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Sediments have an important role in nutrient dynamics by providing sites for denitrification, which, in conjunction with nitrogen (N) fixation and other processes such as dissimilatory nitrate reduction to ammonium (DNRA), can regulate ecosystem N availability. Little Lagoon, Alabama, USA, is affected by anthropogenic perturbations and has N inputs from nitrate (NO sub(3) super(-))-contaminated submarine groundwater discharge. N cycle pathways (N-fixation, denitrification, anammox, and DNRA) and benthic fluxes were measured at three sites over a year to investigate the hypothesis that DNRA, by producing ammonium (NH sub(4) super(+)) as a dissimilatory end product of NO sub(3) super(-) reduction, was a significant process retaining bioavailable fixed N in Little Lagoon. DNRA was found to be an important NO sub(3) super(-) reduction pathway, especially in the summer months (study average: 52.1 mu mol N m super(-2) h super(- 1)) compared to denitrification (study average 7.7 mu mol N m super(-2) h super(- 1)). Sulfidic sediments during the majority of the year interfere with denitrification and result in decreased N loss through denitrification. The conversion of NO sub(3) super(-) to the more biologically preferred form of N, NH sub(4) super(+) via DNRA, means that NO sub(3) super(-) reduction has considerable nourishing potential and likely will affect both ecosystem function and services. These results indicate that NO sub(3) super(-) reduction may contribute to, rather than counteract, eutrophication during warm summers in estuaries receiving high N inputs. As estuarine sediments provide a key ecosystem function of N removal, excess anthropogenic N input jeopardizes their ability to effectively remove reactive N in Little Lagoon and likely in other impacted ecosystems.

Estuaries are hotspots of intense biogeochemical cycling that regulate land-ocean exchanges and support a broad range of ecosystem services. They are a particularly important, still under-resolved, component of the global carbon cycle and often flash points for local socioeconomic conflicts. The mesotidal Guadalquivir estuary is fed by one of the Iberian Peninsula's largest rivers, has a long history of anthropogenic manipulation, and hosts a surrounding population of over 1.7 million people. Monthly sampling of water biogeochemical properties (pigments, nutrients, alkalinity, pH, dissolved oxygen, and organic matter) was carried out in the estuary at 12 stations along its length between November 2007 and August 2009. pCO sub(2) and dissolved inorganic carbon were calculated from total alkalinity and pH, allowing air-water fluxes (FCO sub(2)) and land-ocean transport to be estimated. The spatial distribution of oxygen concentration and suspended materials led to divide the system in three zones, the inner estuary (IE), the middle estuary (ME), and the lower estuary (LE), with a minimum oxygen zone and a maximum turbidity zone being found in the IE and ME, respectively. CO sub(2) exchange pattern defined the estuary as a strong source being the IE the major contributor. Thus, estuarine waters were CO sub(2) oversaturated with respect to the atmosphere during most of the study period, with average annual FCO sub(2) values being 66.9 plus or minus 18.6, 29.4 plus or minus 20.3, and 3.4 plus or minus 8.1 mmol C m super(-2) day super(-1) in the IE, ME, and LE, respectively. The average annual CO sub(2) flux to the atmosphere was 36.4 plus or minus 11.7 mol C m super(-2) year super(-1). The present study reinforces the heterotrophic status of the estuary in relation to the carbon system variable description.

The compound-specific stable carbon isotope compositions ( delta super(13)C) of leaf wax n-alkanes from two short sediment cores recovered off the Pearl River estuary (PRE) were analyzed to check for their capability of indicating decadal scale catchment environmental change. Sedimentary long-chain n-alkanes exhibited an odd-over-even predominance, with a maximum at n-C sub(29) or n-C sub(31), indicating their leaf wax origin was from vascular plants. The delta super(13)C values of C sub(29) and C sub(31) n-alkane in all the sediment samples were in the range of -28.8ppt to -31.2ppt, consistent with the C sub(3) plant-dominated vegetation in the Pearl River catchments. The time series of delta super(13)C records from the two cores were comparable and displayed a decreasing trend from the early 20th century to the end of the 1970s, followed by a reversal in that change leading to continued increase for ca. 15 years. After being corrected for the effect of atmospheric CO sub(2) rise and delta super(13)C sub(atm) decline, the delta super(13)C sub(29) records largely retained their raw changing pattern; the post-1980 increase being more conspicuous. The slightly decreasing trend in corrected delta super(13)C records before around 1980 may have been caused by an increase in precipitation, whereas the subsequent increase of delta super(13)C is likely associated with the observed dry climate and/or intensive anthropogenic deforestation. Our results thus demonstrate that leaf wax n-alkanes buried in the sediments off the PRE may well reflect change in the regional climate and/or human activity in the river catchments over the past century.

