species only or mixed species were conducted in spring with 1 yr old adults and in summer with 3 mo old juveniles. Adult M. affinis (initial dry mass 1.6 mg) took up ∼5 times more 14 C than P. femorata (initial dry mass 1.7 mg), indicating that M. affinis depended more on fresh phytodetritus, while P. femorata had significantly higher 13 C: 15 N uptake, showing a greater reliance of this species on aged organic matter from the deep sediment. In experiments, adult P. femorata consistently fed at depth in the sediment, whereas adult M. affinis modified feeding depth depending on food quality. Juveniles (0.1 mg initial dry mass) of both species had similar tracer uptake and fed both on surface and subsurface sediment, suggesting greater potential for interspecific food competition in juveniles than in adults. Juveniles of both species had higher mass-specific 14 C uptake than adults. Single species treatments had higher 14 C uptake than mixed treatments in both adults and juveniles, indicating food competition at the higher density of the mixed treatments.