Mid Bay Research Articles

Interannual variability of air-water CO2 flux in a large eutrophic estuary

Air-water CO2 fluxes in estuarine environments are characterized by high interannual variability, in part due to hydrological variability that alters estuarine carbonate chemistry through multiple physical and biogeochemical processes. To understand the relative contributions of these varied controls on interannual air-water CO2 fluxes in the mainstem Chesapeake Bay, we implemented both hindcast and scenario simulations using a coupled physical-biogeochemical model. Significant spatiotemporal variability in bay-wide fluxes was found over a 10-year period (1996-2005), where the mainstem Bay was primarily a net CO2 sink, except in drought periods. Sensitivity scenario results suggested substantial effects of riverine nutrient and organic matter (OM) inputs to CO2 flux variations. The high correlations between riverine inputs and upper-Bay fluxes were due to elevated respiration under increased OM inputs. The interannual flux variations in the lower Bay was mostly regulated by the nutrient inputs. Both nutrient and OM inputs contributed to the flux variability in the mid Bay. It is found that the interannual CO2 flux of the mainstem was most sensitive to riverine nutrient inputs associated with the hydrological changes. For each hindcast simulation we computed the ratio of organic carbon turnover time to water residence time (λ), a proxy for CO2 efflux potential, and found that the wetter periods had a relatively lower λ. The variability of mainstem CO2 fluxes can be well represented using a generic function of λ. The model results showed that higher river flows would lead to enhanced CO2 sinks into a large eutrophic estuary by promoting net autotrophy.

Seasonal hypoxia and temperature inversions in a tropical bay

AbstractDissolved oxygen (DO) is a critically important ecological variable, and the prevalence of marine hypoxia is expected to increase due to the combined effects of ocean warming and eutrophication. Thermal stress can co‐occur with hypoxia, especially in tropical systems, and can exacerbate its effects. We examine the physical processes that are important in regulating hypoxia and temperature inversions in Bahía Almirante, a semi‐enclosed tropical embayment on the Caribbean coast of Panama. A 10‐yr record of observations at 7 locations within Bahía Almirante reveals seasonal temperature inversions and hypoxia at depth that often co‐occur. These features are more severe and persistent in the back bay, though they occur throughout Bahía Almirante. DO reductions correspond to periods with high freshwater input, including direct precipitation, resulting in strong salinity stratification that isolates bottom waters, allowing biological oxygen demand to draw down DO. Evidence indicates that lateral advection can contribute to reoxygenation events, and the relationship between near bottom DO and bottom salinities in the mid bay and back bay is consistent with deep‐water renewal as the mechanism for bottom water ventilation. These hypoxia and temperature inversion events impact the biological communities of Bahía Almirante, and the physical dynamics that regulate these coincident and persistent stressors for marine organisms are likely present in other shallow, tropical estuaries.

Molecular and optical characterization reveals the preservation and sulfurization of chemically diverse porewater dissolved organic matter in oligohaline and brackish Chesapeake Bay sediments

In this study, we conducted a detailed analysis of porewater downcore chemical properties and porewater dissolved organic matter (PDOM) composition using elemental C, N and S analysis, fluorescence spectroscopy, and ultrahigh resolution mass spectrometry (FT-ICR MS) at two contrasting sites in Chesapeake Bay. The sites, situated in the oligohaline upper bay and in the seasonally hypoxic mesohaline mid bay, receive fundamentally different detrital inputs predominantly from allochthonous and autochthonous sources, respectively. Unsurprisingly, we observed greater molecular oxygenation and degree of aromaticity in downcore PDOM profiles from the upper bay. At the mid bay station, PDOM composition was more indicative of non-aromatic algal-derived material. Unexpectedly, this autochthonous PDOM had lower C:S ratios. Hence, algal-derived organic matter appeared to be readily sulfurized, which was confirmed by quantification of dissolved organic sulfur as well as by qualitative interpretation of FT-ICR MS data. This finding suggests addition reactions of hydrogen sulfide to double bonds in unsaturated, but non-aromatic, organic molecules in autochthonous PDOM. Intriguingly, we also observed increases in humic-like fluorescence and dissolved organic carbon (DOC) concentrations in downcore PDOM profiles from both sites. Given the differences in molecular composition between sites, these results show that humic-like fluorescence can arise from different sources and biogeochemical processes. In the upper bay, we infer that these fluorescence signals reflect solubilization of terrestrially derived organic matter with a high aromatic and polyphenolic composition. By contrast, in the mid bay, these fluorescence peaks correlated negatively with hydrogen sulfide and are more likely linked to bacterial sulfate reduction.

