Benthic nitrogen cycling traversing the Peruvian oxygen minimum zone

Benthic nitrogen (N) cycling was investigated at six stations along a transect traversing the Peruvian oxygen minimum zone (OMZ) at 11°S. An extensive dataset including porewater concentration profiles and in situ benthic fluxes of nitrate (NO₃⁻), nitrite (NO₂⁻) and ammonium (NH₄⁺) was used to constrain a 1-D reaction-transport model designed to simulate and interpret the measured data at each station. Simulated rates of nitrification, denitrification, anammox and dissimilatory nitrate reduction to ammonium (DNRA) by filamentous large sulfur bacteria (e.g. Beggiatoa and Thioploca) were highly variable throughout the OMZ yet clear trends were discernible. On the shelf and upper slope (80-260 m water depth) where extensive areas of bacterial mats were present, DNRA dominated total N turnover (?2.9 mmol N m⁻² d⁻¹) and accounted for ?65% of NO₃⁻ + NO₂⁻ uptake by the sediments from the bottom water. Nonetheless, these sediments did not represent a major sink for dissolved inorganic nitrogen (DIN = NO₃⁻ + NO₂⁻ + NH₄⁺) since DNRA reduces NO₃⁻ and, potentially NO₂⁻, to NH₄⁺. Consequently, the shelf and upper slope sediments were recycling sites for DIN due to relatively low rates of denitrification and high rates of ammonium release from DNRA and ammonification of organic matter. This finding contrasts with the current opinion that sediments underlying OMZs are a strong sink for DIN. Only at greater water depths (300-1000 m) did the sediments become a net sink for DIN. Here, denitrification was the major process (?2 mmol N m⁻² d⁻¹) and removed 55-73% of NO₃⁻ and NO₂⁻ taken up by the sediments, with DNRA and anammox accounting for the remaining fraction. Anammox was of minor importance on the shelf and upper slope yet contributed up to 62% to total N₂ production at the 1000 m station. The results indicate that the partitioning of oxidized N (NO₃⁻, NO₂⁻) into DNRA or denitrification is a key factor determining the role of marine sediments as DIN sinks or recycling sites. Consequently, high measured benthic uptake rates of oxidized N within OMZs do not necessarily indicate a loss of fixed N from the marine environment.     

Benthic metabolism and nutrient cycling in Boston Harbor, Massachusetts

To gain insight into the importance of the benthos in carbon and nutrient budgets of Boston Harbor and surrounding bays, we measured sediment-water exchanges of oxygen, total carbon dioxide (DIC), nitrogen (ammonium, nitrate+nitrite, urea, N₂O), silicate, and phosphorus at several stations in different sedimentary environments just prior to and subsequent to cessation of sewage sludge disposal in the harbor. The ratio of the average annual DIC release to O₂ uptake at three primary stations ranged from 0.84 to 1.99. Annual average DIC:DIN flux ratios were consistently greater than predicted from the Redfield ratio, suggesting substantial losses of mineralized N. The pattern was less clear for P: some stations showed evidence that the sediments were a sink for P while others appeared to be a net source to the water column over the study period. In general, temporal and spatial patterns of respiration, nutrient fluxes, and flux ratios were not consistently related to measures of sediment oxidation-reduction status such as Eh or dissolved sulfide. Sediments from Boston Harbor metabolize a relatively high percentage (46%) of the organic matter inputs from phytoplankton production and allochthonous inputs when compared to most estuarine systems. Nutrient regeneration from the benthos is equivalent to 40% of the N, 29% of the P, and more than 60% of the Si demand of the phytoplankton. However, the role of the benthos in supporting primary production at the present time may be minor as nutrient inputs from sewage and other sources exceed benthic fluxes of N and P by 10-fold and Si by 4-fold. Our estimates of denitrification from DIC:DIN fluxes suggests that about 45% of the N mineralized in the sediments is denitrified, which accounts for about 17% of the N inputs from land.     

