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Anammox, i.e. the anaerobic oxidation of NH4 + with NO2 - to N2, has redefined our understanding of nitrogen cycling in aquatic ecosystems. The isotope pairing technique (IPT) is the dominant tool for quantifying denitrification in intact sediments, but it cannot distinguish anammox from denitrification as sources of N2 and may, where anammox is significant, lead to large errors in the estimate of true N2 production. In a previous study, the IPT was revised in theory and a solution was proposed whereby the parameter r14, i.e. the ratio of 14 NO3 - to 15 NO3 - in the NO3 - reduction zone is used to correct the IPT in the presence of anammox. We begin by exploring the limitations of the 2 indirect techniques previously proposed for estimating r14. The first, based on the contribution of anammox to N2 production (ra) in sediment slurry incubations, underestimates anammox and cannot fully correct the IPT. The second, derived from the production of 15 N-N2 gas as a function of 15 NO3 - concentration, although valid in sieved sediment, was ineffective in natural intact sediments. In con- trast, a newly developed direct technique based on the 15 N-labelling of N2O, corrects the IPT in the presence of significant anammox (48% of N2 formation) in natural sediment and can even distinguish between anammox and denitrification in estuarine sediment with a lower anammox contribution (21%). We contrast these findings with sediments where anammox is minimal (

A diagenetic model is presented which considers nitrate reduction by both denitrification and Dissimilatory Nitrate Reduction to Ammonium (DNRA). This work builds on an existing model (Kelly-Gerreyn et al 1999; Mar Ecol Prog Ser 177:37-50). Previous models have assumed nitrate reduction to be solely due to denitrification. This paper questions the reliability of this assumption in coastal areas and suggests that DNRA can account for a high proportion of nitrate reduction. Data from a North Sea estuary (the lower Gt. Ouse, Norfolk, UK) containing high nutrient concentrations (mean 406 μM NO 3 - ) are used to derive a relationship between temperature and the proportioning of nitrate reduction driven by nitrate from the overlying water into denitrification and DNRA. The relationship is assumed to apply to total nitrate reduction. The result is a function which shows that DNRA and denitrification occur at all temperatures but that DNRA is the favoured pathway at the extremes of the observed temperatures ( 17°C) while denitrification is favoured only in a narrow range of temperatures (14 to 17°C). The mechanism is probably an adaptive response of different nitrate-reducing bacteria to temperature. This temperature relationship is implemented in the model and used to successfully simulate both observed rates of uncoupled denitrification (4 to 228 μmolN m -2 h -1 ), denitrification fuelled by nitrate in the overlying water (D w ), and calculated rates of DNRA fuelled by nitrate in the overlying water (DNRA w ) (measured nitrate flux - measured D w rate). In contrast, standard diagenetic formulae for nitrate reduction (i.e. by denitrification only) cannot satisfactorily reproduce the D w rates observed in these sediments. It is concluded that temperature is an important controlling factor for partitioning nitrate reduction into DNRA and denitrification in the lower Gt. Ouse sediments. This temperature effect implies that during an extended warm summer in temperate estuaries receiving high nitrate inputs, nitrate reduction may contribute to, rather than counteract, a eutrophication event. Diagenetic models of the nitrogen cycle in coastal areas should include DNRA.

Seasonal measurements of organic matter mineralisation by oxygen uptake and denitrification were carried out from July 1996 to March 1998 along a -200 km transect of the River Thames estuary, UK. There was a distinct gradient of decreasing rates of organic matter mineralisation seaward, which was related to the concentration of suspended solids and sedimentary organic carbon (C) at each site. There was clear seasonality and highest rates of oxygen uptake (10 056 pmol O 2 m -2 h -1 ) at the muddy sites, but lower rates and non-temperature-dependent oxygen uptake at the sandier sites. Denitrification, both that driven by nitrate from the overlying water (D w ) and that coupled to nitrification in the sediment (D n ), followed a similar trend to oxygen uptake, from negligible rates of approximately 1 pmol N m -2 h -1 for both D w and D n at the furthest offshore site, Site 12, to 11 407 and 8209 pmol N m -2 h -1 , respectively, at the inner muddy Site 1. The Thames estuary is heterotrophic and a very efficient organic C filter, trapping and remineralising 77% of its organic C-input. Attenuation of fluvial nitrate loads was regulated by freshwater flow. Minimal attenuation (3 %) occurred during peak flows (i.e. during periods of shortest freshwater flushing time) and > 100 % attenuation during periods of lowest freshwater flow (longest flushing times). Including the sewage treatment works (STWs) nitrate load in this calculation reduced the degree of attenuation of the nitrate load to, on average, 11%. Annual rates of D w and D n for an inner area of 125 km 2 were 112 and 85 Mmol N yr -1 , respectively, with a total rate of 196 Mmol N yr -1 (2744 t), which was equivalent to 9% of the total dissolved inorganic nitrogen (DIN) load for 1995-96. A mean denitrification rate (D w ) of 0.64 mol N m -2 yr -1 , based on measurements in 4 east coast estuaries, was used to estimate a total rate of denitrification for the entire area of UK east coast estuaries. The total rate of 0.81 Gmol N yr -1 represented 16% attenuation of the total fluvial discharge of nitrate (-6 Gmol N yr -1 ) to the UK's east coast estuaries (1995-96) and hence a 16% reduction in the UK nitrate load to the North Sea.

