Nereis diversicolor and copper contamination effect on the erosion of cohesive sediments: A flume experiment

The effect of bioturbation on the erodability of natural and manipulated copper spiked sediments (3 μmol Cu g⁻¹ dw) was investigated using sediments collected in the Tagus estuary and Nereis diversicolor (900 ind m⁻²). The input of particulate matter and Cu into the water column as a result of erosion was quantified in an annular flume at 7 shear velocities (1–13 cm s⁻¹). The biogeochemical characteristics of the sediment were analysed in depth down to 8 cm. Cu contamination elicited lower levels of eroded matter and lower shear strength profiles. Eroded matter and sediment shear strength values were higher (up to 1.7 kg m⁻²) in the presence of N. diversicolor, whose effect was less pronounced under contamination. Sediment erodability was not only related to hydrodynamics but was highly affected by the biogeochemical characteristics and contamination of the sediments.     

Potentially mobile pools of phosphorus and silicon in sediment from the Bay of Brest: Interactions and implications for phosphorus dynamics

Competitive interactions between silicate and phosphate at ligand exchange sites in the sediment surface layer may increase the release of phosphorus (P) from the sediment into the water column. In this study, the role of silicon (Si) in the release of P from the sediment surface layer was studied in a marine estuarine environment, the Bay of Brest, with the aid of a sequential sediment fractionation procedure developed for P, and the addition of inorganic or diatom-bound Si to surface sediment samples in vitro. The potentially mobile pools of P in the surface sediment (loosely bound P + Fe/Al-bound-P) amounted to 5.0 μmol g⁻¹ dry sed., 42% of the total extractable and 33% of the total amount of P in the sediment, while the similarly extracted pools of Si were bigger (ca. 20 μmol g⁻¹ dry sed., 50% of the total extractable Si). Additions of inorganic Si increased the concentration of dissolved P in the sediment interstitial water in a bottle experiment, and the addition of both inorganic Si and cultivated diatoms to intact sediment cores increased the outward flux of dissolved P. Model calculations based on the regression equation from the bottle experiment and Si and P water column data showed that the sedimentation of spring diatoms could cause Si pulses to the sediment which would produce a P flux to the water column of ca. 44 μmol m⁻² d⁻¹. Field data from the bay show that in spring, decreases in P and Si and an increase in chl a due to diatom production are often followed by a small separate P peak which may be caused by Si-induced P fluxes from the sediment surface.     

The fate of production in the central Arctic Ocean - top-down regulation by zooplankton expatriates?

We estimated primary and bacterial production, mineral nutrients, suspended chlorophyll a (Chl), particulate organic carbon (POC) and nitrogen (PON), abundance of planktonic organisms, mesozooplankton fecal pellet production, and the vertical flux of organic particles of the central Arctic Ocean (Amundsen basin, 89-88° N) during a 3 week quasi-Lagrangian ice drift experiment at the peak of the productive season (August 2001). A visual estimate of ≈15% ice-free surface, plus numerous melt ponds on ice sheets, supported a planktonic particulate primary production of 50-150 mg C m⁻² d⁻¹ (mean 93 mg C m⁻² d⁻¹, n = 7), mostly confined to the upper 10 m of the nutrient replete water column. The surface mixed layer was separated from the rest of the water column by a strong halocline at 20 m depth. Phototrophic biomass was low, generally 0.03-0.3 mg Chl m⁻³ in the upper 20 m and <0.02 mg Chl m⁻³ below, dominated by various flagellates, dinoflagellates and diatoms. Bacterial abundance (typically 3.7-5.3 × 10⁵, mean 4.1 × 10⁵ cells ml⁻¹ in the upper 20 m and 1.3-3.7 × 10⁵, mean 1.9 × 10⁵ cells ml⁻¹ below) and Chl concentrations were closely correlated (r = 0.75). Mineral nutrients (3 μmol NO₃ l⁻¹, 0.45 μmol PO₄ l⁻¹, 4-5 μmol SiO₄ l⁻¹) were probably not limiting the primary production in the upper layer. Suspended POC concentration was ∼30-105 (mean 53) mg C m⁻³ and PON ∼5.4-14.9 (mean 8.2) mg N m⁻³ with no clear vertical trend. The vertical flux of POC in the upper 30-100 m water column was ∼37-92 (mean 55) mg C m⁻² d⁻¹ without clear decrease with depth, and was quite similar at the six investigated stations. The mesozooplankton biomass (≈2 g DW m⁻², mostly in the upper 50 m water column) was dominated by adult females of the large calanoid copepods Calanus hyperboreus and Calanus glacialis (≈1.6 g DW m⁻²). The grazing of these copepods (estimated via fecal pellet production rates) was ≈15 mg C m⁻² d⁻¹, being on the order of 3% and 20% of the expected food-saturated ingestion rates of C. hyperboreus and C. glacialis, respectively. The stage structure of these copepods, dominated by adult females, and their unsatisfied grazing capacity during peak productive period suggest allochthonous origin of these species from productive shelf areas, supported by their long life span and the prevailing surface currents in the Arctic Ocean. We propose that the grazing capacity of the expatriated mesozooplankton population would match the potential seasonal increase of primary production in the future decreased ice perspective, diminishing the likelihood of algal blooms.     

