Gerhardt F. Riedel

Shellfish Face Uncertain Future in High CO2 World: Influence of Acidification on Oyster Larvae Calcification and Growth in Estuaries

Background Human activities have increased atmospheric concentrations of carbon dioxide by 36% during the past 200 years. One third of all anthropogenic CO2 has been absorbed by the oceans, reducing pH by about 0.1 of a unit and significantly altering their carbonate chemistry. There is widespread concern that these changes are altering marine habitats severely, but little or no attention has been given to the biota of estuarine and coastal settings, ecosystems that are less pH buffered because of naturally reduced alkalinity. Methodology/Principal Findings To address CO2-induced changes to estuarine calcification, veliger larvae of two oyster species, the Eastern oyster (Crassostrea virginica), and the Suminoe oyster (Crassostrea ariakensis) were grown in estuarine water under four pCO2 regimes, 280, 380, 560 and 800 µatm, to simulate atmospheric conditions in the pre-industrial era, present, and projected future concentrations in 50 and 100 years respectively. CO2 manipulations were made using an automated negative feedback control system that allowed continuous and precise control over the pCO2 in experimental aquaria. Larval growth was measured using image analysis, and calcification was measured by chemical analysis of calcium in their shells. C. virginica experienced a 16% decrease in shell area and a 42% reduction in calcium content when pre-industrial and end of 21st century pCO2 treatments were compared. C. ariakensis showed no change to either growth or calcification. Both species demonstrated net calcification and growth, even when aragonite was undersaturated, a result that runs counter to previous expectations for invertebrate larvae that produce aragonite shells. Conclusions and Significance Our results suggest that temperate estuarine and coastal ecosystems are vulnerable to the expected changes in water chemistry due to elevated atmospheric CO2 and that biological responses to acidification, especially calcifying biota, will be species-specific and therefore much more variable and complex than reported previously.

Behavior of mercury in the Patuxent River estuary

An overview of a comprehensive study of the behavior and fate of mercury in the estuarine Patuxent River is presented. Total Hg (HgT) and methylmercury (MeHg) exhibited weakly non-conservative behavior in the estuary. Total Hg concentrations ranged from 6 ng L-1 in the upper reaches of the sub-urbanized tidal freshwater river to <0.5 ng L-1 in the mesohaline lower estuary. Filterable (0.2 µm) HgT ranged from 0.2 to 1.5 ng L-1. On average, MeHg accounted for <5% of unfiltered HgT and <2% of filterable HgT. Dissolved gaseous section Hg (DGHg) concentrations were highest (up to 150 pg L-1) in the summer in the mesohaline, but were not well correlated with primary production or chlorophyll a, demonstrating the complex nature of Hg0 formation and cycling in an estuarine environment. Organic matter content appeared to control the HgT content of sediments, while MeHg in sediments was positively correlated with HgT and organic matter, and negatively correlated with sulfide. MeHg in sediments was low (0.1 to 0.5% of HgT). Preliminary findings suggest that net MeHg production within sediments exceeds net accumulation. Although HgT in pore waters increased with increasing sulfide, bulk MeHg concentrations decreased. The concentration of MeHg in sediments was not related to the concentration of HgT in pore waters. These observations support the hypothesis that sulfide affects the speciation and therefore bioavailability of dissolved and/or solid-phase Hg for methylation. Comparison with other ecosystems, and the negative correlation between pore water sulfide and sediment MeHg, suggest that sulfide limits production and accumulation of MeHg in this system.

Assessing selenium cycling and accumulation in aquatic ecosystems

We conducted a joint experimental research and modeling study to develop a methodology for assessing selenium (Se) toxicity in aquatic ecosystems. The first phase of the research focused on Se cycling and accumulation. In the laboratory, we measured the rates and mechanisms of accumulation, transformation, and food web transfer of the various chemical forms of Se that occur in freshwater ecosystems. Analytical developments helped define important Se forms. We investigated lower trophic levels (phytoplankton and bacteria) first before proceeding to experiments for each successive trophic component (invertebrates and fish). The lower trophic levels play critical roles in both the biogeochemical cycling and transfer of Se to upper trophic levels. The experimental research provided the scientific basis and rate parameters for a computer simulation model developed in conjunction with the experiments. The model includes components to predict the biogeochemical cycling of Se in the water column and sediments, as well as the accumulation and transformations that occur as Se moves through the food web. The modeled processes include biological uptake, transformation, excretion, and volatilization; oxidation and reduction reactions; adsorption; detrital cycling and decomposition processes; and various physical transport processes within the water body and between the water column and sediments. When applied to a Se-contaminated system (Hyco Reservoir), the model predicted Se dynamics and speciation consistent with existing measurements, and examined both the long-term fate of Se loadings and the major processes and fluxes driving its biogeochemical cycle.