Dynamic Energy Budget modelling to predict eastern oyster growth, reproduction, and mortality under river management and climate change scenarios

Eastern oysters growing in deltaic Louisiana estuaries in the northern Gulf of Mexico must tolerate considerable salinity variation from natural climate variability (e.g., rainfall and stream run-off pushing isohalines offshore; tropical storms pushing isohalines inshore) and man-made diversions and siphons releasing freshwater from the Mississippi River. These salinity variations are predicted to increase with future climate change because of the increased frequency of stronger storms and also in response to proposed large-scale river diversions. Increased Mississippi River flow into coastal estuaries from river diversions, along with potential changes in rainfall and stream run-off from climate change will alter spatial and temporal salinity patterns. In this study we used an individual Dynamic Energy Budget model to predict growth and reproductive potential of eastern oysters across observed and simulated salinity gradients corresponding to different climate and river management scenarios. We used validated model outputs of salinity from a coupled hydrology-hydrodynamic model to assess the current impacts of Davis Pond diversion discharge on oysters located downstream. Under a high diversion discharge scenario oyster growth potential was reduced by 9%, 4%, and 1% in Upper, Mid, and Lower Bay locations, respectively, as compared to a limited discharge year. Reproductive outputs decreased by 34% and 2% in the Upper and Lower Bay locations, respectively, and increased by 2% at the Mid Bay site. In scenarios combining predicted increased temperature with the effect of diversions, all oysters located in the Upper and Mid Bay sites died due to severe summer conditions (high temperatures combined with low salinity). Overall, oysters in down-estuary locations, influenced by both estuarine river management and gulf conditions demonstrated significant tolerance to changing salinity and temperature conditions from diversions alone and when combined with climate change. In contrast, oysters located up-estuary, and exposed to more extreme salinity impacts from river management, demonstrated potentially lethal impacts through direct mortality, and reduced sustainability through decrease in reproductive output. These predictions at the individual level may translate into less sustainable populations in the most extreme scenarios; restoration and production plans may benefit from accounting for these impacts on reproductive output particularly as decision makers seek to restore critical oyster areas.

Evaluation of Seismic Performance of Improved Rocking Concentrically Braced-frames with Zipper Columns

Concentrically braced frames (CBFs) as one of well-known stiff and common lateral force resisting systems often show limited ductility capacity under severe earthquakes. This study proposes rocking zipper braced frame (RZBF) to improve the drift capacity of CBFs which is based on combination of rocking behavior and zipper columns. In the RZBF system, rocking behavior permit the braced frame to uplift during the earthquake and then restoring force induced through post-tensioned bars self-center the frame to its initial state. Also, zipper columns can decrease the concentration of damage by distributing the unbalance force at the mid bay over the frame’s height. To assess the performance of RZBF, a comparison study is carried out considering CBF, rocking concentrically braced frame, zipper braced frame and RZBF. For this purpose, some frames structures are designed and nonlinear time history analysis conduct under a set of earthquake records. Seismic responses such as roof drift ratio, gap opening at the column-base interface, forces of top story braces and post-tensioned bars are taken into consideration. The results show that the proposed RZBF has better performance among the others and zipper columns can improve the behavior of rocking systems.

Factors Controlling Seasonal Phytoplankton Dynamics in the Delaware River Estuary: an Idealized Model Study

Phytoplankton biomass in estuaries is controlled by complex biological and chemical processes that control growth and mortality, and physical processes that control transport and dilution. The effects of these processes on phytoplankton blooms were systematically analyzed, focusing on identifying the dominant controlling factors out of river-induced flushing, tidal dispersion, nutrient limitation, and light limitation. To capture the physical processes related to flow and sediment dynamics, we used the idealized width-averaged iFlow model. The model was extended with a nutrient-phytoplankton module to capture the essential biological-chemical processes. The model was applied to the Delaware River Estuary for the productive months of March to November. Model results were compared with field observations. It was found that phytoplankton blooms cannot form in the lower bay due to tidal dispersion, as water from the estuary and coastal ocean mix in early spring, and due to local effects of nitrogen limitation in summer. In the middle to upper bay, sediment-induced deterioration of the light climate limits the growth but allows for blooms in the mid bay, while no blooms can form in the turbidity maximum zone in the upper estuary. Further upstream in the tidal river, the effects of river-induced flushing dominate in early spring and prevent bloom formation. In the summer and fall, lower river discharges and higher growth rates at higher temperatures allow blooms to form and persist. Analysis of the connectivity between mid bay and tidal river blooms showed that coastal ocean phytoplankton may contribute to mid bay blooms, but do not penetrate beyond the turbidity maximum zone.