New production in the subarctic Pacific Ocean: Net changes in nitrate concentrations, rates of nitrate assimilation and accumulation of particulate nitrogen

Changes in water column nitrate and particulate nitrogen (PN) concentrations and rates of nitrate assimilation at 50°N 145°W were measured over a four-month interval for 1984, 1987 and 1988. Rates of nitrate depletion in the upper 80m of the water column averaged 12mg N m⁻²d⁻¹, but most of the net depletion occurred during May when rates were high (75mg N m⁻²d⁻¹) compared to later in the year. Particulate nitrogen (collected on GF/F filters) increased 2- to 3-fold during the month of May and accounted for 30–60% of the net nitrate depletion for May. Mean rates of PN accumulation for the 4-month intervals were 2.4mg N m⁻²d⁻¹ and accounted for about 20% of the net nitrate depletion. Rates of nitrate assimilation (measured in incubation bottles with ¹⁵N) averaged 45.0±4.5mg N m⁻²d⁻¹ (mean±SD), and appeared to decrease between May and September. A good correspondence between in situ and incubation estimates of nitrate assimilation was found for the 4-month comparison, but not for the month of May when net changes in nitrate concentrations were greatest. Vertical and horizontal inputs of nitrate are about the same order of magnitude as biological removal, thus the high inout of nitrate into the euphotic zone contributed to the continuously high nitrate concentrations in this region. Seasonal changes in nitrate and PN were significant and need to be considered in comparisons of new and export production.     

Nitrogen uptake and primary productivity rates in the Mid-Atlantic Bight (MAB)

Nutrient concentrations, primary productivity, and nitrogen uptake rates were measured in coastal waters of the Mid-Atlantic Bight over a two-year period that included measurements from all four seasons. In order to assess carbon productivity and nitrogen demand within the context of the physical environment, the region was divided into three distinct hydrographic regimes: the Chesapeake and Delaware Bay outflow plumes (PL), the southern Mid-Atlantic shelf influenced by the Gulf Stream (SS), and the mid-shelf area to the north of the Chesapeake Bay mouth (MS). Annual areal rates of total nitrogen (N) uptake were similar across all regions (10.9 ± 2.1 mol N m⁻² y⁻¹). However, annual areal rates of net primary productivity were higher in the outflow plume region (43 mol C m⁻² y⁻¹), than along the Mid-Atlantic shelf and in areas influenced by the Gulf Stream (41 and 34 mol C m⁻² y⁻¹, respectively). Rates of net primary productivity were not well correlated with Chl a concentrations and were uncoupled with net N uptake rates. Seasonally averaged annual areal rates of net primary productivity for the Mid-Atlantic Bight measured in this study were higher than those calculated in previous decades and provide important validation information for biogeochemical models and satellite remote sensing algorithms developed for the region.     

Spatial and temporal changes in estuarine water quality during a post-flood hypoxic event

A major fish kill occurred in the Richmond River estuary in January 2008 due to oxygen depletion following extensive overbank flooding. This paper examines spatial and temporal changes in the chemistry of main channel waters, thereby identifying the primary sources of deoxygenating water. Over 40 km of the mid- to lower estuary main channel was deoxygenated within seven days of the flood peak. Hypoxia was confined to downstream of the confluences with mid-estuary backswamp basins and occurred during the later phase of the flood recession. Water chemistry at key locations in the estuary indicated elevated concentrations of redox sensitive species associated with acid sulfate soils (ASS) during the hypoxic period. Peak concentrations of Fe²⁺ up to 18.2 μmol L⁻¹, dissolved Mn up to 4.3 μmol L⁻¹, chemical oxygen demand (COD) up to 2052 μmol L⁻¹, dissolved organic carbon (DOC) up to 960 μmol L⁻¹ and elemental S⁰ up to 4.7 μmol L⁻¹ were found in the backswamp discharge confluences and mid-estuary main channel locations. The geochemical signature of main channel floodwaters identifies anaerobic decomposition of floodplain vegetation in ASS backswamps as a primary process leading to generation of hypoxic waters. The transport of these hypoxic floodwaters to the estuary has been accelerated and prolonged by extensive floodplain drainage, thereby enhancing the magnitude and duration of estuarine deoxygenation.     

