Abstract. This paper presents new results from high temporal resolution observations over two years (2007 and 2008) from instrumented moorings deployed in the central North Sea, at the Oyster Grounds and on the northern slope of Dogger Bank (North Dogger). The water column was stratified in the summer at both sites, leading to limited exchange of the water in the bottom mixed layer. Data from these moorings revealed the variable nature of summer oxygen depletion at the Oyster Grounds. The combination of in situ and ship-based measurements allowed the physical and biological conditions leading to decreasing dissolved oxygen concentrations in bottom water to be examined. In 2007 and 2008, the concentration of dissolved oxygen in the bottom water at both sites was observed to decrease throughout the summer period after the onset of stratification. Depleted dissolved oxygen concentration (6.5 mg l −1 , 71% saturation) was measured at the North Dogger, a site which is not significantly influenced by anthropogenic nutrient inputs. Lower oxygen saturation (5.2 mg l −1 , 60% saturation) was measured for short durations at the Oyster Grounds. The seasonal increase in bottom water temperature accounted for 55% of the decrease in dissolved oxygen concentration at the Oyster Grounds compared to 10% at North Dogger.Dissolved oxygen concentration in bottom water at the Oyster Grounds was shown to be strongly influenced by short term events including storms and pulses of particulate organic matter input. In contrast, dissolved oxygen concentration in bottom water at the North Dogger reflected longer seaCorrespondence to: N. Greenwood (naomi.greenwood@cefas.co.uk) sonal processes such as a gradual temperature increase over the summer and a more steady supply of particulate organic matter to the bottom mixed layer. The differences between the study sites shows the need for an improved understanding of the mechanisms driving these processes if the use of oxygen in marine management and ensuring ecosystem health is to be meaningful and successful in the future. These high frequency observations provide greater understanding of the nature of the depletion in bottom oxygen concentration in the North Sea.

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Nitrogen fluxes through the lower estuary of the river Great Ouse, England: the role of the bottom sediments 'Department of Biological Sciences. University of Essex, Colchester C 0 4 3SQ, United Kingdom 'Centre for Environment Fisheries and Aquaculture Sciences, Pakefield Road, Lowestoft, Suffolk NR33 OHT, United Kingdom ABSTRACT: Sediment-water nutrient exchange, oxygen uptake, denitrification (acetylene blockage) and pore water nutnent concentration profiles were measured at intertidal sediment (predominantly silWclays) sites in the lower estuary of the nver Great Ouse, England. Sedments were consistent sinks for No3-(310 pm01 m-2 h-', mean sites 4 to 9) and O2 (2800 pm01 m-2 h-', mean sites 4 to 7 ) , sources of NH,' (270 p o l m-2 h-', mean sites 4 to 9) but neutral with respect to NOz-and urea Oxygen uptake was significantly correlated (p c 0.05) 14th seasonal temperature. Nitrate exchange became saturated at No3-concentrations > 400 pM, at a rate of about 400 pm01 NO3-m-' h-'. Denitrification accounted annually for 46% of the NO,-exchanged into the sediments and approached asymptotic rates during spnng and summer at NO3-concentrations > 400 yM. Of the total N flux through the sediments, NH,' efflux accounted for 51 %, whist 49% was converted to gases, compared to >90% in the upper estuary Freshwater flushlng tunes were calculated for a defined area of the estuary and ranged from 20.5 d in June to 3.25 d in November. Attenuation of the riverine total oxidlsed nitrogen (NO,-+ NO2-) load to the estuary ranged from 1 % in the middle of winter to 56'::) at the height of summer and annually the sehments denitrified 38.4 Mm01 N. Recycling of nitrogen in the sedirnents, via N O 3 arnmonification (calculated by difference) and organic ammonification, generated an annual NH,' efflux of 128 Mm01 N, equivalent to 22% of the primary production N requirement in the Wash.

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 294.4 µmol N m-3 h-1. In the deep chlorophyll maximum at the thermocline, primary production was dominated by nitrate uptake, with an average of 35.0 µmol N m-3 h-1, relative to ammonium uptake, with an average of 24.6 µmol N m-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 (782-1275 µmol O2 m-2 h-1 µmol N m-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 68.8 µmol m-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 2.07 mg l-1 (64.7 µmol l-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

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