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.     

Sources of faecal contamination in the Seine estuary (France)

The waters of the Seine river estuary, located in a highly anthropogenicized area in the northern part of France, are of poor microbiological quality; the concentrations of faecal bacteria usually exceed the European Union bathing and recreational water directives. The aim of the present study was to identify the main sources of the faecal pollution of the Seine estuary in order to help define priorities for management and sanitation efforts. Budgets of faecal coliform (FC) inputs to the estuary were established for various hydrological conditions. Main sources of FC were the outfalls of the treated effluents of the wastewater treatment plants (WWTPs) located along the estuary, the faecal bacteria brought in through the tributaries of the Seine estuary, and the faecal bacteria transported by the Seine river flow at the estuary entrance at Poses dam. In order to quantify these inputs, FC were enumerated during sampling campaigns conducted for various hydrological conditions in the Seine at the entrance of the estuary, in the tributaries close to their confluence with the estuary, and in the effluents of some WWTPs located along the estuary. The importance of the flux of FC transported by the Seine river flow at the estuary entrance at Poses dam decreased from 92% of the total FC input when the flow rate was high (717 m³ s⁻¹) to 5% when flow rate was low (143 m³ s⁻¹). The release of the domestic wastewaters of the large city of Paris located 120 km upstream from the entrance of the estuary was mainly responsible for this microbiological pollution. At low flow rates, the tributaries represent the most important source of FC (64–76% for flow rates of the Seine at Poses at approximately 150 m³ s⁻¹), mainly from the Robec and Eure rivers. The treated wastewater of the WWTPs located along the estuary was the second source of FC for low flow conditions (19–30%); it was less important for high to intermediate flow rate conditions.     

Estimating the benthic population ofDreissena polymorpha and its impact in the lower Seine River, France

Dreissena larval fluxes were studied in the lower stretch of the Seine River in 1996–1998. Fluxes reached 150×10¹² ind d⁻¹, representing a larval concentration of 5,000 ind l⁻¹ in the Seine estuary. We showed that a sampling frequency with a 3-d interval allowed us to adequately estimate the annual production of larvae. The water residence time in the Seine River and estuary is sufficient for theDreissena larvae to complete their cycle and settlement. High abundance of the larvae in the plankton samples from the Seine River and its estuary showed the existence of a large community of benthic adults, known to be powerful filter-feeders. The progenitor population and the geographical extent of the adultDreissena were estimated from cohort analyses of the planktonic larvae. The maximum density ofDreissena was found in the highly channelized part of the estuary (up to 4,500 ind m⁻²). Estimated values were compared with concentration of mussels in the benthic traps and samples. Calculated filtration rates of benthic mussels were compared with those of larvae at different stages and with filtration of the zooplankton community. The impact of theDreissena was much higher than that of zooplankton; the filtration of the larvae exceeded that of adults during short periods of maximum larval emission.     

The effects of multiple stressors on the balance between autotrophic and heterotrophic processes in an estuarine system

Responses of autotrophic and heterotrophic processes to nutrients and trace elements were examined in a series of experimental estuarine food webs of increasing trophic complexity using twenty 1-m³ mesocosms. Nutrients (nitrogen and phosphorus) and trace elements (a mix of arsenic, copper, cadmium) were added alone and in combination during four experimental runs spanning from spring 1997 to spring 1998. Diel changes in dissolved oxygen were used to examine whole system gross primary production (WS-GPP), respiration (WS-RESP), and net ecosystem metabolism (NEM). Nutrient and trace element additions had the greatest effect on WS-GPP, WS-RESP, and NEM; trophic complexity did not significantly affect any of these parameters (p>0.3). Effects of trophic complexity were detected in nutrient tanks where bivalves significantly (p=0.03) reduced WS-GPP. Nutrient additions significantly enhanced WS-GPP and to a lesser extent WS-RESP during most mesocosm runs. The system shifted from net heterotrophy (−17.2±1.8 mmol C m⁻³ d⁻¹) in the controls to net autotrophy (29.1±7.6 mmol C m⁻³ d⁻¹) in the nutrient tanks. The addition of trace elements alone did not affect WS-GPP and WS-RESP to the same extent as nutrients, and their effects were more variable. Additions of trace elements alone consistently made the system more net heterotrophic (−24.9±1.4 mmol C m⁻³ d⁻¹) than the controls. When trace elements were added in combination with nutrients, the nutrient-enriched system became less autotrophic (1.6±3.1 mmol C m⁻³ d⁻¹). The effects of trace elements on NEM occurred primarily through reductions in WS-GPP rather than increases in WS-RESP. Our results suggest that autotrophic and heterotrophic processes respond differently to these stressors.     

