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Up to now, three-dimensional temperature responses in mid-latitude shallow shelf seas to large-scale climate variabilities are still poorly understood. Combining observations and high-resolution numerical simulations, this study investigated the three-dimensional structure of interannual-to-decadal variations in water temperature in the Yellow Sea and underlying mechanisms during 1998‒2021. Both sea surface temperature (SST) and bottom layer temperature (BLT) in the majority of the Yellow Sea experienced decadal trends, which were negative during 1998‒2010 but were more significantly positive during 2011‒2021. Magnitudes of decadal SST trends had seasonal variations, being much larger in winter than in summer, but those of decadal BLT trends had no significant seasonal variations. Temperatures below the thermocline experienced more significant decadal trends than SST in spring‒summer seasons. This caused increasing trends during 1998‒2010 but decreasing trends during 2011‒2021 in summer stratification. Changes in decadal temperature trends were linked to activities of the Siberia High and Western Pacific pattern in winter. During 1998‒2010, strengthening Siberia High and weakening Western Pacific pattern enhanced surface heat loss and thus induced cooling trend throughout the water column in the Yellow Sea in winter. However, increasing surface heat gain in subsequent spring‒summer reduced the surface layer cooling trend and resulted in upper layer experiencing weaker cooling trend than lower layer. Seasonally reversing surface heat flux trends generated increasing summer stratification. During 2011‒2021, opposite but more intense situations occurred, with weakening Siberia High and strengthening Western Pacific pattern causing warming temperature and decreasing summer stratification in the Yellow Sea. [ABSTRACT FROM AUTHOR]

The vertically generalized production model (VGPM) is one of the most important methods for estimating marine net primary productivity (PP) using remote sensing. However, different data sources and parameterization schemes of the input variables for the VGPM can introduce uncertainties to the model results. This study compared the PP results from different data sources and parameterization schemes of three major input variables (i.e., chlorophyll-a concentration ( C o p t ), euphotic depth ( Z e u ), and maximum photosynthetic rate ( P o p t B )) and evaluated the sensitivity of VGPM in the Yellow and Bohai Seas on the inputs. The results showed that the sensitivity in the annual mean PP was approximately 40%. Seasonally, the sensitivity was lowest in the spring (35%), highest in the winter (70%), and approximately 60% in the summer and autumn. Spatially, the sensitivity in nearshore water (water depth < 40 m) was more than 60% and around two times higher than that in deep water areas. Nevertheless, all VGPM results showed a decline trend in the PP from 2003 to 2020 in the Yellow and Bohai Seas. The influence of P o p t B and C o p t was important for the magnitude of annual mean PP. The PP seasonal variation pattern was highly related to the parameterization scheme of P o p t B , whereas the spatial distribution was mostly sensitive to the data sources of C o p t . [ABSTRACT FROM AUTHOR]

Ferromanganese concretions commonly occur in shallow‐water coastal regions worldwide. In the Baltic Sea, they can record information about past and present underwater environments and could be a potential source for critical raw materials. We report on their microstructural characteristics and magnetic properties and link them to their formation mechanisms and environmental significance. Microstructural investigations from nano‐ and micro‐computed tomography, electron microscopy, and micro‐X‐ray fluorescence elemental mapping reveal diverse growth patterns within concretions of different morphologies. Alternating Fe‐ and Mn‐rich growth bands indicate fluctuating redox conditions during formation. Bullet‐shaped magnetofossils, produced by magnetotactic bacteria, are present, which suggests the influence of bacterial activity on concretion formation. Spheroidal concretions, which occur in deeper and more tranquil environments, have enhanced microbial biomineralization and magnetofossil preservation. Conversely, crusts and discoidal concretions from shallower and more energetic environments contain fewer magnetofossils and have a greater detrital content. Our results provide insights into concretion formation mechanisms and highlight the importance of diagenetic processes, oxygen availability, and bacterial activity in the Baltic Sea. Key Points: Magnetic minerals within shallow water Fe‐Mn concretions can provide valuable environmental information about their formationMagnetic properties are linked to specific growth patterns in Baltic Sea Fe‐Mn concretionsSpheroidal and crust/discoidal concretions are dominated by biogenic and pedogenic magnetic phases, respectively [ABSTRACT FROM AUTHOR]

On the Northwest European Shelf rivers provide freshwater to the coastal seas. This coastal freshwater can be misrepresented in ocean models without effective coastal resolution. This leaves an unanswered question; is freshwater retained around Scotland and what affects its variability? Here, we deploy and run an unstructured model with enhanced coastal resolution to simulate the Northwest European Shelf from 1993 to 2019, the Scottish Shelf Water‐Reanalysis Service (SSW‐RS) long‐time run. The unstructured nature of the model grid means it more accurately captures a "bubble" of Coastal Water than a 7 km structured grid model (the Atlantic Margin Model 7 km). Surface salinity in the SSW‐RS shows salinity fronts within 80 km of the coast around west and north Scotland that disintegrates east of Orkney. There are periods characterized by high coastal salinity when freshwater is more actively advected away from the coast. Empirical orthogonal function statistical analysis shows the first two modes in surface salinity account for 66% of the variance. The first mode correlates with North Atlantic Oscillation and the salinity driven velocity variability which change the salinity through advection and diffusion. The second mode correlates with Ekman transport variability where the north of Scotland acts as a wedge causing bipolar dynamics either side. Freshwater is trapped in the west, while saline water from the north reduces the freshwater pathway to the North Sea. This is important for salinity distribution, stratification in the North Sea, marine habitats and frontal transport. Plain Language Summary: The outflow of riverwater into the sea around Scotland is important for marine habitats, nutrient concentrations and the transport of pollution or planktonic larvae. The response of the sea to the sum of all river outflow is often missed in simulations of the ocean and leaves questions about how much water is retained near the coast and when it can escape. In our ocean simulation the Scottish Shelf Water‐Reanalysis Service long run, we tackle this problem with high coastal resolution and an extensive river data set. Our simulation shows an improvement in salinity over a coarser resolution model of the same region around Scotland. Generally fresh, river‐sourced water is retained by a front within 80 km of the coast around Scotland to the west of Orkney but this pattern is disrupted to the east of Orkney. Statistics show the majority of the variability in the freshwater around Scotland goes through stages at the time period of years when is it captured close to the coast and stages when it is more able to move away from the coast. This is found to correlate with density driven currents and the wind speed which uses the north of Scotland as a wedge for the water, causing bipolar dynamics either side. Key Points: Our simulation accurately resolves coastal freshwater due to its variable resolutionA coastal salinity front is found west of Orkney with periodic release of its freshwater east of OrkneyThe movement of freshwater into and away from the coastal region is influenced by salinity driven currents and wind [ABSTRACT FROM AUTHOR]

