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

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

Phytoplankton Growth Balanced by Clam and Zooplankton Grazing and Net Transport into the Low-Salinity Zone of the San Francisco Estuary

We estimated the influence of planktonic and benthic grazing on phytoplankton in the strongly tidal, river-dominated northern San Francisco Estuary using data from an intensive study of the low salinity foodweb in 2006–2008 supplemented with long-term monitoring data. A drop in chlorophyll concentration in 1987 had previously been linked to grazing by the introduced clam Potamocorbula amurensis, but numerous changes in the estuary may be linked to the continued low chlorophyll. We asked whether phytoplankton continued to be suppressed by grazing and what proportion of the grazing was by benthic bivalves. A mass balance of phytoplankton biomass included estimates of primary production and grazing by microzooplankton, mesozooplankton, and clams. Grazing persistently exceeded net phytoplankton growth especially for larger cells, and grazing by microzooplankton often exceeded that by clams. A subsidy of phytoplankton from other regions roughly balanced the excess of grazing over growth. Thus, the influence of bivalve grazing on phytoplankton biomass can be understood only in the context of limits on phytoplankton growth, total grazing, and transport.     

Spectral Irradiance, Phytoplankton Community Composition and Primary Productivity in a Salt Marsh Estuary, North Inlet, South Carolina, USA

We investigated spatial and temporal changes in spectral irradiance, phytoplankton community composition, and primary productivity in North Inlet Estuary, South Carolina, USA. High concentrations of colored dissolved organic matter (CDOM) were responsible for up to 84 % of the attenuation of photosynthetically available radiation (PAR). Green-yellow wavelengths were the predominant colors of light available at the two sampling sites: Clam Bank Creek and Oyster Landing. Vertical attenuation coefficients of PAR were 0.7–2.1 m^?1 with corresponding euphotic zone depths of 1.5–6.7 m. Phytoplankton biomass (as chlorophyll a [chl a]) varied seasonally with a summer maximum of 16 μg chl a l^?1 and a winter minimum of 1.4 μg chl a l^?1. The phytoplankton community consisted mainly of diatoms, prasinophytes, cryptophytes and haptophytes, with diatoms and prasinophytes accounting for up to 67 % of total chl a. Changes in phytoplankton community composition showed strongest correlations with temperature. Light-saturated chl a-specific rates of photosynthesis and daily primary productivity varied with season and ranged from 1.6 to 14 mg C (mg chl a) ^?1?h^?1 (32–803 mg C m^?3?day^?1). Calculated daily rates added up to an annual carbon fixation rate of 84 g C m^?3?year^?1. Overall, changes in phytoplankton community composition and primary productivity in North Inlet showed a strong dependence on temperature, with PAR and spectral irradiance playing a relatively minor role due to short residence times, strong tidal forcing and vertical mixing.     

The asiatic clam (Corbicula fluminea) invasion and system-level ecological change in the Potomac River Estuary near Washington,D.C.

The exotic freshwater clam speciesCorbicula fluminea (Asiatic clam) was first reported in the tidal freshwater Potomac estuary near Washington, D.C., in 1977, and was found in benthic surveys, conducted in 1978, 1982, 1984, 1986, and 1992. In 1981 a tripling of water clarity was reported in the region of the clam beds, followed in 1983 by reapperance of submerged aquatic vegetation (SAV) absent for 50 yr. Submerged aquatic vegetation (SAV) has been surveyed and mapped over the entire Potomac estuary region in almost every year from 1976 to 1993 by aerial photography, as part of the United States Environmental Protection Agency's Chesapeake Bay program. Fish surveys in 1986 found populations increased up to 7× in beds of SAV. Starting in 1984, the Washington, D.C. Christmas Bird Census reported significant increases in several aquatic bird populations both nonmigratory and migratory. An extensive benthic survey in September 1986 estimated a spring-summer population of 8.7×10⁶ kg Asiatic clams (wet weight including shell) in the 5-km region of the Potomac below Washington, D.C. This population was calculated as having the capacity to filter one-third to all of the water in this region of the estuary daily, depending on river flow. The 1986 clam population was smaller than that of 1984 and the 1992 population was 25% of that in 1986. Since 1986, SAV acreage has been decreasing in this area of the Potomac. Aquatic bird populations have declined. Yearly nuisance algae (Microcystis) blooms, which had been absent since 1983, reappeared in 1993. This paper presents evidence to support the theory the invasive Asiatic clam population in the 10 km below Washington, D.C., was responsible for SAV resurgence through filtration affecting turbidity. It suggests the clam populations triggered system-level changes in biota, including increase and decrease in local Potomac estuary populations (SAV, bird, fish, algae) over 10 yr, from 1983 to 1993. Major changes in the Asiatic clam population took place approximately 2 yr before parallel changes in SAV acreage were observed.     