Rates and regulation of nitrogen cycling in seasonally hypoxic sediments during winter (Boknis Eck,SW Baltic Sea): Sensitivity to environmental variables

This study investigates the biogeochemical processes that control the benthic fluxes of dissolved nitrogen (N) species in Boknis Eck – a 28 m deep site in the Eckernförde Bay (southwestern Baltic Sea). Bottom water oxygen concentrations (O_2−BW) fluctuate greatly over the year at Boknis Eck, being well-oxygenated in winter and experiencing severe bottom water hypoxia and even anoxia in late summer. The present communication addresses the winter situation (February 2010). Fluxes of ammonium (NH₄⁺), nitrate (NO₃⁻) and nitrite (NO₂⁻) were simulated using a benthic model that accounted for transport and biogeochemical reactions and constrained with ex situ flux measurements and sediment geochemical analysis. The sediments were a net sink for NO₃⁻ (−0.35 mmol m⁻² d⁻¹ of NO₃⁻), of which 75% was ascribed to dissimilatory reduction of nitrate to ammonium (DNRA) by sulfide oxidizing bacteria, and 25% to NO₃⁻ reduction to NO₂⁻ by denitrifying microorganisms. NH₄⁺ fluxes were high (1.74 mmol m⁻² d⁻¹ of NH₄⁺), mainly due to the degradation of organic nitrogen, and directed out of the sediment. NO₂⁻ fluxes were negligible. The sediments in Boknis Eck are, therefore, a net source of dissolved inorganic nitrogen (DIN = NO₃⁻ + NO₂⁻ + NH₄⁺) during winter. This is in large part due to bioirrigation, which accounts for 76% of the benthic efflux of NH₄⁺, thus reducing the capacity for nitrification of NH₄⁺. The combined rate of fixed N loss by denitrification and anammox was estimated at 0.08 mmol m⁻² d⁻¹ of N₂, which is at the lower end of previously reported values. A systematic sensitivity analysis revealed that denitrification and anammox respond strongly and positively to the concentration of NO₃⁻ in the bottom water (NO₃⁻_BW). Higher O_2−BW decreases DNRA and denitrification but stimulates both anammox and the contribution of anammox to total N₂ production (%R_amx). A complete mechanistic explanation of these findings is provided. Our analysis indicates that nitrification is the geochemical driving force behind the observed correlation between %R_amx and water depth in the seminal study of Dalsgaard et al. (2005). Despite remaining uncertainties, the results provide a general mechanistic framework for interpreting the existing knowledge of N-turnover processes and fluxes in continental margin sediments, as well as predicting the types of environment where these reactions are expected to occur prominently.     

Seasonal cycle of N2O vertical distribution and air-sea fluxes over the continental shelf waters off central Chile (∼36°S)

The continental shelf off central Chile is subject to strong seasonal coastal upwelling and has been recognized as an important outgassing area for, amongst others, N₂O, an important greenhouse gas. Several physical and biogeochemical variables, including N₂O, were measured in the water column from August 2002 to January 2007 at a time series station in order to characterize its temporal variability and elucidate the physical and biogeochemical mechanisms affecting N₂O levels. This 4-year time series of N₂O levels reveals seasonal variability associated basically with hydrographic and oceanographic regimes (i.e., upwelling and non-upwelling). However, a noteworthy temporal evolution of both the vertical distribution and N₂O levels was observed repeatedly throughout the entire study period, allowing us to distinguish three stages: winter/early spring (Stage I), mid-spring/mid-summer (Stage II), and late summer/early autumn (Stage III).Stage I presents low N₂O, the lowest surface saturation ever registered (from 64% saturation) in a period of high O₂, and a homogeneous column driven by strong wind; this distribution is explained by physical and thermodynamic mechanisms. Stage II, with increasing N₂O concentrations, agrees with the appearance of upwelling-favourable wind stress and a strong influence of oxygen-poor, nutrient-rich equatorial subsurface waters (ESSW). The N₂O build-up creates a “hotspot” (up to 2426% N₂O saturation) and enhanced concentrations of (up to 3.97 μM) and (up to 4.6 μM) at the oxycline (4-28 μM) (∼20-40 m depth). Although the dominant N₂O sources could not be determined, denitrification (mainly below the oxycline) appears to be the dominant process in N₂O accumulation. Stage III, with diminishing N₂O concentrations from mid-summer to early autumn, was accompanied by low N/P ratios. During this stage, strong bottom N₂O consumption (from 40% saturation) was suggested to be mainly driven by benthic denitrification.Consistent with the evolution of N₂O in the water column over time, the estimated air-sea N₂O fluxes were low or negative in winter (−9.8 to 20 μmol m⁻² d⁻¹, Stage I) and higher in spring and summer (up to 195 μmol m⁻² d⁻¹, Stage II), after which they declined (Stage III). In spite of the occurrence of ESSW and upwelling events throughout stages II and III, N₂O behaviour should be a response of the biogeochemical evolution associated with biological productivity and concomitant O₂ levels in the water and even in the sediments. The results presented herein confirm that the study area is an important source of N₂O to the atmosphere, with a mean annual N₂O flux of 30.2 μmol m⁻² d⁻¹; however, interannual variability could not yet be properly characterized.     