Abstract Ferromanganese concretions commonly occur in shallow‐water coastal regions worldwide. In the Baltic Sea, they can record information about past and present underwater environments and could be a potential source for critical raw materials. We report on their microstructural characteristics and magnetic properties and link them to their formation mechanisms and environmental significance. Microstructural investigations from nano‐ and micro‐computed tomography, electron microscopy, and micro‐X‐ray fluorescence elemental mapping reveal diverse growth patterns within concretions of different morphologies. Alternating Fe‐ and Mn‐rich growth bands indicate fluctuating redox conditions during formation. Bullet‐shaped magnetofossils, produced by magnetotactic bacteria, are present, which suggests the influence of bacterial activity on concretion formation. Spheroidal concretions, which occur in deeper and more tranquil environments, have enhanced microbial biomineralization and magnetofossil preservation. Conversely, crusts and discoidal concretions from shallower and more energetic environments contain fewer magnetofossils and have a greater detrital content. Our results provide insights into concretion formation mechanisms and highlight the importance of diagenetic processes, oxygen availability, and bacterial activity in the Baltic Sea.

Abstract On the Northwest European Shelf rivers provide freshwater to the coastal seas. This coastal freshwater can be misrepresented in ocean models without effective coastal resolution. This leaves an unanswered question; is freshwater retained around Scotland and what affects its variability? Here, we deploy and run an unstructured model with enhanced coastal resolution to simulate the Northwest European Shelf from 1993 to 2019, the Scottish Shelf Water‐Reanalysis Service (SSW‐RS) long‐time run. The unstructured nature of the model grid means it more accurately captures a “bubble” of Coastal Water than a 7 km structured grid model (the Atlantic Margin Model 7 km). Surface salinity in the SSW‐RS shows salinity fronts within 80 km of the coast around west and north Scotland that disintegrates east of Orkney. There are periods characterized by high coastal salinity when freshwater is more actively advected away from the coast. Empirical orthogonal function statistical analysis shows the first two modes in surface salinity account for 66% of the variance. The first mode correlates with North Atlantic Oscillation and the salinity driven velocity variability which change the salinity through advection and diffusion. The second mode correlates with Ekman transport variability where the north of Scotland acts as a wedge causing bipolar dynamics either side. Freshwater is trapped in the west, while saline water from the north reduces the freshwater pathway to the North Sea. This is important for salinity distribution, stratification in the North Sea, marine habitats and frontal transport.

Monitoring marine primary productivity (PP) is crucial for understanding changes in the marine ecosystem. Based on satellite data and the vertically generalized production model (VGPM), this study investigated the spatiotemporal distribution and long-term trend of PP in the Yellow and Bohai Seas (YBSs) from 2003 to 2020. By using the calibrated satellite data and optimized parameterization scheme, the accuracy of the PP results in the YBSs was significantly improved compared to online PP products. The annual mean PP in the YBSs from 2003 to 2020 was 523.8 mgC / (m 2 · d) , with significant seasonal and interannual differences. Seasonally, PP in the Yellow Sea and the Bohai Sea exhibited bimodal (two peaks in May and October) and unimodal (one peak in June) variation, respectively. The magnitude of mean PP in the YBSs was ranked as spring > summer > autumn > winter, with spring PP (~1000 mgC / (m 2 · d) ) contributing more than 40% of the annual PP. The annual mean PP in the YBSs showed an overall decrease from 2003 to 2020, with a decrease rate of 5–6 mgC / (m 2 · d) / y . The interannual variation of the PP was mainly related to the variability of the chlorophyll-a concentration and was essentially inverse to the phases of the Pacific Decadal Oscillation and the El Niño-Southern Oscillation. [ABSTRACT FROM AUTHOR]

We investigated seasonal variations in the dissolved nutrients and organic matter (DOM) in the Yellow Sea and the East China Sea shelf waters from 2017 to 2021. Dissolved organic nitrogen and phosphorus (DON and DOP) concentrations were the highest in the upper layer (<30 m) during the time of spring and autumn bloom and were the lowest in summer. In spring, DON and DOP contributed up to approximately 90% and 80% of the total dissolved nitrogen (TDN) and phosphorus (TDP), respectively. The C:N:P stoichiometry of the DOM pool in spring and autumn indicates that freshly in-situ produced DOM is N-rich compared to the DOM observed in summer and winter, resulting in a dissolved inorganic nitrogen (DIN)-limited environment in the upper layer of seawater during spring and autumn. Conversely, in summer, the highest C:N ratios (∼27) of the DOM pool revealed the DOM remained after the degradation and formation of the refractory pool by marine bacteria. In winter, all nutrients were vertically uniform due to active vertical mixing. This study highlights notable seasonal variations in the cycling of DON and DOP, leading to changes in their bioavailability and subsequent ecosystem responses in the shelf sea. • The highest DON and DOP concentrations were observed in subsurface chlorophyll maximum layers during the spring bloom. • The C:N:P stoichiometry of DOM was the highest in summer, indicating the largest amount of recalcitrant DOM. • Fresh DOM produced by phytoplankton could be a key nutrient source in low-nutrient, stratified waters during summer. [ABSTRACT FROM AUTHOR]

Over the satellite record, the Barents Sea winter maximum in sea ice extent has declined and was increasingly limited to areas north of the Polar Front after 2005 by warming Atlantic Water (AW) and Barents Sea Water (BSW). Sea ice extent here continues to garner interest, not least because it is associated with extreme winter weather in Europe and Asia. Previous model studies suggest there is a possibility that natural variability will cause southward re‐expansion of the lost sea ice cover but reducing uncertainties requires a better understanding of the processes driving BSW variability. To address questions about BSW variability, we used a high‐resolution model validated with observations over 1985–2014 to calculate the watermass transport, heat, and freshwater budgets within the central Barents Sea, south of the Polar Front. The model shows BSW volume minima events in years centering at 1990 and 2004, meaning a reduction in the Barents Sea's volume reservoir (also termed "memory") of water that is consistent with historical BSW properties. Both events were preceded by extensive winter sea ice and substantial summer net sea ice melt. The event in 2004 was more extreme and led to warming AW occupying a greater volume in the Barents Sea after 2005. The reduced "memory" of BSW volume could impede a return to the more extensive winter sea ice regime and make further reduction in winter sea ice possible. Plain Language Summary: Winter sea ice in the Barents Sea, in the eastern Arctic Ocean, has been in decline, particularly since 2000. The sea ice extent in this region is associated with atmospheric weather conditions during winter in Europe and Asia. An important factor in understanding the sea ice retreat is understanding the variability in Barents Sea temperature and salinity. Using a high‐resolution simulation, the source of variability in Barents Sea Water (BSW) properties has been quantified. This shows two events, 1990 and 2004, when BSW volume reduced, giving less "memory" of previous conditions. The event in 2004 was larger and followed a large net sea ice melt. The volume of BSW present in the Barents Sea did not fully recover after 2005. This could make it more difficult for winter sea ice to return to the region. Key Points: We identify events of minimum dense water volume in the Barents Sea, with the 2002–2005 event being unique with large sea ice importDuring this event, a freshwater anomaly from sea ice melt enhanced the salinity gradient, reducing dense water formation and exportAfter the event in 2006–2014, the proportion of dense water present in the southern Barents Sea remained smaller than during 1985–2002 [ABSTRACT FROM AUTHOR]

