The Arabian Sea harbours one of the three major oxygen minimum zones (OMZs) in the world's oceans, and it alone is estimated to account for ~10–20 % of global oceanic nitrogen (N) loss. While actual rate measurements have been few, the consistently high accumulation of nitrite (NO₂^&minus;) coinciding with suboxic conditions in the central-northeastern part of the Arabian Sea has led to the general belief that this is the region where active N-loss takes place. Most subsequent field studies on N-loss have thus been drawn almost exclusively to the central-NE. However, a recent study measured only low to undetectable N-loss activities in this region, compared to orders of magnitude higher rates measured towards the Omani Shelf where little NO₂^&minus; accumulated (Jensen et al., 2011). In this paper, we further explore this discrepancy by comparing the NO₂^&minus;-producing and consuming processes, and examining the relationship between the overall NO₂^&minus; balance and active N-loss in the Arabian Sea. Based on a combination of ¹⁵N-incubation experiments, functional gene expression analyses, nutrient profiling and flux modeling, our results showed that NO₂^&minus; accumulated in the central-NE Arabian Sea due to a net production via primarily active nitrate (NO₃^&minus;) reduction and to a certain extent ammonia oxidation. Meanwhile, NO₂^&minus; consumption via anammox, denitrification and dissimilatory nitrate/nitrite reduction to ammonium (NH₄⁺) were hardly detectable in this region, though some loss to NO₂^&minus; oxidation was predicted from modeled NO₃^&minus; changes. No significant correlation was found between NO₂^&minus; and N-loss rates (<i>p</i>>0.05). This discrepancy between NO₂^&minus; accumulation and lack of active N-loss in the central-NE Arabian Sea is best explained by the deficiency of labile organic matter that is directly needed for further NO₂^&minus; reduction to N₂O, N₂ and NH₄⁺, and indirectly for the remineralized NH₄⁺ required by anammox. Altogether, our data do not support the long-held view that NO₂^&minus; accumulation is a direct activity indicator of N-loss in the Arabian Sea or other OMZs. Instead, NO₂<sup>&...

The Arabian Sea harbours one of the three major oxygen minimum zones (OMZs) in the world's oceans, and it alone is estimated to account for ~10–20% of global oceanic nitrogen (N) loss. While actual rate measurements have been few, the consistently high accumulation of nitrite (NO₂^&minus;) coinciding with suboxic conditions in the central-northeastern part of the Arabian Sea has led to the general belief that this is the region where active N-loss takes place. Most subsequent field studies on N-loss have thus been drawn almost exclusively to the central-NE. However, a recent study measured only low to undetectable N-loss activities in this region, compared to orders of magnitude higher rates measured towards the Omani shelf where little NO₂^&minus; accumulated (Jensen et al., 2011). In this paper, we further explore this discrepancy by comparing the NO₂^&minus; producing and consuming processes, and examining the relationship between the overall NO₂^&minus; balance and active N-loss in the Arabian Sea. Based on a combination of ¹⁵N-incubation experiments, functional gene expression analyses, nutrient profiling and flux modeling, our results showed that NO₂^&minus; accumulated in the Central-NE Arabian Sea due to a net production via primarily active nitrate (NO₃^&minus;) reduction and to a certain extent ammonia oxidation. Meanwhile, NO₂^&minus; consumption via anammox, denitrification and dissimilatory nitrate/nitrite reduction to ammonium (NH₄⁺) were hardly detectable in this region, though some loss to NO₂^&minus; oxidation was predicted from modeled NO₃^&minus; changes. No significant correlation was found between NO₂^&minus; and N-loss rates (<i>p</i>>0.05). This discrepancy between NO₂^&minus; accumulation and lack of active N-loss in the Central-NE Arabian Sea is best explained by the deficiency of organic matter that is directly needed for further NO₂^&minus; reduction to N₂O, N₂ and NH₄⁺, and indirectly for the remineralized NH₄⁺ required by anammox. Altogether, our data do not support the long-held view that NO₂^&minus; accumulation is a direct activity indicator of N-loss in the Arabian Sea or other OMZs. Instead, NO<sub>...

Marine habitats worldwide are increasingly pressurized by climate change, especially along the Antarctic Peninsula. Well-studied areas in front of rapidly retreating tidewater glaciers like Potter Cove are representative for similar coastal environments and, therefore, shed light on habitat formation and development on not only a local but also regional scale. The objective of this study was to provide insights into habitat distribution in Potter Cove, King George Island, Antarctica, and to evaluate the associated environmental processes. Furthermore, an assessment concerning the future development of the habitats is provided. To describe the seafloor habitats in Potter Cove, an acoustic seabed discrimination system (RoxAnn) was used in combination with underwater video images and sediment samples. Due to the absence of wave and current measurements in the study area, bed shear stress estimates served to delineate zones prone to sediment erosion. On the basis of the investigations, two habitat classes were identified in Potter Cove, namely soft-sediment and stone habitats that, besides influences from sediment supply and coastal morphology, are controlled by sediment erosion. A future expansion of the stone habitat is predicted if recent environmental change trends continue. Possible implications for the Potter Cove environment, and other coastal ecosystems under similar pressure, include changes in biomass and species composition.