Controls on Carbonate System Dynamics in a Coastal Plain Estuary: A Modeling Study

AbstractThe study of acidification in Chesapeake Bay is challenged by the complex spatial and temporal patterns of estuarine carbonate chemistry driven by highly variable freshwater and nutrient inputs. A new module was developed within an existing coupled hydrodynamic‐biogeochemical model to understand the underlying processes controlling variations in the carbonate system. We present a validation of the model against a diversity of field observations, which demonstrated the model's ability to reproduce large‐scale carbonate chemistry dynamics of Chesapeake Bay. Analysis of model results revealed that hypoxia and acidification were observed to cooccur in midbay bottom waters and seasonal cycles in these metrics were regulated by aerobic respiration and vertical mixing. Calcium carbonate dissolution was an important buffering mechanism for pH changes in late summer, leading to stable or slightly higher pH values in this season despite persistent hypoxic conditions. Model results indicate a strong spatial gradient in air‐sea CO2 fluxes, where the heterotrophic upper bay was a strong CO2 source to atmosphere, the mid bay was a net sink with much higher rates of net photosynthesis, and the lower bay was in a balanced condition. Scenario analysis revealed that reductions in riverine nutrient loading will decrease the acid water volume (pH < 7.5) as a consequence of reduced organic matter generation and subsequent respiration, while bay‐wide dissolved inorganic carbon (DIC) increased and pH declined under scenarios of continuous anthropogenic CO2 emission. This analysis underscores the complexity of carbonate system dynamics in a productive coastal plain estuary with large salinity gradients.

Roles of Wind‐Driven Currents and Surface Waves in Sediment Resuspension and Transport During a Tropical Storm

AbstractSatellite remote sensing shows two hot spots of high suspended sediment concentration during the passage of Hurricane Irene (2011) over Chesapeake Bay: the shallow shoals in the mid Bay and the area around the mouth of the estuary. A coupled ocean wave sediment transport model is used to investigate mechanisms driving sediment resuspension and transport during the storm. The model reproduces the observed spatial variations of suspended sediment concentration and surface wave heights in the estuary and shows that both wave‐ and current‐induced shear stresses are important in stirring bottom sediment. In the mid‐Bay region, large wave‐induced shear stress causes sediment resuspension on the shallow shoals, while wind‐driven currents advect the suspended sediment downstream. Around the mouth of the estuary, the combined action of large waves and strong outflows produces high suspended sediment concentration, resulting in the export of ~0.8 Mt of estuarine sediments to the shelf. The storm‐induced sediment resuspension and export could be an important term in the sedimentary budget of an estuary.

Managed nutrient reduction impacts on nutrient concentrations, water clarity, primary production, and hypoxia in a north temperate estuary

Except for the Providence River and side embayments like Greenwich Bay, Narragansett Bay can no longer be considered eutrophic. In summer 2012 managed nitrogen treatment in Narragansett Bay achieved a goal of reducing effluent dissolved inorganic nitrogen inputs by over 50%. Narragansett Bay represents a small northeast US estuary that had been heavily loaded with sewage effluent nutrients since the late 1800s. The input reduction was reflected in standing stock nutrients resulting in a statistically significant 60% reduction in concentration. In the Providence River estuary, total nitrogen decreased from 100 μm to about 40 μm, for example. We tested four environmental changes that might be associated with the nitrogen reduction. System apparent production was significantly decreased by 31% and 45% in the upper and mid Bay. Nutrient reductions resulted in statistically improved water clarity in the mid and upper Bay and in a 34% reduction in summer hypoxia. Nitrogen reduction also reduced the winter spring diatom bloom; winter chlorophyll levels after nutrient reduction have been significantly lower than before the reduction. The impact on the Bay will continue to evolve over the next few years and be a natural experiment for other temperate estuaries that will be experiencing nitrogen reduction. To provide perspective we review factors effecting hypoxia in other estuaries with managed nutrient reduction and conclude that, as in Narragansett Bay, physical factors can be as important as nutrients. On a positive note managed nutrient reduction has mitigated further deterioration in most estuaries.