Quantifying Tidal Movements of the Shore Crab <Emphasis Type="Italic">Carcinus maenas</Emphasis> on to Complex Epibenthic Bivalve Habitats

Many subtidal predators undertake regular tidal migrations into intertidal areas in order to access abundant prey. One of the most productive habitats in soft bottom intertidal systems is formed by beds of epibenthic bivalves such as blue mussels (Mytilus edulis) and Pacific oysters (Crassostrea gigas). In the Dutch Wadden Sea, these bivalves might face substantial predation pressure by the shore crab (Carcinus maenas), which increased considerably in numbers during the last 20 years. However, the quantification of this species on bivalve beds is challenging, since most methods common for quantifying animal abundance in marine habitats cannot be used. This study investigated the potential of two methods to quantify the abundance of C. maenas on 14 epibenthic bivalve beds across the Dutch Wadden Sea. The use of the number of crabs migrating from subtidal towards intertidal areas as a proxy of abundance on bivalve beds yielded unreliable results. In contrast, crabs caught with traps on the beds were correlated with the abundance assessed on the surrounding bare flats by beam trawl and therefore provided usable results. The estimates, however, were only reliable for crabs exceeding 35 mm in carapace width (CW). The application of these estimates indicated that crab abundances on bivalve beds were influenced by the biogenic structure. Beds dominated by oysters attracted many large crabs (> 50-mm CW), whereas abundances of medium-sized crabs (35–50-mm CW) showed no relationship to the oyster occurrence. The combination of traps and trawls is capable of quantifying crab abundance on bivalve beds, which offers the possibility to study biotic processes such as predator-prey interactions in these complex structures in more detail.     

Distribution and Trophic Importance of Anthropogenic Nitrogen in Narragansett Bay: An Assessment Using Stable Isotopes

Narragansett Bay has been heavily influenced by human activities for more than 200 years. In recent decades, it has been one of the more intensively fertilized estuaries in the USA, with most of the anthropogenic nutrient load originating from sewage treatment plants (STP). This will soon change as tertiary treatment upgrades reduce nitrogen (N) loads by about one third or more during the summer. Before these reductions take place, we sought to characterize the sewage N signature in primary (macroalgae) and secondary (the hard clam, Mercenaria mercenaria) producers in the bay using stable isotopes of N (δ¹⁵N) and carbon (δ¹³C). The δ¹⁵N signatures of the macroalgae show a clear gradient of approximately 4‰ from north to south, i.e., high to low point source loading. There is also evidence of a west to east gradient of heavy to light values of δ¹⁵N in the bay consistent with circulation patterns and residual flows. The Providence River Estuary, just north of Narragansett Bay proper, receives 85% of STP inputs to Narragansett Bay, and lower δ¹⁵N values in macroalgae there reflected preferential uptake of ¹⁴N in this heavily fertilized area. Differences in pH from N stimulated photosynthesis and related shifts in predominance of dissolved C species may control the observed δ¹³C signatures. Unlike the macroalgae, the clams were remarkably uniform in both δ¹⁵N (13.2 ± 0.54‰ SD) and δ¹³C (−16.76 ± 0.61‰ SD) throughout the bay, and the δ¹⁵N values were 2–5‰ heavier than in clams collected outside the bay. We suggest that this remarkable uniformity reflects a food source of anthropogenically heavy phytoplankton formed in the upper bay and supported by sewage derived N. We estimate that approximately half of the N in the clams throughout Narragansett Bay may be from anthropogenic sources.     