Tidally-induced shear stress variability above intertidal mudflats in the macrotidal seine estuary

Tidal currents and the spatial variability of tidally-induced shear stress were studied during a tidal cycle on four intertidal mudflats from the fluvial to the marine part of the Seine estuary. Measurements were carried out during low water discharge (<400 m³ s⁻¹) in neap and spring tide conditions. Turbulent kinetic energy, covariance, and logarithmic profile methods were used and compared for the determination of shear stress. The c_TKE coefficient value of 0.19 cited in the literature was confirmed. Shear stress values were shown to decrease above mudflats from the mouth to the fluvial part of the estuary due to dissipation of the tidal energy, from 1 to 0.2 N m⁻² for spring tides and 0.8 to 0.05 N m⁻² for neap tides. Flood currents dominate tidally-induced shear stress in the marine and lower fluvial estuary during neap and spring tides and in the upper fluvial part during spring tides. Ebb currents control tidally-induced shear stress in the upper fluvial part of the estuary during neap tides. These results revealed a linear relationship between friction velocities and current velocities. Bed roughness length values were calculated from the empirical relationship given by Mitchener and Torfs (1996) for each site; these values are in agreement with the modes of the sediment particle-size distribution. The influence of tidal currents on the mudflat dynamics of the Seine estuary was examined by comparing the tidally-induced bed shear stress and the critical erosion shear stress estimated from bed sediment properties. Bed sediment resuspension induced by tidal currents was shown to occur only in the lower part of the estuary.     

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.     

The Contribution of Benthic Nutrient Regeneration to Primary Production in a Shallow Eutrophic Estuary,Weeks Bay,Alabama

Benthic oxygen, dinitrogen, and nutrient fluxes (NH₄⁺, NO₃⁻, and PO₄³⁻) were measured monthly during a 1-year period at two locations in Weeks Bay, a shallow (1.4 m) and eutrophic estuary in Alabama. Gross primary productivity (GPP), ecosystem respiration (R), and net ecosystem metabolism were determined from high-frequency dissolved oxygen measurements. Peak water column NO₃⁻ (55 μM) and chlorophyll a (138 μg/l) concentrations were measured during spring and fall, respectively. Sediments were a net source of NH₄⁺ (102 μmol m⁻² h⁻¹) and PO₄³⁻ (0.9 μmol m⁻² h⁻¹) but a sink for NO₃⁻ (−30 μmol m⁻² h⁻¹). Benthic N₂ fluxes indicated net N fixation (12 μmol N m⁻² h⁻¹). Sediment oxygen demand (0.55 g O₂ m⁻² day⁻¹) accounted for <10% of R (7.3 g O₂ m⁻² day⁻¹). Despite high GPP rates (4.7 g O₂ m⁻² day⁻¹), the estuary was net heterotrophic. Benthic regeneration supplied, on average, 7.5% and 4% of primary productivity N and P demands, respectively. These results contrast with the conventional view that benthic regeneration accounts for a large fraction of phytoplankton nutrient demand in shallow estuaries.     

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.     

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.     

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.     