Sedimentary organic matter mineralisation (oxygen uptake), nitrogen (N) loss (denitrification) and nutrient exchange were measured seasonally in areas of both high and low Enteromorpha sp. and Ulva sp. cover at 6 sites in 2 harbours on the south coast of England. Measurements of macroalgal and phytoplankton photosynthesis were also carried out. Sedimentary carbon (C) and N cycling was most rapid at the sites with a heavy cover of macroalgae. Macroalgae were responsible, on average, for 57% of total dark oxygen uptake, with sediment bacterial respiration accounting for the remaining 43 %. Dark rates of nutrient uptake for Enteromorpha sp. and Ulva sp. were equivalent to 70% of those in the light. Denitrification rates were low (D w [NO 3 - from overlying water] 98%) remained within the system. External inputs of nutrients (N and P) to the harbours may have supported the spring growth of macroalgae, but it seemed unlikely that they were capable of supporting the summer peaks in algal biomass and rapid rates of N turnover. More likely the intense recycling (ammonification) of organically bound N within the sediments, coupled to a minimum loss via denitrification, provided a sustained and sufficient N supply for the macroalgae.

Fluvial and sewage loading of N and P to Liverpool Bay (England) elevated winter con- centrations of dissolved inorganic N (29.2 µM) and P (1.7 µM) and molar ratios of N:P (17.0) and N:Si (6.0) compared to Irish coastal waters (9.5 µM N, 0.8 µM P; N:P 12.0 N:Si 1.9). At the enriched site in Liverpool Bay, enhanced spring production (up to 3165.8 mg C m -2 d -1 ) summer production (471.8 to 971.5 mg C m -2 d -1 ) and biomass (4.1 to 13.6 mg chorophyll m -3 ) was dominated by diatoms. Annual production at this site was estimated as 182 g C m -2 compared to 97 g C m -2 at the Irish coastal sta- tion. Enrichment and shifts in nutrient ratios did not favour flagellate growth compared to growth of diatoms in Liverpool Bay. Low amounts of sediment phytopigments (9.2 to 26 mg m -2 ), low concen- trations of pore water Si (mean, 9.8 µM), and a negligible summer benthic efflux of Si (1.0 µmol m -2 h -1 ) suggested little phytodetrital input to sediments in Liverpool Bay and that summer diatom pro- duction required an allochthonous supply of Si. At the Irish coastal station, coupling between benthic and water-column processes ensured that benthic efflux of Si was sufficient to support the bulk of summer diatom production. Water-column recycling of N was an order of magnitude greater than sediment recycling of N at both coastal sites.