Disentangling fine- and broad- scale effects of habitat on predator–prey interactions

Predator–prey interactions can be influenced by habitat at different spatial scales. In seagrass systems, blade density can provide refugia for prey at fine scales, which are further embedded within broad-scale features such as variation in biotic (e.g., predator assemblages) and abiotic attributes (e.g., turbidity, salinity). Fine-scale effects of seagrass habitats on predator–prey interactions involving invertebrates have been well studied while less is known about their effects on fish as prey. A field experiment was conducted in Tampa Bay, Florida, USA to examine and separate the effects of habitat across fine and broad scales on the relative predation rates of tethered pinfish (Lagodon rhomboides). Artificial seagrass units (ASUs) were used at three levels of blade density and deployed in different locations within the seascape. Predation rates on pinfish decreased with increasing seagrass blade density. The effects of blade density were consistent across locations, but overall mortality was higher in the lower Bay, where the water was less turbid, higher in salinity, and characterized by a different suite of predators compared to the mid Bay. Using controlled-laboratory experiments, it was found that pinfish reduced their activity levels in more turbid water as well as in response to the presence of a common predator in both clear and more turbid waters. Thus, predation rates were influenced by the combined effects of refugia (fine scale), variation in prey behavior (broad scale), and detection by predators (both scales). This study demonstrates the strong influence habitat can have at different spatial scales in mediating predator–prey interactions of mobile species in estuarine environments.

Resource Use of an Aquacultured Oyster (Crassostrea gigas) in the Reverse Estuary Bahía San Quintín, Baja California, México

Shellfish aquaculture is prominent in many coastal and estuarine environments and has both ecological and economic impacts. Bahia San Quintin is a reverse estuary in Baja California, Mexico, where Pacific oysters (Crassostrea gigas) are cultivated. While oysters likely feed heavily on phytoplankton especially during upwelling periods, we hypothesized that other forms of organic matter such as seagrass (Zostera marina) and macroalgae (Ulva spp.) must also be used by the oysters, especially in the most inshore portions of the bay. We measured the carbon and hydrogen stable isotope composition of oysters and their potential food resources at upper, mid, and lower bay sites during upwelling and non-upwelling seasons and applied a Bayesian mixing model to evaluate resource use. Hydrogen isotopes provided a large separation between potential food resources. Although we did not find any strong seasonal effects due to upwelling, there was a strong spatial gradient in resource use. Phytoplankton were most important at the lower (oceanic) site (median resource use for two sampling times, 68 and 79 %) and decreased up the estuary as macroalgae became more important (43 and 56 % at the upper site). At all sites for both sampling times, seagrass was an unimportant resource for oysters. The contrast between high phytoplankton use at the lower site and increased macroalgal use at the upper site is likely due to available resource biomass. Results indicate the adaptability of oysters to varying resource availability and the possibility of a higher system carrying capacity than that based on phytoplankton alone given multiple potential food sources. This study also highlights the utility of hydrogen isotopes in estuarine food web research.

In Situ Hatching Success of Calanoid Copepod Eggs in Hypoxic Sediments of a Coastal Bay

ABSTRACT Choi, K-H.; Jang, M-C.; Shin, H.H.; Lee, W-J., and Shin, K., 2016. In situ hatching success of calanoid copepod eggs in hypoxic sediments of a coastal bay. Hypoxia can have adverse effects on recruitment of marine copepods via its effects on egg hatching and survival of nauplii. This study investigated the effects of hypoxia on in situ hatching success of calanoid copepods in Gamak Bay, Korea, a shallow (9-m) microtidal coastal bay with long water residence times exceeding a month. Plankton emergence trap/chambers were placed into seafloor–surface sediments in July at a hypoxic site (mean = 0.6 mL L−1) in the inner part of the bay and at a normoxic site in the mid bay. Nauplii that hatched and left the sediment were collected daily for 12 days, and the top 1 cm of sediment was preserved at the end of incubation. The number of eggs in the sediment was >3 × 105 eggs m−2 at the hypoxic site, about six times that at the normoxic site. Most of the eggs appeared to be subitaneous. Egg-hatching rate in ...