Tidal variability in benthic silicic acid fluxes and microphytobenthos uptake in intertidal sediment

Silicic acid (DSi) benthic fluxes play a major role in the benthic–pelagic coupling of coastal ecosystems. They can sustain microphytobenthos (MPB) development at the water–sediment interface and support pelagic diatoms when river DSi inputs decrease. DSi benthic fluxes have been studied at the seasonal scale but little is known about their dial variations. This study measured the amplitude of such variations in an intertidal area over an entire tidal cycle by following the alteration of DSi pore water concentrations at regular intervals over the flood/ebb period. Furthermore we independently estimated the potential DSi uptake by benthic diatoms and compared it to the variations of DSi pore water concentrations and fluxes. The microphytobenthos DSi demand was estimated from primary production measurements on cells extracted from the sediment. There were large changes in DSi pore water concentration and a prominent effect of tidal pumping: the DSi flushed out from the sediment at rising tide, occurs in a very short period of time, but plays a far more important role in fueling the ecosystem (800 μmol-Si m⁻² d⁻¹), than diffusive fluxes occurring throughout the rest of the tidal cycle (2 μmol-Si m⁻² d⁻¹). This process is not, to our knowledge, currently considered when describing the DSi cycling of intertidal sediments. Moreover, there was a large potential MPB requirement for DSi (812 μmol-Si m⁻² d⁻¹), similar to the advective flow periodically pumped by the incoming tide, and largely exceeded benthic diffusive fluxes. However, this DSi uptake by benthic diatoms is almost undetectable given the variation of DSi concentration profiles within the sediment.     

Dry atmospheric deposition and diazotrophy as sources of new nitrogen to northwestern Mediterranean oligotrophic surface waters

Atmospheric dry deposition of nitrogen (N) and dinitrogen (N₂) fixation rates were assessed in 2004 at the time-series DYFAMED station (northwestern Mediterranean, 43°25′N, 7°52′E). The atmospheric input was monitored over the whole year. Dinitrogen fixation was measured during different seasonal trophic states (from mesotrophy to oligotrophy) sampled during nine cruises. The bioavailability of atmospherically deposited nutrients was estimated by apparent solubility after 96 h. The solubility of dry atmospheric N deposition was highly variable (from ∼18% to more than 96% of total N). New N supplied to surface waters by the dry atmospheric deposition was mainly nitrate (NO₃⁻) (∼57% of total N, compared to ∼6% released as ammonium (NH₄⁺)). The mean bioavailable dry flux of total N was estimated to be ∼112 μmol m⁻² d⁻¹ over the whole year. The NO₃⁻ contribution (70 μmol NO₃⁻ m⁻² d⁻¹) was much higher than the NH₄⁺ contribution (1.2 μmol NH₄⁺ m⁻² d⁻¹). The N:P ratios in the bioavailable fraction of atmospheric inputs (122.5–1340) were always much higher than the Redfield N:P ratio (16). Insoluble N in atmospheric dry deposition (referred to as “organic” and believed to be strongly related to anthropogenic emissions) was ∼40 μmol m⁻² d⁻¹. N₂ fixation rates ranged from 2 to 7.5 nmol L⁻¹ d⁻¹. The highest values were found in August, during the oligotrophic period (7.5 nmol L⁻¹ at 10 m depth), and in April, during the productive period (4 nmol L⁻¹ d⁻¹ at 10 m depth). Daily integrated values of N₂ fixation ranged from 22 to 100 μmol N m⁻² d⁻¹, with a maximum of 245 μmol N m⁻² d⁻¹ in August. No relationship was found between the availability of phosphorus or iron and the observed temporal variability of N₂ fixation rates. The atmospheric dry deposition and N₂ fixation represented 0.5–6% and 1–20% of the total biological nitrogen demand, respectively. Their contribution to new production was more significant: 1–28% and 2–55% for atmospheric dry deposition and N₂ fixation, respectively. The dry atmospheric input was particularly significant in conditions of water column stratification (16–28% of new production), while N₂ fixation reached its highest values in June (46% of new production) and in August (55%).     