Insights into estuarine benthic dissolved organic carbon (DOC) dynamics using δC-DOC values, phospholipid fatty acids and dissolved organic nutrient fluxes

Benthic dissolved organic carbon (DOC) flux rates and changes in DOC isotope ratios, along with nutrient fluxes, phospholipid fatty acids concentration and carbon isotope ratios were measured in productive estuarine sediments over a diel cycle to determine the mechanisms driving benthic-pelagic coupling of DOC. There was uptake of DOC during the dark and efflux during the light at all sites. DOC uptake rates were related to benthic respiration (dark O₂ uptake) and effluxes were coupled to the trophic status (ratio of production to respiration) of the sediments. Highest uptake and efflux rates were observed at two high nutrient concentration sites. The DOC:DON ratio of water column dissolved organic matter (DOM) decreased during the dark and increased during the light indicating preferential uptake and release of carbon rich dissolved organic matter. The calculated carbon isotope ratio of the DOC taken up by the benthos was significantly more depleted than the bulk water column DOC pool, suggesting preferential uptake of selected components of the water column DOC pool. Generally the isotope ratio of the DOC released during the light was more enriched than that taken up during the dark, which suggests that the benthos has the potential to significantly alter the estuarine DOC pool. Uptake and efflux were coupled to respiration and algal grazing/mineralization, therefore increased nutrient loading may shift the composition of the estuarine DOC pool through changes in the magnitude of benthic DOC fluxes. A combination of biological (diel shifts in DOC production and consumption) and abiotic processes (flocculation) appear to be driving the observed benthic DOC dynamics at the study sites. This study was the first to measure carbon isotopic changes in the water column DOC pool due to benthic processes, and shows that the benthos can alter the estuarine DOC pool through diel differences in DOC uptake and efflux.     

Nutrient Fluxes from Sediments in the San Francisco Bay Delta

In September 2011 and March 2012, benthic nutrient fluxes were measured in the San Francisco Bay Delta, across a gradient from above the confluence of the Sacramento and San Joaquin Rivers to Suisun Bay. Dark and illuminated core incubation techniques were used to measure rates of denitrification, nutrient fluxes (phosphate, ammonium, nitrate), and oxygen fluxes. While benthic nutrient fluxes have been assessed at several sites in northern San Francisco Bay, such data across a Delta–Bay transect have not previously been determined. Average September rates of DIN (nitrate, nitrite, ammonium) flux were net positive across all sites, while March DIN flux indicated net uptake of DIN at some sites. Denitrification rates based on the N₂/Ar ratio approach were between 0.6 and 1.0 mmol m^?2 day^?1, similar to other mesotrophic estuarine sediments. Coupled nitrification–denitrification was the dominant denitrification pathway in September, with higher overlying water nitrate concentrations in March resulting in denitrification driven by nitrate flux into the sediments. Estimated benthic microalgal productivity was variable and surprisingly high in Delta sediments and may represent a major source of labile carbon to this ecosystem. Variable N/P stoichiometry was observed in these sediments, with deviations from Redfield driven by processes such as denitrification, variable light/dark uptake of nutrients by microalgae, and adsorption of soluble reactive phosphorus.     

Benthic nutrient remineralization in a coastal lagoon ecosystem

In situ measurements of the exchange of ammonia, nitrate plus nitrite, phosphate, and dissolved organic phosphorus between sediments and the overlying water column were made in a shallow coastal lagoon on the ocean coast of Rhode Island, U.S.A. The release of ammonia from mud sediments in the dark (20–440 μmol per m² per h) averaged ten times higher than from a sandy tidal flat (0–60 μmol per m² per h), and while mud sediments also released nitrate and phosphate, sandy sediments took up these nutrients. Fluxes of nutrients from mud sediments, but not from sandy areas, markedly increased with temperature. Ammonia release rates for mud sediments in the light (0–350 μmol per m² per h) were lower than those in the dark and it is estimated that some 25% of the ammonia released to the water column on an annual basis may be intercepted by the benthic microfloral community. Estimates of the annual net exchange of nutrients across the sediment-water interface, weighted by sediment type for the lagoon as a whole, showed a release of 450 mmol per m² of ammonia, 5 mmol per m² of phosphate, 5 mmol per m² of dissolved organic phosphorus, and an uptake of 80 mmol per m² of nitrate. Although rates of ammonia and nitrate exchange were comparable to those described for the deeper heterotrophic bottom communities of nearby Narragansett Bay, rates of benthic phosphate release were significantly lower. On an annual basis the Bay benthos released approximately 20 times more inorganic phosphate per unit area than did the lagoon benthos. As a result., the N/P ratio for the flux from the sediments was 74∶1 in the lagoon, compared with 16∶1 in “average” marine plankton and 8∶1 for the benthic flux from Narragansett Bay. The lack of remineralized phosphate in the lagoon, is reflected in water, column phosphate concentrations (always <1 μm) and water column N/P ratios (annual N/P=27) and suggests that the lagoon may show phosphate limitation rather than the nitrogen limitation commonly associated with marine systems.     