Trace element transformation during the development of an estuarine algal bloom

Copper and arsenic underwent large changes in chemical form during the development and senescence of natural phytoplankton blooms in the Patuxent River, a subestuary of Chesapeake Bay in Maryland. Arsenate was rapidly reduced to arsenite and methylated species. At a total arsenic concentration of 20 nmol l−1, arsenate reduction rates ranged from 50 amol cell−1 d−1 to >230 amol cell−1 d−1, with the rate and extent of reduction dependent upon the concentration of arsenic, the dominant phytoplankton present, the season, and the degree of decline in phosphorus concentrations during bloom development. In general, the percentage of organically-associated copper was lowest (20–40% of total copper) during periods of rapid cell growth and highest (60–100% of total copper) during periods of cell decline or periods of dominance by red tide-forming dinoflagellates, a pattern associated with periods of high release of organic compounds during either bloom senescence or dense algal blooms. The end result of biological mediation was to increase the proportion of each element present in a less toxic form, thus affecting the potential toxicity to a natural ecosystem.

Elevated arsenic concentrations in bivalves from the southeast coasts of the USA.

Abstract Since 1986, the National Oceanic and Atmospheric Administration (NOAA) National Status and Trends (NS&T) Program, Mussel Watch Project (MWP) has been analyzing contaminants in bivalves (oysters and mussels) collected along the coastal USA. Compared to the rest of the USA, the oysters collected from sites located along the southeastern coasts, from North Carolina to the Florida panhandle, display high concentrations of arsenic (As) in their soft tissues. In this area, As concentrations can be elevated in sediments and in bivalves, although exact spatial correspondence between the two is infrequent. As concentrations in waters and food (plankton and suspended particles) directly surrounding the mollusks collected in winter are not unusually high. Phosphate deposits and soil pesticide residues are the hypothesized main sources of this As, and the enrichment mechanism appears to result from a mixture of processes including atmospheric deposition, river and aquifer inputs, and ocean up-welling. In the southeast oysters, the large bio-accumulation of As may also be affected by the seasonal cycle of adsorption/solubilization of As observed in several estuarine and coastal areas, by local physico-chemical parameters such as temperature, salinity, and the nature of sediments (e.g. high contents in iron, calcium, phosphate, and organic material). Even at these very high concentrations, the As present in the southeastern oysters does not appear to present a health threat to humans or to marine life.

Uptake of selenium by freshwater phytoplankton

The uptake of three forms of Se, selenate, selenite and selenomethionine, was examined in three species of freshwater algae, Anabaenaflos-aquae (Cyanophyceae), Chlamydomonasreinhardtii (Chlorophyceae), and Cyclotellameneghiania (Bacillariophyceae) in a defined medium using radiotracers at Se concentrations representative of contaminated systems. Based on the relative accumulation by live vs. heat-killed cells, and linear accumulation through time, selenate accumulation by all three species appears to be a physiological process. However, selenite accumulation at these concentrations appears to be due largely to sorption rather than active uptake, as shown by rapid initial accumulation and the fact that accumulation by heat-killed cells was nearly equal to that of dead cells. Both selenate and selenite uptake rates increased linearly with concentration over the range of 1 to 50 µg L−1. Selenomethionine uptake is a biological process with saturable uptake kinetics (Ks ranging from about 2 to 30 µg L−1 Se), with much greater uptake rates than the other two forms, and little inactive sorbtion to heat-killed cells.

