Nitrogen (N) input to the coastal oceans has increased considerably because of anthropogenic activities, however, concurrent increases have not occurred in open oceans. It has been suggested that benthic denitrification in sandy coastal sediments is a sink for this N. Sandy sediments are dynamic permeable environments, where electron acceptor and donor concentrations fluctuate over short temporal and spatial scales. The response of denitrifiers to these fluctuations are largely unknown, although previous observations suggest they may denitrify under aerobic conditions. We examined the response of benthic denitrification to fluctuating oxygen concentrations, finding that denitrification not only occurred at high O2 concentrations but was stimulated by frequent switches between oxic and anoxic conditions. Throughout a tidal cycle, in situtranscription of genes for aerobic respiration and denitrification were positively correlated within diverse bacterial classes, regardless of O2 concentrations, indicating that denitrification gene transcription is not strongly regulated by O2 in sandy sediments. This allows microbes to respond rapidly to changing environmental conditions, but also means that denitrification is utilized as an auxiliary respiration under aerobic conditions when imbalances occur in electron donor and acceptor supply. Aerobic denitrification therefore contributes significantly to N-loss in permeable sediments making the process an important sink for anthropogenic N-inputs.

Observations of waves, setup, and wave-driven mean flows were made on a steep coral forereef and its associated lagoonal system on the north shore of Moorea, French Polynesia. Despite the steep and complex geometry of the forereef, and wave amplitudes that are nearly equal to the mean water depth, linear wave theory showed very good agreement with data. Measurements across the reef illustrate the importance of including both wave transport (owing to Stokes drift), as well as the Eulerian mean transport when computing the fluxes over the reef. Finally, the observed setup closely follows the theoretical relationship derived from classic radiation stress theory, although the two parameters that appear in the model—one reflecting wave breaking, the other the effective depth over the reef crest—must be chosen to match theory to data.

We investigated microbial pathways of nitrogen transformation in highly permeable sediments from the German Bight (South‐East North Sea) by incubating sediment cores percolated with ¹⁵N‐labeled substrates under near in situ conditions. In incubations with added , production of occurred while the sediment was oxic, indicating ammonia oxidation. Similarly, production during incubations indicated nitrite oxidation. Taken together these findings provide direct evidence of high nitrification rates within German Bight sands. The production of ¹⁵N‐N₂ on addition of revealed high denitrification rates within the sediment under oxic and anoxic conditions. Denitrification rates were strongly and positively correlated with oxygen consumption rates, suggesting that denitrification is controlled by organic matter availability. Nitrification and denitrification rates were of the same magnitude and the rapid production of ¹⁵N‐N₂ in incubations with added confirmed close coupling of the two processes. Areal rates of N‐transformation were estimated taking advective transport of substrates into account and integrating volumetric rates over modeled oxygen and nitrate penetration depths, these ranged between 22 μmol N m⁻² h⁻¹ and 94 μmol N m⁻² h⁻¹. Furthermore, results from the ¹⁵N‐labeling experiments show that these subtidal permeable sediments are, in sharp contrast to common belief, a substantial source of N₂O. Our combined results show that nitrification fuels denitrification by providing an additional source of nitrate, and as such masks true N‐losses from these highly eutrophic sediments. Given the widespread occurrence of anthropogenically influenced permeable sediments, coupled benthic nitrification–denitrification might have an important but so far neglected role in N‐loss from shelf sediments.