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Hypoxia in coastal waters is a pressing ecological problem caused by continued eutrophication and climatic change that has widespread consequences for metazoan life and biogeochemical cycles. Numerous studies have investigated the controls on seasonal hypoxia formation and persistence in many of the world's large estuaries and coastal hypoxic zones, but far fewer studies have examined the controls on short-term oxygen variability that leads to diel-cycling hypoxia in shallow-water environments. We utilized a unique, comprehensive (181 stations) record of dissolved oxygen concentrations collected at shallow water sites (primarily < 2 m) at high frequency (15 min) throughout the estuarine complex of the Chesapeake Bay and its tributaries to quantify how internal and external variables co-varied with dissolved oxygen. We used a combination of time-series analysis, harmonic analysis, and machine learning (e.g., classification and regression trees (CART)) approaches to identify spatial patterns in major controls on oxygen variability and the duration of moderate hypoxia. We found that key controls on oxygen variability varied substantially over space. For example, photosynthetically active radiation (PAR) was a strong predictor of oxygen dynamics in the majority of mesohaline waters. In more fetch-exposed stations, wind strongly controlled hypoxic duration, but in eutrophic, inshore locations, chlorophyll a, or turbidity were often better predictors. Specifically, diel oxygen variability was muted in upstream regions characterized by high turbidity. The duration of low oxygen conditions, which we defined conservatively as less than 4.8 mg O2 L−1 (156 µM), was strongly controlled by temperature, and simple projections of regional warming and CART-derived oxygen thresholds suggest that the Bay could experience a 10% increase in this type of hypoxia duration by mid-to-late twenty-first century. The ratio of tidal to biological variability in oxygen was found to increase under conditions of higher turbidity, stronger wind, and lower salinity, but biological variability was typically a factor of two higher than tidal variability. Although chlorophyll-a generated high oxygen concentrations at some locations, those stations with exceptionally high chlorophyll a (> 30 µg L−1) were the most vulnerable to hypoxia. Because conventional water quality modeling frameworks are designed to capture hypoxia on relatively long time scales, these new insights can help inform updated oxygen models to support the management of shallow-water estuaries in the face of managed nutrient reductions and climate change. [ABSTRACT FROM AUTHOR]

Marine cable bacteria (Candidatus Electrothrix) and large colorless sulfur‐oxidizing bacteria (e.g., Beggiatoaceae) are widespread thiotrophs in coastal environments but may exert different influences on biogeochemical cycling. Yet, the factors governing their niche partitioning remain poorly understood. To map their distribution and evaluate their growth constraints in a natural setting, we examined surface sediments across seasons at two sites with contrasting levels of seasonal oxygen depletion in Chesapeake Bay using microscopy coupled with 16S rRNA gene amplicon sequencing and biogeochemical characterization. We found that cable bacteria, dominated by a single phylotype closely affiliated to Candidatus Electrothrix communis, flourished during winter and spring at a central channel site which experiences summer anoxia. Here, cable bacteria density was positively correlated with surface sediment chlorophyll, a proxy of phytodetritus sedimentation. Cable bacteria were also present with a lower areal density at an adjacent shoal site which supports bioturbating macrofauna. Beggiatoaceae were more abundant at this site, where their biomass was positively correlated with sediment respiration, but additionally potentially inhibited by sulfide accumulation which was evident during one summer. A springtime phytodetritus sedimentation event was associated with a proliferation of Beggiatoaceae and multiple Candidatus Electrothrix phylotypes, with cable bacteria reaching 1000 m length cm−2. These observations indicate the potential impact of a spring bloom in driving a hot moment of cryptic sulfur cycling. Our results suggest complex interactions between benthic thiotroph populations, with bioturbation and seasonal oscillations in bottom water dissolved oxygen, sediment sulfide, and organic matter influx as important drivers of their distribution. [ABSTRACT FROM AUTHOR]

