John D. Wang

The fate of river discharge on the continental shelf: 1. Modeling the river plume and the inner shelf coastal current

We study the development and evolution of buoyant river plumes on the continental shelf. Our calculations are based on three-dimensional numerical simulations, where the river runoff is introduced as a volume of zero salinity water in the continuity equation and mixing is provided by the models turbulence closure scheme and wind forcing. In the absence of wind forcing, the modeled river plumes typically consist of an offshore bulge and a coastal current in the direction of Kelvin wave propagation. We propose a plume classification scheme based on a bulk Richardson number, which expresses the relative magnitude of the buoyancy-induced stratification versus the available mixing. When the ratio of the discharge and shear velocities is greater (less) than 1, the plume is categorized as supercritical (subcritical); that is, the width of the bulge is greater (less) than the width of the coastal current. Supercritical plumes are often characterized by a meandering pattern along the coastal current, caused by a baroclinic instability process. For a given discharge, subcritical plumes are produced by large mixing and/or shallow water depths. In the presence of wind forcing, the favorable conditions for offshore removal of coastal low-salinity waters include high river runoff and strong upwelling-favorable wind stress. When the rivers are treated as individual sources of freshwater (“point source” behavior), the wind-driven flow may exhibit substantial spatial variability. Under the above removal conditions, strong offshore transport takes place in “jetlike” flow regions within the river plume, in contrast to the downwind acceleration of adjacent waters. When the rivers are treated as a long “line source” of freshwater, the plume region resembles a coastal low-salinity band, and the above removal conditions trigger offshore transport that is most pronounced at the “head” of the source.

Presence of Pathogens and Indicator Microbes at a Non-Point Source Subtropical Recreational Marine Beach

ABSTRACT Swimming in ocean water, including ocean water at beaches not impacted by known point sources of pollution, is an increasing health concern. This study was an initial evaluation of the presence of indicator microbes and pathogens and the association among the indicator microbes, pathogens, and environmental conditions at a subtropical, recreational marine beach in south Florida impacted by non-point sources of pollution. Twelve water and eight sand samples were collected during four sampling events at high or low tide under elevated or reduced solar insolation conditions. The analyses performed included analyses of fecal indicator bacteria (FIB) (fecal coliforms, Escherichia coli, enterococci, and Clostridium perfringens), human-associated microbial source tracking (MST) markers (human polyomaviruses [HPyVs] and Enterococcus faecium esp gene), and pathogens (Vibrio vulnificus, Staphylococcus aureus, enterovirus, norovirus, hepatitis A virus, Cryptosporidium spp., and Giardia spp.). The enterococcus concentrations in water and sand determined by quantitative PCR were greater than the concentrations determined by membrane filtration measurement. The FIB concentrations in water were below the recreational water quality standards for three of the four sampling events, when pathogens and MST markers were also generally undetectable. The FIB levels exceeded regulatory guidelines during one event, and this was accompanied by detection of HPyVs and pathogens, including detection of the autochthonous bacterium V. vulnificus in sand and water, detection of the allochthonous protozoans Giardia spp. in water, and detection of Cryptosporidium spp. in sand samples. The elevated microbial levels were detected at high tide and under low-solar-insolation conditions. Additional sampling should be conducted to further explore the relationships between tidal and solar insolation conditions and between indicator microbes and pathogens in subtropical recreational marine waters impacted by non-point source pollution.

The fate of river discharge on the continental shelf: 2. Transport of coastal low‐salinity waters under realistic wind and tidal forcing

A three-dimensional numerical simulation of shelf circulation is presented. We employ realistic forcing for the Southeast U.S. Continental Shelf during the spring season. We show that the strongest offshore transport of river-induced, coastal, low-salinity waters and associated materials occurs near the surface. The preferred mean pathway is in the northeastward direction, and it takes about 2 months to cross the entire shelf. Owing to the mean direction of surface transport and the topography of the South Atlantic Bight shelf, the preferred location for springtime removal is off Charleston, South Carolina, and presumably in the vicinity of the Charleston Bump. The transport and fate of the river-induced, coastal, low-salinity waters during the spring season are determined by (1) the stratification of nearshore waters, which is due to the high river runoff and causes the decoupling between “near-surface” and “near-bottom” layers; (2) the prevailing northeastward winds, which cause significant offshore transport within the shallow near-surface Ekman layer; and (3) the tidally induced bottom stirring (M2 tides). Comparison of model and data time series of currents shows very good agreement. Standard deviations of the model and data-computed empirical orthogonal functions are almost identical, while the respective variance-conserving spectra agree both in amplitude and phase.

