Benthic metabolism and nutrient cycling along an estuarine salinity gradient

Benthic metabolism and nutrient exchange across the sediment-water interface were examined over an annual cycle at four sites along a freshwater to marine transect in the Parker River-Plum Island Sound estuary in northeastern Massachusetts, U.S. Sediment organic carbon content was highest at the freshwater site (10.3%) and decreased along the salinity gradient to 0.2% in the sandy sediments at the marine end of the estuary. C:N ratios were highest in the mid estuary (23:1) and lowest near the sea (11:1). Chlorophyll a in the surface sediments was high along the entire length of the estuary (39–57 mg chlorophyll a m⁻²) but especially so in the sandy marine sediments (172 mg chlorophyll a m⁻²). Chlorophyll a to phaeophytin ratios suggested most chlorophyll is detrital, except at the sandy marine site. Porewater sulfide values varied seasonally and between sites, reflecting both changes in sulfate availability as overlying water salinity changed and sediment metabolism. Patterns of sediment redox potential followed those of sulfide. Porewater profiles of inorganic N and P reflected strong seasonal patterns in remineralization, accumulation, and release. Highest porewater NH₄ ⁺ values were found in upper and mid estuarine sediments, occasionally exceeding 1 mM N. Porewater nitrate was frequently absent, except in the sandy marine sediments where concentrations of 8 μM were often observed. Annual average respiration was lowest at the marine site (13 mmol O₂ m⁻² d⁻¹ and 21 mmol TCO₂ m⁻² d⁻¹) and highest in the mid estuary (130 mmol O₂ m⁻² d⁻¹ and 170 mmol TCO₂ m⁻² d⁻¹) where clam densities were also high. N₂O and CH₄ fluxes were low at all stations throughout the year: Over the course, of a year, sediments varied from being sources to sinks of dissolved organic C and N, with the overall spatial pattern related closely to sediment organic content. There was little correlation between PO₄ ³⁻ flux and metabolism, which we attribute to geochemical processes. At the two sites having the lowest salinities, PO₄ ³⁻ flux was directed into the sediments. On average, between 22% and 32% of total system metabolism was attributable to the benthos. The mid estuary site was an exception, as benthic metabolism accounted for 95% of the total, which is attributable to high densities of filter-feeding clams. Benthic remineralization supplied from less than 1% to over 190% of the N requirements and 0% to 21% of the P requirements of primary producers in this system. Estimates of denitrification calculated from stoichiometry of C and N fluxes ranged from 0% for the upper and mid estuary site to 35% for the freshwater site to 100% of sediment organic N remineralization at the marine site. We hypothesize that low values in the upper and mid estuary are attributable to enhanced NH₄ ⁺ fluxes during summer due to desorption of exchangeable ammonium from rising porewater salinity. NH₄ ⁺ desorption during summer may be a mechanism that maintains high rates of pelagic primary production at a time of low inorganic N inputs from the watershed.     

Planktonic and whole system metabolism in a nutrient-rich estuary (the Scheldt estuary)

Planktonic gross primary production (GPP), community respiration (CR), and nitrification (NIT) were measured monthly in the Scheldt estuary by the oxygen incubation method in 2003. No significant evolution of planktonic GPP was observed since the 1990s with high rates in the freshwater area (salinity 0; 97±65 mmol C m⁻² d⁻¹) decreasing seaward (22–37 mmol C m⁻² d⁻¹). A significant decrease of NIT was observed with regard to previous investigations although this process still represents up to 20% of total organic matter production in the inner estuary. Planktonic CR was highest in the inner estuary and seemed to be mainly controlled by external organic matter inputs. Planktonic net community production was negative most of the time in the estuary with values ranging from −300 to 165 mmol C m⁻² d⁻¹. Whole estuary net ecosystem production (NEP) was investigated on an annual scale using the results mentioned above and published benthic metabolic rates. A NEP of −39±8 mmol C m⁻² d⁻¹ was estimated, which confirms the strong heterotrophic status of this highly nutrified estuary. NEP rates were computed from June to December 2003 to compare with results derived from the Land-Ocean Interaction in the Coastal Zone budgeting procedure applied to dissolved inorganic phosphorus and carbon (DIP and DIC). DIP budgets failed to provide realistic estimates in the inner estuary where abiotic processes account for more than 50% of the nonconservative DIP flux. DIC budgets predicted a much lower NEP than in situ incubations (−109±31 versus −42±9 mmol C m⁻² d⁻¹) although, as each approach is associated with several critical assumptions, the source of this discrepancy remains unclear.     