The object of this article is to define the facilities construction for the Avaldsnes section of the Johan Sverdrup field in the North Sea. Based on the experiences of oil and gas companies, the applicability of various offshore oil and gas field structures will be analyzed. According to the results, the most relevant field facilities scenarios were formed. Each scenario was technically and economically assessed. The technical assessment consisted of the evaluation of loads under the different climate conditions on the offshore oil and gas facility, along with resistance against overturning and sliding. The economic assessment included the calculation of the key economic indicators of various scenarios and comparing them. The integrated assessment indicated that the combination of several subsea production modules is the most cost-effective field facilities scenario to implement in the Avaldsnes area of the Johan Sverdrup field in the North Sea. [ABSTRACT FROM AUTHOR]

Primary production dynamics are strongly associated with vertical density profiles, which dictate the depth of stratification and mixed layers. Climate change and artificial structures (e.g. windfarms) are likely to modify the strength of stratification and vertical distribution of nutrient fluxes, especially in shelf seas where fine scale processes are important drivers, affecting the vertical distribution of phytoplankton. To understand the effect of physical changes on primary production, identifying the linkage between density and phytoplankton profiles is essential. Here, the ecological relevance of eight density layers (DLs) obtained by multiple methods that define three different portions of the pycnocline (above, centre, below) was evaluated to identify a valuable proxy for subsurface Chlorophyll-a (Chl-a mg m-3) concentrations. The associations of subsurface Chl-a with surface and deep mixing were investigated by hypothesizing the occurrence at the same depth of any DL and the maximum Chl-a layer (DMC) using Spearman correlation, linear regression, and a Major Axis analysis. Out of 1237 observations of the water column exhibiting a pycnocline, 78% reported DMCs above the bottom mixed layer depth (BMLD). This suggests that the BMLD is a boundary trapping Chl-a in shallow waters (≤ 120 m). BMLD constantly described Chl-a vertical distribution despite surface mixing indicators, suggesting a significant contribution of deep mixing processes in supporting subsurface production under specific conditions (e.g. prolonged stratification, tidal cycle, and bathymetry). Using BMLD for defining subsurface Chl-a could be a valuable tool for understanding the spatiotemporal variability of Chl-a in shelf seas, representing a potential variable for ecological assessments. [ABSTRACT FROM AUTHOR]

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]

Marine uptake of carbon dioxide reduces the accumulation of carbon dioxide in the atmosphere. Continental shelf seas are essential for carbon uptake from the atmosphere, but are also highly variable environments, for which uncertainties of carbon budget estimates are large. Recent studies indicate that their carbon sink capacity is weakening. A way to reduce the uncertainty of carbon budgets is to increase our observational capacity, for example through FerryBox installations on Ships-of-Opportunity. Here, we compare FerryBox observations in the North Sea for the fall seasons of 2019 and 2020. We show that short-lived mesoscale events can be characterized when the sampling resolution is adequately high, and that these events cause changes in essential environmental variables on the same magnitude as seasonal cycles. Whether advective or biological in origin, these events rapidly lowered seawater pCO2 by 8–10% and influenced the carbon uptake capacity. We demonstrate the importance of resolving and integrating the variability of these smaller features in regional carbon budget assessments and advocate for the tuning of models in order to capture this small-scale variability.

Monitoring marine primary productivity (PP) is crucial for understanding changes in the marine ecosystem. Based on satellite data and the vertically generalized production model (VGPM), this study investigated the spatiotemporal distribution and long-term trend of PP in the Yellow and Bohai Seas (YBSs) from 2003 to 2020. By using the calibrated satellite data and optimized parameterization scheme, the accuracy of the PP results in the YBSs was significantly improved compared to online PP products. The annual mean PP in the YBSs from 2003 to 2020 was 523.8 mgC/(m2·d), with significant seasonal and interannual differences. Seasonally, PP in the Yellow Sea and the Bohai Sea exhibited bimodal (two peaks in May and October) and unimodal (one peak in June) variation, respectively. The magnitude of mean PP in the YBSs was ranked as spring > summer > autumn > winter, with spring PP (~1000 mgC/(m2·d)) contributing more than 40% of the annual PP. The annual mean PP in the YBSs showed an overall decrease from 2003 to 2020, with a decrease rate of 5–6 mgC/(m2·d)/y. The interannual variation of the PP was mainly related to the variability of the chlorophyll-a concentration and was essentially inverse to the phases of the Pacific Decadal Oscillation and the El Niño-Southern Oscillation.

The object of this article is to define the facilities construction for the Avaldsnes section of the Johan Sverdrup field in the North Sea. Based on the experiences of oil and gas companies, the applicability of various offshore oil and gas field structures will be analyzed. According to the results, the most relevant field facilities scenarios were formed. Each scenario was technically and economically assessed. The technical assessment consisted of the evaluation of loads under the different climate conditions on the offshore oil and gas facility, along with resistance against overturning and sliding. The economic assessment included the calculation of the key economic indicators of various scenarios and comparing them. The integrated assessment indicated that the combination of several subsea production modules is the most cost-effective field facilities scenario to implement in the Avaldsnes area of the Johan Sverdrup field in the North Sea.