Chronic food limitation of egg production in populations of copepods of the genusAcartia in the San Francisco estuary

Egg production of planktonic copepods, is commonly measured as a proxy for secondary production in population dynamics studies and for quantifying food limitation. Although limitation of copepod egg production by food quantity or quality is common in natural waters, it appears less common or severe in estuaries where food concentrations are often high. San Francisco Estuary, California, has unusually low concentrations of chlorophyll compared to other estuaries. We measured egg production rates of three species ofAcartia, with dominate the zooplankton biomass at salinity above 15 psu, on 36 occasions during 1999–2002. Egg production was determined by incubating up to 40 freshly collected individual copepods for 24 h in 140 ml of ambient water. Egg production was less than 10 eggs female⁻¹ d⁻¹ most of the year, but as high as 52 eggs female⁻¹ d⁻¹ during month-long spring phytoplankton blooms. Egg production was a saturating function of total chlorophyll concentration with a mean of 30 eggs female⁻¹ d⁻¹ above a chlorophyll concentration of 12±6 mg chl m⁻³. We take chlorophyll to be a proxy for total food ofAcartia, known to feed on microzooplankton as well as phytoplankton. These findings, together with long-term records of chlorophyll, concentration and earlier studies of abundance of nauplius larvae in the estuary, imply chronic food limitation ofAcartia species, with sufficient food for maximum egg production <10% of the time over the last 25 yr. These results may show the most extreme example of food limitation of copepod reproduction in any temperate estuary. They further support the idea that estuaries may provide suitable habitat forAcartia species by virtue of other factors than high food concentration.     

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.     

Patterns of mangrove forest structure and soil nutrient dynamics along the Shark River estuary, Florida

The basal area and productivity of managrove wetlands are described in relation to selected soil properties to understand the general pattern of optimum forest stature at the mouth of estuaries in the Everglades, such as the Shark River Slough, Florida (U.S.). The basal area of mangroves decreases from 40.4 m² ha⁻¹ and 39.7 m² ha⁻¹ at two stations 1.8 km and 4.1 km from the estuary mouth to 20.7 m² ha⁻¹ and 19.6 m² ha⁻¹ at two sites 9.9 km and 18.2 km from the mouth, respectively. The gradient in basal area at these four sites is mostly the result of approximately 34 yr of growth since Hurricane Donna. Wood productivity is higher in the lower estuary (10.7 Mg ha⁻¹ yr⁻¹ and 12.0 Mg ha⁻¹ yr⁻¹) than in the upper estuary (3.2 Mg ha⁻¹ yr⁻¹ and 4.2 Mg ha⁻¹ yr⁻¹). Porewater salinity among these four mangrove sites during seasonal sampling in 1994 and 1995 ranged from 1.6 g kg⁻¹ to 33.5 g kg⁻¹, while sulfide was generally<0.15 mM at all sites. These soil values indicate that abiotic stress cannot explain the decrease in forest structure along this estuarine gradient. Concentrations of nitrogen (N) and phosphorus (P) are more closely related to patterns of forest development, with higher soil fertility at the mouth of the estuary as indicated by higher concentrations of extractable ammonium, total soil P, and available P, along with higher ammonium production rates. The more fertile sites of the lower estuary are dominated by Laguncularia racemosa, whereas the less fertile sites in the intermediate and upper estuary are dominated by Rhizophora mangle. Relative N mineralization per unit of total N is higher in the lower estuary and is related positively to concentrations of available P, indicating the importance of turnover rates and nutrient interactions to soil fertility. Concentrations of Ca-bound P per volume soil in the lower estuary is 40-fold higher than in the upper estuary, and along with an increase in residual P in the upper estuary, indicate a shift from mineral to organic P along the estuarine gradient. Mineral inputs to the mouth of Shark River estuary from the Gulf of Mexico (rather than upland inputs) apparently control the patterns of mangrove structure and productivity.     