Benthic Oxygen Consumption and Organic Matter Turnover in Organic-poor, Permeable Shelf Sands

The high permeability of sediments and strong near-bottom currents cause seawater to infiltrate the surface layers of Middle Atlantic Bight shelf deposits. In this study, sandy sediment cores from 11 to 12 m water depth were percolated with filtered seawater on shipboard. Sedimentary oxygen consumption (SOC) increased non-linearly with pore water flow, approaching maximum rates of 120 mmol m⁻² d⁻¹ (May 2001) or 75 mmol m⁻² d⁻¹(July 2001). The addition of acetate to the inflowing water promptly enhanced the release of dissolved inorganic carbon (DIC) from the cores. DIC production rates were a linear function of acetate concentration, ranging from 100 to 300 mmol m⁻² d⁻¹ without substrate addition to 572 mmol m⁻² d⁻¹ with 100 mM acetate. The sediments also hydrolyzed a glucose pseudopolymer, and the liberated glucose prompted an increase of SOC. Our results suggest that decomposition rates of organic matter in permeable sands can exceed those of fine-grained, organic-rich deposits, when water currents cause advective interstitial flow, supplying the subsurface microbial community with degradable material and electron acceptors. We conclude that the highly permeable sand beds of the Middle Atlantic Bight are responsive within minutes to hours and efficiently operate as biocatalytical filters.     

Calcite dissolution driven by benthic mineralization in the deep-sea: in situ measurements of Ca2+, pH,pCO2 and O2

In situ measured microprofiles of Ca²⁺, pCO₂, pH and O₂ were performed to quantify the CaCO₃ dissolution and organic matter mineralization in marine sediments in the eastern South Atlantic. A numerical model simulating the organic matter decay with oxygen was used to estimate the calcite dissolution rate. From the oxygen microprofiles measured at four stations along a 1300-m isobath of the eastern African margin and one in front of the river Niger at a water depth of 2200 m the diffusive oxygen uptake (DOU) and oxygen penetration depth (OPD) was calculated. DOU rates were in the range of 0.3 to 3 mmol m⁻² d⁻¹ and showed a decrease with increasing water depth, corresponding to an increase in OPD. The calculated amount of degradated organic matter is in the range of 1 to 8.5 gC m⁻² a⁻¹. The metabolic CO₂, released from mineralization of the organic matter drives calcite dissolution in these sediments overlain by calcite-supersaturated water. Fluxes across the sediment water interface calculated from the in situ Ca²⁺ microprofiles were 0.6 mmol m⁻² d⁻¹ for two stations at a water depth of 1300 m. The ratio of calcite dissolution flux and organic C degradation is 0.53 and 0.97, respectively. The microprofiles indicate that CO₂ produced within the upper oxic sediment layer dissolves up to 85% of the calcite rain to the seafloor. Modeling our O₂, pH and Ca²⁺ profiles from one station predicted a calcite dissolution rate constant for this calcite-poor site of 1000 mol kgw⁻¹ a⁻¹ (mol per kg water and year), which equals 95% d⁻¹. This rate constant is at the upper end of reported in situ values.     