The BEACHES Study: health effects and exposures from non-point source microbial contaminants in subtropical recreational marine waters

BACKGROUND Microbial water-quality indicators, in high concentrations in sewage, are used to determine whether water is safe for recreational purposes. Recently, the use of these indicators to regulate recreational water bodies, particularly in sub/tropical recreational marine waters without known sources of sewage, has been questioned. The objectives of this study were to evaluate the risk to humans from exposure to subtropical recreational marine waters with no known point source, and the possible relationship between microbe densities and reported symptoms in human subjects with random-exposure assignment and intensive individual microbial monitoring in this environment. METHODS A total of 1303 adult regular bathers were randomly assigned to bather and non-bather groups, with subsequent follow-up for reported illness, in conjunction with extensive environmental sampling of indicator organisms (enterococci). RESULTS Bathers were 1.76 times more likely to report gastrointestinal illness [95% confidence interval (CI) 0.94-3.30; P = 0.07]; 4.46 times more likely to report acute febrile respiratory illness (95% CI 0.99-20.90; P = 0.051) and 5.91 times more likely to report a skin illness (95% CI 2.76-12.63; P < 0.0001) relative to non-bathers. Evidence of a dose-response relationship was found between skin illnesses and increasing enterococci exposure among bathers [1.46 times (95% CI 0.97-2.21; P = 0.07) per increasing log(10) unit of enterococci exposure], but not for gastrointestinal or respiratory illnesses. CONCLUSIONS This study indicated that bathers may be at increased risk of several illnesses relative to non-bathers, even in the absence of any known source of domestic sewage impacting the recreational marine waters. There was no dose-response relationship between gastroenteritis and increasing exposure to enterococci, even though many current water-monitoring standards use gastroenteritis as the major outcome illness.

A water quality modeling study of non-point sources at recreational marine beaches

A model study was conducted to understand the influence of non-point sources including bather shedding, animal fecal sources, and near shore sand, as well as the impact of the environmental conditions, on the fate and transport of the indicator microbe, enterococci, at a subtropical recreational marine beach in South Florida. The model was based on an existing finite element hydrodynamic and transport model, with the addition of a first order microbe deactivation function due to solar radiation. Results showed that dog fecal events had a major transient impact (hundreds of Colony Forming Units/100 ml [CFU/100 ml]) on the enterococci concentration in a limited area within several hours, and could partially explain the high concentrations observed at the study beach. Enterococci released from beach sand during high tide caused mildly elevated concentration for a short period of time (ten to twenty of CFU/100 ml initially, reduced to 2 CFU/100 ml within 4 h during sunny weather) similar to the average baseline numbers observed at the beach. Bather shedding resulted in minimal impacts (less than 1 CFU/100 ml), even during crowded holiday weekends. In addition, weak current velocity near the beach shoreline was found to cause longer dwelling times for the elevated concentrations of enterococci, while solar deactivation was found to be a strong factor in reducing these microbial concentrations.

Circulation On the Continental Shelf of the Southeastern United States. Part I: Subtidal Response to Wind and Gulf Stream Forcing During Winter

Abstract Subtidal current and sea level response to wind and Gulf Stream forcing are investigated for the South Atlantic Bight shelf during winter conditions. Low-frequency flow variability in the outer shelf results primarily from wavelike meanders and eddies in the Gulf Stream front that occur in a 2-day to 2-week period band. Current meter derived vertically integrated momentum balances indicated that these large amplitude flow events are in approximate geostrophic balance with baroclinic pressure gradients induced by northward propagating Gulf Stream disturbances. Low-frequency flow at midshelf is primarily a local Ekman response to wind forcing. Cross-shelf momentum balance for the total water column is between the along-shelf geostrophic current and the cross-shelf barotropic pressure gradient resulting from wind induced sea level changes at the coast. This balance holds for both mean and fluctuating parts of the flow, with the along-shelf barotropic current lagging sea level by 6 to 12 hours and al...