Accurately resolving coastal Total Water Levels (TWL) is crucial for socio-economic and environmental reasons. Recent efforts in satellite altimetry and numerical modeling have improved accuracy over near-shore areas. In this study we used data from tide gauges (TGs), SAR-mode altimetry from two satellites [Sentinel-3A (S3) and CryoSat-2 (C2)], and a state-of-the-art high-resolution regional coupled environmental prediction model (Amm15) to undertake an inter-comparison between the observations and the model. The aim is to quantify our capability to measure TWL around the United Kingdom coast, and to quantify the capacity of the model to represent coastal TWL. Results show good agreement between the satellite and TG data [the mean correlation (R) over seventeen TGs between June 2016 and September 2017 is 0.85 for S3 and 0.80 for C2]. The satellite-model comparison shows that the variability is well captured (R = 0.98 for both satellite), however, there is an offset (−0.23 m for S3, −0.15 m for C2) between the satellite and model data, that is near-constant across the domain. This offset is partly attributed to the difference in the reference level used by the satellites and the model, and residual differences linked to the temporal resolution of the model. The best agreement between model and satellite is seen away from the coast, further than 3–4 km offshore. However, even within the coastal band, R remains high, ∼0.95 (S3) and ∼0.96 (C2). In conclusion, models are still essential to represent TWL in coastal regions where there is no cover from in-situ observations, but satellite altimeters can now provide valuable observations that are reliable much closer to the coast than before.

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The seasonally stratified continental shelf seas are highly productive, economically important environments which are under considerable pressure from human activity. Global dissolved oxygen concentrations have shown rapid reductions in response to anthropogenic forcing since at least the middle of the twentieth century. Oxygen consumption is at the same time linked to the cycling of atmospheric carbon, with oxygen being a proxy for carbon remineralisation and the release of CO2. In the seasonally stratified seas the bottom mixed layer (BML) is partially isolated from the atmosphere and is thus controlled by interplay between oxygen consumption processes, vertical and horizontal advection. Oxygen consumption rates can be both spatially and temporally dynamic, but these dynamics are often missed with incubation based techniques. Here we adopt a Bayesian approach to determining total BML oxygen consumption rates from a high resolution oxygen time-series. This incorporates both our knowledge and our uncertainty of the various processes which control the oxygen inventory. Total BML rates integrate both processes in the water column and at the sediment interface. These observations span the stratified period of the Celtic Sea and across both sandy and muddy sediment types. We show how horizontal advection, tidal forcing and vertical mixing together control the bottom mixed layer oxygen concentrations at various times over the stratified period. Our muddy-sand site shows cyclic spring-neap mediated changes in oxygen consumption driven by the frequent resuspension or ventilation of the seabed. We see evidence for prolonged periods of increased vertical mixing which provide the ventilation necessary to support the high rates of consumption observed. [ABSTRACT FROM AUTHOR]

Inorganic nutrients and phytoplankton (chlorophyll- a concentration and community variables) were studied under a hydrologic scenario defined by a joint topographic-hydrodynamic upwelling event and an algal bloom in the southeast Gulf of Mexico. To do so three oceanographic cruises (GOMEX series) were undertaken throughout the Yucatan shelf during: September 11th-21st of 2010, September 23rd-October 3 rd of 2011, and November 29th-December 9th, 2012 (named as GOMEX-1, GOMEX-2 and GOMEX-3, respectively). We aim to assess the spatial inorganic nutrients and Chl-a patterns corresponding to each cruise to explore potential biochemical consequences of the temporal variability of the vertical inflow in the eastern shelf boundary (Cape Catoche: CC) and further spatial propagation of water featuring this hydrographic signature onto a shallow shelf, marked also by a high algal bloom incidence. This framework allows tackling the implications of the interplay between coastal variability and oceanographic processes on the phytoplankton biomass and community parameters. The spatial pattern of surficial Chl-a, as well as sub-surface maxima of ≈10 mgm−3 concurrent with the 22.5 °C isotherm over the southeastern shelf area of CC supports the fact that vertical nutrients supply is removed from the euphotic layer through phytoplankton uptake. This scenario indicates a biochemical setting consistent with rapid transfer of external resources advected from deep levels, capable not only to enhance phytoplankton growth, but also to change its species composition. Vertical mixing conditions associated with N-NW winds (locally known as "nortes") and a lack of upwelling scenario during GOMEX-3 in 2012 lead to relatively more homogeneous nitrate spatial distribution with overall high concentrations in deep, oceanic areas subject to nutrients entrainment. The distribution of specific richness among diatoms and dinoflagellates and relative abundance within such groups, as well as Chl- a concentrations are considered normal with regards to what has been observed in other studies for the Gulf of Mexico. The potential implications of large-scale, inter-annual climatic processes (e.g., El Niño-Southern Oscillation) on marine resources relevant to regional primary productivity variability (changes in the phytoplankton community) are discussed. • The vertical nutrients supply under an upwelling event is removed from the euphotic layer through phytoplankton uptake. • The upwelling induced a peak surficial Chl-a maxima ≈10 mgm-3 concurrent with the 22.5 isotherm over the SE GoM. • This scenario enhance also changes in phytoplankton species composition. • Vertical mixing conditions brought about by N-NW north winds lead to relatively homogeneous nitrate spatial distribution. • The phytoplankton abundances and Chl-a concentrations are considered normal in the Gulf of Mexico. [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]

Using cruise observations before and after the typhoon Chebi in August 2013 and those without the typhoon in July 2012, this study investigates variations in current structure, nutrient distribution, and transports disturbed by a typhoon in a typical coastal upwelling zone east of Hainan Island in the northwestern South China Sea. The results show that along-shore northeastward flow dominates the coastal ocean with a volume transport of 0.64 × 106 m3/s in the case without the typhoon. The flow reversed southwestward, with its volume transport halved before the typhoon passage. After the typhoon passage, the flow returned back northeastward except the upper layer in waters deeper than 50 m and the total volume transport decreased to 0.10 × 106 m3/s. For the cross-shelf component, the flow kept shoreward, while transports crossing the 50 m isobath decreased from 0.25, 0.12 to 0.06 × 106 m3/s in the case without the typhoon as well as before and after typhoon passage, respectively. For the along-shore/cross-shelf nutrient transports, SiO32− has the largest value of 866.13/632.74 μmol/s per unit area, NO3− half of that, and PO43− and NO2− one order smaller in the offshore water without the typhoon. The values dramatically decreased to about one-third for SiO32−, NO3−, and PO43− after the typhoon, but changed little for NO2−. The disturbed wind field and associated Ekman flow and upwelling process may explain the variations in the current and nutrient transports after the typhoon.