Denitrification and the stoichiometry of nutrient regeneration in Waquoit Bay, Massachusetts

To determine the removal of regenerated nitrogen by estuarine sediments, we compared sediment N₂ fluxes to the stoichiometry of nutrient and O₂ fluxes in cores collected in the Childs River, Cape Cod, Massachusetts. The difference between the annual PO₄ ³⁻ (0.2 mol P m⁻² yr⁻¹) and NH₄ ⁺ (1.6 mol N m⁻² yr⁻¹) flux and the Redfield N∶P ratio of 16 suggested an annual deficit of 1.5 mol N m⁻² yr⁻¹. Denitrification predicted from O₂∶NH₄ ⁺ flux ratios and measured as N₂ flux suggested a nitrogen sink of roughly the same magnitude (1.4 mol N m⁻² yr⁻¹). Denitrification accounted for low N∶P ratios of benthic flux and removed 32–37% of nitrogen inputs entering the relatively highly nutrient loaded Childs River, despite a relatively brief residence time for freshwater in this system. Uptake of bottom water nitrate could only supply a fraction of the observed N₂ flux. Removal of regenerated nitrogen by denitrification in this system appears to vary seasonally. Denitrification efficiency was inversely correlated with oxygen and ammonium flux and was lowest in summer. We investigated the effect of organic matter on denitrification by simulating phytoplankton deposition to cores incubated in the lab and by deploying chambers on bare and macroaglae covered sediments in the field. Organic matter addition to sediments increased N₂ flux and did not alter denitrification efficiency. Increased N₂ flux co-varied with O₂ and NH₄ ⁺ fluxes. N₂ flux (261±60 μmol m⁻² h⁻¹) was lower in chambers deployed on macroalgal beds than deployed on bare sediments (458±70 μmol m⁻² h⁻¹), and O₂ uptake rate was higher in chambers deployed on macroalgal beds (14.6±2.2 mmol m⁻² h⁻¹) than on bare sediments (9.6±1.5 mmol m⁻² h⁻¹). Macroalgal cover, which can retain nitrogen in the system, is a link between nutrient loading and denitrification. Decreased denitrification due to increasing macroalgal cover could create a positive feedback because decreasing denitrification would increase nitrogen availability and could increase macroalgae cover.     

Benthic Phosphorus Dynamics in the Gulf of Finland, Baltic Sea

Benthic fluxes of soluble reactive phosphorus (SRP) and dissolved inorganic carbon (DIC) were measured in situ using autonomous landers in the Gulf of Finland in the Baltic Sea, on four expeditions between 2002 and 2005. These measurements together with model estimates of bottom water oxygen conditions were used to compute the magnitude of the yearly integrated benthic SRP flux (also called internal phosphorus load). The yearly integrated benthic SRP flux was found to be almost 10 times larger than the external (river and land sources) phosphorus load. The average SRP flux was 1.25?±?0.56?mmol?m^?2?d^?1 on anoxic bottoms, and ?0.01?±?0.08?mmol?m^?2?d^?1 on oxic bottoms. The bottom water oxygen conditions determined whether the SRP flux was in a high or low regime, and degradation of organic matter (as estimated from benthic DIC fluxes) correlated positively with SRP fluxes on anoxic bottoms. From this correlation, we estimated a potential increase in phosphorus flux of 0.69?±?0.26?mmol?m^?2?d^?1 from presently oxic bottoms, if they would turn anoxic. An almost full annual data set of in situ bottom water oxygen measurements showed high variability of oxygen concentration. Because of this, an estimate of the time which the sediments were exposed to oxygenated overlying bottom water was computed using a coupled thermohydrodynamic ocean?Csea and ecosystem model. Total phosphorus burial rates were calculated from vertical profiles of total phosphorus in sediment and sediment accumulation rates. Recycling and burial efficiencies for phosphorus of 97 and 3%, respectively, were estimated for anoxic accumulation bottoms from a benthic mass balance, which was based on the measured effluxes and burial rates.     