Evaluation of conventional and alternative monitoring methods for a recreational marine beach with nonpoint source of fecal contamination.

The objectives of this work were to compare enterococci (ENT) measurements based on the membrane filter, ENT(MF) with alternatives that can provide faster results including alternative enterococci methods (e.g., chromogenic substrate (CS), and quantitative polymerase chain reaction (qPCR)), and results from regression models based upon environmental parameters that can be measured in real-time. ENT(MF) were also compared to source tracking markers (Staphylococcus aureus, Bacteroidales human and dog markers, and Catellicoccus gull marker) in an effort to interpret the variability of the signal. Results showed that concentrations of enterococci based upon MF (<2 to 3320 CFU/100 mL) were significantly different from the CS and qPCR methods (p < 0.01). The correlations between MF and CS (r = 0.58, p < 0.01) were stronger than between MF and qPCR (r ≤ 0.36, p < 0.01). Enterococci levels by MF, CS, and qPCR methods were positively correlated with turbidity and tidal height. Enterococci by MF and CS were also inversely correlated with solar radiation but enterococci by qPCR was not. The regression model based on environmental variables provided fair qualitative predictions of enterococci by MF in real-time, for daily geometric mean levels, but not for individual samples. Overall, ENT(MF) was not significantly correlated with source tracking markers with the exception of samples collected during one storm event. The inability of the regression model to predict ENT(MF) levels for individual samples is likely due to the different sources of ENT impacting the beach at any given time, making it particularly difficult to to predict short-term variability of ENT(MF) for environmental parameters.

Estimation of enterococci input from bathers and animals on a recreational beach using camera images.

Enterococci, are used nationwide as a water quality indicator of marine recreational beaches. Prior research has demonstrated that enterococci inputs to the study beach site (located in Miami, FL) are dominated by non-point sources (including humans and animals). We have estimated their respective source functions by developing a counting methodology for individuals to better understand their non-point source load impacts. The method utilizes camera images of the beach taken at regular time intervals to determine the number of people and animal visitors. The developed method translates raw image counts for week days and weekend days into daily and monthly visitation rates. Enterococci source functions were computed from the observed number of unique individuals for average days of each month of the year, and from average load contributions for humans and for animals. Results indicate that dogs represent the larger source of enterococci relative to humans and birds.

Pore water transport of enterococci out of beach sediments.

Enterococci are used to evaluate the safety of beach waters and studies have identified beach sands as a source of these bacteria. In order to study and quantify the release of microbes from beach sediments, flow column systems were built to evaluate flow of pore water out of beach sediments. Results show a peak in enterococci (average of 10% of the total microbes in core) released from the sand core within one pore water volume followed by a marked decline to below detection. These results indicate that few enterococci are easily removed and that factors other than simple pore water flow control the release of the majority of enterococci within beach sediments. A significantly larger quantity and release of enterococci were observed in cores collected after a significant rain event suggesting the influx of fresh water can alter the release pattern as compared to cores with no antecedent rainfall.

Environmental controls, oceanography and population dynamics of pathogens and harmful algal blooms: connecting sources to human exposure

Coupled physical-biological models are capable of linking the complex interactions between environmental factors and physical hydrodynamics to simulate the growth, toxicity and transport of infectious pathogens and harmful algal blooms (HABs). Such simulations can be used to assess and predict the impact of pathogens and HABs on human health. Given the widespread and increasing reliance of coastal communities on aquatic systems for drinking water, seafood and recreation, such predictions are critical for making informed resource management decisions. Here we identify three challenges to making this connection between pathogens/HABs and human health: predicting concentrations and toxicity; identifying the spatial and temporal scales of population and ecosystem interactions; and applying the understanding of population dynamics of pathogens/HABs to management strategies. We elaborate on the need to meet each of these challenges, describe how modeling approaches can be used and discuss strategies for moving forward in addressing these challenges.