Geology topics

C.C. Gilmour

Publications and source records attributed to C.C. Gilmour.

6 recordsLinked to original sources

USGS2011

Dissolved organic matter in the Florida everglades: Implications for ecosystem restoration

Dissolved organic matter (DOM) in the Florida Everglades controls a number of environmental processes important for ecosystem function including the absorption of light, mineral dissolution/precipitation, transport of hydrophobic compounds (e.g., pesticides), and the transport and reactivity of metals, such as mercury. Proposed attempts to return the Everglades to more natural flow conditions will result in changes to the present transport of DOM from the Everglades Agricultural Area and the northern conservation areas to Florida Bay. In part, the restoration plan calls for increasing water flow throughout the Everglades by removing some of the manmade barriers to flow in place today. The land- and water-use practices associated with the plan will likely result in changes in the quality, quantity, and reactivity of DOM throughout the greater Everglades ecosystem. The authors discuss the factors controlling DOM concentrations and chemistry, present distribution of DOM throughout the Everglades, the potential effects of DOM on key water-quality issues, and the potential utility of dissolved organic matter as an indicator of success of restoration efforts.

Florida↗

USGS2007

Whole-ecosystem study shows rapid fish-mercury response to changes in mercury deposition

Methylmercury contamination of fisheries from centuries of industrial atmospheric emissions negatively impacts humans and wild-life worldwide. The response of fish methylmercury concentrations to changes in mercury deposition has been difficult to establish because sediments/soils contain large pools of historical contamination, and many factors in addition to deposition affect fish mercury. To test directly the response of fish contamination to changing mercury deposition, we conducted a whole-ecosystem experiment, increasing the mercury load to a lake and its watershed by the addition of enriched stable mercury isotopes. The isotopes allowed us to distinguish between experimentally applied mercury and mercury already present in the ecosystem and to examine bioaccumulation of mercury deposited to different parts of the watershed. Fish methylmercury concentrations responded rapidly to changes in mercury deposition over the first 3 years of study. Essentially all of the increase in fish methylmercury concentrations came from mercury deposited directly to the lake surface. In contrast, <1% of the mercury isotope deposited to the watershed was exported to the lake. Steady state was not reached within 3 years. Lake mercury isotope concentrations were still rising in lake biota, and watershed mercury isotope exports to the lake were increasing slowly. Therefore, we predict that mercury emissions reductions will yield rapid (years) reductions in fish methylmercury concentrations and will yield concomitant reductions in risk. However, a full response will be delayed by the gradual export of mercury stored in watersheds. The rate of response will vary among lakes depending on the relative surface areas of water and watershed.

Proceedings of the National Academy of Sciences of↗

USGS2001

Constants for mercury binding by organic matter isolates from the Florida Everglades

Dissolved organic matter (DOM) has been implicated as an important complexing agent for Hg that can affect its mobility and bioavailability in aquatic ecosystems. However, binding constants for natural Hg-DOM complexes are not well known. We employed a competitive ligand approach to estimate conditional stability constants for Hg complexes with DOM isolates collected from Florida Everglades surface waters. The isolates examined were the hydrophobic fraction of DOM from a eutrophic, sulfidic site (F1-HPoA) and the hydrophilic fraction from an oligotrophic, low-sulfide site (2BS-HPiA). Our experimental determinations utilized overall octanol-water partitioning coefficients (D ow ) for 203 Hg at 0.01 M chloride and across pH and DOM concentration gradients. Use of this radioisotope allowed rapid determinations of Hg concentrations in both water and octanol phases without problems of matrix interference. Conditional stability constants (I = 0.06, 23°C) were log K′ = 11.8 for F1-HPoA and log K′ = 10.6 for 2BS-HPiA. These are similar to previously published stability constants for Hg binding to low-molecular-weight thiols. Further, F1-HPoA showed a pH-dependent decline in D ow that was consistent with models of Hg complexation with thiol groups as the dominant Hg binding sites in DOM. These experiments demonstrate that the DOM isolates are stronger ligands for Hg than chloride ion or ethylenediamine-tetraacetic acid. Speciation calculations indicate that at the DOM concentrations frequently measured in Everglades, 20 to 40 μM, significant complexation of Hg by DOM would be expected in aerobic (sulfide-free) surface waters.