Microbial respiration in the Levantine Sea: evolution of the oxidative processes in relation to the main Mediterranean water masses

First data on microbial respiration in the Levantine Sea are reported with the aim of assessing the distribution of oxidative processes in association with the main Mediterranean water masses and the changing physical structure determined by the Eastern Mediterranean Transient. Respiratory rates, in terms of metabolic carbon dioxide production, were estimated from measured electron transport system activities in the polygonal area of the Levantine Sea (32.5–36.5 N Latitude, 26.0–30.25 E Longitude) and at Station Geo’95, in the Ionian Sea (35°34.88 N; 17°14.99 E). At the Levantine Sea, the mean carbon dioxide production rate decreased from the upper to the deeper layers and varied from 22.0±12.4 μg C h⁻¹ m⁻³ in the euphotic layer to 1.30±0.5 μg C h⁻¹ m⁻³ in the depth range between 1600 and 3000 m. Significant differences were found among upper, intermediate and bottom layers. The euphotic zone supported a daily carbon dioxide production of 96.6 mg C d⁻¹ m⁻² while the aphotic zone (between 200 and 3000 m) sustained a 177.1 mg C d⁻¹ m⁻² carbon dioxide production. In Station Geo’95, the carbon dioxide production rates amounted to 170.4 and 102.2 mg C d⁻¹ m⁻² in the euphotic and aphotic zones, respectively. The rates determined in the identified water masses showed a tight coupling of respiratory processes and Mediterranean circulation patterns. The increasing respiratory rates in the deep layers of the Levantine Sea are explained by the introduction of younger waters recently formed in the Aegean Sea.     

In vitro simulation of oxic/suboxic diagenesis in an estuarine fluid mud subjected to redox oscillations

Estuarine turbidity maxima (ETMs) are sites of intense mineralisation of land-derived particulate organic matter (OM), which occurs under oxic/suboxic oscillating conditions owing to repetitive sedimentation and resuspension cycles at tidal and neap-spring time scales. To investigate the biogeochemical processes involved in OM mineralisation in ETMs, an experimental set up was developed to simulate in vitro oxic/anoxic oscillations in turbid waters and to follow the short timescale changes in oxygen, carbon, nitrogen, and manganese concentration and speciation. We present here the results of a 27-day experiment (three oxic periods and two anoxic periods) with an estuarine fluid mud from the Gironde estuary. Time courses of chemical species throughout the experiment evidenced the occurrence of four distinct characteristic periods with very different properties. Steady oxic conditions were characterised by oxygen consumption rates between 10 and 40 μmol L⁻¹ h⁻¹, dissolved inorganic carbon (DIC) production of 9–12 μmol L⁻¹ h⁻¹, very low NH₄⁺ and Mn²⁺ concentrations, and constant NO₃⁻ production rates (0.4 - 0.7 μmol L⁻¹ h⁻¹) due to coupled ammonification and nitrification. The beginning of anoxic periods (24 h following oxic to anoxic switches) showed DIC production rates of 2.5–8.6 μmol L⁻¹ h⁻¹ and very fast NO₃⁻ consumption (5.6–6.3 μmol L⁻¹ h⁻¹) and NH₄⁺ production (1.4–1.5 μmol L⁻¹ h⁻¹). The latter rates were positively correlated to NO₃⁻ concentration and were apparently caused by the predominance of denitrification and dissimilatory nitrate reduction to ammonia. Steady anoxic periods were characterised by constant and low NO₃⁻ concentrations and DIC and NH₄⁺ productions of less than 1.3 and 0.1 μmol L⁻¹ h⁻¹, respectively. Mn²⁺ and CH₄ were produced at constant rates (respectively 0.3 and 0.015 μmol L⁻¹ h⁻¹) throughout the whole anoxic periods and in the presence of nitrate. Finally, reoxidation periods (24–36 h following anoxic to oxic switches) showed rapid NH₄⁺ and Mn²⁺ decreases to zero (1.6 and 0.8–2 μmol L⁻¹ h⁻¹, respectively) and very fast NO₃⁻ production (3 μmol L⁻¹ h⁻¹). This NO₃⁻ production, together with marked transient peaks of dissolved organic carbon a few hours after anoxic to oxic switches, suggested that particulate OM mineralisation was enhanced during these transient reoxidation periods. An analysis based on C and N mass balance suggested that redox oscillation on short time scales (day to week) enhanced OM mineralisation relative to both steady oxic and steady anoxic conditions, making ETMs efficient biogeochemical reactors for the mineralisation of refractory terrestrial OM at the land-sea interface.     