Abstract The geochemistry of sedimentary organic matter (SOM) in coastal areas is complex due to its multiple sources and intricate hydrological features. In this study, the biogenic element concentrations and stable carbon (δ13C) and nitrogen (δ15N) isotopic compositions in the coastal surface sediments of northern Shandong Peninsula, along with some parameters related to water quality, were analyzed to investigate the temporal-spatial variations in SOM and the processes that control its distribution. The results revealed that marine autogenous organic matter is a major contributor to SOM, accounting for 75.4 ± 3.3%, 60.8 ± 6.6% and 67.4 ± 10.3% in August and November 2015 and March 2016, respectively. In summer, TOC and TN concentrations were significantly higher than those in autumn and spring. The relatively high abundances of SOM were found in the offshore areas in summer and spring, which was contrary to those in autumn. Riverine discharge, nutrients, primary productivity and dissolved oxygen (DO) dynamics could all influence the composition and contents of SOM in different seasons. In summer, high primary productivity and hypoxia condition led to high SOM values. In comparison, SOM contents were relatively low due to sufficient DO in bottom water in autumn and spring. Dissolved nutrients in seawater could affect the accumulation of autogenous organic matter by impacting upon primary production. In summer, nitrate in surface water had the most obvious effects on autogenous organic carbon (AOC) and may be the principal factor of limiting the growth of phytoplankton. In autumn, nitrate as well as dissolved silicate had more effects on AOC storage. However, phosphate had the most obvious influence on AOC storage in spring. Highlights • Spatiotemporal variations of carbon and nitrogen in surface sediments of coastal northern Shandong Peninsula were analyzed. • The average concentrations of carbon and nitrogen in summer were significantly higher than those in spring and autumn. • The marine derived organic carbon on average accounted for 60.8–75.4% of TOC. • The parameters such as nutrients and dissolved oxygen had significant effects on the distribution of TOC. [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 bulk organic matter composition (total organic carbon (TOC) content and δ 13 C TOC ) and composition of isoprenoid and branched glycerol dialkyl glycerol tetraethers (GDGT) in surface sediments from 43 stations in the Berau River delta (east Kalimantan, Indonesia), including two coast-shelf transects and stations within the river mouth, were examined to reveal the spatial heterogeneity in these parameters in order to assess the impact of a tropical river loaded with suspended matter on the sedimentary organic matter in the shelf system. The high-resolution study showed that, despite the extensive transport of eroded soil material by the river to the sea, terrestrial organic matter and brGDGTs are only deposited on a relatively small part of the shelf. The concentrations of brGDGTs are highest (up to 120 μg g −1 TOC) in sediments deposited in and close to the mouth of the Berau River and their distribution indicates that they represent a mixture of soil-derived and river in-situ produced brGDGTs. Crenarchaeol concentrations reach 700 μg g −1 TOC in sediments deposited on the outer shelf due to Thaumarchaeotal production in shelf waters. This results in a strong gradient (0.93–0.03) in the BIT index, with high values in the river mouth and low values on the shelf. The decline in the BIT index is caused by both decreasing concentrations of the brGDGTs and increasing concentrations of crenarchaeol. The BIT index shows a highly significant but non-linear relationship with δ 13 C TOC . On the shelf, in the area not under the direct influence of the Berau River, cyclic brGDGTs become relatively dominant, most probably due to in-situ production in the alkaline pore waters of the surface sediments. The spatial heterogeneity of sources of brGDGTs on the Berau shelf complicates the use of brGDGTs as temperature proxies. Application of the global soil calibration to sedimentary mixtures of brGDGTs in the river-influenced area of the shelf results in a severe underestimation of mean annual air temperature (MAT) by 6 °C. This is due to the mixed origin of the brGDGTs, which are not only derived from soil erosion but, likely, also from riverine production, as has been observed for other river systems. Comparison of the Berau shelf other shelf systems indicates that in-situ production of brGDGTs in shelf sediments is a widespread phenomenon that is especially pronounced at water depths of ca. 50–300 m. It is hypothesized that this is so because benthic in-situ production of heterotrophic brGDGT-producing bacteria is fueled by the higher delivery of fresh organic matter to these sediments as the consequence of higher primary productivity in shelf waters and a decreased mineralization due to the relatively short settling times of particles on the shelf. For palaeoclimatic studies of marine shelf sediments the application of brGDGTs as proxies is severely complicated by the heterogeneity of sources of brGDGTs. Comparison of the brGDGT composition of soils with those of shelf sediments may assist in deciding if sedimentary brGDGTs are predominantly derived from soil erosion. Several methods to do so are discussed. [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]

An optical model is developed using experimental data of Inherent Optical Properties (IOP) from oceanic, coastal and productive lagoon waters in order to calculate vertical and spectral profiles of the average cosine in a wide variety of waters within coastal and shelf-sea environments. The results are compared with those generated using a radiative transfer numerical model based on the invariant imbedding technique (HydroLight) with realistic depth-dependent IOPs and appropriate surface and bottom boundary conditions and the results from three existing models ( Haltrin, 1998 ; Timofeyeva, 1971 ; Talaulikar et al., 2014 ). The average cosine predicted by the new model shows good agreement with the values obtained directly from radiative transfer calculations for a broad range of the IOPs and solar zenith conditions. Good correlations with excellent linearity with significantly low errors demonstrate a good deal of confidence of the model for accomplishing further applications. Since knowledge of spatial and temporal structures of the average cosine is of great importance to our understanding of the particle dynamics of pelagic ecosystems and coastal processes, efforts were made to apply the present model to both multispectral MODIS-Aqua imagery and hyperspectral (HICO) images acquired over the Arabian Sea and coastal lagoons of the Bay of Bengal dominated by river plumes and phytoplankton blooms. Maps of the average cosine derived from these data demonstrated significant changes in the magnitude and spectral behavior of the average cosine (from nearly featureless to strong spectral features and inflections) from different water types. Substantial changes in its spatial and spectral structures associated with highly productive waters, phytoplankton blooms and sediment plumes, as compared with open ocean areas, are well supported by the theoretical and experimental studies. The advantages of the new model in comparison with existing models are its capability to predict the vertical, spatial, spectral and temporal structures of average cosine with greater accuracy. [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]

A three dimensional hydrodynamic model with a coupled carbonate speciation sub-model is used to simulate large additions of CO 2 into the North Sea, representing leakages at potential carbon sequestration sites. A range of leakage scenarios are conducted at two distinct release sites, allowing an analysis of the seasonal, inter-annual and spatial variability of impacts to the marine ecosystem. Seasonally stratified regions are shown to be more vulnerable to CO 2 release during the summer as the added CO 2 remains trapped beneath the thermocline, preventing outgasing to the atmosphere. On average, CO 2 injected into the northern North Sea is shown to reside within the water column twice as long as an equivalent addition in the southern North Sea before reaching the atmosphere. Short-term leakages of 5000 tonnes CO 2 over a single day result in substantial acidification at the release sites (up to -1.92 pH units), with significant perturbations (greater than 0.1 pH units) generally confined to a 10 km radius. Long-term CO 2 leakages sustained for a year may result in extensive plumes of acidified seawater, carried by major advective pathways. Whilst such scenarios could be harmful to marine biota over confined spatial scales, continued unmitigated CO 2 emissions from fossil fuels are predicted to result in greater and more long-lived perturbations to the carbonate system over the next few decades. [ABSTRACT FROM AUTHOR]

