Comparing rate and depth of feeding in benthic deposit-feeders: a test on two amphipods, Monoporeia affinis (Lindström) and Pontoporeia femorata Kröyer

Abstract

Feeding depth is a critical aspect of habitat use by deposit-feeders. There is evidence of two partly separated food webs in infaunal benthos, where one relies directly on newly sedimented fresh phytodetritus, available mostly at the sediment surface, while the other primarily uses older organic matter in the sediment. We have developed a procedure for objective experimental identification of these two feeding types without relying on assumptions based on morphology. The rate and depth of feeding in the two deposit-feeding amphipods Monoporeia affinis and Pontoporeia femorata were studied in two laboratory experiments using 14C-labelled diatoms ( Skeletonema costatum) as food. Experiments were performed in spring and summer, with diatoms added on top and mixed down into the sediment in the spring experiment, and either on top or mixed into the sediment in the summer experiment. Treatments included the two species both in sympatry and allopatry, and at field-relevant densities between 1000 and 2600 individuals m −2. An overall greater 14C-assimilation was found for M. affinis, confirming that M. affinis has a higher ingestion rate and metabolism than P. femorata. In the spring experiment, M. affinis had similar 14C-assimilation both in treatments with labelled algae in the sediment and with labelled algae on the sediment surface, while P. femorata had ∼4× greater assimilation when the algae were mixed into the sediment. The summer experiment showed the same pattern for P. femorata, but M. affinis had ∼3× greater assimilation from algae spread on the sediment surface. No significant differences were found between densities or between sympatric and allopatric treatments indicating that competition between the two species had little effect under the conditions of the experiments. We conclude that the two species differ in feeding depth, with M. affinis more of a surface deposit-feeder and P. femorata almost exclusively a subsurface deposit-feeder, suggesting that they use partly different food resources. Depending on their local abundance, the two amphipods will affect mineralisation and bioturbation rates differently.