Transport of juvenile gem clams (Gemma gemma) in headland wake

Accumulation of bivalve recruits in the bottom convergence at the center of coastal eddies has been suggested as a possible mechanism resulting in locally abundant adult populations. We investigated transport of juvenile gem clams (Gemma gemma) in a headland wake to determine whether they accumulated, and where. Velocity measurements during three flood tides showed that a wake consistently formed, but that flow speeds were too slow to transport juvenile clams to the eddy center. Instead, the clams were deposited just inside the wake perimeter, where shear velocities decreased to levels below critical erosion velocities of the clams. This result demonstrated that accumulation in a coastal flow separation can occur even in the absence of a well-defined eddy or a strong bottom convergence. Juvenile gem clams were carried, probably as bedload, to regions in the wake dominated by sediments with similar grain sizes, rather than similar fall velocities, suggesting that bedload transport was particularly dependent on particle diameter in this flow regime. Adult gem clam populations tended to be locally abundant in regions receiving transported juveniles, but clam transport on any specific flood tide was not sufficient to fully predict the adult distributions.     

Trace metal bioaccumulation in the shells of mussels and clams at deep-sea hydrothermal vent fields

The bioaccumulation of trace metals in the carbonate shells of mussel and clams was investigated at seven hydrothermal vent fields of the Mid-Atlantic Ridge (Menez Gwen, Snake Pit, Rainbow, and Broken Spur) and the Eastern Pacific (9°N and 21°N at the East Pacific Rise and the southern trough of Guaymas Basin). Mineralogical analysis showed that the carbonate skeletons of the mytilid mussel Bathymodiolus sp. and the vesicomyid clam Calyptogena m. are composed mainly of calcite and aragonite, respectively. The first data were obtained for the content of a variety of elements in the bivalve carbonate shells from various hydrothermal vent sites. The analysis of the chemical compositions (including Fe, Mn, Zn, Cu, Cd, Pb, Ag, Ni, Cr, Co, As, Se, Sb, and Hg) of 35 shell samples and 14 water samples from the mollusk biotopes revealed the influences of environmental conditions and some biological parameters on the bioaccumulation of metals. Bivalve shells from hydrothermal fields with black smokers are enriched in Fe and Mn by a factor of 20–30 relative to the same species from the Menez Gwen low-temperature vent site. It was shown that the essential elements Fe, Mn, Ni, and Cu were more actively accumulated during the early ontogeny of the shells. The high concentration factors of most metals (n × 10²−n × 10⁴) indicate an efficient accumulation function of bivalve carbonate shells. Passive metal accumulation owing to adsorption on the shell surface was estimated to be no higher than 50% of the total amount, varying from 14% for Fe to 46% for Mn.     

How the Distribution of Anthropogenic Nitrogen Has Changed in Narragansett Bay (RI,USA) Following Major Reductions in Nutrient Loads

Over the past decade, nitrogen (N) loads to Narragansett Bay have decreased by more than 50%. These reductions were, in large part, the direct result of multiple wastewater treatment facility upgrades to tertiary treatment, a process which employs N removal. Here, we document ecosystem response to the N reductions and assess how the distribution of sewage N in Narragansett Bay has changed from before, during, and shortly after the upgrades. While others have observed clear responses when data were considered annually, our seasonal and regional comparisons of pre- and post-tertiary treatment dissolved inorganic nitrogen (DIN) concentrations and Secchi depth data, from bay-wide surveys conducted periodically from the early 1970s through 2016, resulted in only a few subtle differences. Thus, we sought to use stable isotope data to assess how sewage N is incorporated into the ecology of the Bay and how its distribution may have changed after the upgrades. The nitrogen (δ¹⁵N) and carbon (δ¹³C) stable isotope measurements of particulate matter served as a proxy for phytoplankton, while macroalgae served as short-term integrators of water column bio-available N, and hard clams (Mercenaria mercenaria) as integrators of water column production. In contrast to other estuarine stable isotope studies that have observed an increased influence of isotopically lower marine N when sewage N is reduced, the opposite has occurred in Narragansett Bay. The tertiary treatment upgrades have increased the effluent δ¹⁵N values by at least 2‰. The plants and animals throughout Narragansett Bay have similarly increased by 1–2‰, on average. In contrast, the δ¹³C values measured in particulate matter and hard clams have declined by about the same amount. The δ¹⁵N results indicated that, even after the N reductions, sewage N still plays an important role in supporting primary and secondary production throughout the bay. However, the δ¹³C suggests that overall net production in Narragansett Bay has decreased. In the 5 years after the major wastewater treatment facilities came on-line for nutrient removal, oligotrophication has begun but sewage remains the dominant source of N to Narragansett Bay.     