Knowledge gaps in tropical Southeast Asian seagrass systems

Seagrasses are habitats with significant ecological and economic functions but we have limited knowledge of seagrasses in Southeast Asia, the hypothesized centre-of-origin for tropical seagrasses. There have been only 62 ISI-cited publications on the seagrasses of Southeast Asia in the last three decades and most work has been in few sites such as Northwest Luzon in the Philippines and South Sulawesi in Indonesia. Our understanding of the processes driving spatial and temporal distributions of seagrass species here has focussed primarily on backreef and estuarine seagrass meadows, with little work on forereef systems. We used Pulau Tinggi, an island off the southeast coast of Peninsular Malaysia, as an example of a subtidal forereef system. It is characterized by a community of small and fast growing species such as Halophila ovalis (mean shoot density 1454.6 ± 145.1 m⁻²) and Halodule uninervis (mean shoot density 861.7 ± 372.0 m⁻²) growing in relatively low light conditions (mean PAR 162.1 ± 35.0 μmol m⁻² s⁻¹ at 10 m depth to 405.8 ± 99.0 μmol m⁻² s⁻¹ at 3 m water depth) on sediment with low carbonate (mean 9.24 ± 1.74 percentage dry weight), organic matter (mean 2.56 ± 0.35 percentage dry weight) and silt-clay content (mean 2.28 ± 2.43 percentage dry weight). The literature reveals that there is a range of drivers operating in Southeast Asian seagrass systems and we suggest that this is because there are various types of seagrass habitats in this region, i.e. backreef, forereef and estuary, each of which has site characteristics and ecological drivers unique to it. Based on our case study of Pulau Tinggi, we suggest that seagrasses in forereef systems are more widespread in Southeast Asia than is reflected in the literature and that they are likely to be driven by recurring disturbance events such as monsoons, sediment burial and herbivory.     

Effects of irradiance on benthic and water column processes in a Gulf of Mexico estuary: Pensacola Bay, Florida, USA

We examined the effect of light on water column and benthic fluxes in the Pensacola Bay estuary, a river-dominated system in the northeastern Gulf of Mexico. Measurements were made during the summers of 2003 and 2004 on 16 dates distributed along depth and salinity gradients. Dissolved oxygen fluxes were measured on replicate sediment and water column samples exposed to a gradient of photosynthetically active radiation. Sediment inorganic nutrient (NH₄⁺, NO₃⁻, PO₄³⁻) fluxes were measured. The response of dissolved oxygen fluxes to variation in light was fit to a photosynthesis–irradiance model and the parameter estimates were used to calculate daily integrated production in the water column and the benthos. The results suggest that shoal environments supported substantial benthic productivity, averaging 13.6 ± 4.7 mmol O₂ m⁻² d⁻¹, whereas channel environments supported low benthic productivity, averaging 0.5 ± 0.3 mmol O₂ m⁻² d⁻¹ (±SE). Estimates of baywide microphytobenthic productivity ranged from 8.1 to 16.5 mmol O₂ m⁻² d⁻¹, comprising about 16–32% of total system productivity. Benthic and water column dark respiration averaged 15.2 ± 3.2 and 33.6 ± 3.7 mmol O₂ m⁻² d⁻¹, respectively Inorganic nutrient fluxes were generally low compared to relevant estuarine literature values, and responded minimally to light exposure. Across all stations, nutrient fluxes from sediments to the water column averaged 1.11 ± 0.98 mmol m⁻² d⁻¹ for NH₄⁺, 0.58 ± 1.08 mmol m⁻² d⁻¹ for NO₃⁻, 0.01 ± 0.09 mmol m⁻² d⁻¹ for PO₄³⁻. The results of this study illustrate how light reaching the sediments is an important modulator of benthic nutrient and oxygen dynamics in shallow estuarine systems.     