[1] We applied a hydrogeological numerical simulation to investigate spatial and temporal variations in groundwater flow and discharge at a well-studied tidal flat margin in the German Wadden Sea. The simulation was forced by high-resolution sea level boundary conditions. Accuracy of the simulation was checked by comparing simulated with measured pressure heads obtained from direct burial pressure sensor measurements. Overall discharge calculated from the simulation for a time period of almost 12 month averages 0.97 m 3 per meter of margin length per tide but may vary significantly, most pronounced with the spring-neap tide cycle. The simulation predicts that the maximum hydraulic gradient at ebb tide controls the magnitude of discharge, whereas duration of ebb tide is less important. Additionally, simulated and measured pressure gradients can be used to investigate the influence of varying lithology on the groundwater flow regime. The presence of a low permeable Holocene silt layer, ubiquitous in the study area, creates a layered unconfined/semiconfined aquifer system. Both parts of the aquifer are hydraulically connected to the deeply incising tidal creek but show a different response to the tidal forcing. Groundwater flow in the upper (unconfined) margin sediments can be described as a circulation of seawater, where inflow and outflow are coupled to changes in saturation. Sediments below the confining layer release groundwater from storage during falling and ebb tide and recharge during rising and high tide. We can show that tidally driven water exchange between semiconfined aquifers and coastal waters may have a major impact on total groundwater discharge.Citation: Riedel, T., K. Lettmann, M. Beck, and H.-J. Brumsack (2010), Tidal variations in groundwater storage and associated discharge from an intertidal coastal aquifer,

Marine oxygen deficient zones (ODZs) have long been identified as sites of fixed nitrogen (N) loss. However, the mechanisms and rates of N loss have been debated, and traditional methods for measuring these rates are labor‐intensive and may miss hot spots in spatially and temporally variable environments. Here we estimate rates of heterotrophic nitrate reduction, heterotrophic nitrite reduction (denitrification), nitrite oxidation, and anaerobic ammonium oxidation (anammox) at a coastal site in the eastern tropical South Pacific (ETSP) ODZ based on high‐resolution concentration and natural abundance stable isotope measurements of nitrate (NO3−) and nitrite (NO2−). These measurements were used to estimate process rates using a two‐step inverse modeling approach. The modeled rates were sensitive to assumed isotope effects for NO3− reduction and NO2− oxidation. Nevertheless, we addressed two questions surrounding the fates of NO2− in the ODZ: (1) Is NO2− being primarily reduced to N2 or oxidized to NO3− in the ODZ? and (2) what are the contributions of anammox and denitrification to NO2− removal? Depth‐integrated rates from the model suggest that 72–88% of the NO2− produced in the ODZ was oxidized back to NO3−, while 12–28% of NO2− was reduced to N2. Furthermore, our model suggested that 36–74% of NO2− loss was due to anammox, with the remainder due to denitrification. These model results generally agreed with previously measured rates, though with a large range of uncertainty, and they provide a long‐term integrated view that compliments incubation experiments to obtain a broader picture of N cycling in ODZs.

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Suspended particulate matter (SPM) fluxes and dynamics are investigated in the East Frisian Wadden Sea using a coupled modeling system based on a hydrodynamical model [the General Estuarine Transport Model (GETM)], a third-generation wave model [Simulating Waves Nearshore (SWAN)], and a SPM module attached to GETM. Sedimentological observations document that, over longer time periods, finer sediment fractions disappear from the Wadden Sea Region. In order to understand this phenomenon, a series of numerical scenarios were formulated to discriminate possible influences such as tidal currents, wind-enhanced currents, and wind-generated surface waves. Starting with a simple tidal forcing, the considered scenarios are designed to increase the realism step by step to include moderate and strong winds and waves and, finally, to encompass the full effects of one of the strongest storm surges affecting the region in the last hundred years (Storm Britta in November 2006). The results presented here indicate that moderate weather conditions with wind speeds up to 7.5 m/s and small waves lead to a net import of SPM into the East Frisian Wadden Sea. Waves play only a negligible role during these conditions. However, for stronger wind conditions with speeds above 13 m/s, wind-generated surface waves have a significant impact on SPM dynamics. Under storm conditions, the numerical results demonstrate that sediments are eroded in front of the barrier islands by enhanced wave action and are transported into the back-barrier basins by the currents. Furthermore, sediment erosion due to waves is significantly enhanced on the tidal flats. Finally, fine sediments are flushed out of the tidal basins due to the combined effect of strong erosion by wind-generated waves and a longer residence time in the water column because of their smaller settling velocities compared to coarser sediments.