The CE‐QUAL‐ICM (Corps of Engineers Integrated Compartment Water Quality Model) eutrophication model was applied in a 21‐year simulation of Chesapeake Bay water quality, 1985‐2005. The eutrophication ...model is part of a larger model package and is forced, in part, by models of atmospheric deposition, watershed flows and loads, and hydrodynamics. Results from the model are compared with observations in multiple formats including time series plots, cumulative distribution plots, and statistical summaries. The model indicates only one long‐term trend in computed water quality: light attenuation deteriorates circa 1993 through the end of the simulation. The most significant result is the influence of physical processes, notably stratification and associated effects (e.g., anoxic volume), on computed water quality. Within the application period, physical effects are more important determinants of year‐to‐year variability in computed water quality than external loads.
The Conowingo Reservoir is situated on the Susquehanna River, immediately upstream of Chesapeake Bay, the largest estuary in the United States. Sedimentation in the reservoir provides an unintended ...benefit to the bay by preventing sediments, organic matter, and nutrients from entering the bay. The sediment storage capacity of the reservoir is nearly exhausted, however, and the resulting increase in loading of sediments and associated materials is a potential threat to Chesapeake Bay water quality. In response to this threat, the Lower Susquehanna River Watershed Assessment was conducted. The assessment indicates the reservoir is in a state of “dynamic equilibrium” in which sediment loads from the upstream watershed to the reservoir are balanced by sediments leaving the reservoir. Increased sediment loads are not a threat to bay water quality. Increased loads of associated organic matter and nutrients are, however, detrimental. Bottom‐water dissolved oxygen declines of 0.1 to 0.2 g m−3 are projected as a result of organic matter oxidation and enhanced eutrophication. The decline is small relative to normal variations but results in violations of standards enforced in a recently enacted total maximum daily load. Enhanced reductions in nutrient loads from the watershed are recommended to offset the decline in water quality caused by diminished retention in the reservoir. The assessment exposed several knowledge gaps that require additional investigation, including the potential for increased loading at flows below the threshold for reservoir scour and the nature and reactivity of organic matter and nutrients scoured from the reservoir bottom.
Core Ideas
Reservoir sedimentation prevents sediments from entering Chesapeake Bay.
Reservoir sediment storage capacity is nearly exhausted.
Added sediment loads are not a threat to bay water quality.
Associated organic matter and nutrients are detrimental to bay water quality.
Environmental benefits are one of the motivations for management restoration of depleted bivalve populations. We describe a series of linked modules for benefits calculation. The modules include: ...oyster (
Crassostrea virginica
) bioenergetics, materials transport via the tidal prism, and benefits quantification. Quantified benefits include carbon, nitrogen, and phosphorus removal and shell production. The modules are demonstrated through application to the Great Wicomico River, a tributary of Chesapeake Bay, USA. Oysters on seven reefs (total area 2.8 × 10
5
m
2
) are calculated to remove 15.2, 6.2, and 0.2 tons per annum of carbon, nitrogen, and phosphorus, respectively, from the Great Wicomico. Oyster mortality contributes 108 tons per annum dry weight shell to the reefs.
In 2020, the Chesapeake Bay Program moved to offset impacts from climate change for the 30‐year period from 1995 through 2025 by having its seven watershed jurisdictions (Delaware, Maryland, New ...York, Pennsylvania, Virginia, West Virginia, and the District of Columbia) apply additional nutrient pollutant reduction practices. The climate change assessment was performed with integrated models of the Chesapeake watershed, airshed, and estuary. Scenarios run for the years 2025, 2035, 2045, and 2055 estimated effects from the different future climatic conditions. This article presents the results of that assessment and is intended to provide a guide to assist other modeling practitioners in assessing climate change impacts in coastal watersheds. Major influences of climate change that were quantified include increases in precipitation volume, potential evapotranspiration, watershed nutrient loads, tidal water temperature, and sea level. Minor influences quantified in the climate change analysis include changes in nutrient speciation and increases in wet deposition of nitrogen, CO2, rainfall intensity, tidal wetland loss, up‐estuary salt intrusion, and phytoplankton biomass. To offset climate change impacts from 1995 to 2025 on water quality, the scenarios indicate an additional 2.3 million and 0.3 million kg of nitrogen and phosphorus per annum, respectively, will need to be reduced beyond what is called for in the Chesapeake Total Maximum Daily Load.
Nutrient load allocations and subsequent reductions in total nitrogen and phosphorus have been applied in the Chesapeake watershed since 1992 to reduce hypoxia and to restore living resources. In ...2010, sediment allocations were established to augment nutrient allocations supporting the submerged aquatic vegetation resource. From the initial introduction of nutrient allocations in 1992 to the present, the allocations have become more completely applied to all areas and loads in the watershed and have also become more rigorously assessed and tracked. The latest 2010 application of nutrient and sediment allocations were made as part of the Chesapeake Bay total maximum daily load and covered all six states of the Chesapeake watershed. A quantitative allocation process was developed that applied principles of equity and efficiency in the watershed, while achieving all tidal water quality standards through an assessment of equitable levels of effort in reducing nutrients and sediments. The level of effort was determined through application of two key watershed scenarios: one where no action was taken in nutrient control and one where maximum nutrient control efforts were applied. Once the level of effort was determined for different jurisdictions, the overall load reduction was set watershed‐wide to achieve dissolved oxygen water quality standards. Further adjustments were made to the allocation to achieve the James River chlorophyll‐a standard.