The description and calibration of a reaction-diffusion model of early diagenesis are presented. Unlike previous models it has been developed for a temperate latitude estuary (Gt Ouse, UK) impacted by high nitrate concentrations (annual mean 700 μM). Five variables, O 2 , NO 3 - , NH 4 + , SO 4 2- and S 2- , are modelled from the steady state distributions of bulk total organic carbon (TOC) (i.e. a 1-G model). Three methods for deriving the first order rate constant, k, for TOC mineralisation are tested: (1) data calculated k values [i.e. (depth integrated total mineralisation rate) + (depth integrated TOC inventory)]; (2) an exponential formulation, k z = k 0 e -αz (k 0 = k at sediment surface, a = reactivity coefficient of decrease, z = depth); and (3) use of separate k values for individual mineralisation pathways. Method 1 underestimates observed fluxes of solutes across the sediment-water interface (SWI) by up to an order of magnitude. This is due to an inappropriate use of the calculated kin the model. The calculation of k yields an overall net value which implicitly accounts for all factors acting on mineralisation. Such factors (e.g. oxidant limitation of organic decay) are explicitly modelled. Consequently, k is significantly reduced by factors applied to it in the model which have previously been accounted for in the calculation. In Method 2, measured NO 3 - fluxes are overestimated by up to a factor of 7. To reproduce measured benthic oxygen demands and sulphate reduction rates, a cannot be simultaneously fitted to the NO 3 - fluxes. The high overlying NO 3 - concentrations result in model denitrification that cannot reproduce the degree of limitation that actually occurs. Method 3 reproduces the data (i.e. both stoichiometrically derived mineralisation rates and measured solute fluxes at the SWI) to a high degree (r > 0.99, p < 0.001), but at the expense of increasing the degrees of freedom in the model and conceptual simplicity. These results cast doubt over the universal applicability of diagenetic models for estuarine systems exposed to high NO 3 - concentrations. It is concluded that the use of commonly calculated first order rate constants (Method 1) and the frequently used exponential function (Method 2) in diagenetic models cannot be relied upon to reproduce observations in high NO 3 - estuaries. Previous stoichiometric calculations suggested that all of the measured ammonium fluxes across the SWI in the Gt Ouse could be accounted for with oxygen, nitrate and sulphate reduction alone. With these latter processes the model (Method 3) underestimates the observed ammonium fluxes by up to 44 % at 3 out of 4 sites. This suggests that other mineralisation pathways (e.g. nitrate ammonification) are active in the Great Ouse sediments.

The impact of the spring bloom on benthic remineralisation rates was studied in offshore waters of the western Irish Sea during 1998. Initiation of the spring bloom coincided with the onset of thermal stratification at the end of April. Peak chlorophyll biomass (6.48 mg chl m -3 ) and algal standing stock (145.0 mg chl m -2 ) were measured on May 11, and the bloom lasted approximately 1 mo. Sediment oxygen uptake increased to 1658 μmol m -2 h -1 for a short period in early May, which was followed by increased denitrification (up to 48 μmol N m -2 h -1 ) in late May and early July. Subsequently sulphate reduction increased (up to 83 μmol SO 4 2- m -2 h -1 ) in early July. Efflux of nitrate remained low but constant (10 μmol m -2 h -1 ) throughout the study, accounting for 27 % of ammonified organic N, with the remaining 63% being denitrified. Sediment depth profiles of chlorophyll and phaeopigment showed deep (>30 cm) mixing of phytodetritus into the sediment in early July, which may explain the high rates of sulphate reduction measured at this time. Evidence of a large phytodetrital flux to the benthos during spring 1998 was limited. Deepening of chlorophyll isopleths suggested sinking of algae in early May, although sediment pigment concentrations indicated a continual but low input. Benthic oxygen consumption represented 46 % of total spring phytoplankton production or 61% of new production and approximately balanced the calculated flux of detrital carbon (C) to the benthos. The impact of detrital C (assumed to be largely phytodetritus immediately after the spring bloom) on the benthos was short lived, being rapidly remineralised, and in turn little spring production would have been available for secondary production, e.g. Nephrops norvegica. In such years, benthic production, particularly that of N. norvegica, must be supported by phytoplankton production which takes place after the bloom or by detrital C which is advected into the area following the breakdown of the gyre.

The fate of nitrogen (N) in the bottom sediments of the upper Great Ouse estuary, England, was examined over the course of a year. The sedirnents were consistent sinks for NO3from the overlying river water, and were weak sources of NH,'. Simultaneous measurements of oxygen uptake, nutrient exchange and sulphate reduction, when combined with the measured C:N ratios of the sediment organic matter, permitted calculation of the amount of N released within the sediment by organic matter mineralisation. With the exception of a site w ~ t h thixotropic sediment, at all other sites the amount of inorganic N entering the sedlment by transport from the overlying water and by internal ammonification of organic matter was not matched by measured exports of N from the sedirnents. We calculate that >90% of the flux of N through the sediment was lost as gases, and that 50'L of the N ammonified from organic matter must have been converted to gases by coupled nitrification-denitrification within the sediments. When compared to the total flux of N through the entire estuary, any N loss by denitrification in the sediments of the upper estuary was minor (-1 X) because of the small surface area of sediment to freshwater flow