The Agulhas Bank is a productive shelf sea, supporting important fish stocks, nursery grounds, and spawning sites. Few studies have examined the dynamics of primary production and the physio-chemical conditions that support this productivity during autumn. We report from a 14-day, 51-station survey of the central and eastern (21-27°E) Agulhas Bank in March 2019, during which we examined water-column structure, macronutrients, chlorophyll- a (total and size-fractionated), diatom cell counts and Net Primary Production (NPP). East to west trends were observed, with surface mixed layers (SML) and stratification increasing to the west. Euphotic zones were deeper than the SML, with SML irradiance conditions indicative of favorable light conditions for NPP. On average, surface waters contained ∼1.2 μmol N L−1 of nitrate (nitrate + nitrite; NO 3) and ∼3 μmol Si L−1 of silicic acid, which contrasts with nutrient deficient subtropical source waters. Surface chlorophyll- a ranged from 0.3 to 5.1 mg m−3, with high values inshore and near the shelf break. Nanoplankton (2–20 μm) dominated size-fractionated chlorophyll- a , with microplankton (>20 μm) contributions increasing to the west. Measurements of NPP were collected at seven stations, ranging from 0.3 to 1.1 g C m−2 d−1, with a statistically significant relationship between integrated NPP and surface chlorophyll- a allowing further estimates of NPP (0.1–1.1 g C m−2 d−1). We estimated nitrogen-demand to support NPP, with a comparison to surface NO 3 indicating ample nutrients to support daily NPP. Around half of the stations possessed a Subsurface Chlorophyll Maximum (SCM), with chlorophyll- a ranging from 1.7 to 10.3 mg m−3. Characteristics of the SCM (depth, light level, chlorophyll-to-carbon ratios) showed east to west variability, implying that the mechanisms of SCM formation ranged from in-situ growth (east) to photo-acclimation (west). • Net primary production (NPP) ranged from 0.3 to 1.1 g C m−2 d−1 during March on the Agulhas Bank. • Surface mixed layers on the bank had abundant irradiance and nitrate to support NPP rates. • Subsurface chlorophyll maxima (SCM) occurred at 47% of sites, containing 1.7–10.3 mg m−3. • SCM deepened east-to-west, linked to in-situ growth (east) and photo-acclimation (west). [ABSTRACT FROM AUTHOR]

Spatial and seasonal changes in benthic fluxes of dissolved silica (DSi) across the coastal zones were investigated in the southern Baltic Sea. Measurements were performed using ex situ incubations of sediment cores with natural benthic assemblages. Obtained benthic fluxes ranged from the uptake of −1.11 mmol m−2 d−1 in summer to a release of 6.79 mmol m−2 d−1 in autumn, while in situ concentrations in the bottom water were the lowest in autumn (1 μmol L−1) and the highest in winter (up to 58 μmol L−1). Sediments with high mud content had the highest fluxes in autumn, intermediate in spring, and the lowest in winter whereas no clear seasonal patterns were detected for sandy sediments. Generalized Linear Models indicated that in shallow enclosed areas biological factors, particularly presence of Chironomidae larvae, explained DSi fluxes variation while in open areas the environmental factors, such as organic matter quantity and quality, were key explanatory variables. In all studied environments (enclosed lagoon, open bay, and open coastal zone) DSi benthic fluxes represent an important component of the marine Si cycle. Further, the total yearly load of DSi from two major rivers (Vistula and Oder) and from the sediments, to the southern Baltic Sea reaches 258 kt y−1 of which benthic fluxes may constitute up to 34%. [Display omitted] • Spatial and seasonal changes in dissolved silica benthic fluxes were investigated. • There were significant differences in fluxes rates between sands and sandy muds. • Biological factors controlled benthic fluxes in the shallow enclosed areas. • Environmental factors mainly explained flux variation in the open-sea areas. • Benthic fluxes constitute a significant part of silica budget in the coastal areas. [ABSTRACT FROM AUTHOR]

Within the next few decades, large underwater structures of thousands of wind turbines in the northern European shelf seas will substantially increase the amount of habitat available for mobile demersal megafauna. As a first indication of the possible effects of this large scale habitat creation on faunal stocks settling on hard substrata, we compared selected taxa of the mobile demersal megafauna (decapods and fish) associated with the foundation of an offshore research platform (a wind-power foundation equivalent) with those of five shipwrecks and different areas of soft bottoms in the southern German Bight, North Sea. When comparing the amount of approximately 5000 planned wind-power foundations (covering 5.1 × 106 m2 of bottom area) with the existing number of at least 1000 shipwrecks (covering 1.2 × 106 m2 of bottom area), it becomes clear that the southern North Sea will provide about 4.3 times more available artificial hard substratum habitats than currently available. With regard to the fauna found on shipwrecks, on soft substrata and on the investigated wind-power foundation, we predict that the amount of added hard substrata will allow the stocks of substrata-limited mobile demersal hard bottom species to increase by 25–165% in that area. The fauna found at the offshore platform foundations is very similar to that at shipwrecks. Megafauna abundances at the foundations, however, are lower compared to those at the highly fractured wrecks and are irregularly scattered over the foundations. The upper regions of the platform construction (5 and 15 m depth) were only sparsely colonized by mobile fauna, the anchorages, however, more densely. The faunal assemblages from the shipwrecks and the foundations, respectively, as well as from the soft bottoms clearly differed from each other. We predict that new wind-power foundations will support the spread of hard bottom fauna into soft bottom areas with low wreck densities. [ABSTRACT FROM AUTHOR]

This paper shows that the sub-surface light regime in the offshore North Sea varies spatially and seasonally between different ecohydrodynamic regions, which is likely to have important implications for primary production and carbon and nutrient fluxes in different areas of the North Sea. Measurements of downward irradiance were collected using different instruments (i.e. water column-profiling instruments, semi-autonomous moorings, and remote sensing) at three ecohydrodynamically distinct sites in the North Sea: in the southern Bight (SB), at the Oyster Grounds (OG) and north of the Dogger Bank (ND). The ND site was the deepest, and had the lowest and least variable light attenuation coefficients (mean K(PAR) = 0.11 m). The onset of the phytoplankton spring bloom was earlier than at the other two sites. In summer, ND had low K(PAR) ~ 0.07 m and light penetration was shifted towards blue-green wavelengths (490-560 nm), with water itself being one of the strongest contributors to overall attenuation. In contrast, the SB site was characterised by the highest and most variable values of K(PAR) (mean = 0.54 m), comparable to near-coastal waters, and the spring bloom started almost a month later than at the ND site. The vertical variability of the attenuation coefficient and the strong PAR attenuation in the blue region of the spectrum were the result of higher concentrations of phytoplankton, CDOM and SPM, due to riverine inputs, shallow depth and strong tidal mixing. The OG site showed intermediate conditions between the ND and SB sites with a mean K(PAR) = 0.23 m, and deepest penetration of irradiance in the green region of the spectrum at 560 nm. The implications of these results for phytoplankton growth and ecosystem modelling are discussed. [ABSTRACT FROM AUTHOR]