Going West: Nutrient Limitation of Primary Production in the Northern Gulf of Mexico and the Importance of the Atchafalaya River

To investigate controls on phytoplankton production along the Louisiana coastal shelf, we mapped salinity, nutrient concentrations (dissolved inorganic nitrogen (DIN) and phosphorus (P_i), silicate (Si)), nutrient ratios (DIN/P_i), alkaline phosphatase activity, chlorophyll and ¹⁴C primary productivity on fine spatial scales during cruises in March, May, and July 2004. Additionally, resource limitation assays were undertaken in a range of salinity and nutrient regimes reflecting gradients typical of this region. Of these, seven showed P_i limitation, five revealed nitrogen (N) limitation, three exhibited light (L) limitation, and one bioassay had no growth. We found the phytoplankton community to shift from being predominately N limited in the early spring (March) to P limited in late spring and summer (May and July). Light limitation of phytoplankton production was recorded in several bioassays in July in water samples collected after peak annual flows from both the Mississippi and Atchafalaya Rivers. We also found that organic phosphorus, as glucose-6-phosphate, alleviated P limitation while phosphono-acetic acid had no effect. Whereas DIN/P_i and DIN/Si ratios in the initial water samples were good predictors of the outcome of phytoplankton production in response to inorganic nutrients, alkaline phosphatase activity was the best predictor when examining organic forms of phosphorus. We measured the rates of integrated primary production (0.33?C7.01 g C m^?2 d^?1), finding the highest rates within the Mississippi River delta and across Atchafalaya Bay at intermediate salinities. The lowest rates were measured along the outer shelf at the highest salinities and lowest nutrient concentrations (<0.1 ??M DIN and Pi). The results of this study indicate that P_i limitation of phytoplankton delays the assimilation of riverine DIN in the summer as the plume spreads across the shelf, pushing primary production over a larger region. Findings from water samples, taken adjacent the Atchafalaya River discharge, highlighted the importance of this riverine system to the overall production along the Louisiana coast.     

Net Ecosystem Metabolism and Nonconservative Fluxes of Organic Matter in a Tropical Mangrove Estuary, Piauí River (NE of Brazil)

Net ecosystem metabolism (NEM) was measured in the Piauí River estuary, NE Brazil. A mass balance of C, N, and P was used to infer its sources and sinks. Dissolved inorganic carbon (DIC) concentrations and fluxes were measured over a year along this mangrove dominated estuary. DIC concentrations were high in all estuarine sections, particularly at the fluvial end member at the beginning of the rainy season. Carbon dioxide concentrations in the entire estuary were supersaturated throughout the year and highest in the upper estuarine compartment and freshwater, particularly at the rainy season, due to washout effects of carbonaceous soils and different organic anthropogenic effluents. The estuary served as a source of DIC to the atmosphere with an estimated flux of 13 mol CO₂ m^?2 year^?1. Input from the river was 46 mol CO₂ m^?2 year^?1. The metabolism of the system was heterotrophic, but short periods of autotrophy occurred in the lower more marine portions of the estuary. The pelagic system was more or less balanced between auto- and heterotrophy, whereas the benthic and intertidal mangrove region was heterotrophic. Estimated annual NEM yielded a total DIC production in the order of 18 mol CO₂ m^?2 year^?1. The anthropogenic inputs of particulate C, N, and P, dissolved inorganic P (DIP), and DIC were significant. The fluvial loading of particulate organic carbon and dissolved inorganic nitrogen (DIN) was largely retained in two flow regulation and hydroelectric reservoirs, promoting a reduction of C:N and C:P particulate ratios in the estuary. The net nonconservative fluxes obtained by a mass balance approach revealed that the estuary acts as a source of DIP, DIN, and DIC, the latter one being almost equivalent to the losses to the atmosphere. Mangrove forests and tidal mudflats were responsible for most of NEM rates and are the main sites of organic decomposition to sustain net heterotrophy. The main sources for this organic matter are the fluvial and anthropogenic inputs. The mangrove areas are the highest estuarine sources of DIP, DIC, and DIN.     