Environmental Geochemistry of Swan Lake Inlet,Rongcheng Bay,the Yellow Sea of China

The Swan Lake Inlet, the State Primary Wildlife Protection Area, is a lagoon-inlet system located in the Rongcheng Bay, Shandong Peninsula, China. It has been undergoing development for aquaculture and tourism. In the summer of 1999, a study on the environment of the Swan Lake Inlet was carried out. The concentrations of the major elements and trace elements Fe, Al, Pb, Zn, Cd, Cu, Cr, Mn and P have been measured by ICP-AES and graphite furnace atomic adsorption spectrometry. The sources and distribution of the elements in the Swan Lake Inlet have been discussed. It is concluded that the Swan Lake Inlet has not been subjected to significant environmental pollution. The chemical results show that the dissolved oxygen (DO) contents are generally normal. At some locations DO solubility appears to be >100 %. The BOD₅ ( five-day biochemical oxygen demand) values are generally <4 mg/L and COD (chemical oxygen demand) 3～4 mg/L. The seawater N, P and Si contents are lower than the Class I water type specified by the Chinese National Standard of Water Quality. The low nutrient distribution reflects little discharge from land, therefore lacking of nutrient supply.     

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

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

Balance of Catchment and Offshore Nutrient Loading and Biogeochemical Response in Four New Zealand Coastal Systems: Implications for Resource Management

Nutrient mass balance analyses are a way of obtaining ‘whole system’ viewpoints on coastal biogeochemical functions and their forcing. Seasonal mass balances are presented for four large bay systems in New Zealand (NZ), with the aim of showing how they can inform coastal management. Freshwater volumes, and surface and groundwater, wastewater and atmospheric inorganic and organic nitrogen (N) and phosphorus (P) were balanced with levels of salinity, N and P from ocean surveys, used to determine non-conservative N and P fluxes and, via stoichiometry, carbon (C) fluxes. For Golden and Tasman Bays and Hauraki Gulf, exchange with adjacent shelf waters usually dominated total N supply (80–85%). In contrast, for the Firth of Thames, 51% of total N and 85% of dissolved inorganic N supply originated from its agricultural catchment. Net ecosystem metabolism (NEM; balance of autotrophy and heterotrophy) of Golden and Tasman Bays and Hauraki Gulf was usually nearly balanced. In contrast, Firth NEM was highly seasonally variable, often exhibiting strong heterotrophy coincident with expression of respiration-related stressors (low O₂ and high DIC/low pH). Denitrification accounted for about 51% of total N export across the four systems, signifying its importance as a eutrophication-regulating ecosystem service. Budgets made 12 years apart in the Firth showed decreased denitrification efficiency, coincident with large increases in system N and phytoplankton. The findings for land-ocean nutrient balance, NEM and denitrification showed how mass balance budgeting can inform coastal management, including inventories of nutrient inputs, balances of oceanic and terrestrial nutrient loading, and potential for risk associated with biogeochemical responses.     

