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We examined the net exchange of total mercury (THg) and methylmercury (MeHg) between a tidal marsh and its adjacent estuary over a 1-year period from August 2007 to July 2008. Our objectives were to estimate the importance of tidal salt marshes as sources and sinks of mercury within the Chesapeake Bay system, and to examine the hydrologic and biogeochemical controls on mercury fate and transport in tidal marshes. Tidal flows and water chemistry were measured at an established tidal flume at the mouth of the principal tidal creek of a 3-ha marsh section at the Smithsonian Environmental Research Center. Fluxes were estimated by combining continuous tidal flow measurement for the entire study year, with discrete, hourly, flow-weighted measurements of filterable and particulate THg and MeHg, dissolved organic carbon (DOC), and suspended particulate matter (SPM) made over 20 tidal cycles during the year. We found that the marsh was a relatively small net tidal source of MeHg, mainly during the warmer growing season. We also confirmed that the marsh was a substantial source of DOC to the adjacent estuary. DOC was a significant predictor of both filterable THg and MeHg fluxes. However, although the marsh was a source of filterable THg, it was overall a net sink for THg because of particulate trapping. The net per-area annual flux of MeHg from tidal marshes is greater than other MeHg pathways within Chesapeake Bay. The annual load of MeHg from tidal marshes into Chesapeake Bay, however, is likely small relative to fluvial fluxes and efflux from bottom sediment. This study suggests that MeHg production within the tidal marsh has greater consequences for biota inhabiting the marsh than for the efflux of MeHg from the marsh. [ABSTRACT FROM AUTHOR]

The article discusses the development of statistical models that portend stream nitrate concentration from watershed land cover in Chesapeake Bay, Virginia in 2011. The buffer model was analyzed to measure differences within physiographic provinces in the potentials for nitrate removal in buffers and nitrate loss from croplands. Model predictions for the watershed study offered approximates of nitrate removals attained with the existing buffer and cropland distributions.

We estimated net anthropogenic phosphorus inputs (NAPI) in the Chesapeake Bay region. NAPI is an index of phosphorus pollution potential. NAPI was estimated by quantifying all phosphorus inputs and outputs for each county. Inputs include fertilizer applications and non-food phosphorus uses, while trade of food and feed can be an input or an output. The average of 1987, 1992, 1997, and 2002 NAPI for individual counties ranged from 0.02 to 78.46 kg P ha−1 year−1. The overall area-weighted average NAPI for 266 counties in the region was 4.52 kg P ha−1 year−1, indicating a positive net phosphorus input that can accumulate in the landscape or can pollute the water. Large positive NAPI values were associated with agricultural and developed land cover. County area-weighted NAPI increased from 4.43 to 4.94 kg P ha−1 year−1 between 1987 and 1997 but decreased slightly to 4.86 kg P ha−1 year−1 by 2002. Human population density, livestock unit density, and percent row crop land combined to explain 83% of the variability in NAPI among counties. Around 10% of total NAPI entering the Chesapeake Bay watershed is discharged into Chesapeake Bay. The developed land component of NAPI had a strong direct correlation with measured phosphorus discharges from major rivers draining to the Bay ( R 2 = 0.81), however, the correlation with the simple percentage of developed land was equally strong. Our results help identify the sources of P in the landscape and evaluate the utility of NAPI as a predictor of water quality. [ABSTRACT FROM AUTHOR]

Efforts to quantify the effects of riparian buffers on watershed nutrient discharges have been confounded by a commonly used analysis, which estimates buffer potential as the percentage of forest or wetland within a fixed distance of streams. Effective landscape metrics must instead be developed based on a clear conceptual model and quantified at the appropriate spatial scale. We develop new metrics for riparian buffers in two stages of increasing functional specificity to ask: (1) Which riparian metrics are more distinct from measures of whole watershed land cover? (2) Do functional riparian metrics provide different information than fixed-distance metrics? (3) How do these patterns vary within and among different physiographic settings? Using publicly available geographic data, we studied 503 watersheds in four different physiographic provinces of the Chesapeake Bay Drainage. In addition to traditional fixed-distance measures, we calculated mean buffer width, gap frequency, and measures of variation in buffer width using both ‘unconstrained2019; metrics and 2018;flow-path2019; metrics constrained by surface topography. There were distinct patterns of relationship between watershed and near-stream land cover in each physiographic province and strong correlations with watershed land cover confounded fixed-distance metrics. Flow-path metrics were more independent of watershed land cover than either fixed-distance or unconstrained measures, but both functional metrics provided greater detail, interpretability, and flexibility than the fixed-distance approach. Potential applications of the new metrics include exploring the potential for land cover patterns to influence water quality, accounting for buffers in statistical nutrient models, quantifying spatial patterns for process-based modeling, and targeting management actions such as buffer restoration. [ABSTRACT FROM AUTHOR]