Biogeochemistry of Major Redox Elements and Mercury in a Tropical Reservoir Lake (Petit Saut,French Guiana)

The hydroelectric reservoir of Petit Saut, French Guiana, was created in 1994–1995 by flooding 350 km² of tropical forest. When sampled in 1999, the lake exhibited a permanent stratification separating the 3–5 m thick, oxygenated epilimnion from the anoxic hypolimnion. The rate of anaerobic organic carbon mineralization below the oxycline was on the order of 1 μmol C m⁻² s⁻¹ and did not show a pronounced difference between wet and dry seasons. Methanogenesis accounted for 76–83% of anaerobic carbon mineralization, with lesser contributions of sulfate reduction and dissimilatory iron reduction. Upward mixing of reduced inorganic solutes explained 90% of the water column O ₂ demand during the dry season, while most O ₂ consumption during the wet season was coupled to aerobic respiration of organic matter synthesized in the surface waters. Inorganic mercury species represented 10–40% of total dissolved mercury in the epilimnion, but were of relatively minor importance (≤10%) in the anoxic portion of the water column. Net production of soluble organic mercury compounds in the flooded soils and anoxic water column did not vary significantly between wet and dry seasons. Methylmercury accounted for about 15% of total dissolved mercury below the oxycline. Its estimated net production rate, 0.04 mg m⁻² yr⁻¹, is of the same order of magnitude as values reported for contaminated lakes and flooded terrestrial ecosystems.     

The Effects of Salinity on Nitrogen Losses from an Oligohaline Estuarine Sediment

Benthic respiration, sediment–water nutrient fluxes, denitrification and dissimilatory nitrate reduction to ammonium (DNRA) were measured in the upper section of the Parker River Estuary from 1993 to 2006. This site experiences large changes in salinity over both short and long time scales. Sediment respiration ranged from 6 to 52 mmol m⁻² day⁻¹ and was largely controlled by temperature. Nutrient fluxes were dominated by ammonium fluxes, which ranged from a small uptake of −0.3 to an efflux of over 8.2 mmol N m⁻² day⁻¹. Ammonium fluxes were most highly correlated with salinity and laboratory experiments demonstrated that ammonium fluxes increased when salinity increased. The seasonal pattern of DNRA closely followed salinity. DNRA rates were extremely low in March, less than 0.1 mmol m⁻² day⁻¹, but increased to 2.0 mmol m⁻² day⁻¹ in August. In contrast, denitrification rates were inversely related to salinity, ranging from 1 mmol m⁻² day⁻¹ during the spring and fall to less than 0.2 mmol m⁻² day⁻¹ in late summer. Salinity appears to exert a major control on the nitrogen cycle at this site, and partially decouples sediment ammonium fluxes from organic matter decomposition.     

Primary production in Long Island sound

Daily and annual integrated rates of primary productivity and community respiration were calculated using physiological parameters measured in oxygen-based photosynthesis-irradiance (P-I) incubations at 8 stations throughout central and western Long Island Sound (cwLIS) during the summer and autumn of 2002 and 2003 and the late spring of 2003. Each calculation takes into account actual variations in incident irradiance over the day and underwater irradiance and standing stock with depth. Annual peak rates, ±95% confidence interval of propagated uncertainty in each measurement, of gross primary production (GPP, 1,730±610 mmol O₂ m⁻² d⁻¹), community respiration (R_c, 1,660±270 mmol O₂ m⁻² d⁻¹), and net community production (NCP, 1,160±1,100 mmol O₂ m⁻² d⁻¹) occurred during summer at the western end of the Sound. Lowest rates of GPP (4±11 mmol O₂ m⁻² d⁻¹), R_c (−50±300 mmol O₂ m⁻² d⁻¹), and NCP (−1,250±270 mmol O₂ m⁻² d⁻¹) occurred during late autumn-early winter at the outer sampled stations. These large ranges in rates of GPP, R_c, and NCP throughout the photic zone of cwLIS are attributed to seasonal and spatial variability. Algal respiration (R_a) was estimated to consume an average of 5% to 52% of GPP, using a literature-based ratio of R_a:R_c. From this range, we established that the estimated R_a accounts for approximately half of GPP, and was used to estimate daily net primary production (NPP), which ranged from 2 to 870 mmol O₂ m⁻² d⁻¹ throughout cwLIS during the study. Annual NPP averaged 40±8 mol O₂ m⁻² yr⁻¹ for all sampled stations, which more than doubled along the main axis of the Sound, from 32±14 mol O₂ m⁻² yr⁻¹ at an eastern station to 82±25 mol O₂ m⁻² yr⁻¹ at the western-most station. These spatial gradients in productivity parallel nitrogen loads along the main axis of the Sound. Daily integrals of productivity were used to test and formulate a simple, robust biomass-light model for the prediction of phytoplankton production in Long Island Sound, and the slope of the relationship was consistent with reports for other systems.     