Coastal marine organisms experience dynamic pH and dissolved oxygen (DO) conditions in their natural habitats, which may impact their susceptibility to long-term anthropogenic changes. Robust characterizations of all temporal scales of natural pH and DO fluctuations in different marine habitats are needed; however, appropriate time series of pH and DO are still scarce. We used multiyear (2008-2012), high-frequency (6 min) monitoring data to quantify diel, seasonal, and interannual scales of pH and DO variability in a productive, temperate tidal salt marsh (Flax Pond, Long Island, US). pH sub(NBS) and DO showed strong and similar seasonal patterns, with average (minimum) conditions declining from 8.2 (8.1) and 12.5 (11.4)mg l super(-1) at the end of winter to 7.6 (7.2) and 6.3 (2.8)mg l super(-1) in late summer, respectively. Concomitantly, average diel fluctuations increased from 0.22 and 2.2 mg l super(-1) (February) to 0.74 and 6.5 mg l super(-1) (August), respectively. Diel patterns were modulated by tides and time of day, eliciting the most extreme minima when low tides aligned with the end of the night. Simultaneous in situ pCO sub(2) measurements showed striking fluctuations between 330 and 1,200 (early May), 2,200 (mid June), and 4,000 mu atm (end of July) within single tidal cycles. These patterns also indicate that the marsh's strong net heterotrophy influences its adjacent estuary by 'outwelling' acidified and hypoxic water during ebb tides. Our analyses emphasize the coupled and fluctuating nature of pH and DO conditions in productive coastal and estuarine environments, which have yet to be adequately represented by experiments.

Repeated surveys of the Kennebec estuary, a macrotidal river estuary in Maine, USA, between 2004 and 2008 found spatial and temporal variability both in sources of carbon dioxide (CO sub(2)) to the estuary and the air-sea flux of estuary CO sub(2). On an annual basis, the surveyed area of the Kennebec estuary had an area-weighted average partial pressure of CO sub(2) (pCO sub(2)) of 559 mu atm. The area-weighted average CO sub(2) flux to the atmosphere was 3.54 mol C m super(-2) year super(-1). Overall, the Kennebec estuary was an annual source of 7.2x10 super(7) mol CO sub(2) to the atmosphere. Distinct seasonality in estuarine pCO sub(2) was observed, with shifts in the seasonal pattern evident between lower and higher salinities. Fluxes of CO sub(2) from the estuary were elevated following two summertime storms, and inputs of riverine CO sub(2) outweighed internal estuarine CO sub(2) inputs in nearly all months. River and estuarine inputs of CO sub(2) represented 68 and 32 % of the total CO sub(2) contributions to the estuary, respectively. This study examines the variability of CO sub(2) in a large New England estuary, and highlights the comparatively high contribution of CO sub(2) from riverine sources.

Data on optical properties such as diffuse attenuation coefficient K sub(d)(PAR), beam attenuation coefficient (c sub(p)) and the optically active constituents (OACs) CDOM, Chl-a and suspended particulate matter were obtained in a Danish temperate coastal plain estuary (56 degree N) and a Vietnamese tropical ria (12 degree N) at high discharges. The major difference was the spatial distribution of the optical properties against distance, best described by significant power functions in the ria, compared to significant linear functions in the coastal plain. It was hypothesized that estuarine morphometry could explain this spatial distribution. Partition and multiple regression analyses showed that Chl-a governed K sub(d)(PAR) and beam attenuation coefficient in both estuaries. Significant, high correlations were obtained by multiple regression analyses in the estimation of K sub(d)(PAR) and beam attenuation coefficients in the two estuaries using OACs as input parameters. It is concluded that there are no large differences in OAC concentrations between the two estuaries. The spatial distributions of OACs and optical properties were significantly different and governed by the estuary morphometry, i.e. a power distribution in the tropical ria and a linear function in the temperate coastal plain estuary, and applicable to similar estuary types.

Present-day altered distribution of the natural habitats in the Ebro Delta is consequence of intensive human settlement in the last two centuries. We developed spatial predictive models of potential natural wetland habitats of the Ebro Delta based on ecogeographical predictors and presence/pseudo-absence data for each habitat. The independent variables (i.e. elevation, distance from the coast, distance from the river and distance from the inner border) were analysed using Generalized Additive Models (GAMs). Elevation and the distance from the coast appeared as key predictors in most of the coastal habitats (coastal lagoons, sandy environments, Salicornia-type marshes and reed beds), whereas distances from the river and from the inner border were relevant in the most terrestrial or inland habitats (salt meadows, Cladium-type marshes and riparian vegetation). Our findings suggest that the most inland habitats (i.e. Cladium-type marshes, salt meadows and riparian vegetation) would have undergone a severe reduction (higher than 90%), whereas in the most coastal habitats (coastal lagoons, sandy environments, Salicornia-type marshes) the reduction in relation to their potential distribution would be around 70%. This modelling approach can be applied to other deltaic areas, since all them share a similar topography.