Oyster (<Emphasis Type="Italic">Crassostrea virginica</Emphasis>) Aquaculture Shifts Sediment Nitrogen Processes toward Mineralization over Denitrification

Filter-feeding bivalves, like oysters, couple pelagic primary production with benthic microbial processes by consuming plankton from the water column and depositing unassimilated material on sediment. Conceptual models suggest that at low to moderate oyster densities, this deposition can stimulate benthic denitrification by providing denitrifying bacteria with organic carbon and nitrogen (N). While enhanced denitrification has been found at oyster reefs, data from oyster aquaculture are limited and equivocal. This study measured seasonal rates of denitrification, as well as dissimilatory nitrate reduction to ammonium (DNRA), and dissolved inorganic N fluxes at a rack and bag eastern oyster (Crassostrea virginica) aquaculture farm. Consistent with models, denitrification was enhanced within the farm, with an average annual increase of 350% compared to a reference site. However, absolute denitrification rates were low relative to other coastal systems, reaching a maximum of 19.2 μmol m^?2 h^?1. Denitrification appeared to be nitrate (NO₃ ^?) limited, likely due to inhibited nitrification caused by sediment anoxia. Denitrification may also have been limited by competition for NO₃ ^? with DNRA, which accounted for an average of 76% of NO₃ ^? reduction. Consequently, direct release of ammonium (NH₄ ⁺) from mineralization to the water column was the most significant benthic N pathway, with seasonal rates exceeding 900 μmol m^?2 h^?1 within the farm. The enhanced N processes were spatially limited however, with significantly higher rates directly under oysters, compared to in between oyster racks. For commercial aquaculture farms like this, with moderate oyster densities (100–200 oysters m^?2), denitrification may be enhanced, but nonetheless limited by biodeposition-induced sediment anoxia. The resulting shift in the sediment N balance toward processes that regenerate reactive N to the water column rather than remove N is an important consideration for water quality.     

Benthic fluxes and porewater concentration profiles of dissolved organic carbon in sediments from the North Carolina continental slope

Numerous studies of marine environments show that dissolved organic carbon (DOC) concentrations in sediments are typically tenfold higher than in the overlying water. Large concentration gradients near the sediment–water interface suggest that there may be a significant flux of organic carbon from sediments to the water column. Furthermore, accumulation of DOC in the porewater may influence the burial and preservation of organic matter by promoting geopolymerization and/or adsorption reactions. We measured DOC concentration profiles (for porewater collected by centrifugation and “sipping”) and benthic fluxes (with in situ and shipboard chambers) at two sites on the North Carolina continental slope to better understand the controls on porewater DOC concentrations and quantify sediment–water exchange rates. We also measured a suite of sediment properties (e.g., sediment accumulation and bioturbation rates, organic carbon content, and mineral surface area) that allow us to examine the relationship between porewater DOC concentrations and organic carbon preservation. Sediment depth-distributions of DOC from a downslope transect (300–1000 m water depth) follow a trend consistent with other porewater constituents (ΣCO₂ and SO₄²⁻) and a tracer of modern, fine-grained sediment (fallout Pu), suggesting that DOC levels are regulated by organic matter remineralization. However, remineralization rates appear to be relatively uniform across the sediment transect. A simple diagenetic model illustrates that variations in DOC profiles at this site may be due to differences in the depth of the active remineralization zone, which in turn is largely controlled by the intensity of bioturbation. Comparison of porewater DOC concentrations, organic carbon burial efficiency, and organic matter sorption suggest that DOC levels are not a major factor in promoting organic matter preservation or loading on grain surfaces. The DOC benthic fluxes are difficult to detect, but suggest that only 2% of the dissolved organic carbon escapes remineralization in the sediments by transport across the sediment-water interface.     

Denitrification and N2O production in near-shore marine sediments

Methods were developed for determining rates of denitrification in coastal marine sediments by measuring the production of N₂ from undisturbed cores incubated in gas-tight chambers. Denitrification rates at summer temperatures (23°C) in sediment cores from Narragansett Bay, Rhode Island, were about 50μmol N₂m^?2 hr^?1. This nitrogen flux is equal to approximately one-half of the NH⁺₄flux from the sediments at this temperature and is of the magnitude necessary to account for the anomalously low N/P and anomalously high O/N ratios often reported for benthic nutrient fluxes. The loss of fixed nitrogen as N₂ during the benthic remineralization of organic matter, coupled with the importance of benthic remineralization processes in shallow coastal waters may help to explain why the availability of fixed nitrogen is a major factor limiting primary production in these areas. Narragansett Bay sediments are also a source of N₂O, but the amount of nitrogen involved was only about 0.2 μmol m^?2 hr^?1 at 23°C.     