Florida↗

USGS2000

Methyl-mercury degradation pathways: A comparison among three mercury impacted ecosystems

We examined microbial methylmercury (MeHg) degradation in sediment of the Florida Everglades, Carson River (NV), and San Carlos Creek (CA), three freshwater environments that differ in the extent and type of mercury contamination and sediment biogeochemistry. Degradation rate constant ( k deg ) values increased with total mercury (Hg t ) contamination both among and within ecosystems. The highest k deg 's (2.8−5.8 d - 1 ) were observed in San Carlos Creek, at acid mine drainage impacted sites immediately downstream of the former New Idria mercury mine, where Hg t ranged from 4.5 to 21.3 ppm (dry wt). A reductive degradation pathway (presumably mer -detoxification) dominated degradation at these sites, as indicated by the nearly exclusive production of 14 CH 4 from 14 C-MeHg, under both aerobic and anaerobic conditions. At the upstream control site, and in the less contaminated ecosystems (e.g. the Everglades), k deg 's were low (≤0.2 d - 1 ) and oxidative demethylation (OD) dominated degradation, as evident from 14 CO 2 production. k deg increased with microbial CH 4 production, organic content, and reduced sulfur in the Carson River system and increased with decreasing pH in San Carlos Creek. OD associated CO 2 production increased with pore-water SO 4 2- in Everglades samples but was not attributable to anaerobic methane oxidation, as has been previously proposed. This ecosystem comparison indicates that severely contaminated sediments tend to have microbial populations that actively degrade MeHg via mer -detoxification, whereas OD occurs in heavily contaminated sediments as well but dominates in those less contaminated.

Environmental Science & Technology↗

USGS1999

Mercury methylation in periphyton of the Florida Everglades

Trophic accumulation of mercury (Hg) in aquatic ecosystems is of global concern due to health effects associated with eating fish with elevated Hg levels. The methylated form of Hg bioaccumulates so it is important to understand how inorganic Hg is transformed to methylmercury in the environment. Here, a new site for Hg methylation, the periphyton communities that are prevalent in the Florida Everglades, is described. It is hypothesized that periphyton communities that support an active microbial sulfur cycle support Hg methylation. This new methylation site has implications for trophic transfer of methylmercury since periphyton can be the base of the food web in aquatic ecosystems.

Limnology and Oceanography↗

USGS1998

Methyl mercury dynamics in littoral sediments of a temperate seepage lake

The sites and rates of methyl mercury (MeHg) production and transport in littoral zone sediments were investigated at Pallette Lake in northern Wisconsin. In littoral areas where groundwater inflow occurs, sulfate supply from groundwater creates profiles of electron acceptors (sulfate) and donors (methane, sulfide) that are reversed from those found in sediments whose sulfate supply is delivered from overlying water. The highest MeHg concentrations in porewaters and the maximal advective MeHg flux rates (4.5-61.7 ng ·m -2 ·day -1 ) were observed in the spring, while highest bulk phase concentrations occur later in the summer. These estimated MeHg fluxes are greater than the mean areal production rates estimated previously for the water column and are similar to the atmospheric flux. Gross MeHg production was measured using the addition of 203 Hg as a tracer to sediments. The depth at which maximal 203 Hg methylation occurred coincided with the observed maximums in solid-phase and porewater MeHg concentrations. Because input, advection, and accumulation of MeHg in these sediments were measured directly, an independent estimate of MeHg production could be made and compared with 203 Hg-derived rates. This comparison suggests that that the 203 Hg tracer method provides reasonable estimates of gross methylation rates and that a substantial fraction of solid-phase Hg is available for methylation

Canadian Journal of Fisheries and Aquatic Sciences↗