Microelectrode Study of Oxygen Uptake and Organic Matter Decomposition in The Sediments of Xiamen Western Bay

Sediment cores were sampled from Xiamen Western Bay at five sites during the summer and winter of 2006 and Hg–Au microelectrodes were used to make on board measurements of the concentration gradients of dissolved oxygen, Mn²⁺, and Fe²⁺ within the sediments. The O₂ concentrations decreased sharply from about 200 μmol L⁻¹ in the bottom seawater to zero within a depth of a few millimeters into the sediment. Dissolved Mn²⁺ was detected below the oxic zones with peak concentrations up to 600 μmol L⁻¹, whereas dissolved Fe²⁺ had peak concentrations up to 1,000 μmol L⁻¹ in deeper layers. The elemental contents of organic carbon and nitrogen within the sediments were analyzed and their C/N ratios were in the range of 9.0 to 10.1, indicative of heavy terrestrial origin. Sediments from two sites near municipal wastewater discharge outlets had higher organic contents than those from the other sites. These high organic contents corresponded to shallow O₂ penetration depths, high dissolved Mn²⁺ and Fe²⁺ concentrations, and negative redox potentials within the sediments. This indicated that the high organic matter content had promoted microbial respiration within the sediments. Overall, the organic content did not show any appreciable decrease with increasing sediment depths, so a quadratic polynomial function was used to fit the curve of O₂ profiles within the sediments. Based on the O₂ profiles, O₂ fluxes across the seawater and sediment interface were estimated to be in the range 6.07 to 14.9 mmol m⁻² day⁻¹, and organic carbon consumption rates within the surface sediments were estimated to be in the range 3.3 to 20.8 mgC cm⁻³ a⁻¹. The case demonstrated that biogeochemistry within the sediments of the bay was very sensitive to human activities such as sewage discharge.     

Temporal variability in summertime bottom hypoxia in shallow areas of Mobile Bay,Alabama

This paper addresses temporal variability in bottom hypoxia in broad shallow areas of Mobile Bay, Alabama. Time-series data collected in the summer of 2004 from one station (mean depth of 4 m) exhibit bottom dissolved oxygen (DO) variations associated with various time scales of hours to days. Despite a large velocity shear, stratification was strong enough to suppress vertical mixing most of the time. Bottom DO was closely related to the vertical salinity gradient (ΔS). Hypoxia seldom occurred when ΔS (over 2.5 m) was <2 psu and occurred almost all the time when ΔS was >8 psu in the absence of extreme events like hurricanes. Oxygen balance between vertical mixing and total oxygen demand was considered for bottom water from which oxygen demand and diffusive oxygen flux were estimated. The estimated decay rates at 20°C ranging between 0.175–0.322 d⁻¹ and the corresponding oxygen consumption as large as 7.4 g O₂ m⁻² d⁻¹ fall at the upper limit of previously reported ranges. The diffusive oxygen flux and the corresponding vertical diffusivity estimated for well mixed conditions range between 8.6–9.5 g O₂ m⁻² d⁻¹ and 2.6–2.9 m² d⁻¹, respectively. Mobile Bay hypoxia is likely to be associated with a large oxygen demand, supported by both water column and sediment oxygen demands, so that oxygen supply from surface water during destratification events would be quickly exhausted to return to hypoxic conditions within a few hours to days after destratification events are terminated.     