This article presents some advantages using a shape-preserving total variation diminishing (TVD) advection scheme in an ecosystem model. The superbee flux-limiter has been used to the second-order Lax-Wendroff advection scheme to make it TVD. We performed simulations for three shelf sea regions with different characteristic time scales, namely, the North Sea, the Barents Sea, and the Baltic Sea. To explore the advantages, we also performed reference runs with the much simpler and computationally cheaper upwind advection scheme. Frontal structures are much better resolved with the TVD scheme. The bottom salinity in the Baltic Sea stays at realistic values throughout the 10-year simulation with the TVD scheme, while with the upwind scheme, it drifts towards lower values and the permanent haline stratification in the Baltic is almost completely eroded within one seasonal cycle. Only when applying TVD for both the vertical and horizontal advections the model succeeded to preserve haline stratification in the decadal simulation. Lower trophic level patterns are far better reproduced with the TVD scheme, and for the estimated cod larval survival, the advantages seem to be even stronger. Simulations using the TVD-derived prey fields identified distinct regions such as Dogger Bank to favor potential larvae survival (PLS), which did not appear as particularly favorable in the upstream simulations. The TVD scheme needs about 25 % more time on the central processing unit (CPU) in case of a pure hydrodynamic setup with only two scalar state variables (Barents Sea application). The additional CPU time cost increases for a coupled physical-biological model application with a large number of state variables. However, this is more than compensated by all the advantages found, and, hence, we conclude that it is worthwhile to use the TVD scheme in our ecosystem model. [ABSTRACT FROM AUTHOR]

Sea surface temperature satellite imagery and a regional hydrodynamic model are used to investigate the variability and structure of the Liverpool Bay thermohaline front. A statistically based water mass classification technique is used to locate the front in both data sets. The front moves between 5 and 35 km in response to spring-neap changes in tidal mixing, an adjustment that is much greater than at other shelf-sea fronts. Superimposed on top of this fortnightly cycle are semi-diurnal movements of 5-10 km driven by flood and ebb tidal currents. Seasonal variability in the freshwater discharge and the density difference between buoyant inflow and more saline Irish Sea water give rise to two different dynamical regimes. During winter, when cold inflow reduces the buoyancy of the plume, a bottom-advected front develops. Over the summer, when warm river water provides additional buoyancy, a surface-advected plume detaches from the bottom and propagates much larger distances across the bay. Decoupled from near-bed processes, the position of the surface front is more variable. Fortnightly stratification and re-mixing over large areas of Liverpool Bay is a potentially important mechanism by which freshwater, and its nutrient and pollutant loads, are exported from the coastal plume system. Based on length scales estimated from model and satellite data, the erosion of post-neap stratification is estimated to be responsible for exporting approximately 19% of the fresh estuarine discharge annually entering the system. Although the baroclinic residual circulation makes a more significant contribution to freshwater fluxes, the episodic nature of the spring-neap cycle may have important implications for biogeochemical cycles within the bay. [ABSTRACT FROM AUTHOR]

The UK National Oceanography Centre has maintained an observatory in Liverpool Bay since August 2002. Over 8 years of observational measurements are used in conjunction with regional ocean modelling data to describe the physical and dynamical oceanography of Liverpool Bay and to validate the regional model, POLCOMS. Tidal dynamics and plume buoyancy govern the fate of the fresh water as it enters the sea, as well as the fate of its sediment, contaminants and nutrient loads. In this context, an overview and summary of Liverpool Bay tidal dynamics are presented. Freshwater forcing statistics are presented showing that on average the bay receives 233 m s. Though the region is salinity controlled, river input temperature is shown to significantly modulate the plume buoyancy with a seasonal cycle. Stratification strongly influences the region's dynamics. Data from long-term moored instrumentation are used to analyse the stratification statistics that are representative of the region. It is shown that for 65% of tidal cycles, the region alternates between being vertically mixed and stratified. Plume dynamics are diagnosed from the model and are presented for the region. The spring-neap modulation of the plume's westward extent, between 3.5 ° W and 4° W, is highlighted. The rapid eastward erosion of the plume during spring tides is identified as a potentially important freshwater mixing mechanism. Novel climatological maps of temperature, salinity and density from the CTD surveys are presented and used to validate numerical simulations. The model is found to be sensitive to the freshwater forcing rates, temperature and salinities. The existing CTD survey grid is shown to not extend sufficiently near the coast to capture the near coastal and vertically mixed component the plume. Instead the survey grid captures the westward spreading, shallow and transient, portion of the plume. This transient plume feature is shown in both the long-term averaged model and observational data as a band of stratified fluid stretching between the mouth of the Mersey towards the Isle of Man. Finally the residual circulation is discussed. Long-term moored ADCP data are favourably compared with model data, showing the general northward flow of surface water and southward trajectory of bottom water. [ABSTRACT FROM AUTHOR]

Abstract: We investigate a role for vertical migration in stratified coastal water, where the swimming speed is generally significantly less than the typical turbulent fluctuations in a tidally-mixed bottom layer. In our modelling approach we use a k-ɛ turbulence model to describe the physical forcing, a Lagrangian random walk model to describe the vertical displacement of individual cells in response to turbulence and due to cell motility, and a phytoplankton growth model to direct the swimming behaviour of the phytoplankton according to their light and nutrient requirements. The model results show how the cells form a stable subsurface chlorophyll maximum (SCM) at the base of the thermocline where episodic tidal turbulence causes erosion of part of the SCM biomass into the bottom mixed layer (BML). We then focus on the question of whether an ability to swim (weakly, compared to typical bottom layer turbulent intensities) provides any advantage by allowing return to the SCM. Our results show that tidal turbulence in the BML helps both motile and neutrally-buoyant cells by periodically pushing them into the base of the thermocline. Motile cells then have the advantage that they can swim further into the thermocline towards higher light which also reduces the likelihood of being re-mixed back into the BML. [Copyright &y& Elsevier]