Corrosion Behavior of ASTM A416 Steel in Simulated Pore Solution

Cable used in post-tensioned bridges usually consists of multiple strands encased within a plastic sleeve. The annulus or space between the steel tendon and the plastic sleeve is filled with grout, which is intended to provide corrosion protection for the steel. Often the grout does not provide adequate protection due to a number of factors, including insufficient grout-cover, i.e., air voids; insufficient grout quality and/or strength; faulty grouting procedure; or inadequate waterproofing sealant. Because post-tensioned concrete members rely on tensile strength of prestressed steel to resist loading, the loss of even a few wires or strands could have catastrophic results. And yet, because these members are encased in grout, a method to detect corrosion or corrosion risk does not yet exist. Since post-tensioning technology is still relatively new, these problems were not evident until the 1980’s. The first instance occurred in 1980 when the southern outer roof of the Berlin Congress Hall collapsed 23 years after it was constructed. Soon thereafter, two bridges were found with similar serious corrosion issues – the Taf Fawr Bridge on A470 in Wales, England and the Angel Road Bridge on the A406 North Circular in London, England.1 The Taf Fawr Bridge was demolished in 1986, and the Angel Road Bridge was significantly retrofitted in 1982. In 1985, the single-span segmental post-tensioned Ynys-y-Gwas Bridge in Wales collapsed as a result of corrosion of longitudinal tendons at its segmented joints. This structure was only 32 years old, and there had been no previous indication of distress prior to collapse. In 1992, the British Department of Transportation conducted a study on these corrosion issues and concluded that there was no method that could guarantee complete corrosion prevention. Later that year, post-tensioned bridges were effectively banned in the United Kingdom.2 The United Kingdom was not the only country with post-tensioned bridge issues. The post-tensioned Melle Bridge, which was built in Belgium in 1956, collapsed in 1992. In this instance, the bridge had been inspected, load tested, re-waterproofed, declared adequate, and just restored to service two years prior to its collapse. More recently, the Saint Stefano Bridge in Italy (1999) and the Lowe’s Motor Speedway footbridge in North Carolina (2000) collapsed due to similar corrosion-related failures.3 Locally, in Florida, corrosion in post-tensioned bridges is a major concern as well. The first reported observation of post-tension corrosion was at the 18-year old Niles Channel Bridge in the Keys (1999).4 Similar issues were reported at the 7-year old Mid Bay Bridge in the Western Panhandle (2001)5 and the 15-year old Sunshine Skyway Bridge in Tampa (2002).6 As part of an effort to develop an impedance-based sensor for detecting corrosion of pre-stressed tendons, experiments were conducted using rotating and stationary disk electrodes fashioned out of the tendon steel and immersed in a simulated alkaline pore solution. A model for the impedance response was developed that accounted for a porous electrode behavior that gradually disappeared over a period of 36 hours. In some cases, a small corrosion rate could be discerned from the impedance response, but, in general, the metal was stable under these conditions. References Proverbio, E. and Bonaccorsi, L. M. (2002). Failure of prestressing steel induced by crevice corrosion in prestressed concrete structures. Proceedings 9th International Conference on the durability of building materials and components, Brisbane, Australia, March 17-21. Lewis, A. (1996). A moratorium lifted. Concrete, November/December, 25-27. Goins, D. (2000). Motor speedway bridge collapse caused by corrosion. Materials Performance, 36 (7) 18-19. Sagüés, A. A., Kranc, S. C., and Hoehne, R. H (2000). Initial development of methods for assessing condition of post-tensioned tendons of segmental bridges. Final Report Florida Department of Transportation, Tallahassee, FL, May. Hartt, W. H. (2002). Corrosion evaluation of post-tensioned tendons on the mid bay bridge in Destin, FL. Final Report, Florida Department of Transportation, Tallahassee, FL, April 15. Sagüés, A., Powers, R. G., and Wang, H (2008). Mechanism of corrosion of steel strands in post tensioned grouted assemblies. NACE International, paper no. 03312.

Profiling gene expression to distinguish the likely active diazotrophs from a sea of genetic potential in marine sediments

Nitrogen (N) cycling microbial communities in marine sediments are extremely diverse, and it is unknown whether this diversity reflects extensive functional redundancy. Sedimentary denitrifiers remove significant amounts of N from the coastal ocean and diazotrophs are typically regarded as inconsequential. Recently, N fixation has been shown to be a potentially important source of N in estuarine and continental shelf sediments. Analysis of expressed genes for nitrite reductase (nirS) and a nitrogenase subunit (nifH) was used to identify the likely active denitrifiers and nitrogen fixers in surface sediments from different seasons in Narragansett Bay (Rhode Island, USA). The overall diversity of diazotrophs expressing nifH decreased along the estuarine gradient from the estuarine head to an offshore continental shelf site. Two groups of sequences related to anaerobic sulphur/iron reducers and sulphate reducers dominated libraries of expressed nifH genes. Quantitative polymerase chain reaction (qPCR) and quantitative reverse transcription polymerase chain reaction (qRT-PCR) data shows the highest abundance of both groups at a mid bay site, and the highest nifH expression at the head of the estuary, regardless of season. Several potential environmental factors, including water temperature, oxygen concentration and metal contamination, may influence the abundance and nifH expression of these two bacterial groups.

