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Ocean deoxygenation threatens ocean productivity, carbon cycling and marine ecosystems. Shelf seas are highly dynamic regions, which contributes to their high productivity and also makes monitoring and constraining their oxygen status a challenge. Here, using the temperate Celtic shelf sea (April and July 2015) as a case study, we present high‐resolution ocean glider observations of turbulence and biogeochemical parameters, demonstrating the potential of these autonomous platforms for environmental monitoring. We estimate vertical turbulent oxygen fluxes be 25% higher in summer than in spring, due to the presence of subsurface chlorophyll and associated oxygen maxima at the seasonal thermocline. We demonstrate that glider‐based estimates were able to constrain similar bottom layer respiration rates as those derived from traditional ship‐based measurements. We suggest ocean gliders are useful monitoring tools that can aid sustainable management of shelf sea ecosystems. Plain Language Summary: Oxygen levels in the ocean are decreasing. Oxygen is needed by almost all life in the oceans, thus low oxygen levels can result in dramatic changes to marine ecosystems. The decrease in oxygen levels is particularly alarming in the coastal ocean or "shelf sea" (the region between the land and the deep open ocean), which supports the majority of global fisheries (over 90%). Therefore, there is both an urgent societal and an environmental need to better understand processes influencing oxygen levels in the coastal ocean, such as physical water circulation and mixing, and biological oxygen production and consumption. Here we present turbulent mixing data collected over 40 days in a typical shelf sea using an unmanned, autonomous underwater vehicle (AUV) called an ocean glider. We use this data combined with oxygen data to calculate the contribution of physical oxygen fluxes to the observed change in oxygen, and from this deduce how much of the change was driven by biology. We prove that AUVs may be used as an effective method for monitoring oxygen dynamics and that this can aid responsible marine management in shelf seas. Key Points: Glider observations are able to integrate physical and biogeochemical processes influencing oxygen depletion in shelf seasMixing and advection replenish respired oxygen in the bottom mixed layerThe presence of a subsurface oxygen maxima results in larger oxygen fluxes to the bottom mixed layer in summer compared to spring [ABSTRACT FROM AUTHOR]

Abstract Vertical structures of the water column, especially the surface mixed layer and the pycnocline layer, are important in the mediation of vertical transportation of oceanic matter, momentum, heat and salt. Field observations were conducted to investigate vertical structures and mixed layer depth (MLD) in the shelf sea of the northern South China Sea, where the Pearl River plume enters with distinct extents in different seasons. A total of six cruises were performed in spring, summer and autumn from 2006 to 2016. Four different types of vertical structures, A-α, A-β, B and C types, were identified in the upper water layer. The A-α type was characterized by a gradual increase in density with depth, while the A-β type was characterized by an increase in density in a very thin surface layer (<10 m) with a sub-layer of density uniformity. The B type was defined as an almost uniform layer over the surface water column. The C type was defined as an entirely uniform layer throughout the whole water column. Both the A-α and A-β types occurred in spring and summer. The B type appeared in all three seasons, and the C type mostly appeared in spring and autumn. The MLD was shallower in summer and deeper in autumn. The A-α type could change to the B type under strong wind energy. Two 24-h mooring surveys, one within the Pearl River plume and the other beyond the plume zone, were conducted to examine the MLD temporal variations. The A-α and B types prevailed in and beyond the plume zone, respectively. The A-α type resulted from the strong stratification formed by freshwater and sea water, while the B type was mainly induced by air-sea interface buoyancy fluxes. We proposed that the A-α type should be considered in studies of the mixing and transporting of matter, salt, heat and energy in all the global costal seas. Highlights • We characterized four types of vertical structures in the shelf sea. • The formations of the four water types were discussed. • The river plume zone had no surface mixed layer with vertically uniform density under calm condition. [ABSTRACT FROM AUTHOR]

Shelf sediments play a vital role in global biogeochemical cycling and are particularly important areas of oxygen consumption and carbon mineralisation. Total benthic oxygen uptake, the sum of diffusive and faunal mediated uptake, is a robust proxy to quantify carbon mineralisation. However, oxygen uptake rates are dynamic, due to the diagenetic processes within the sediment, and can be spatially and temporally variable. Four benthic sites in the Celtic Sea, encompassing gradients of cohesive to permeable sediments, were sampled over four cruises to capture seasonal and spatial changes in oxygen dynamics. Total oxygen uptake (TOU) rates were measured through a suite of incubation experiments and oxygen microelectrode profiles were taken across all four benthic sites to provide the oxygen penetration depth and diffusive oxygen uptake (DOU) rates. The difference between TOU and DOU allowed for quantification of the fauna mediated oxygen uptake and diffusive uptake. High resolution measurements showed clear seasonal and spatial trends, with higher oxygen uptake rates measured in cohesive sediments compared to the permeable sediment. The significant differences in oxygen dynamics between the sediment types were consistent between seasons, with increasing oxygen consumption during and after the phytoplankton bloom. Carbon mineralisation in shelf sediments is strongly influenced by sediment type and seasonality. [ABSTRACT FROM AUTHOR]

