Effects of the coexisting Baltic amphipods Monoporeia affinis and Pontoporeia femorata on the fate of a simulated spring diatom bloom

A laboratory experiment was performed to quantify the fate of diatom phytodetritus and how this is affected by the presence of benthic amphipods. A Baltic Sea spring bloom sedimentation event was simulated by adding 14C-labeled diatoms (Skeletonema costatum) to microcosms with varying densities of the amphipods Monoporeia affinis and Pontoporeia femorata, as well as to microcosms without amphipods, where the sediment was disturbed mechanically. After 1 mo of incubation, 51 to 77% of the added diatom carbon was still in the sediment; 2 mo later 49 to 66% remained. The effect of amphipods on the fate of the phytodetritus differed between species. At near-field density, M. affinis incorporated 6 to 11% of the added 14C, P. femorata only 1.2%. The results indicate that burrowing slows mineralization, presumably by mixing organic material to anoxic depths in the sediment. The effect of P. femorata on mineralization could not be distinguished from the effect of mechanical stirring. M. affinis feeding and respiration resulted in a significant increase in mineralization; at low densities this compensated for the mixing effect, at high densities M. affinis feeding resulted in enhanced net mineralization.