Seasonal variations in dimethylsulfoniopropionate and dimethylsulfide concentrations in relation to the plankton community in the St. Lawrence Estuary

Weekly variations in total dimethylsulfoniopropionate (DMSPt) and dimethylsulfide (DMS) were investigated in relation to the phytoplankton assemblage from spring to fall 1994 at a coastal fixed station in the St. Lawrence Estuary. DMSPt and DMS concentrations showed a strong seasonality and were tightly coupled in time. Maximum concentrations of DMSPt and DMS were observed in July and August, during a period of warm water and low nutrient concentrations. Seasonal maxima of 365.4 nmol l⁻¹ for DMSPt and 14.2 nmol l⁻¹ for DMS in early August coincided with the presence of many phytoplankton species, such as Alexandrium tamarense, Dinophysis acuminata, Gymnodinium sp., Heterocapsa rotundata, Protoperidinium ovatum, Scrippsiella trochoidea, Chrysochromulina sp. (6 μm), Cryptomonas sp. (6 μm), a group of microflagellates smaller than 5 μm (mf < 5), many tintinnids, and Mesodinium rubrum. The abundance of mf < 5 followed the general trend of DMS concentrations. The temporal occurrence of high P. ovatum abundance and DMSPt concentrations suggests that this heterotrophic dinoflagellate can either synthesize DMSP or acquire it from DMSP-rich prey. The calculated sea-to-air DMS flux reached a maximum of 8.36 μmol ⁻² d⁻¹ on August 1. The estimated annual emission from the St. Lawrence Estuary is 77.2 tons of biogenic sulfur to the atmosphere.     

Hydrodynamic response of the Breton Sound estuary to pulsed Mississippi River inputs

Pulsed re-introduction of Mississippi River water into the deltaic plain has been proposed as a wetland restoration strategy for coastal Louisiana. In this study, the hydrodynamic response of the Breton Sound estuary to a two-week pulse of Mississippi River water via the Caernarvon river diversion structure was investigated using a barotropic, three-dimensional, Finite-Volume Coastal Ocean Model (FVCOM). The numerical model was driven by tidal and subtidal forcing at the open Gulf boundary, freshwater discharge from the Caernarvon river diversion structure, as well as wind stress at the water surface. After successfully validating the model with field observations, three numerical experiments were run to assess the response of current, water level, and marsh flooding to different diversion discharge scenarios. The three scenarios considered were: a pulsed scenario of ∼200 m³ s⁻¹ corresponding to the actual diversion discharge in March 2001, a constant discharge scenario of 40 m³ s⁻¹ corresponding to the annually averaged discharge of 2001, and a scenario with no discharge. Numerical simulation results indicated that constant 40 m³ s⁻¹ discharge caused little change in wetland inundation comparing to the no discharge case and, thus, inter-exchange between deep channels and the wetlands was not improved by this rate of diversion discharge. In contrast, the two-week ∼200 m³ s⁻¹ discharge caused enhanced water exchange between wetlands and adjacent water bodies, substantially increasing water velocity in the bayous and channels of the upper estuary. These effects occurred in the estuary to about 20–25 km from the diversion structure, and caused a noticeable increase in down-estuary residual current with a significant reduction of local estuarine residence times for the whole estuary. Beyond 30 km from the diversion structure, the impact of high water discharge was small and the hydrodynamics was mostly controlled by tides and wind.     

Effects of upwelling,tides and biological processes on the inorganic carbon system of a coastal lagoon in Baja California

The role of coastal lagoons and estuaries as sources or sinks of inorganic carbon in upwelling areas has not been fully understood. During the months of May–July, 2005, we studied the dissolved inorganic carbon system in a coastal lagoon of northwestern Mexico during the strongest period of upwelling events. Along the bay, different scenarios were observed for the distributions of pH, dissolved inorganic carbon (DIC) and apparent oxygen utilization (AOU) as a result of different combinations of upwelling intensity and tidal amplitude. DIC concentrations in the outer part of the bay were controlled by mixing processes. At the inner part of the bay DIC was as low as 1800 μmol kg⁻¹, most likely due to high water residence times and seagrass CO₂ uptake. It is estimated that 85% of San Quintín Bay, at the oceanic end, acted as a source of CO₂ to the atmosphere due to the inflow of CO₂-rich upwelled waters from the neighboring ocean with high positive fluxes higher than 30 mmol C m⁻² d⁻¹. In contrast, there was a net uptake of CO₂ and HCO₃⁻ by the seagrass bed Zostera marina in the inner part of the bay, so the pCO₂ in this zone was below the equilibrium value and slightly negative CO₂ fluxes of −6 mmol C m⁻² d⁻¹. Our positive NEP and ΔDIC values indicate that Bahía San Quintín was a net autotrophic system during the upwelling season during 2005.     