Hydrodynamic modeling and ecohydrological analysis of river inflow effects on Apalachicola Bay, Florida, USA

This paper presents an integrated hydrodynamic modeling and probability analysis approach to assess the long-term effects of changing river inflows on the estuarine ecosystem. The probability analysis method, which is popularly used in advanced hydrological frequency analysis of river flows and rainfalls, has been applied to analyze the effects of changing inflow on salinity and thus on oyster ecology in Apalachicola Bay. Long-term salinity data were predicted through the application of a calibrated 3D hydrodynamic model under two river inflow conditions over a 10-year period. The first flow represents the historic flow. The 2nd flow condition, called Scenario-1, represents a regulated flow scenario to account for the potential increasing upstream water demands. Two stations, Mid Bay and Dry Bar, in the bay were selected to examine the estuarine responses. Under the historic flow condition, the maximum probability salinity at Dry Bar in the rich oyster reef is near 24 ppt, within the optimal salinity range for oyster growth of 16–26 ppt (Harned et al., 1996); the maximum probability salinity at Mid Bay station is 27 ppt, beyond the optimal salinity for oyster growth in mid-bay area where there is no oyster reef around. While it is difficult to examine the difference between two scenarios by conventional time series analysis of river flows and salinity, probability analysis reasonably characterizes and quantifies the changes of river flow and salinity patterns over the 10-year period. The Scenario-1 has caused the increase of the probability in low flows. Higher probability of low flows for the regulated flow scenario shortens the period of optimal salinity in the oyster reef, and cause substantial increase of exceedance probability of higher salinity in the oyster reef to the level beyond the optimal salinity range for oyster growth. The probability analysis approach has demonstrated its advantage for the risk assessments of the long-term estuarine ecohydrological effects under various regulated inflow scenarios to support estuarine water resources managements.

Methylmercury production in sediments of Chesapeake Bay and the mid-Atlantic continental margin

Methylmercury (MeHg) concentration and production rates were studied in bottom sediments along the mainstem of Chesapeake Bay and on the adjoining continental shelf and slope. Our objectives were to 1) observe spatial and temporal changes in total mercury (HgT) and MeHg concentrations in the mid-Atlantic coastal region, 2) investigate biogeochemical factors that affect MeHg production, and 3) examine the potential of these sediments as sources of MeHg to coastal and open waters. Estuarine, shelf and slope sediments contained on average 0.5 to 1.5% Hg as MeHg (% MeHg), which increased significantly with salinity across our study site, with weak seasonal trends. Methylation rate constants ( k meth), estimated using enriched stable mercury isotope spikes to intact cores, showed a similar, but weaker, salinity trend, but strong seasonality, and was highly correlated with % MeHg. Together, these patterns suggest that some fraction of MeHg is preserved thru seasons, as found by others [Orihel, D.M., Paterson, M.J., Blanchfield, P.J., Bodaly, R.A., Gilmour, C.C., Hintelmann, H., 2008. Temporal changes in the distribution, methylation, and bioaccumulation of newly deposited mercury in an aquatic ecosystem. Environmental Pollution 154, 77] Similar to other ecosystems, methylation was most favored in sediment depth horizons where sulfate was available, but sulfide concentrations were low (between 0.1 and 10 μM). MeHg production was maximal at the sediment surface in the organic sediments of the upper and mid Bay where oxygen penetration was small, but was found at increasingly deeper depths, and across a wider vertical range, as salinity increased, where oxygen penetration was deeper. Vertical trends in MeHg production mirrored the deeper, vertically expanded redox boundary layers in these offshore sediments. The organic content of the sediments had a strong impact on the sediment:water partitioning of Hg, and therefore, on methylation rates. However, the HgT distribution coefficient ( K D) normalized to organic matter varied by more than an order of magnitude across the study area, suggesting an important role of organic matter quality in Hg sequestration. We hypothesize that the lower sulfur content organic matter of shelf and slope sediments has a lower binding capacity for Hg resulting in higher MeHg production, relative to sediments in the estuary. Substantially higher MeHg concentrations in pore water relative to the water column indicate all sites are sources of MeHg to the water column throughout the seasons studied. Calculated diffusional fluxes for MeHg averaged ∼ 1 pmol m − 2 day − 1 . It is likely that the total MeHg flux in sediments of the lower Bay and continental margin are significantly higher than their estimated diffusive fluxes due to enhanced MeHg mobilization by biological and/or physical processes. Our flux estimates across the full salinity gradient of Chesapeake Bay and its adjacent slope and shelf strongly suggest that the flux from coastal sediments is of the same order as other sources and contributes substantially to the coastal MeHg budget.

