Profiles of oxygen and nitrate were measured with microsensors in sediment surrounding burrow structures of the polychaete Nereis diversicolor in a shallow mudflat in Limfjorden, Denmark. Rates and spatial distribution of reactions involving oxygen and nitrate (oxygen consumption, nitrification and nitrate reduction) in the vicinity of burrow structures and surface sediment were calculated and identified from concentration profiles. The burrow walls of the natural N. diversicolor population increased the surface area of the sediment -water interface several-fold and introduced oxic microniches with high metabolic activity into otherwise anoxic sediment. Nitrification and nitrate reduction were spatially separated processes, with nitrification restricted to the 0.9 to 1.5 mm thick oxic zone and nitrate-reduction occurring as a strictly anaerobic process in the nitratecontaining layers next to the oxic zone. At a density of 2560 individuals m -2 , burrow walls of N. diversicolor account for 12 to 40% of bulk sediment oxygen uptake and 50 to 77% and 58 to 82% of bulk nitrification and nitrate reduction, respectively. Thus, burrow structures of macro-infauna are potentially a major sediment compartment involved in nitrogen cycling and may promote loss of bioavailable nitrogen from the sediment.

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Sulfate reduction rates and various parameters related to the sulfur cycling were investigated in situ and experimentally by a mm approach in sediment surrounding burrows of the polychaete Arenicola marina. Sulfate reduction rates were immediately affected in a 5 to 15 mmthick zone surrounding the tail shaft. Rates were depressed by 52% in the burrow wall and increased with distance towards ambient rates. Experimental use of artificially ventilated burrows showed that the reduction was mediated by introduction of oxygenated surface water by irrigation, which suppressed possible stimulating effect of increase in organic carbon availability from mucus secretion by the worm. Furthermore, irrigation and sediment reworking by A. marina increased the content of biological reactive Fe(III) in the feeding funnel and in a narrow zone around the tail shaft, creating environments suitable for dissimilative Fe(III) reduction. Total reduced sulfide pools showed that reoxidation of reduced sulfide species occurred along the burrow and in the feeding funnel of A. marina. Reoxidation in the tail shaft region was mediated by irrigation, while the reduction in the feeding funnel probably occurred due to the combined effect of irrigation and sediment reworking. In general, A. marina has a great impact on sediment sulfur biogeochemistry, suggesting that bioturbation depresses sulfate reduction and increases the importance of Fe(III) as an electron acceptor in carbon oxidation. It is also indicated that A. marina reduces permanent burial of reduced sulfide and lowers the steady-state level of total reduced sulfide in the sediment.

A new 33 P-tracer technique was used to define low-level phosphate (Pi) uptake kinetics for a dominant tropical seagrass, Thalassia testudinum. We established seagrass Pi uptake kinetics at high (western Bay) and low (eastern Bay) nutrient sites in a fine-grained carbonate lagoon of south Florida (Florida Bay), where P limitation of seagrass has been documented. Sediment Pi adsorption kinetics were also investigated to test whether carbonate sediments could sequester Pi to the threshold levels we established for T. testudinum, defined as S min or the physiological threshold value where Pi uptake is positive. At Pi levels characteristic of Florida Bay (Յ0.26 mol L Ϫ1), leaf and root Pi uptake was linear with increasing levels of Pi, and similar affinities (␣) were found for leaves and roots (0.12-0.30 and 0.10-0.20 mol g dry weight Ϫ1 h Ϫ1). S min for roots and leaves was extremely low (0.004-0.009 mol L Ϫ1 ) regardless of the P status of the site. While T. testudinum was able to take up Pi at nanomolar levels, the uptake rates were insufficient for plant requirements. Thus, sediment pools or transient fluxes of P to the water column must sustain high seagrass production rates in the Bay. Sediment adsorption-desorption equilibrium for Pi was 10-fold lower in the eastern versus western Bay sites, meeting Ͻ10% and Ͼ87% of the P demand for T. testudinum, respectively, a result that might account for the reported P limitation of seagrass biomass and production in eastern Florida Bay.