The purpose of this research was to investigate the potential causes of low oxygen levels in the bottom water of the Oyster Grounds region of the shallow southern North Sea, an area which provides suitable conditions for low oxygen levels to develop. At the end of the summer stratified period, relevant biogeochemical processes were investigated using a combination of sedimentary and water column rate measurements. Phytoplankton nitrate and ammonium uptake was measured throughout the water column using 15N labelled isotopes and showed ammonium uptake dominated in the upper and bottom mixed layer with a maximum 294.4μmolNm−3 h−1. In the deep chlorophyll maximum at the thermocline, primary production was dominated by nitrate uptake, with an average of 35.0μmolNm−3 h−1, relative to ammonium uptake, with an average of 24.6μmolNm−3 h−1. This high relative nitrate uptake will in part result in exportable new production to the isolated bottom mixed layer and sediments, as opposed to regenerated ammonium driven uptake. This biomass export was indicated by significant benthic oxygen consumption rates in the stratified region (782–1275μmolO2 m−2 h−1 μmolNm−3 h−1) long after the end of the spring bloom. The sediments were also an active net source of nitrate, ammonium, phosphate and silicate into the bottom mixed layer of 4.4, 8.4, 2.3 and 68.8μmolm−2 h−1, respectively. The export of new production within the thermocline to the bottom mixed layer and the consequent sediment oxygen consumption in the isolated bottom mixed layer in the Oyster Grounds are expected to have contributed to the low bottom water oxygen concentrations of 2.07mgl−1 (64.7μmoll−1) measured. The long stratified period associated with this low oxygen is predicted to occur more regularly in the future and continued monitoring of this ecologically important region is therefore essential if the causes of these potentially damaging low oxygen levels are to be fully understood. [Copyright &y& Elsevier]

Abstract: The general circulation in the North Sea and Skagerrak is simulated using the hybrid coordinate ocean model (HYCOM). Although HYCOM was originally developed for simulations of the open ocean, it has a design which should make it applicable also for coastal and shallow shelf seas. Thus, the objective of this study has been to examine the skills of the present version of HYCOM in a coastal shelf application, and to identify the areas where HYCOM needs to be further developed. To demonstrate the capability of the vertical coordinate in HYCOM, three experiments with different configurations of the vertical coordinate were carried out. In general, the results from these experiments compares quite well with in situ and satellite data, and the water masses and the general circulation in the North Sea and Skagerrak is reproduced in the simulations. Differences between the three experiments are small compared to other errors, which are related to a combined effect of model setup and properties of the vertical mixing scheme. Hence, it is difficult to quantify which vertical coordinate configuration works best for the coastal region. It is concluded that HYCOM can be used for simulations of coastal and shelf seas, and further suggestions for improving the model results are given. Since HYCOM also works well in open ocean and basin scale simulations, it may allow for a realistic modelling of the transition region between the open ocean and coastal shelf seas. [Copyright &y& Elsevier]

Previously published stable isotopic data on benthic foraminiferal species from a Holocene sequence in the Celtic Sea have been interpreted in terms of the progressive replacement of a tidally mixed by a stratified water mass. Offsets in the δ18O data between Ammonia batavus and Quinqueloculina seminulum were attributed to a 'seasonal effect' in which these two species were hypothesized to have calcified at different times of the year. The aims of this study were to test the hypotheses (1) that benthic foraminiferal stable isotope records from across the Celtic Sea front reflect seasonal stratification and (2) that offsets in the oxygen isotope record between different species are the result of the postulated seasonal effect. Hypothesis 1 was tested through investigation of live and dead benthic foraminiferal and bottom-water δ18O and δ13C sampled in transects across the Celtic Sea front from mixed through frontal to stratified water masses. Measurements of bottom-water salinity enabled a mixing-line equation to be developed for this area enabling quantitative reconstructions of bottom-water temperature from the isotopic data. Samples from stratified settings are characterized by heavier δ18Oforam and lighter δ13Cforam values than the mixed samples. Offsets in δ18Oforam between A. batavus and Q. seminulum support the notion of the seasonal effect. A. batavus produces values close to equilibrium while Q. seminulum overestimates temperature by up to 2°C and this might explain some of the offset observed between the two species observed in the palaeodata. Comparison of the δ18Oforam data with measured seasonal temperature cycles from mixed and stratified localities in the Celtic Sea demonstrates that, while most foraminifera calcify during the summer months, different species calcify at, or are preserved from, different times within this warm part of the seasonal cycle; Q. seminulum calcifies during September when peak bottom-water temperatures occur, while A. batavus calcifies during September in stratified localities, but during spring or early summer in mixed localities. This study confirms the interpretation of the δ18O palaeodata from the Celtic Sea as a palaeostratification record and demonstrates that δ18O data from shelf-sea cores can be used to supplement benthic foraminiferal assemblages as a tool for reconstructing the long-term dynamics of seasonal stratification. [ABSTRACT FROM AUTHOR]

Published records of the Holocene evolution of seasonal stratification in the Celtic Sea (NW European shelf) have been based on benthic proxies, notably benthic foraminifera and associated stable isotopic data. We have investigated organic-walled dinoflagellate cyst assemblages from a well-dated Holocene sequence from the central Celtic Sea in order to assess the signal from this planktonic proxy and to reconstruct paired bottom- and surface-water conditions through time. This sequence has, on the basis of the benthic proxies, been interpreted previously as a record of the replacement of tidally mixed water by stratified water associated with tidal-front migration during the early Holocene. Interpretation of the dinocyst record has been facilitated by a parallel study of the distribution of cysts from Celtic Sea surface sediments and their relationship with seasonal water masses. The dinocyst stratigraphy indicates mixed-water conditions during the early Holocene consistent with reduced water depths (hence lowered sea level) over the core site. The first significant change in the dinocyst assemblages is recorded at around 6650 cal. years BP and indicates a transition from mixed-frontal conditions to seasonal stratification. This interpretation of frontal migration is consistent with changes in the benthic foraminiferal assemblages and associated stable isotopes at the same core depth. From 6650 to 3600 cal. years BP, the significant occurrence of Bitectatodinium tepikiense accompanied by Spiniferites elongatus is attributed to strong seasonality, with winter sea-surface temperatures possibly below 5°C. Another transition at 3600 cal. years BP is attributed to a reduction in seasonality generated by milder winter conditions linked to a stronger influence of the thermohaline circulation over the studied area. This transition is not recorded by the benthic proxies and is attributed to climate forcing rather than to any change in tidal dynamics. It is notable that many mires in western Britain record distinct wet shifts contemporary with this change. [ABSTRACT FROM AUTHOR]