Mine water-induced gradients in sediment metals and arcellacean assemblages in a boreal freshwater bay (Petkellahti, Finland)

Sediment chemistry and arcellacean community composition were analyzed to study the ecological effects of copper mine-derived acidic and metal-rich waters in a freshwater bay in eastern Finland. To track spatial and temporal changes in the bay, 32 mine-impacted (top) and pre-impact (bottom) samples were analyzed from 16 short sediment cores taken along a transect extending from the pollution source. In addition, short cores from the impacted and reference sites were studied. Recent sediments displayed a geochemical gradient from the inner bay, where mine water is discharged, to the outer bay. Inner bay sites were enriched with C, S, and Fe, whereas Mn was depleted, but has precipitated in the outer bay. Among the heavy metals, Cu, Al, Zn, and Ni concentrations had increased in the inner and mid bay, while Co and Cd concentrations had increased at mid-bay sites. The change from the natural to mine-impacted condition was also seen in faunal gradients. According to ordination and distance measures, the greatest changes in species composition occurred in the inner bay, with fairly small changes in the outer bay. Numerical methods suggested that metals (Cu, Pb, Al, Zn, Cr), redox-sensitive elements (Fe, Mn), organic carbon and nutrients could be related to changes in arcellacean assemblages. Geochemical changes in the impacted core started at ~20 cm with increases in S alternating with peaks in Cr and Mg. Heavy metal concentrations increased markedly at 10 cm, after the active mining period, suggesting the beginning of acid mine drainage. Geochemical changes at ~20 cm were already apparent in the arcellacean assemblages, but the most notable change coincided with the geochemical shift at 10 cm, with signs of decreased pH. Numerical methods suggest that mining-related metals Co, Cu, Zn and Ni co-vary with arcellaceans, but Al appears to behave independently with respect to the species data.

The impact of changing climate on phenology, productivity, and benthic–pelagic coupling in Narragansett Bay

The timing and magnitude of phytoplankton blooms have changed markedly in Narragansett Bay, RI (USA) over the last half century. The traditional winter–spring bloom has decreased or, in many years, disappeared. Relatively short, often intense, diatom blooms have become common in spring, summer, and fall replacing the summer flagellate blooms of the past. The annual and summer mean abundance (cell counts) and biomass (chl a) of phytoplankton appear to have decreased based on almost 50 years of biweekly monitoring by others at a mid bay station. These changes have been related to warming of the water, especially during winter, and to increased cloudiness. A significant decline in the winter wind speed may also have played a role. The changes in the phenology of the phytoplankton and the oligotrophication of the bay appear to have decreased greatly the quantity and (perhaps) quality of the organic matter being deposited on the bottom of the bay. This decline has resulted in a very much reduced benthic metabolism as reflected in oxygen uptake, nutrient regeneration, and the magnitude and direction of the net flux of N 2 gas. Based on many decades of standard weekly trawls carried out by the Graduate School of Oceanography, the winter biomass of bottom feeding epibenthic animals has also declined sharply at the mid bay station. After decades of relatively constant anthropogenic nitrogen loading (and declining phosphorus loading), the fertilization of the bay will soon be reduced during May–October due to implementation of advanced wastewater treatment. This is intended to produce an oligotrophication of the urban Providence River estuary and the Upper Bay. The anticipated decline in the productivity of the upper bay region will probably decrease summer hypoxia in that area. However, it may have unanticipated consequences for secondary production in the mid and lower bay where climate-induced oligotrophication has already much weakened the historically strong benthic–pelagic coupling.