Marsh-water column interactions in two Louisiana estuaries. II. Nutrient dynamics

The exchange of dissolved nutrients between marshes and the inundating water column was measured using throughflow marsh flumes built, in two microtidal Louisiana estuaries: the Barataria Basin estuary and Fourleague Bay. The flumes were sampled between September 1986 and April 1988, coincident with an extended period of low sea level on the Louisiana coast. The Barataria Basin estuary is in the later, deteriorating stage of the deltaic cycle, characterized by low freshwater inputs and subsiding marshes. Both brackish and saline marshes supplied dissolved organic nitrogen (DON), inorganic nitrogen (ammonium + nitrate + nitrite = DIN), dissolved organic carbon (DOC), and total nitrogen (as total Kjeldahl nitrogen = TKN) to the water column. The export of DIN is probably related to the N accumulated in earlier stages of deltaic development and released as these marshes deteriorate. Coastal brackish marshes of Fourleague, Bay, part of an accreting marsh system in an early, developmental stage of the deltaic cycle, exported TKN to the open water estuary in all samplings. This marsh apparently acted as a short-term buffer of DIN by taking up NH₄ ⁺ in spring, when baywide concentrations were high, and supplying DIN to the estuary in summer and fall, when concentrations, in the bay were lower. Differences in phosphorus (P), DOC, and DON fluxes between these two estuaries were also observed. The Fourleague Bay site exported soluble reactive phosphorus (SRP) and total phosphorus (TP) and imported DOC. This P export may be related to remobilization of sediment-bound riverine P by the reducing, soils of the marshes. Fluxes of SRP at the Barataria Basin sites were variable and low while DOC was imported. Most imports of dissolved nutrients were correlated with higher upstream [source] concentrations, and flux rates were fairly consistent throughout the tide. Dissolved nutrient exports, did not correlate with upstream concentrations, though, and in many cases the flux was dominated by early, flood tide nutrient release. This pulsed behavior may be caused by rapid diffusion from the sediments early in the tidal cycle, when the sediment-water concentration gradient is largest. Interestuary differences were also seen in particulate organic matter fluxes, as the Fourleague Bay marsh exported POC and PON during all samplings while Barataria Basin imported these nutrients. In general, the magnitude and direction of nutrient exchanges in Louisiana marshes, seem to reflect the deltaic successional stage of the estuary.     

Bottom-water Hypoxia Effects on Sediment–Water Interface Nitrogen Transformations in a Seasonally Hypoxic,Shallow Bay (Corpus Christi Bay,TX, USA)

Bottom-water hypoxia effects on sediment–water interface nitrogen (N) transformations in Corpus Christi Bay (TX, USA) were examined using continuous-flow intact sediment core incubations. Sediment cores were collected from three sites in August 2002 (summer hypoxia) and April 2003 (normoxia). Oxygen (O₂) and hydrogen sulfide (H₂S) depth profiles were generated with microelectrodes. Membrane inlet mass spectrometry was used to measure sediment O₂ demand and net N₂ flux and combined with isotope pairing to determine potential denitrification and N fixation. Potential dissimilatory nitrate reduction to ammonium (DNRA) was measured using high-performance liquid chromatography. Sediment O₂ penetration depths ranged from 5 to 10 mm. H₂S ranged from being present in overlying water and throughout the sediment column in August to not detectable in overlying water or sediment in April. Sediment O₂ demand was higher during bottom-water normoxia conditions versus hypoxia. Sediments were a significant source of \textNH_\text4^{\text +} {\text{NH}}_{\text{4}}^{\text{ + }} to overlying water during hypoxia but not during normoxia. Net N₂ fixation was observed at one station in August and all stations in April. Denitrification rates were significantly higher during hypoxia at two of three sites. Potential DNRA was observed during both oxic states, but rates were significantly higher during hypoxia, which may reflect sulfide enhancement and absence of cation exchange with ^\text14 \textNH_\text4^{\text +} ^{{\text{14}}} {\text{NH}}_{\text{4}}^{\text{ + }} . DNRA may contribute to formation and maintenance of bottom-water hypoxic events in this system. These results show that N transformation pathways and rates change when bottom-water O₂ concentrations drop to hypoxic levels. Since south Texas is a semiarid region with few episodic runoff events, these results indicate that Corpus Christi Bay sediments are a N source most of the year, and denitrification may drive N limitation between episodic runoff events.     