Importance of submarine groundwater discharge as a source of nutrients for the Neuse River Estuary,North Carolina

Seepage rate and chemical composition of groundwater discharge entering the Neuse River Estuary (NRE) were quantified over an annual cycle from July 2005 through June 2006. Lee type seepage meters were deployed at eight locations within the NRE to quantify the amount of submerged groundwater discharge (SGD) entering the system. Sediment porewater nitrate (NO₃ ⁻), ammonium (NH₄ ⁺), and phosphate (PO₄ ⁻³) were also quantified at each of these locations to determine groundwater chemical composition. Seepage rates for the system ranged from 0.004 to 0.035 m³ m⁻² d⁻¹. Both the average and median value for the system-wide SGD were 0.01 m³ m⁻²d⁻¹. There were no significant differences between upstream and downstream seepage rates or between those at the north and south side of the estuary. Seepage rates varied greatly in time and space. Discharging groundwater was NO₃ ⁻ deplete but highly enriched in NH₄ ⁺. Porewater PO₄ ⁻³ levels varied but were usually present below Redfield values due to NH₄ ⁺ enrichment. SGD nutrient loading represented a small part of watershed nitrogen and phosphorus loading, 0.8% and 1.0%, respectively.     

Flushing of dense, hypoxic water from a cavity of the Swan River estuary, Western Australia

Flushing of dense water from cavities of the upper reaches of the Swan River estuary in Western Australia was investigated using measured salinity and dissolved oxygen profiles and a two-dimensional, laterally averaged hydrodynamic model (TISAT). Seasonal flushing of dense, hypoxic bottom waters from a relatively deep site took place over ∼3 days at the onset of winter in 1994. Model simulations of the purging of this dense water did not correspond closely with changes in the densimetric Froude number. Purging, expressed as depth of the halocline as a fraction of the total cavity depth, occurred when the simulated mean horizontal velocity at 2 m depth (top of cavity) changed from negative to strongly positive, indicating arrest of upstream flow and continuous downstream flow. This corresponded to freshwater discharge of about 50 m³ s⁻¹. Oxygen depletion of bottom waters was closely related to stratification. Oxygen dynamics at the onset of winter river flow was analysed using an exponential decay model, assuning that there was no net inflow or outflow across the halocline and thus no vertical transport of oxygen during a period of strong stratification. The rate constant for oxygen decay at Ron Courtney Island (RCI) was estimated to be 0.232 d⁻¹ for this period. Bottom waters at RCI declined to less than 1 mg 1⁻¹ prior to complete flushing through increased river flows. This study provided in sights to how freshwater flows may be allocated to maintain suitable oxygen levels in the bottom waters of estuarine cavities.     

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.     

Net aboveground primary production and soil properties of floating and attached freshwater tidal marshes in the Río de la Plata estuary,Argentina

In the lower delta of the Paraná River, at the head of the Río de la Plata estuary (Argentina), we compared net aboveground primary production (NAPP) and soil properties of the dominant macrophyteScirpus giganteus (Kunth) in a floating and an attached marsh community. Both marshes are tidally influenced but in different ways. The floating marsh site is relatively isolated from tidal influences because its ability to float makes it resistant to overland flow and to sediment inputs from the estuary. The attached marsh lacks the capacity to float and receives sediment supplies from the estuary through overland flow. These hydrologic differences are reflected in lower mineral content in sediments of the floating marsh. Using a leaf tagging technique, estimated NAPP was 1,109 ± 206 g m⁻² yr⁻¹ for the floating marsh and 1,866 ±258 g m⁻² yr⁻¹ for the attached marsh. We attribute the lower NAPP of the floating marsh to isolation from sediment input from overland flow.     