Topographically generated fronts, very nearshore oceanography and the distribution and settlement of mussel larvae and barnacle cyprids

Flow patterns adjacent to shore may prevent or aid shoreward migration of benthic invertebrate larvae. We hypothesized that a front at the mouth of Sunset Bay, Oregon, prevents shoreward dispersal of larvae, significantly altering settlement of mussel larvae and barnacle cyprids. Settlement was measured at three sets of moorings (three moorings per site) distributed across the front at Sunset Bay. From 6 July to 4 September 2000, samples were collected roughly every other day. Concurrently, we made vertical zooplankton tows adjacent to each mooring site and collected physical oceanographic data. During upwelling-favorable winds, the front was always present at the bay mouth, separating significantly cooler, saltier and denser offshore water from that within the bay. During downwelling winds, the front broke down and we found no significant difference in the surface physical oceanographic parameters across the bay mouth. During upwelling, the concentration of mussel larvae was higher seaward of the front than landward, but there was no significant difference in concentration during downwelling, suggesting that the front may act as a barrier to the shoreward dispersal of mussels. Mussel settlement was too low and sporadic to allow statistical analysis. There was no difference in cyprid concentrations across the bay mouth whether the front was present or not. Cyprid settlement was, however, nearly an order of magnitude lower at moorings seaward of the front than at those landward. A significant cross-correlation was found between settlement at the offshore mooring and tidal range (r=0.464, lag=0 days) and between settlement at the mid and inner moorings and downwelling winds (r=0.532 mid bay, r=0.532 inner bay, lag=0 days). Seaward of the front, settlement varied with tidal range, while landward of the front, most settlement occurred as brief pulses during downwelling winds, periods when the front was not present. We found large differences in the distribution of cyprids, and mussel larvae and cyprid settlement relative to the front; larval distributions and settlement varied with upwelling versus downwelling winds and was due to differences in the very nearshore (i.e. within 100‐1000 m of shore) coastal oceanography.

Planktonic and bacterial respiration along an estuarine gradient: responses to carbon and nutrient enrichment

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 30:251-261 (2003) - doi:10.3354/ame030251 Planktonic and bacterial respiration along an estuarine gradient: responses to carbon and nutrient enrichment Erik M. Smith*, W. Michael Kemp University of Maryland Center for Environmental Science, Horn Point Laboratory, PO Box 775, Cambridge, Maryland 21613, USA *Present address: Département des Sciences Biologiques, Université du Québec à Montréal, CP 8888, succursale Centre-Ville, H3C 3P8 Montréal, Québec, Canada. Email: smith.erik@uqam.ca ABSTRACT: Previous observations have revealed that rates of plankton community respiration vary consistently along the main salinity gradient of Chesapeake Bay, USA, with maximum values occurring in the mesohaline region and significantly lower rates in both the oligohaline and polyhaline regions. This study tested the hypothesis that spatial variations in respiration are the result of differential resource limitation of the bacterial communities along the land-sea gradient. Effects of organic carbon and inorganic nutrients on microbial (<3 μm) respiration rate were assessed in short-term (6 h) enrichment bioassays. Responses of bacterioplankton to enrichment were also investigated by means of re-growth culture experiments. Both sets of experiments were conducted during summer in 3 regions along the main salinity gradient of the estuary. Within the re-growth cultures, there was little variation in, or effect of enrichment on, specific growth rates, which were roughly 2 d-1 in all cultures. In contrast, estimates of cell-specific respiration varied 2-fold along the salinity gradient (from 3.3 to 6.9 fg O2 cell-1 h-1 in the Upper and Mid Bay regions, respectively) and showed significant responses to substrate enrichment. Resulting estimates of bacterial growth efficiencies exhibited similar variability, ranging from 20.4 to 41.3%, with spatial variations in efficiencies apparently related to nutrient concentrations. Results of substrate enrichment in both short-term bioassays and bacterioplankton re-growth cultures demonstrated a pattern of stimulation by organic carbon in the Upper Bay and by inorganic nutrients in the Lower Bay, with no significant response to substrate enrichment in the Mid Bay. Patterns of bacterioplankton response to enrichment in the nutrient-poor polyhaline region suggested a propensity for the bacterial community to maximize growth, rather than efficiency, via changes in specific respiration rates. KEY WORDS: Respiration · Bacterioplankton · Nutrient effects · Growth efficiency · Estuaries Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 30, No. 3. Online publication date: January 23, 2003 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2003 Inter-Research.