BackgroundBioturbators affect multiple biogeochemical interactions and have been suggested as suitable candidates to mitigate organic matter loading in marine sediments. However, predicting the effects of bioturbators at an ecosystem level can be difficult due to their complex positive and negative interactions with the microbial community.Methodology/Principal FindingsWe quantified the effects of deposit-feeding sea cucumbers on benthic algal biomass (microphytobenthos, MPB), bacterial abundance, and the sediment–seawater exchange of dissolved oxygen and nutrients. The sea cucumbers increased the efflux of inorganic nitrogen (ammonium, NH4 +) from organically enriched sediments, which stimulated algal productivity. Grazing by the sea cucumbers on MPB (evidenced by pheopigments), however, caused a net negative effect on primary producer biomass and total oxygen production. Further, there was an increased abundance of bacteria in sediment with sea cucumbers, suggesting facilitation. The sea cucumbers increased the ratio of oxygen consumption to production in surface sediment by shifting the microbial balance from producers to decomposers. This shift explains the increased efflux of inorganic nitrogen and concordant reduction in organic matter content in sediment with bioturbators.Conclusions/SignificanceOur study demonstrates the functional role and potential of sea cucumbers to ameliorate some of the adverse effects of organic matter enrichment in coastal ecosystems.

We measured in situ photosynthesis of benthic microbial mats at various depths in Lake Hoare, a permanently ice-covered lake of the McMurdo Dry Valleys, Antarctica, using oxygen (O 2 ) microelectrodes. We further investigated the vertical distribution and activity of pigments in the microbial mats using an imaging pulseamplitude-modulated fluorometer. Microbial mats to at least 16.6-m water depth are net producers of O 2 during the summer period. Net O 2 production ranges from 100-500 mmol m 22 h 21 at incident downwelling irradiances of photosynthetically active radiation (PAR) of 1.0-4.6 mmol quanta m 22 s 21 . Photosynthesis of mat-forming cyanobacteria and diatoms occurs at all lake depths at or close to maximum efficiency. We measured absorption by the pigment arrays at a single water depth and, by assuming that absorption is water-depth invariant, we estimated an area-specific maximum community quantum yield of 0.073 mol carbon per mol photons. A community compensation irradiance of 0.1 mmol quanta m 22 s 21 was estimated, reflecting extreme shade acclimation. These results confirm estimates previously derived from laboratory gas-exchange measurements and imply that even minor changes in the intensity of the incident downwelling irradiance of PAR caused by, for example, changes in the transparency of the ice cover or the optical properties of the water column can significantly alter rates of benthic carbon fixation. In situ measurements were confined to mats with flat surfaces. Laboratory measurements at the surface of mats with pinnacled surfaces revealed a complex small-scale chemical structure at the mat-water interface.

Colonisation by macrobenthos after defaunation d u e to anoxia and sulphide accumulation was studied in 2 field experiments, performed at shallow sites in the southwestern Baltic Sea. In the first experiment (Wismar Bight) a n area was initially covered with dark foil which caused azoic anoxic sediments with high sulphide concentrations (up to 3.4 mm01 I-' at 2 cm depth). After foil removal sulphide concentrations in the upper sediment layer decreased, but a t 10 cm depth high concentrations of 1.65 mm01 I-' sulphide remained even after 69 d. Within 3 d , adults of species living in the upper sediment layer, e.g. the ostracod Cyprideis torosa, appeared as first immigrants. After 1 mo the density, species composition, diversity and evenness of the macrobenthlc community in the experunental area were not significantly different from the reference area. In the second experiment (Hiddensee Island) the colonisation of an area defaunated by a thick cover of decaying algae Fucus vesiculosus was investigated. After removal of the algal mat, recovery of the experimental area (decrease of sulphlde concentrations, colonisation pattern of macrofauna, development of community parameters, recovery tune) was similar to that described for the first colonisation expenment. Laboratory tolerance expennlents revealed that species such a s C. torosa that returned first to the experimental areas also showed highest survival rates under hypoxia and sulphide. In contrast, the amphipod Corophium volutator, the most sensitive species to hypoxia and sulphide, was one of the last animals to reach both experimental areas. The macrobenthic community consists mainly of deposit feeding species with a high tolerance for temporary hypoxic conditions and frequent exposure to sulphide. The few less tolerant species have the capacity to produce large populations within short time periods (e.g. the oligochaete Paranais litoralis) andlor have a high mobility (e.g C. volutator). At these shallow sites in the Baltic Sea a combined set of abiotic and biotic factors seems to favour rapid recovery of a stress-preconditioned macrobenthos from recurring small-scale hypoxic and sulphidic periods which dominate the ecosystem as structuring factors.

Benthic autotrophs in oligotrophic rivers must adapt to and modify their hydrodynamic environment to balance the conflicting requirements of minimal drag (to minimize detachment risks) and maximal exposure to turbulent flow (to maximize nutrient acquisition). We explored flow -organism interactions using the benthic, freshwater alga Didymosphenia geminata. D. geminata forms large mats in swift, oligotrophic alluvial rivers. The physical properties that allow D. geminata to resist detachment and proliferate under these harsh conditions are unknown. We transplanted cobbles with attached D. geminata mats from a riverbed to a flume and used velocimetry and microelectrode profiling to measure hydrodynamic and transport conditions above and within the mats over a wide range of flows. We then removed the mats from the cobbles and repeated the velocimetry measurements. Experiment results indicated that D. geminata mats reduce form-induced stresses and near-bed turbulent velocity fluctuations, which may reduce the risk of detachment. D. geminata mats also increase turbulent shear stress just above mat surfaces, which may enhance water column -mat solute exchange. High friction associated with flow at mat surfaces leads to very low velocities and predominantly diffusive transport within mats, which may in turn favor the retention of solutes derived from organic matter within and below mats. Enhanced mass transfer at mat surfaces and effective solute retention in mat matrices suggest a mechanism by which D. geminata cells acquire nutrients from different sources: advectiondominated transport of water-column nutrients to cells at mat surfaces, and diffusion-dominated transport from decomposing organic matter within mats, with minimal advective losses.

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