The Conowingo Reservoir is situated at the lower terminus of the Susquehanna ‐‐‐River watershed, immediately above Chesapeake Bay. Since construction, the reservoir has been filling with sediment to ...the point where storage capacity is nearly exhausted. The potential for release of accumulated sediments, organic matter, and nutrients, especially through the action of storm scour, causes concern for water quality in Chesapeake Bay. We used hydrodynamic and eutrophication models to examine the effects of watershed loads and scour loads on bay water quality under total maximum daily load conditions. Results indicate that increased suspended solids loads are not a threat to bay water quality. For most conditions, solids scoured from the reservoir settle out before the season during which light attenuation is critical. The organic matter and nutrients associated with the solids are, however, detrimental. This material settles to the estuary bottom and is mineralized in bed sediments. Carbon diagenesis spurs oxygen consumption in bottom sediments and in the water column via release of chemical oxygen demand. The nutrients are recycled to the water column and stimulate algal production. As a result of a scour event, bottom‐water dissolved oxygen declines up to 0.2 g m−3, although the decline is 0.1 g m−3 or less when averaged over the summer season. Surface chlorophyll increases 0.1 to 0.3 mg m−3 during the summer growing season.
Core Ideas
Reservoir sedimentation can adversely impact water quality downstream.
Infilling of Conowingo Reservoir results in increased sediments and nutrients passed through to Chesapeake Bay.
Sediments are not a threat to water quality in Chesapeake Bay.
Nutrients and organic matter associated with sediments contribute to eutrophication in Chesapeake Bay.
Eutrophication and hypoxia represent an ever-growing stressor to estuaries and coastal ecosystems due to population growth and climate change. Understanding water quality dynamics in shallow water ...systems is particularly challenging due to the complex physical and biogeochemical dynamics and interactions among them. Within shallow waters, benthic microalgae can significantly contribute to autotrophic primary production, generate organic matter, increase dissolved oxygen consumption, and alter nutrient fluxes at the sediment–water interface, yet they have received little attention in modeling applications. A state-of-the-art modeling system, the Semi-Implicit Cross-Scale Hydroscience Integrated System Model (SCHISM), coupled with the Integrated Compartment Model (ICM) of water quality and benthic microalgae, has been implemented in the Corsica River estuary, a tributary to Chesapeake Bay, to study benthic microalgal impact on water quality in shallow water systems. The model simulation has revealed a broad impact of benthic microalgae, ranging from sediment–water interface fluxes to water column dynamics, and the effects are observed from near-field to far-field monitoring stations. High-frequency variability and non-linearity dominate benthic microalgal dynamics, sediment oxygen demand, and nutrient fluxes at the sediment–water interface. Resource competition and supply determine the spatial scope of benthic microalgal impacts on far-field stations and the whole estuary system. Our study shows that benthic microalgae are a significant factor in shallow water dynamics that needs adequate attention in future observation and modeling applications.
We investigated the hypothesis that effects of cultural eutrophication can be reversed through natural resource restoration via addition of an oyster module to a predictive eutrophication model. We ...explored the potential effects of native oyster restoration on dissolved oxygen (DO), chlorophyll, light attenuation, and submerged aquatic vegetation (SAV) in eutrophic Chesapeake Bay. A tenfold increase in existing oyster biomass is projected to reduce system-wide summer surface chlorophyll by approximately 1 mg m⁻³, increase summer-average deep-water DO by 0.25 g m⁻³, add 2100 kg C (20%) to summer SAV biomass, and remove 30,000 kg d⁻¹ nitrogen through enhanced denitrification. The influence of oyster restoration on deep extensive pelagic waters is limited. Oyster restoration is recommended as a supplement to nutrient load reduction, not as a substitute.
► A mass-balance model of the carbonate cycle is added to a eutrophication model. ► The model is applied to a tidal freshwater system plagued by algal blooms, the Potomac River, USA. ► Sensitivity ...analysis shows hydrology is the most significant determinant of pH in this system.
The pH of the freshwater portion of the Potomac River estuary attains 9–10.5, driven by photosynthesis during cyanobacteria blooms. Processes which contribute to elevated pH are examined by adding a mass-balance model of the carbonate cycle to an existing eutrophication model. Four new variables are added to the model suite: alkalinity, total inorganic carbon, total calcium, and calcium carbonate. The pH is computed from these four quantities via equilibrium kinetics. The model is employed in a continuous simulation of the years 1994–2000. Emphasis in examination of model results is placed on the tidal fresh portion of the system where elevated pH is an environmental concern. Model sensitivity analysis indicates hydrology has the greatest influence on pH. During low-flow periods, residence time is lengthy allowing ample time for algal production to occur. The production stimulates net uptake of TIC, and results in enhanced pH.
The Chesapeake Bay, USA, suffers from multiple water quality impairments including poor water clarity. A management strategy aimed at improving water clarity through reduction of nutrient and solids ...loads to the bay is under development. The strategy is informed through the use of the Chesapeake Bay Environmental Modeling Package. We describe herein aspects of the model devoted to suspended solids, a major contributor to poor water clarity. Our approach incorporates a dynamic model of inorganic solids into an eutrophication model, in order to account for interactions between physical and biotic factors which influence suspended solids transport and fate. Solids budgets based on the model indicate that internal production of organic solids is the largest source of suspended solids to the mainstem bay. Scenario analysis indicates that control of solids loads reduces solids concentration in the vicinity of the loading sources. Control of nutrient loads provides more widespread but lesser reductions in suspended solids.