Phosphorus Budget of the Sundarban Mangrove Ecosystem: Box Model Approach

Phosphorus (P) cycling in mangroves plays an important role in productivity but the magnitude of atmospheric input in the mangrove P budget is still uncertain. This study applied a box model approach to assess P budget in the Indian Sundarban, the world’s largest mangrove ecosystem for conceptual understanding of P cycling and for better representation of transport and transformation of P within the mangrove ecosystem. The P content in the sediment (0.19–0.67 μg g^?1) was found much below its maximum retention capacity (322 μg g^?1) and was lower than the mean marine sediment (669 μg g^?1). The C:N and C:P ratios were correlated (r ² = 0.66, P < 0.01) and the major fraction of available P was recycled within the organic structure of mangrove ecosystem, thus maintaining productivity through conservation strategies. Atmospheric input accounted for 56.7% of total P input (16.06 Gg year^?1) and 50% of total P output (14.7 Gg year^?1) was attributed to plant uptake. Budget closing or unaccounted P (1.36 Gg) was only 8.5% of the total input. Two feedback pathways, i.e., input of P from dust fallout and biochemical mineralization of organic matter, significantly affected P availability. The findings of the study suggest that atmospheric deposition is of major importance as a natural and/or anthropogenic forcing function in the Sundarban mangrove system.     

Evidence for Declines in the Native Leukoma staminea as a Result of the Intentional Introduction of the Non-native Venerupis philippinarum in Coastal British Columbia,Canada

On the west coast of British Columbia (BC), Canada, the intertidal regions are under ever increasing pressure to be used primarily for near-bottom mariculture practices. These include seeding the foreshore with the nonindigenous Venerupis philippinarum (Manila clam) followed by the application of antipredator netting. The Manila is confamilial with the indigenous Leukoma staminea (littleneck clam), and seeding could possibly provide a competitive advantage for the Manila leading to the extirpation of the littleneck within coastal BC. Over two survey years, nine and seven farm-reference paired beaches (18 and 14 beaches) from three geographically distinct regions of BC were sampled for abundances and size class structure of the Manila and littleneck clam. Several lines of evidence suggest that the Manila is replacing the native littleneck; (1) comparison of average abundances of the littleneck versus Manila showed a significantly greater number of Manila on both farmed and reference sites across all three regions (p?<?0.05; years I and II); (2) as distance between farmed and reference sites increased, numbers of littlenecks on reference sites also increased suggesting that close proximity to farmed sites increases the effect of seeding on numbers of the native species; (3) comparison of percent similarity of the population structure as determined by size class distribution for the Manila versus the littleneck clam indicated that intertidal reference sites in close proximity to farmed sites tended to be more similar to each other with respect to similar Manila population structure versus that of the littleneck (r?=?0.4; p?=?0.08); (4) within the region where active farming is the most aggressive (Baynes Sound), Manila clams are the dominant bivalve on all sampled intertidal regions; and (5) where the dispersal of “wild” (not-seeded) Manila clam is prevented because of thermal tolerance limits, the littleneck is the dominant bivalve. Ecological consequences of such species replacement on intertidal function are as yet known.     

Factors Controlling Phytoplankton Biomass in a Subtropical Coastal Lagoon: Relative Scales of Influence

Patterns in phytoplankton biomass are essential to understanding estuarine ecosystem structure and function and are the net result of various gain and loss processes. In this study, patterns in phytoplankton biomass were explored in relation to a suite of potentially regulating factors in a well-flushed, subtropical lagoon, the Matanzas River Estuary (MRE) in northeast Florida. We examined temporal variability in water temperature, light availability, nutrient concentrations, phytoplankton productivity, and phytoplankton standing stock over 8 years (2003–2010) and explored relationships among variables through correlation analysis. Laboratory experiments in the spring and summer of 2009 quantified phytoplankton growth rates, nutrient limitation potential, and zooplankton grazing rates. The potential influence of oyster grazing was also examined by scaling up population metrics and filtration rate estimates. Results indicated that phytoplankton biomass in the study area was relatively low mainly due to a combination of low temperature and light availability in the winter and consistent tidal water exchange and bivalve grazing throughout the year. Relatively low levels of phytoplankton standing stock and small inter-annual variability within the MRE reflect a balance between gain and loss processes which provide a degree of resilience of the system to natural and anthropogenic influences.     

Oysters and the Chesapeake Bay ecosystem: A case for exotic species introduction to improve environmental quality?