Pathways of arsenic uptake and incorporation in estuarine phytoplankton and the filter-feeding invertebrates Eurytemora affinis, Balanus improvisus and Crassostrea virginica

Arsenic uptake from water and from phytoplankton was followed in the copepod Eurytemora affinis and the barnacle Balanus improvisus collected from the Patuxent River estuary, Chesapeake Bay, eastern coast of the USA in 1987, and in the oyster Crassostrea virginica obtained from a hatchery on the shore of Chesapeake Bay in 1987. Dissolved arsenic was readily taken up by phytoplankton and by shell material of B. improvisus and C. virginica; however, no dissolved arsenic was incorporated into the invertebrate tissues. When E. affinis, B. improvisus and C. virginica were fed phytoplankton containing elevated arsenic contents, significant arsenic incorporation occurred. Juvenile B. improvisus incorporated relatively more arsenic than adults of all three species. Compared to the 100 to 200% increase in arsenic content by phytoplankton exposed to dissolved arsenic, the 25 to 50% increase in these invertebrate species via trophic transfer is relatively small. Even though the trophic pathway for arsenic transfer is the major one for higher trophic levels within an ecosystem, the potential for direct arsenic impact to trophic levels other than phytoplankton appears to be minimal.

The effect of biological and physical disturbances on the transport of arsenic from contaminated estuarine sediments

From the distribution of dissolved and solid arsenic species in a contaminated estuarine sediment and measured rates of flux of the various arsenic species we propose an empirical model for the cycling of arsenic between sediments and water column. The chemical form of arsenic in the sediment was largely determined by the redox state of the sediment. Arsenite was the dominant dissolved and solid species in the deeper reduced sediment, and arsenate was dominant in the oxidized surface layer. Arsenite in the interstitial water diffused toward the surface layer, where it was mostly oxidized to arsenate prior to leaving the sediments. Some arsenate adsorbed to the surface sediments and produced a surface layer enriched in arsenic. Small concentrations of methyl and dimethyl arsenic were produced in the sediments, and these also diffused into the overlying water. Nereis succinea, a burrowing polychaete, affected distribution and flux of arsenic from the sediments by its production of irrigated burrows. These burrows increased both the effective surface area of the sediment and the diffusion of arsenic by a factor of five. When the relative effects of the activities of Nereis succinea and physical resuspension are compared, results indicate that although physical resuspension can produce large pulses of materials from contaminated sediments, continuous biological activity is likely to be more important in the mobilization of contaminants from sediments in many estuarine environments.

Biogeochemical control on the flux of trace elements from estuarine sediments: effects of seasonal and short-term hypoxia

Abstract A long-term (162-day) study of fluxes of trace elements (Mn, As, Cu, Cd) was conducted with intact sediment cores collected from Baltimore Harbor, MD. Under hypoxic conditions large amounts of Mn initially fluxed out of the sediment; however, the rate of Mn flux diminished substantially over time. No Mn flux was seen under oxic conditions. After an initial ‘pulse’, As flux held steady through the hypoxic period. Under oxic conditions, As flux was very low initially and increased near the end of the experiment, with greater fluxes from formerly hypoxic sediments. Initially, fluxes of Cu and Cd were stimulated by hypoxic conditions; however, after a few days, flux of either was completely inhibited. Fluxes of both Cu and Cd occurred under oxic conditions and after the conclusions of hypoxic periods. At the average flux rates measured under oxic conditions, benthic fluxes of Cu and Cd were comparable to point sources, and storm-water runoff inputs to Baltimore Harbor, and significantly greater than atmospheric inputs. Benthic fluxes of As were estimated to be less than storm-water runoff, but considerably higher than point sources or other inputs. ©

The annual cycle of arsenic in a temperate estuary

Dissolved concentrations of four forms of arsenic: arsenite, arsenate, monomethylarsenic, and dimethylarsenic, were measured in the Patuxent River Estuary near Benedict, Maryland, over two annual cycles. In each year, total arsenic concentrations peaked in the summer, in late July and August, while minimum concentrations occurred in winter. Except in late winter, aresenate was a predominant form of arsenic present. In late winter and spring, dimethylarsenic was also a predominant form. A period of monomethylarsenic abundance occurred in summer, following the predominance of dimethylarsenic. Arsenite occurred irregularly in spring. Concurrent temperature and salinity measurements in indicate that total arsenic concentrations rose before the summer increase in salinity, suggesting an arsenic source other than the end members, the Patuxent River or Chesapeake Bay.