Differential Effects of Bivalves on Sediment Nitrogen Cycling in a Shallow Coastal Bay

In coastal ecosystems, suspension-feeding bivalves can remove nitrogen though uptake and assimilation or enhanced denitrification. Bivalves may also retain nitrogen through increased mineralization and dissimilatory nitrate reduction to ammonium (DNRA). This study investigated the effects of oyster reefs and clam aquaculture on denitrification, DNRA, and nutrient fluxes (NO _x , NH₄ ⁺, O₂). Core incubations were conducted seasonally on sediments adjacent to restored oyster reefs (Crassostrea virginica), clam aquaculture beds (Mercenaria mercenaria) which contained live clams, and bare sediments from Smith Island Bay, Virginia, USA. Denitrification was significantly higher at oyster reef sediments and clam aquaculture site than bare sediment in the summer; however, DNRA was not enhanced. The clam aquaculture site had the highest ammonium production due to clam excretion. While oyster reef and bare sediments exhibited seasonal differences in rate processes, there was no effect of season on denitrification, or dissimilatory nitrate reduction to ammonium (DNRA) or ammonium flux at the clam aquaculture site. This suggests that farm management practices or bivalve metabolism and excretion may override seasonal differences. When water column nitrate concentration was elevated, denitrification increased in clam aquaculture site and oyster reef sediments but not in bare sediment; DNRA was only stimulated at the clam aquaculture site. This, along with a significant and positive relationship between denitrification and sediment organic matter, suggests that labile carbon limited nitrate reduction at the bare sediment site. Bivalve systems can serve as either net sinks or sources of nitrogen to coastal ecosystems, depending mainly on the type of bivalve, location, and management practices.     

A Mini-Review of the Contribution of Benthic Microalgae to the Ecology of the Continental Shelf in the South Atlantic Bight

Benthic microalgae (BMA) inhabit the upper few centimeters of shelf sediments. This review summarizes the current information on BMA communities in the South Atlantic Bight (SAB) region of the Southeastern US continental shelf to provide insights into the potential role of these communities in the trophodynamics and biogeochemical cycling in shelf waters. Benthic irradiance is generally 2–6% of surface irradiance in the SAB region, providing sufficient light to support BMA primary production over 80–90% of the shelf width. BMA biomass greatly exceeds that of integrated phytoplankton biomass in the overlying water column on an areal basis. The SAB appears to have lower BMA biomass, but higher production than most temperate continental shelves. Annual production estimates average 101 and 89 g C m^?2 year^?1 for 5–20 and >?20 depth intervals, respectively. However, high variation in rates and biomass in time and space make comparisons between studies difficult. Submarine groundwater discharge (SGD) rather than the water column or in situ N regeneration from organic matter maybe the major “new” N source for BMA. The estimated supply of N (1.2 mmol N m^?2 day^?1) by SGD closely approximates the rates needed to support BMA primary production (3.1 to 1.6 mmol N m^?2 day^?1) in the sediments of the SAB. Identifying the source(s) of fixed N supporting the BMA community is essential for understanding the carbon dynamics and net ecosystem metabolism within the large area (76,000 km²) of the continental shelf in the SAB as well other temperate shelves worldwide.     

Nitrogen dynamics in nontidal littoral sediments: Role of microphytobenthos and denitrification

Previous measurements from cool microtidal temperate areas suggest that microphytobenthic incorporation of nitrogen (N) exceeds N removal by denitrification in illuminated shallow-water sediments. The present study investigates if this is true also for fully nontidal sediments in the Baltic Sea., Sediment-water fluxes of inorganic (DIN) and, organic nitrogen (DON) and oxygen, as well as denitrification, were measured in early autumn and spring, in light and dark, at four sites representing different sediment types. All sediments were autotrophic during the daytime both in the autumn and spring. On a 24-h time scale, they were autotrophic in the spring and heterotrophic in early autumn. Sediments funcitoned as sources of DIN and DON during the autumn and sinks during the spring, with DON fluxes dominating or being as important as DIN fluxes. Microphytobenthos (MPB) activity controlled fluxes of both DIN and DON. Significant differences between sites were found, although sediment type (sand or silt) had no consistent effect on the magnitude of MPB production or nutrient fluxes. The clearest effect related to sediment type was found for denitrification, although only in the autumn, with higher rates in silty sediments. Estimated N assimilation by MPB, based on both net primary production (0.7–6.5 mmol N m⁻² d⁻¹) and on 80% of gross primary production (1.9–9.4 mmol N m⁻² d⁻¹) far exceeded measured rates of denitrification (0.01–0.16 mmol N m⁻² d⁻¹). A theoretical calculation showed that MPB may incorporate between 40% and 100% of the remineralized N, while denitrification removes, <5%. MPB assimilation of N appears to be a far more important N consuming process than denitrification in these nontidal, shallow-water sediments.     