Restoration of the Chesapeake Bay ecosystem has been a priority for residents and governments of the bay watershed for the past decade. One obstacle in the efforts to “save the bay” has been continuing nutrient enrichment from agricultural and sewer runoff. The attainability of a mandated 40% nutrient reduction goal has yet to be seen. Furthermore, disappearance of certain organisms may have had an adverse effect on the resilience of the ecosystem. The Eastern oyster (Crassostrea virginica), once abundant in Chesapeake Bay, was a vital part of the food web, processing excess phytoplankton and depositing materials on the bottom. Over harvesting and disease have decimated the native oyster population. The introduction of an exotic species, the Japanese oyster (Crassostrea gigas), may be a way to reestablish a robust oyster community in the bay. The literature on the role of bivalve molluscs in estuarine ecosystems shows that they are an essential part of healthy estuaries around the world. A comparison ofC. virginica andC. gigas in terms of temperature and salinity tolerance and resistance to disease shows thatC. virginica is ideally adapted to conditions in Chesapeake Bay, but it is unable to stave off the endemic diseases, whereasC. gigas is adapted to conditions in the lower bay only but is much less susceptible to the same diseases. We conclude that the potential introduction ofC. gigas to Chesapeake Bay would be limited by the Japanese species’ physiological requirements but that the revitalization of a bivalve population is imperative to the restoration of ecosystem function.     

Sources and Exchange of Particulate Organic Matter in an Estuarine Mangrove Ecosystem of Xuan Thuy National Park, Vietnam

The spatio-temporal variations in stable isotope signatures (??¹³C and ??¹⁵N) and C/N ratios of particulate organic matter (POM), and physicochemical parameters in a creek water column were examined in an estuarine mangrove ecosystem of Xuan Thuy National Park, Vietnam. The objective was to examine the factors influencing creek water properties, and the sources and exchange of POM in this important mangrove ecosystem. The diel and seasonal variations in water temperature, flow velocity, pH, dissolved oxygen, and salinity demonstrated that tidal level, season, and biological factors affected the creek water properties. Mangroves had relatively low ??¹⁵N and very low ??¹³C values, with respective average values of 1.5?±?0.9?? and ?28.1?±?1.4??. The low mangrove leaf ??¹⁵N indicated minor anthropogenic nitrogen loading to the mangrove forests. A significant positive correlation between POM?C??¹³C and salinity along the axis of Ba Lat Estuary, Red River, indicated that marine phytoplankton (??¹³C value, ?21.4?±?0.5??) was the predominant source of POM at the estuary mouth. Based on the co-variation of ??¹³C and C/N ratios, marine phytoplankton and mangrove detritus were predominant in POM of major creeks and small creeks, respectively. During the diurnal tidal cycle, the dynamics of POM were affected by sources of organic matter, tidal energy, and seasonal factors. The contribution of mangrove detritus to POM reached a maximum at the low tide and was enhanced during the rainy season, whereas marine phytoplankton contribution was highest at high tide.     

From Headwaters to Coast: Influence of Human Activities on Water Quality of the Potomac River Estuary

The natural aging process of Chesapeake Bay and its tributary estuaries has been accelerated by human activities around the shoreline and within the watershed, increasing sediment and nutrient loads delivered to the bay. Riverine nutrients cause algal growth in the bay leading to reductions in light penetration with consequent declines in sea grass growth, smothering of bottom-dwelling organisms, and decreases in bottom-water dissolved oxygen as algal blooms decay. Historically, bay waters were filtered by oysters, but declines in oyster populations from overfishing and disease have led to higher concentrations of fine-sediment particles and phytoplankton in the water column. Assessments of water and biological resource quality in Chesapeake Bay and tributaries, such as the Potomac River, show a continual degraded state. In this paper, we pay tribute to Owen Bricker’s comprehensive, holistic scientific perspective using an approach that examines the connection between watershed and estuary. We evaluated nitrogen inputs from Potomac River headwaters, nutrient-related conditions within the estuary, and considered the use of shellfish aquaculture as an in-the-water nutrient management measure. Data from headwaters, nontidal, and estuarine portions of the Potomac River watershed and estuary were analyzed to examine the contribution from different parts of the watershed to total nitrogen loads to the estuary. An eutrophication model was applied to these data to evaluate eutrophication status and changes since the early 1990s and for comparison to regional and national conditions. A farm-scale aquaculture model was applied and results scaled to the estuary to determine the potential for shellfish (oyster) aquaculture to mediate eutrophication impacts. Results showed that (1) the contribution to nitrogen loads from headwater streams is small (about 2 %) of total inputs to the Potomac River Estuary; (2) eutrophic conditions in the Potomac River Estuary have improved in the upper estuary since the early 1990s, but have worsened in the lower estuary. The overall system-wide eutrophication impact is high, despite a decrease in nitrogen loads from the upper basin and declining surface water nitrate nitrogen concentrations over that period; (3) eutrophic conditions in the Potomac River Estuary are representative of Chesapeake Bay region and other US estuaries; moderate to high levels of nutrient-related degradation occur in about 65 % of US estuaries, particularly river-dominated low-flow systems such as the Potomac River Estuary; and (4) shellfish (oyster) aquaculture could remove eutrophication impacts directly from the estuary through harvest but should be considered a complement—not a substitute—for land-based measures. The total nitrogen load could be removed if 40 % of the Potomac River Estuary bottom was in shellfish cultivation; a combination of aquaculture and restoration of oyster reefs may provide larger benefits.     

