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The San Francisco Bay (California, USA) is a water body listed as impaired because of Hg contamination in sport fish for human consumption, as well as possible effects on resident wildlife. A legacy of Hg mining in local watersheds and Hg used in Au mining in the Sierra Nevada (USA) has contributed to contamination seen in the bay, with additional more recent and ongoing inputs from various sources. Methylmercury is the species of Hg most directly responsible for contamination in biota, so better understanding of its sources, loads, and processes was sought to identify the best means to reduce impacts. A regional scale model of San Francisco Bay was developed to characterize major methylmercury inputs and processes. The model was used to evaluate the potential impact of uncertainties in estimates for methylmercury loading pathways and environmental processes, identify major data gaps, and explore management prospects for reducing methylmercury contamination. External loading pathways considered in the mass balance include methylmercury loads entering via atmospheric deposition to the bay surface, and discharges from the Sacramento/San Joaquin Delta, local watersheds, municipal wastewater, and fringing wetlands. Internal processes considered include exchange between bed and suspended sediments and the water column, in situ production and demethylation, biological uptake, and losses via hydrologic transport to the ocean through the Golden Gate. In situ sediment methylation and demethylation were dominant sources and losses determining ambient steady-state concentrations in the model, with changes in external loads and export causing smaller changes. Better information on methylation and demethylation is thus most critical to improving understanding of methylmercury balances and management. Environ. Toxicol. Chem. 2011;30:88-96. © 2010 SETAC [ABSTRACT FROM AUTHOR]

Nitrogen and phosphorus budgets were developed for four sub-catchments in the Richmond River catchment for two study years. The catchmentis used for a variety of farming pursuits including dairying, beef, cropping, fruit, nuts, forestry, and sugar cane. Each subcatchment varies in hydrology, the proportion of each land use, and the population density which enabled a unique opportunity to study fluxes and storage associated with a variety of environmental factors. Total loadings entering each sub-catchment varied from 12 to 57 kg ha-1yr-1 for nitrogen and 0.25 to 6.6 kg ha-1 yr-1 for phosphorus with little inter-annual variation. Averaged across the whole catchment, nitrogen fixation (47%) dominated the inputs; fertiliser (26%) and rainfall (21%) made up the next largest inputs. Fertiliser inputs dominated the phosphorus budget (65.5%); rainfall and manures making up 13% and 12% respectively. Produce dominated the outputs of both nitrogen and phosphorus from the four sub-catchments being greater than the riverine export. The delivery of nitrogen to catchment streams ranged from <1 to 24% of the total inputs and the delivery of phosphorus to catchment streams ranged from <1 to 39%. Storage of phosphorus in catchment soils varied between -0.32 and4.46 kg ha-1 yr-1. When denitrification and volatilisation were estimated using data from other studies, storage ofnitrogen ranged from 1 to 24 kg ha-1 yr-1. Despite the episodic nature of runoff in the sub-tropical Richmond Rivercatchment, the magnitude of nutrient fluxes and storage appear similar to other catchments of the world which have mixed land use and relatively low catchment nutrient loadings. [ABSTRACT FROM AUTHOR]

Nitrogen and phosphorus loads in the sub-tropical Richmond River estuary were quantified and material budgets were developed over two years of contrasting freshwater discharge. During both years >74% of thenitrogen and >84% of the phosphorus load entered the estuary during one month when flooding occurred in the catchment. Due to larger flood magnitude, loads during the 1995/96 year were 3.3 and 2.5 times greater than during the 1994/95 year for nitrogen and phosphorus respectively. During floods the estuarine basin was completely flushed of brackish water and the majority of the nutrient loads passed directly through the estuary. The nutrient load retained in the estuary during floods was inversely proportional to flood magnitude. Annual budgets show that >97% of the nutrient load entering the estuary was from diffuse catchment sources; precipitation, urban runoff, and sewage were negligible. Less than 2.5% of the nitrogen and <5.4% of the phosphorus loads entering the estuary were retained in sediments. During dry seasons the estuary became a net sink for nitrogen input from the ocean and the estuarine sediments remained a net source of phosphorus to the water column and ocean. The process of flood scouring is likely to be the cleansing mechanism responsible for maintaining water quality both on an annual basis and over the last 50 years and may also be responsible for potential nitrogen limitation. The sub-tropical Richmond River estuary contrasts with the majority of temperate systems ofNorth America and Europe which typically have lower inter- and intra-annual nutrient load variability, longer and less variable flushing times, and greater nutrient retention. [ABSTRACT FROM AUTHOR]