The stability of stratified flows at locations in the Clyde, Irish and Celtic Seas on the UK Continental Shelf is examined. Flows are averaged over periods of 12–30 min in each hour, corresponding to the times taken to obtain reliable estimates of the rate of dissipation of turbulent kinetic energy per unit mass, ε . The Taylor–Goldstein equation is solved to find the maximum growth rate of small disturbances to these averaged flows, and the critical gradient Richardson number, Ri c . The proportion of unstable periods where the minimum gradient Richardson number, Ri min , is less than Ri c is about 35%. Cases are found in which Ri c < 0.25; 37% of the flows with Ri min < 0.25 are stable, and Ri c < 0.24 in 68% of the periods where Ri min < 0.25. Marginal conditions with 0.8 < Ri min /Ri c < 1.2 occur in 30% of the periods examined. The mean dissipation rate at the level where the fastest growing disturbance has its maximum amplitude is examined to assess whether the turbulence there is isotropic and how it relates to the wave–turbulence boundary. It is concluded that there is a background level of dissipation that is augmented by instability; instability of the averaged flow does not account for all the turbulence observed in mid-water. The effects of a horizontal separation of the measurements of shear and buoyancy are considered. The available data do not support the hypothesis that the turbulent flows observed on the UK shelf adjust rapidly to conditions that are close to being marginal, or that flows in a particular location and period of time in one sea have stability characteristics that are very similar to those in another. [ABSTRACT FROM AUTHOR]

Tidal fronts are associated with the transition between homogeneous and vertically stratified water and are characterized by the simultaneous availability of light and nutrients that enhance the growth of marine productivity. We study the variability in the position of two tidal fronts located in Patagonia Argentina: the San Matías and Valdés fronts. The rate of tidal dissipation in these regions is among the highest of the world oceans. The study is based on the analysis of over 1200 satellite derived sea surface temperature images. The results indicate that the mean monthly position of both fronts is strongly linked to the characteristics of the bottom topography. In response to increasing surface heat flux the fronts displace toward shallower areas. Similarly, a slight displacement towards deeper waters is observed when the heat flux decreases. High frequency variability is revealed by the standard deviation around monthly averages. At the mouth of the San Matías gulf, the front location variability in the spring–neap cycle is around 10 km, while east of the Valdés Peninsula the fortnightly cycle is masked by high frequency fluctuations (~30 km) governed by meanders and meso-scale filaments. A simple conceptual model is proposed which suggests that the mean frontal position is determined by the bottom topography while its seasonal variability is driven by the surface heat flux, and the front intensity is modulated by the spring–neap transition. [ABSTRACT FROM AUTHOR]

Coastal shelf seas are zones of intense nutrient cycling, where a strong coupling between the sediment and the water column enhances primary productivity. To identify factors that control the strength of this benthic-pelagic coupling we measured sediment characteristics, solute fluxes, and porewater nutrient profiles in spring along a south - north transect in the North Sea crossing distinct regions: the shallow Oyster Grounds closest to the Dutch shore, the shallow Dogger Bank, the 80-m deep central North Sea, and the 150-m deep Fladen Grounds between the north of Scotland and Norway. The data were used to constrain rates of different mineralization processes with the 1-D diagenetic model (OMEXDIA). Surprisingly, we found no major differences in the biogeochemical signature along the 670 km long North Sea transect, despite sediments ranging in median grainsize from 25 to 217 μm, and a permeability range >3 orders of magnitude. Total carbon mineralization ranged between 4 and 13.5 mmol C m−2 d−1, and decreased significantly northward. Oxic mineralization was the dominant mineralization process in all studied sites. Finest, least permeable sediments were found in the Fladen Grounds where highest denitrification rates were recorded, linked to high nitrate concentrations in the overlying water. The coarsest, most permeable sediments of the shallow Dogger Bank represented a transition area between the Oyster Grounds, where oxic mineralization was highest (75–90%), and the central North Sea samples, where anoxic mineralization increased relative to oxic mineralization due to higher bioturbation rates (oxic: 59–72%, anoxic: 27–39%). Overall, denitrification rates increased, while phosphorus removal tended to decrease northward. This contrasting behaviour in nitrogen and phosphorus removal was identified as a possible cause for decreasing DIN:DIP ratios in the water column towards the north. • The combination of field measurements and diagenetic modelling showed rapid mineralization of fresh organic matter in the sediments of a region of the North Sea extending from 100 km from the Dutch coast up to the Fladen Grounds in the northern part of the North Sea. • This resulted in an overall low nutrient build-up, despite significant differences in sediment characteristics and environmental conditions. • The latitudinal gradient showed a northward decrease in total mineralization rates. • Additional northward gradients showed an increased removal of nitrogen and decreased removal of phosphorus. [ABSTRACT FROM AUTHOR]