Potential of anthracite,dolomite, limestone and pozzolan as reactive media for de-icing salt removal from road runoff

De-icing salt (NaCl) application is a common practice during winter road maintenance in northern countries, compromising ecosystem services. Ecoengineering facilities, including reactive filter beds, are becoming an effective strategy for road runoff management. Different materials (anthracite coal, dolomite, limestone, and pozzolan) were tested as reactive media of filter beds. Characterization showed that dolomite has the best physical properties (resistance to fragmentation, porosity) for filter bed construction and maintenance, followed by limestone. NaCl removal efficiency was investigated in batch at different concentrations (150–5000 mg L^?1 Cl). Removal efficiency substantially varied depending on the element (Cl or Na), the material, and the added NaCl concentration. At the lowest NaCl concentration, Cl removal was higher onto anthracite (48 %) and dolomite (59 %); but greater Na removal was reached onto limestone (54 %) and pozzolan (67 %). At higher concentrations, Cl removal was similar (anthracite), decreased (dolomite), or increased (limestone, pozzolan); and Na removal increased (anthracite) or decreased (dolomite, limestone, pozzolan). Parallel experiments at 4 °C showed lower NaCl removal, anthracite being the most efficient. Practical applicability was evaluated in columns using synthetic runoff solution (NaCl and metals). NaCl removal efficiency was much lower in column assays with respect to batch. The highest NaCl removal was reached onto dolomite, followed by limestone. Metals were successfully removed, generally remaining over time in a wide range (41–89 % Cd, 78–97 % Ni, 44–88 % Cu, and 3–83 % Zn) depending on the material, being pozzolan the least efficient. Further studies including a combination of materials would be of high interest.     

Predation on soft-shell clams,Mya arenaria,by the common mummichog,Fundulus heteroclitus

Invertebrate predation has been cited as the major factor determining post-settling survival of the soft-shell clam, Mya arenaria. Feeding studies on common mummichogs, Fundulus heteroclitus, in Essex Bay, Massachusetts, from July 1976 to June 1978 showed soft-shell clams to be an important diet item in fish greater than 55 mm TL. Biweekly quantitative seining from 16 April to 24 September 1977 revealed predation peaks in the spring and fall corresponding to March–April and June–July spawning periods for M. arenaria. In spring and fall 1977 and spring 1978, 38.5% of the male and 52.8% of the female mummichogs sampled over 55 mm TL averaged 6.77 and 7.32 clams per stomach respectively. The maximum number of clams per stomach was 49 (108 mm female), and 47 (100 mm male). Quantitative seining at low tide yielded mummichog densities from 0.35 to 6.04 fish/m². Combining mummichog density estimates with soft-shell clam predation data gave a possible consumption of 546,000 M. arenaria per km low tide shoreline per day during peak predation periods. Results indicate mummichog predation may equal or exceed invertebrate predation as a major cause of small (<12 mm) soft-shell clam mortality in the Gulf of Maine.