Physical Controls on Spatial Variability in Decomposition of Organic Matter in Lake Kinneret,Israel

High resolution chemical data collected during summer 2003 indicate that the lower water mass (LWM) of the thermally stratified Lake Kinneret (LK) can be subdivided into three layers: a benthic boundary layer (BBL), overlain by the hypolimnion (HYP), and on top, the lower part of the metalimnion (ME-L). After onset of thermal stratification, the BBL is the first layer that turns anoxic, followed shortly afterward by the ME-L, while the HYP remains oxic and has relatively higher pH until later in summer. Thus, during the early summer, the HYP forms an oxygen-containing layer in-between two DO-deficient layers. Somewhat later, the HYP is characterized by still having significant levels of nitrate NO₃, while in both adjacent layers nitrate is already removed through denitrification. The mechanisms controlling the gradual decline of dissolved oxygen (DO) in the HYP during the summer were studied. The seasonal mean lake-wide vertical eddy diffusion coefficient in this layer, evaluated from heat flux measurements, is approximately 4 × 10⁻⁶ m² s⁻¹. The vertical oxygen flux due to diffusion from within the HYP toward its oxygen-deficient upper and lower boundaries accounts for most of the slow summer decline in DO in this layer. A smaller portion of this decline can be attributed to in-layer respiratory processes. The low turbidity, relatively high pH, and slow accumulation rate of NH₄ in the HYP support the notion that the slower mineralization processes occurring in this layer result from relatively low ambient concentrations of biodegradable organic matter, most probably due to the short residence time of the particles settling through this layer.     

Suspended Matter,Chl-a,CDOM, Grain Sizes,and Optical Properties in the Arctic Fjord-Type Estuary,Kangerlussuaq, West Greenland During Summer

Optical constituents as suspended particulate matter (SPM), chlorophyll (Chl-a), colored dissolved organic matter (CDOM), and grain sizes were obtained on a transect in the arctic fjord-type estuary Kangerlussuaq (66°) in August 2007 along with optical properties. These comprised diffuse attenuation coefficient of downwelling PAR (K _d(PAR)), upwelling PAR (K _u(PAR)), particle beam attenuation coefficient (c _p), and irradiance reflectance R(−0, PAR). PAR is white light between 400 and 700 nm. The estuary receives melt water from the Greenland Inland Ice and stations covered a transect from the very high turbid melt water outlet to clear marine waters. Results showed a strong spatial variation with high values as for suspended matter concentrations, CDOM, diffuse attenuation coefficient K _d(PAR), particle beam attenuation coefficients (c _p), and reflectance R(−0, PAR) at the melt water outlet. Values of optical constituents and properties decreased with distance from the melt water outlet to a more or less constant level in central and outer part of the estuary. There was a strong correlation between inorganic suspended matter (SPMI) and diffuse attenuation coefficient K _d(PAR) (r ² = 0.92) and also for particle beam attenuation coefficient (c _p; r ² = 0.93). The obtained SPMI specific attenuation—K _d^*(PAR) = 0.13 m² g⁻¹ SPMI—and the SPMI specific particle beam attenuation—c _p^* = 0.72 m² g⁻¹—coefficients were about two times higher than average literature values. Irradiance reflectance R(−0, PAR) was comparatively high (0.09−0.20) and showed a high (r ² = 0.80) correlation with K _u(PAR). Scattering dominated relative to absorption—b(PAR)/a(PAR) = 12.3. Results strongly indicated that the high values in the optical properties were related to the very fine particle sizes (mean = 2–6 μm) of the suspended sediment. Data and results are discussed and compared to similar studies from both temperate and tropical estuaries.     

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.     