Distribution of nitrifying activity in the Seine River (France) from Paris to the estuary

The distribution of nitrification has been measured with the H¹⁴CO₃ ⁻ incorporation method in the Seine River and its estuary during summer conditions. The Seine River below Paris receives large amounts of ammonium through wastewater discharge. In the river itself, this ammonium is only slowly nitrified, while in the estuary nitrification is rapid and complete. We show that this contrasting behavior is related to the different hydrosedimentary conditions of the two systems, as nitrifying bacteria are associated with suspended particles. In the river, particles and their attached bacteria either rapidly settle or have a sestonic behavior. Because of the short residence times of the water masses, the slow growing nitrifying population has no time to develop sufficiently to nitrify the available ammonium. The estuary is characterized by strong tidal dynamics. Particles settle and are resuspended continuously with the strong current inversions of ebb and flood. As a result of these dynamics, particles and their attached nitrifying bacteria experience longer residence times in a temporary suspended state than the water masses themselves, providing to slow growing nitrifying bacteria the opportunity to develop a large population capable of nitrifying all the available ammonium.     

Phytoplankton Biomass and Production in Subtropical Hong Kong Waters: Influence of the Pearl River Outflow

The size-fractionated phytoplankton biomass and primary production were investigated in four contrasting areas of Hong Kong waters in 2006. Phytoplankton biomass and production varied seasonally in response to the influence of the Pearl River discharge. In the dry season, the phytoplankton biomass and production were low (<42 mg chl m⁻² and <1.8 g C m⁻² day⁻¹) in all four areas, due to low temperatures and dilution and reduced light availability due to strong vertical mixing. In contrast, in the wet season, in the river-impacted western areas, the phytoplankton biomass and production increased greater than five-fold compared to the dry season, especially in summer. In summer, algal biomass was 15-fold higher than in winter, and the mean integrated primary productivity (IPP) was 9 g C m⁻² day⁻¹ in southern waters due to strong stratification, high temperatures, light availability, and nutrient input from the Pearl River estuary. However, in the highly flushed western waters, chl a and IPP were lower (<30 mg m⁻² and 4 g C m⁻² day⁻¹, respectively) due to dilution. The maximal algal biomass and primary production occurred in southern waters with strong stratification and less flushing. Spring blooms (>10 μg chl a L⁻¹) rarely occurred despite the high chl-specific photosynthetic rate (mostly >10 μg C μg chl a ⁻¹ day⁻¹) as the accumulation of algal biomass was restricted by active physical processes (e.g., strong vertical mixing and freshwater dilution). Phytoplankton biomass and production were mostly dominated by the >5-μm size fraction all year except in eastern waters during spring and mostly composed of fast-growing chain-forming diatoms. In the stratified southern waters in summer, the largest algal blooms occurred in part due to high nutrient inputs from the Pearl River estuary.     

Water quality analysis of a freshwater diversion at Caernarvon, Louisiana

Since 1991, Mississippi River water has been diverted at Caernarvon, Louisiana, into Breton Sound estuary. Breton Sound estuary encompasses 1100 km² of fresh and brackish, rapidly subsiding wetlands. Nitrite + nitrate, total Kjeldahl nitrogen, ammonium, total phosphorus, total suspended sediments, and salinity concentrations were monitored at seven locations in Breton Sound from 1988 to 1994. Statistical analysis of the data indicated decreased total Kjeldahl nitrogen with associated decrease in total nitrogen, and decreased salinity concentrations in the estuary due to the diversion. Spring and summer water quality transects indicated rapid reduction of nitrite + nitrate and total suspended sediment concentration as diverted Mississippi River water entered the estuary, suggesting near complete assimilation of these constituents by the ecosystem. Loading rates of nitrite + nitrate (5.6–13.4 g m⁻² yr⁻¹), total nitrogen (8.9–23.4 g m⁻² yr⁻¹), and total phosphorus (0.9–2.0 g m⁻² yr⁻¹) were calculated along with removal efficiencies for these constituents (nitrite + nitrate 88–97%; total nitrogen 32–57%; total phosphorus 0–46%). The low impact of the diversion on water quality in the Breton Sound estuary, along with assimilation of TSS over a very short distance, suggests that more water may be introduced into the estuary without detrimental affects. This would be necessary if freshwater diversions are to be used to distribute nitrients and sediments into the lower reaches of the estuary, in an effort to compensate for relative sea-level rise, and reverse the current trend of rapid loss of wetlands in coastal Louisiana.