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

Abstract

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 14 C-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 14 C, 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.

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

“…The 5-fold higher 14 C uptake by adult M. affinis compared with adult P. femorata demonstrates a preference for surface feeding on fresh material by M. affinis, in agreement with previous experimental studies (van de Bund et al 2001, Byrén et al 2002. The uptake of both δ 13 C and δ 15 N, which demonstrates subsurface feeding on aged phytodetritus, was significantly greater in P. femorata than in M. affinis, indicating that the former relies more on old organic material found at depth in the sediment.…”

“…This may indicate direct competition for food at higher density, as reported in previous experimental studies , van de Bund et al 2001, rather than a crowding effect causing interference with feeding. No indication of competition was found at similar amphipod densities in a study where fresh 14 C-labelled algae had been mixed down into the sediment (Byrén et al 2002), and both amphipod species had about the same uptake from subsurface feeding.…”

“…The radioactive carbon isotope 14 C is one of the most frequently used tracers in biological research and is commonly used in feeding or food web studies (Rudnick 1989, Widbom & Frithsen 1995, van de Bund et al 2001. Stable isotope ratios, e.g.…”

“…After a bloom settles to the bottom sediment, diagenetic processes soon start to change its chemical composition (Graf 1987). Due to bioturbation by deposit-feeders, some organic matter is often mixed down into deeper layers of the sediment (Hansen & Blackburn 1992, where anoxia slows the rate of decomposition (Andersen 1996, Bianchi et al 2000, van de Bund et al 2001). There are indications that benthic deposit-feeders differ in their use of freshly deposited and older organic matter in the sediment (Rudnick 1989, Widbom & Frithsen 1995, Ólafsson & Elmgren 1997, Byrén et al 2002.…”

Settling blooms of filamentous cyanobacteria as food for meiofauna assemblages

“…Around 0.3% 6 0.2% and 0.3% 6 0.1% were found in meiofauna tissues, while 1.4% 6 0.5% and 1.1% 6 0.7% were incorporated in macrofauna individuals for the Aphanizomenon and Nodularia treatments, respectively. This experiment did not use CO 2 traps, but in similar radiolabeling experiments with diatoms, about 35% of the radioactivity was released by respiration in 1 month (Ó lafsson et al 1999;Van De Bund et al 2001). …”