Sulfate reduction and porewater chemistry in a gulf coast<Emphasis Type="Italic">Juncus roemerianus</Emphasis> (Needlerush) marsh

Sulfate reduction rates were measured over the course of a year in the sediments of aJuncus roemerianus marsh located in coastal Alabama. Sulfate reduction rates were typically highest in the surface 0–2 cm and at depths corresponding to peak belowground biomass of the plants. The highest volume-based sulfate reduction rate measured was 1,350 μmol liter-sediment⁻¹ d⁻¹ in September 1995. Areal sulfate reduction rates (integrated to 20 cm depth) were strongly correlated to sediment temperature and varied seasonally from 15.2 mmol SO ₄ ²⁻ m⁻² d⁻¹ in January 1995 to 117 mmol SO ₄ ²⁻ m⁻² d⁻¹ in late August 1995. Despite high sulfate reduction rates porewater dissolved sulfide concentrations were low (<73 μM), indicating rapid sulfide oxidation or precipitation. Sulfate depletion data indicated that net oxidation of sediment sulfides occurred in March through May, following a period of infrequent tidal flooding and during a period of high plant production. Porewater Fe(II) reached very high levels (maximum of 969 μM; mean for all dates was 160 μM), particularly during periods of high sulfate reduction. The annual sulfate reduction rate integrated over the upper 20 cm of sediment was 22.0 mol SO ₄ ²⁻ m⁻² yr⁻¹, which is among the highest rates measured in a wetland ecosystem. Based on literature values of net primary production inJ. roemerianus marshes, we estimate that an amount equivalent to 16% to 90% of the annual belowground production may be remineralized through sulfate reduction.     

Carbon Cycling and the Coupling Between Proton and Electron Transfer Reactions in Aquatic Sediments in Lake Champlain

We used fine-scale porewater profiles and rate measurements together with a multiple component transport–reaction model to investigate carbon degradation pathways and the coupling between electron and proton transfer reactions in Lake Champlain sediments. We measured porewater profiles of O₂, Mn²⁺, Fe²⁺, HS⁻, pH and pCO₂ at mm resolution by microelectrodes, and profiles of NO₃ ⁻, SO₄ ²⁻, NH₄ ⁺, total inorganic carbon (DIC) and total alkalinity (TA) at cm resolution using standard wet chemical techniques. In addition, sediment–water fluxes of oxygen, DIC, nitrate, ammonium and N₂ were measured. Rates of gross and net sulfate reduction were also measured in the sediments. It is shown that organic matter (OM) decomposes via six pathways: oxic respiration (35.2%), denitrification (10.4%), MnO₂ reduction (3.6%), FeOOH reduction (9.6%), sulfate reduction (14.9%), and methanogenesis (26.4%). In the lake sediments, about half of the benthic O₂ flux is used for aerobic respiration, and the rest is used for the regeneration of other electron acceptors produced during the above diagenetic reactions. There is a strong coupling between O₂ usage and Mn²⁺ oxidation. MnO₂ is also an important player in Fe and S cycles and in pH and TA balance. Although nitrate concentrations in the overlying water were low, denitrification becomes a quantitatively important pathway for OM decomposition due to the oxidation of NH₄ ⁺ to NO₃ ⁻. Finally, despite its low concentration in freshwater, sulfate is an important electron acceptor due to its high efficiency of internal cycling. This paper also discusses quantitatively the relationship between redox reactions and the porewater pH values. It is demonstrated here that pH and pCO₂ are sensitive variables that reflect various oxidation and precipitation reactions in porewater, while DIC and TA profiles provide effective constraints on the rates of various diagenetic reactions.     