Inorganic nitrogen dynamics in intertidal rocky biofilms and sediments of the Douro River estuary (Portugal)

In this study rates of oxygen, ammonium (NH₄ ⁺), nitrate (NO₃ ⁻), nitrite (NO₂ ⁻), and nitrous oxide (N₂O) fluxes, nitrogen (N) fixation, nitrification, and denitrification were compared between two intertidal sites for which there is an abundant global literature, muddy and sandy sediments, and two sites representing the rocky intertidal zone where biogeochemical processes have scarcely been investigated. In almost all sites oxygen production rates greatly exceeded oxygen consumption rates. During daylight, NH₄ ⁺ and NO₃ ⁻ uptake rates together with ammonification could supply the different N requirements of the primary producer communities at all four sites; N assimilation by benthic or epilithic primary producers was the major process of dissolved inorganic nitrogen (DIN) removal; N fixation, nitrification, and denitrification were minor processes in the overall light DIN cycle. At night, distinct DIN cycling processes took place in the four environments, denitrification rates ranged from 9 ± 2 to 360 ± 30 μmol N₂ m⁻² h⁻¹, accounting for 10–48% of the water column NO₃ ⁻ uptake; nitrification rates varied from 0 to 1712 ± 666 μmol NH₄ ⁺ m⁻² h⁻¹. A conceptual model of N cycle dynamics showed major differences between intertidal sediment and rocky sites in terms of the mean rates of DIN net fluxes and the processes involved, with rocky biofilm showing generally higher fluxes. Of particular significance, the intertidal rocky biofilms released 10 times the amount of N₂O produced in intertidal sediments (up to 17 ± 6 μmol N₂O m⁻² h⁻¹), representing the highest N₂O release rates ever recorded for marine systems. The biogeochemical contributions of intertidal rocky substrata to estuarine and coastal processes warrant future detailed investigation.     

Experimental nutrient enrichment causes complex changes in seagrass, microalgae, and macroalgae community structure in Florida Bay

We examined the spatial extent of nitrogen (N) and phosphorus (P) limitation of each of the major benthic primary producer groups in Florida Bay (seagrass, epiphytes, macroalgae, and benthic microalgae) and characterized the shifts in primary producer community composition following nutrient enrichment. We established 24 permanent 0.25-m² study plots at each of six sites across. Florida Bay and added N and P to the sediments in a factorial design for 18 mo. Tissue nutrient content of the turtlegrassThalassia testudinum revealed a spatial pattern in P limitation, from severe limitation in the eastern bay (N:P>96:1), moderate limitation in two intermediate sites (approximately 63:1), and balanced with N availability in the western bay (approximately 31:1). P addition increasedT. testudinum cover by 50–75% and short-shoot productivity by up to 100%, but only at the severely P-limited sites. At sites with an ambient N:P ratio suggesting moderate P limitation, few seagrass responses to nutrients occurred. Where ambientT. testudinum tissue N:P ratios indicated N and P availability was balanced, seagrass was not affected by nutrient addition but was strongly influenced by disturbance (currents, erosion). Macroalgal and epiphytic and benthic microalgal biomass were variable between sites and treatments. In general, there was no algal overgrowth of the seagrass in enriched conditions, possibly due to the strength of seasonal influences on algal biomass or regulation by grazers., N addition had little effect on any benthic primary producers throughout the bay. The Florida Bay benthic primary producer community was P limited, but P-induced alterations of community structure were not uniform among primary producers or across Florida Bay and did not always agree with expected patterns of nutrient limitation based on stoichiometric predictions from field assays ofT. testudinum tissue, N:P ratios.     

Atmospheric input of inorganic nitrogen to Delaware Bay

The coastal waters of the mid-Atlantic region of the United States receive inputs of atmospheric pollutants as a consequence of being located downwind from major industrial and urban emissions. These inputs are potentially the largest received by any marine area of the country. Of current interest is the atmospheric input of dissolved inorganic nitrogen (DIN = NO₃ ^?+NH₄ ⁺). We have conducted a first-order examination of the magnitude of atmospheric DIN deposition relative to other large-scale inputs for Delaware Bay, a partially urbanized mid-Atlantic coastal plain estuary. The following loading terms: direct atmospheric deposition, indirect atmospheric loading, urban point discharges, fluvial input, benthic flux, and salt marsh export were evaluated. On an annual basis, municipal-industrial effluents provide a dominant source (ca. 40%) of the DIN inputs to the estuary. Total (wet plus dry) atmospheric deposition accounts for about 15% of the total annual DIN inputs. However, during summer, which is characterized by low river-flow and seasonally maximum atmospheric loading, this figure increases to around 25%. Although atmospheric input can satisfy only a fraction of the primary production demands, this summer flux may represent an ecologically important source of external DIN, half of which is directly deposited to surface photic zones where it is readily available for biological uptake.     

