James G. Sanders

EFFECTS OF ARSENIC SPECIATION AND PHOSPHATE CONCENTRATION ON ARSENIC INHIBITION OF SKELETONEMA COSTATUM (BACILLARIOPHYCEAE)

Arsenate is taken up readily by Skeletonema costatum (Greville) Cleve due to its chemical similarity to phosphate, and it inhibits primary productivity at concentrations as low as 67 nM when the phosphate concentration is low. A phosphate enrichment of greater than 0.3 μM alleviates this inhibition; however, the arsenate stress causes an increase in the cells requirement for phosphorus. Arsenite is also toxic to Skeletonema at similar concentrations. Methylated species, such as dimethylarsinic acid, did not affect cell productivity at the levels examined. Thus, the reduction and methylation of arsenate to dimethylarsinic acid by the cell produces a stable, non‐toxic compound.

Arsenic geochemistry in Chesapeake Bay: Dependence upon anthropogenic inputs and phytoplankton species composition

Arsenic is not conservative in Chesapeake Bay. Inputs of man-derived arsenic, of the order of 100 kg d−1, cause substantial positive deviation from theoretical dilution. The chemical form of the arsenic varies both seasonally and along the axis of the bay. During winter, arsenic is present only as arsenate. During summer, substantial quantities of reduced and methylated forms are present in different areas, indicative of separate formation processes. Arsenite, present in low-salinity regions, may have been formed by chemical reduction in anoxic, subsurface waters and then mixed into the surface layer. Methylated arsenicals correlate highly with algal standing stocks. One particular form, methylarsonate, is highly correlated with the dominant algal genus, Chroomonas. As both arsenic reactivity and toxicity are altered by transformation of chemical form, the observed variations in arsenic speciation have considerable geochemical and ecological significance.

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.

Microbial role in the demethylation and oxidation of methylated arsenicals in seawater

Abstract Mixed bacterial cultures isolated from an estuarine system are able to demethylate and oxidize dime thy larsinic acid to arsenate. The measured oxidation rate averaged approximately 1 ng DMA·l −1 ·day −1 , which is similar to the rate of DMA production due to algal reduction and methylation in marine systems. Bacteria, therefore, may be a prime mediator in the cycling of arsenic in surface waters.

A watershed perspective on nutrient enrichment, science, and policy in the Patuxent River, Maryland: 1960–2000

The Patuxent River, Maryland, is a nutrient-overenriched tributary of the Chesapeake Bay. Nutrient inputs from sewage outfalls and nonpoint sources (NPS) have grown substantially during the last four decades, and chlorophylla levels have increased markedly with concomitant reductions in water quality and dissolved oxygen concentrations. The Patuxent has gained national attention because it was one of the first river basins in the U.S. for which basin-wide nutrient control standards were developed. These included a reduction in NPS inputs and a limit on both nitrogen (N) and phosphorus (P) loadings in sewage discharges intended to return the river to 1950s conditions. Full implementation of point source controls occurred by 1994, but population growth and land-use changes continue to increase total nutrient loadings to the river. The present paper provides the perspectives of scientists who participated in studies of the Patuxent River and its estuary over the last three decades, and who interacted with policy makers as decisions were made to develop a dual nutrient control strategy. Although nutrient control measures have not yet resulted in dramatic increases in water quality, we believe that without them, more extensive declines in water quality would have occurred. Future reductions will have to come from more effective NPS controls since future point source loading will be difficult to further reduce with present technology. Changing land use will present a challenge to policy makers faced with sprawling population growth and accelerated deforestation.

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.

Nutrient Enrichment Drives Gulf of Mexico Hypoxia

During most summers over the past 30 years, bottom dissolved oxygen across a large area of the Louisiana and upper Texas continental shelf declined to concentrations too low (hypoxia) for most fish and large invertebrate animals to survive. This area is one of the best known “dead zones” proliferating around the world [Diaz and Rosenberg, 2008]. During July 2008, hypoxic bottom waters extended across 20,720 square kilometers (Figure 1), but they were probably even more extensive because winds from Hurricane Dolly mixed the waters off Texas before the survey could be completed.

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. ©

Temporal and spatial patterns of trace elements in the Patuxent River: A whole watershed approach

Trace element distributions, partitioning, and speciation were examined at 15 sites in the Patuxent River watershed from May 1995 through October 1997 to determine possible sources of trace elements to the river and estuary, to examine the relationship of the trace element discharges to freshwater discharges as well as to land use and geographic region, to validate previous estimates of loadings to the river, and to provide baseline data for trace elements in the Patuxent River watershed and estuary. Six freshwater sites were examined, representing different basins and geographic provinces, and nine sites along the estuarine salinity gradient. Subregions within the watershed varied considerably in concentrations and areal yields for some elements. Concentrations of As, Cd, Ni, Pb, and Zn were elevated in the Coastal Plain sites compared to the Piedmont sites, while Cu and Hg were more evenly distributed. Cadmium, Cu, Hg, Ni, Pb, and Zn showed overall positive correlations with river flow while As and methylHg (meHg) showed negative correlations with river flow. Concentrations of trace elements in the estuarine portion of the river were generally low, and consistent with mixing between Patuxent River water with elevated concentrations and the lower concentrations of the Chesapeake Bay. Interesting features included a local Cd maximum in the low salinity region of the estuary, probably caused by desorption from suspended sediments, and a significant input of water containing high As concentrations from the Chesapeake Bay and from As being released from bottom sediments in summer. Comparisons between the estimated annual flux of trace elements and the estimates of suspected source terms (atmospheric deposition, urban runoff, and known point sources) suggest that, except for Hg, direct atmospheric deposition is small compared to fluvial loads. Current estimates of trace element inputs from point sources or from urban runoff are inadequate for comparison with other sources, because of inappropriate techniques and/or unacceptably high detection limits. A complete examination of trace element dynamics in the Patuxent River (and in other coastal systems) will require better data for these potential sources.