Dissolution of Carbonate Sediments Under Rising <Emphasis Type="Italic">p</Emphasis>CO<Subscript>2</Subscript> and Ocean Acidification: Observations from Devil’s Hole,Bermuda

Rising atmospheric pCO₂ and ocean acidification originating from human activities could result in increased dissolution of metastable carbonate minerals in shallow-water marine sediments. In the present study, in situ dissolution of carbonate sedimentary particles in Devil’s Hole, Bermuda, was observed during summer when thermally driven density stratification restricted mixing between the bottom water and the surface mixed layer and microbial decomposition of organic matter in the subthermocline layer produced pCO₂ levels similar to or higher than those levels anticipated by the end of the 21st century. Trends in both seawater chemistry and the composition of sediments in Devil’s Hole indicate that Mg-calcite minerals are subject to selective dissolution under conditions of elevated pCO₂. The derived rates of dissolution based on observed changes in excess alkalinity and estimates of vertical eddy diffusion ranged from 0.2 mmol to 0.8 mmol CaCO₃ m⁻² h⁻¹. On a yearly basis, this range corresponds to 175–701 g CaCO₃ m⁻² year⁻¹; the latter rate is close to 50% of the estimate of the current average global coral reef calcification rate of about 1,500 g CaCO₃ m⁻² year⁻¹. Considering a reduction in marine calcification of 40% by the year 2100, or 90% by 2300, as a result of surface ocean acidification, the combination of high rates of carbonate dissolution and reduced rates of calcification implies that coral reefs and other carbonate sediment environments within the 21st and following centuries could be subject to a net loss in carbonate material as a result of increasing pCO₂ arising from burning of fossil fuels.     

Benthic Respiration and Inorganic Nutrient Fluxes in the Estuarine Region of Patos Lagoon (Brazil)

In situ benthic flux chamber experiments were performed during late austral spring and early summer of 1996 at eleven nearshore locations in the southern Patos Lagoon, Brazil. The Patos Lagoon is the largest lagoonal system in South America and is a very important nursery ground for local fin fish and shell fish fisheries. These are the first benthic flux measurements made in Patos Lagoon and they suggest that remineralizationwithin the sediments may dominate the recycling of organic matter and nutrients in thelagoon. Measured oxygen benthic fluxes (45–160 mmol m^-2 d^-1) are sufficientto remineralize reported mean water column carbon fixation while phosphate and fixednitrogen benthic fluxes (-0.4–2 and -1.1–4.2 mmol m^-2 d^-1, respectively)are sufficient to supply 100% and 25% of the required water column nutrient demand,respectively. Although of limited areal and temporal coverage, these initial studiesdemonstrate that sediments play a major role in the metabolism and nutrient cyclingwithin the Patos Lagoon Estuary and that future studies of lagoonal biogeochemistrymust consider exchange with the bottom.     

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.     

Determination of denitrification in the Chesapeake Bay from measurements of N2 accumulation in bottom water

This study demonstrates the feasibility of using direct N₂ measurements in an estuary for determination of denitrification. High precision measurements of dinitrogen: argon ratios (N₂∶Ar) were made by membrane inlet mass spectrometry on water samples taken along the length of the Chesapeake Bay in July and October 2004. The N₂∶Ar ratio in low salinity surface water was elevated relative to air saturation by 0.3–0.5% with no systematic change along the length of the Bay. N₂∶Ar in high salinity bottom water exhibited a linear increase in the landward direction along a 144-km longitudinal section. In this section of the Bay covering 20% of the main stem, the bottom water salinity was statistically uniform and the increase in N₂∶Ar was in the direction of net residual current flow. The system was analyzed as a capped river with the assumption that N₂ entered the water from the underlying sediment where denitrification is known to take place. The rate of denitrification needed to support the measured increase in N₂ was calculated using an average residual current velocity and water column depth. The increase in N₂ with distance (0.046μmol N l⁻¹ km⁻¹) equated to an average denitrification flux of 73 μmol N m⁻² h⁻¹. N₂ fluxes determined on sediment cores taken from the source and terminus regions of the delineated water mass were 45±23 and 83±39 μmol N m⁻² hr⁻¹, respectively, which were not statistically different from the whole system estimate. The measured change in oxygen concentration within the bottom water was used to estimate nitrogen remineralization and the efficiency of denitrification. Denitrification efficiency (nitrogen denitrified/nitrogen remineralized) was estimated to be in the range of 22–28% for the bottom water sediment system and 30–37% considering the sediment zone alone.