Simultaneous solution for water,heat and salt balances in a Mediterranean coastal lagoon (Mar Menor,Spain)

A modelling approach is proposed to evaluate the environmental dynamics of coastal lagoons. The water, heat and salt balances are addressed simultaneously, providing a better estimation of evaporation and water exchanges. Compared to traditional approaches, the model presented accounts for the effects of water salinity, heat storage and net energy advected in the water body. The model was applied daily to the Mar Menor coastal lagoon (SE Spain) from 2003 through 2006. Water exchanges with the Mediterranean Sea were estimated based on the monthly trend of the lagoon salinity and were correlated with monthly averages of wind speed. The mean daily water exchange with the sea was 1.77 hm³ d⁻¹. This exchange accounted for only 1% of the heat losses in the lagoon heat balance, and it is the most important flow in the water balance. The mean annual evaporation flux amounted to 101.3 W m⁻² (3.55 mm d⁻¹), while the sensible heat flux amounted to 19.7 W m⁻², leading to an annual Bowen ratio on the order of 0.19. To validate the model, daily water temperatures were predicted based on the daily heat balance of the water body and were compared with remote sensing data from water surface standard products.     

Sulfate reduction in Black Sea sediments: in situ and laboratory radiotracer measurements from the shelf to 2000 m depth

Sulfate reduction rate measurements by the ³⁵SO₄²⁻ core injection method were carried out in situ with a benthic lander, LUISE, and in parallel by shipboard incubations in sediments of the Black Sea. Eight stations were studied along a transect from the Romanian shelf to the deep western anoxic basin. The highest rates measured on an areal basis for the upper 0–15 cm were 1.97 mmol m⁻² d⁻¹ on the shelf and 1.54 mmol m⁻² d⁻¹ at 181 m water depth just below the chemocline. At all stations sulfate reduction rates decreased to values <3 nmol cm⁻³ d⁻¹ below 15 cm depth in the sediment. The importance of sulfate reduction relative to the total mineralization of organic matter was very low, 6%, on the inner shelf, which was paved with mussels, and increased to 47% on the outer shelf at 100 m depth. Where the oxic–anoxic interface of the water column impinged on the sea floor at around 150 m depth, the contribution of sulfate reduction increased from >50% just above the chemocline to 100% just below. In the deep sea, mean sulfate reduction rates were 0.6 mmol m⁻² d⁻¹ corresponding to an organic carbon oxidation of 1.3 mmol m⁻² d⁻¹. This is close to the mean sedimentation rate of organic carbon over the year in the western basin. A comparison with published data on sulfate reduction in Black Sea sediments showed that the present results tend to be higher in shelf sediments and lower in the deep-sea than most other data. Based on the present water column H₂S inventory and the H₂S flux out of the sediment, the calculated turnover time of H₂S below the chemocline is 2100 years.     

Influence of ENSO variability on sinking-particle fluxes in the northeastern equatorial Pacific

A time-series sediment trap was operated from July 2003 to July 2008 at a station located in the 10°N thermocline ridge of the northeastern equatorial Pacific (10°30′N, 131°20′W), with the aim of understanding variations in natural background sinking-particle flux and the influence on such fluxes of ENSO (El Niño-Southern Oscillation). Each one of weak El Niño, moderate El Niño and moderate La Niña were observed during the monitoring period. During non-ENSO periods, total mass fluxes varied from 4.1 to 36.9 mg m⁻² d⁻¹, with a distinct seasonal variation, ranging from an average flux of 14.0 mg m⁻² d⁻¹ in the warm season (June-November) to 25.3 mg m⁻² d⁻¹ in the cold season (December-May). This seasonal fluctuation was characterized by a distinct difference in CaCO₃ flux between the two seasons. The enhanced particle fluxes during the cold season are attributed to the supply of nutrient-enriched subsurface water by wind-driven vertical mixing, supported by a simultaneous reduction in sea surface temperature and enhanced trade winds. The weak El Niño event occurred in the monitoring period had no recognizable effect on particle fluxes in the study area, but the moderate El Niño event was accompanied by a significant reduction in particle fluxes to 60% of the average background value in the warm season. In contrast, particle fluxes during the moderate La Niña increased to a maximum value of 129.9 mg m⁻² d⁻¹, almost three times the average background value. Organic carbon and biogenic silica fluxes were most sensitive to the El Niño and La Niña conditions. The observed variations of particle fluxes are synchronized with those of chlorophyll-a, suggesting primary productivity for the main cause of flux change. The present data indicate that marked seasonal variability in background fluxes commonly exceeds the variability associated with ENSO and post-ENSO signals, which should be taken into account when evaluating the influence of ENSO on sinking particle fluxes in the 10°N thermocline ridge area.