Comparison of dissolved inorganic and organic carbon yields and fluxes in the watersheds of tropical volcanic islands,examples from Guadeloupe (French West Indies)

Organic matter is an important factor that cannot be neglected when considering global carbon cycle. New data including organic matter geochemistry at the small watershed scale are needed to elaborate more constrained carbon cycle and climatic models. The objectives are to estimate the DOC and DIC yields exported from small tropical watersheds and to give strong constraints on the carbon hydrodynamic of these systems. To answer these questions, we have studied the geochemistry of eleven small watersheds around Basse-Terre volcanic Island in the French West Indies during different hydrological regimes from 2006 to 2008 (i.e. low water level versus floods). We propose a complete set of carbon measurements, including DOC and DIC concentrations, δ¹³C data, and less commonly, some spectroscopic indicators of the nature of organic matter. The DOC/DIC ratio varies between 0.07 and 0.30 in low water level and between 0.25 and 1.97 during floods, indicating that organic matter is mainly exported during flood events. On the light of the isotopic composition of DOC, ranging from ? 32.8 to ? 26.2‰ during low water level and from ? 30.1 to ? 27.2‰ during floods, we demonstrate that export of organic carbon is mainly controlled by perennial saprolite groundwaters, except for flood events during which rivers are also strongly influenced by soil erosion. The mean annual yields ranged from 2.5 to 5.7 t km^? 2 year^? 1 for the DOC and from 4.8 to 19.6 t km^? 2 year^? 1 for the DIC and exhibit a non-linear relationship with slopes of watersheds. The flash floods explain around 60% of the annual DOC flux and between 25 and 45% of the DIC flux, highlighting the important role of these extreme meteorological events on global carbon export in small tropical volcanic islands. From a carbon mass balance point of view the exports of dissolved carbon from small volcanic islands are important and should be included in global organic carbon budgets.     

Variability of dissolved organic carbon in sediments of a seagrass bed and an unvegetated area within an estuary in Southern Texas

Pore-water dissolved organic carbon (PWDOC) concentrations were examined in vegetated and bare sediments of aHalodule wrightii seagrass bed, and in a mud bottom sediment of a southern Texas estuary. Temporal variability was examined at diel (dawn and noon) and bimonthly time scales. Distribution patterns of PWDOC were compared with physical, chemical, and biological factors thought to exert control on PWDOC. Concentration of PWDOC, bacterial production, and resultant PWDOC turnover times displayed statistically significant spatial and temporal variability. Concentration of PWDOC ranged from 14 mg C 1^?1 to 107 mg C 1^?1 of pore water, or 9–71 μg C cm^?3 wet sediment. PWDOC was more variable and was approximately 5 times higher than DOC concentrations in the water column. Low PWDOC concentrations (mean = 14.6 μg C cm^?3) and high bacterial production rates (mean = 1.92 μg C cm^?3 h^?1) were observed at the mud station, whereas PWDOC concentrations were high (mean = 24.6 μg C cm^?3) and bacterial production rates were low (mean = 0.43 μg C cm^?3 h^?1) at the bare station. PWDOC turnover times (T_t), assuming 50% bacterial growth efficiency (1–840 h) were shortest at the mud station (mean=13 h) and longest at the bare station (mean=180 h). In the overlying water column, T_t values were longer, ranging from 1,000–10,000 h. PWDOC concentrations were 25% higher in vegetated sediments than in neighboring bare sediments. This difference was probably due to inputs of labile photosynthetic excretia, since bacterial production rates in vegetated sediments displayed significant diel variability and were 4 times greater than that of bare sediments. Based upon the entire data set, PWDOC was significantly related to macrofaunal biomass, sediment POC, sediment C:N ratios, and oxygen metabolism, but was significantly correlated only to the latter two variables in stepwise multiple regression. Our findings suggest that organism activities and detrital quality are the major determinants controlling variability in PWDOC.