Salt marsh restoration is hypothesized to provide shoreline stabilization, increased fish habitat, and organic carbon subsidies for estuarine food webs. Organic carbon comes from diverse primary producers that differ in carbon fixation rates and areal extent within wetland systems. This study was designed to obtain some of the first estimates of the relative contribution of different primary producers to total organic carbon production within open water and tidally flooded wetlands of the northern San Francisco Estuary (SEE). Carbon fixation rates of phytoplankton, microphytobenthos, and low marsh emergent vegetation were measured in two natural and four restoring wetlands in 2004. Areal (m super(2)) rates of carbon fixation were greatest for low marsh vegetation, while phytoplankton and microphytobenthos rates were one and two orders of magnitude lower, respectively. However, when areal production rates were scaled to the amount of habitat available for each primary producer group, the relative importance of each group varied by location. Given that each primary producer group supports a different subset of estuarine consumers, the type of food subsidy desired should influence the amount open water channel, mudflat and low marsh area restored. Large-scale wetland restoration activities should consider the types of primary producers likely to occupy restored habitats when estimating future food web impacts.

Dinitrogen (N sub(2)) and/or nitrous oxide (N sub(2)O) are produced through denitrification, anaerobic ammonium oxidation (anammox) or nitrification in sediments, of which entangled processes complicate the absolute rate estimations of gaseous nitrogen production from individual pathways. The classical isotope pairing technique (IPT), the most common super(15)N nitrate enrichment method to quantify denitrification, has recently been modified by different researchers to (1) discriminate between the N sub(2) produced by denitrification and anammox or to (2) provide a more accurate denitrification rate under considering production of both N sub(2)O and N sub(2). In case 1, the revised IPT focused on N sub(2) production being suitable for the environments of a low N sub(2)O-to-N sub(2) production ratio, while in case 2, anammox was neglected. This paper develops a modified method to refine previous versions of IPT. Cryogenic traps were installed to separately preconcentrate N sub(2) and N sub(2)O, thus allowing for subsequent measurement of the two gases generated in one sample vial. The precision is better than 2% for N sub(2) (m/z 28, m/z 29 and m/z 30), and 1.5% for N sub(2)O (m/z 44, m/z 45 and m/z 46). Based on the six m/z peaks of the two gases, the super(15)N nitrate traceable processes including N sub(2) and N sub(2)O from denitrification and N sub(2) from anammox were estimated. Meanwhile, N sub(2)O produced by nitrification was estimated via the production rate of unlabeled super(44)N sub(2)O. To validate the applicability of our modified method, incubation experiments were conducted using sediment cores taken from the Danshuei Estuary in Taiwan. Rates of the aforementioned nitrogen removal processes were successfully determined. Moreover, N sub(2)O yield was as high as 66%, which would significantly bias previous IPT approaches if N sub(2)O was not considered. Our modified method not only complements previous versions of IPT but also provides more comprehensive information to advance our understanding of nitrogen dynamics of the water-sediment interface.

Over the past three decades, Narragansett Bay has undergone various ecological changes, including significant decreases in water column chlorophyll a concentrations, benthic oxygen uptake, and benthic nutrient regeneration rates. To add to this portrait of change, we measured the net flux of N sub(2) across the sediment-water interface over an annual cycle using the N sub(2)/Ar technique at seven sites in the bay for comparison with measurements made decades ago. Net denitrification rates ranged from about 10-90 mu mol N sub(2)-N m super(-2) h super(-1) over the year. Denitrification rates were not significantly different among sites and had no clear correlation with temperature. Net nitrogen fixation (-5 to -650 mu mol N sub(2)-N m super(-2) h super(-1)) was measured at three sites and only observed in summer (June-August). Neither denitrification nor nitrogen fixation exhibited a consistent relationship with sediment oxygen demand or with fluxes of nitrite, nitrate, ammonium, total dissolved inorganic nitrogen, or dissolved inorganic phosphate across all stations. In contrast to the mid-bay historical site where denitrification rates have declined, denitrification rates in the Providence River Estuary have not changed significantly over the past 30 years.