We examined responses of water‐column conditions, sediment‐water fluxes, ecosystem metabolism, and nutrient export in the Back River estuary during the past three decades following multiple phases of nutrient load reductions from a large wastewater treatment plant. Total nitrogen (TN) loads from the treatment plant declined from 7000 kg N d−1 in the mid‐1980s to 1500 kg N d−1 following the implementation of enhanced nutrient removal in late 2017. Total phosphorus (TP) loads declined by ~90% from peaks in the mid‐1980s–1990s and have been stable ever since. In response, TN and TP concentrations measured since 1985 show declines that generally mirror that of loads from the treatment plant, and box model computations suggest significant reductions in nutrient export to adjacent Chesapeake Bay. As a consequence, water‐column chlorophyll a (Chl a) concentrations have declined modestly over the record, despite inter‐annual variability. This reduction in Chl a coincided with a reduced frequency of nitrogen and phosphorus concentrations that would saturate phytoplankton growth, as well as reductions in ecosystem gross primary production and respiration derived from high‐frequency oxygen time‐series. Sediment‐water fluxes of dissolved nitrogen, phosphorus, and oxygen, as well as associated sediment concentrations of nitrogen, phosphorus, and carbon also declined over the record. These temporal patterns were reproduced in a 35‐yr model simulation that suggests a relatively rapid response to reduced organic matter deposition changes. Finally, the recycling of ammonium for a given TN load declined substantially, consistent with high observed rates of denitrification, indicating that well‐mixed estuaries can recover relatively rapidly in response to nutrient remediation. [ABSTRACT FROM AUTHOR]

Acidification can present a stress on organisms and habitats in estuaries in addition to hypoxia. Although oxygen and pH decreases are generally coupled due to aerobic respiration, pH dynamics may be more complex given the multiple modes of buffering in the carbonate system. We studied the seasonal cycle of dissolved oxygen (DO), pH, dissolved inorganic carbon, total alkalinity, and calcium ion (Ca2+) along the main channel of Chesapeake Bay from May to October in 2016. Contrary to the expected co‐occurrence of seasonal DO and pH declines in subsurface water, we found that the pH decline ended in June while the DO decline continued until August in mid‐Chesapeake Bay. We discovered that aerobic respiration was strong from May to August, but carbonate dissolution was minor in May and June and became substantial in August, which buffered further pH declines and caused the seasonal DO and pH minima mismatch. The rate of calcium carbonate (CaCO3) dissolution was not primarily controlled by the saturation state in bottom water, but was instead likely controlled by the supply of CaCO3 particles. The seasonal variability of Ca2+ addition in the mid‐bay was connected to Ca2+ removal in the upper bay, and the timing of high carbonate dissolution coincided with peak seasonal biomass of upper Bay submerged aquatic vegetation. This study suggests a mechanism for a novel decoupling of DO and pH in estuarine waters associated with CaCO3, but future studies are needed to fully investigate the seasonality of physical transport and cycling of CaCO3. [ABSTRACT FROM AUTHOR]

Chesapeake Bay has a long history of nutrient pollution resulting in degraded water quality. However, we report improvements in chlorophyll a in surface waters and dissolved oxygen in bottom waters at one of three estuarine stations in the Choptank tributary of Chesapeake Bay. We updated a previous nutrient budget for the estuary constructed for reference year 1998 using rates of atmospheric deposition, inputs of watershed diffuse sources (primarily agriculture), and discharges of point sources (primarily human waste) for reference year 2017. Parallel trends suggest that improvements in water quality at the one station were likely due to 20% reductions in direct atmospheric deposition on the estuary's surface and 78–95% reductions in wastewater N and P due to installation of tertiary treatment. The agricultural sector, the dominant source of N and P, appeared to provide little contribution to improved water quality during this period. Although efforts to reduce nutrient losses from agriculture are common throughout the Choptank basin, widespread reductions from agricultural diffuse sources could make large contributions to improved water quality at all stations in the estuary. The response in the Choptank is similar to those observed elsewhere in the USA, Europe, Australia, and New Zealand due to improved wastewater treatment. Similar to our findings, the upper Potomac River of Chesapeake Bay saw improvements driven by reductions in atmospheric deposition. Unfortunately, few studies elsewhere have shown improvements in water quality due to agricultural management. The data presented here indicate that public and industrial investments in reductions of atmospheric emissions and upgrades to wastewater treatment plants have improved estuarine water quality in the Choptank. [ABSTRACT FROM AUTHOR]

Vertebrate and invertebrate taxa that serve as forage for predators play a critical role in coastal ecosystems by linking lower trophic levels to ecologically and economically valuable predators in upper trophic levels. We analyzed long-term data from multiple monitoring surveys in Chesapeake Bay and its major tributaries to evaluate patterns and relative abundances of forage at two spatial scales and to identify drivers of forage availability. Time series of forage abundances showed positive spatial correlation, becoming increasingly similar with declining distance between tributaries and regions of Chesapeake Bay. Statistical models were fit to identify relationships among forage taxa and climatic, biological, and environmental variables. Annual abundance indices of many forage taxa were higher in years when spring water temperatures warmed slowly, as indicated by an annual 5 °C degree-day warming index that represented the rate of warming during late winter/early spring. Forage indices also were related (in taxon-specific ways) to winter–spring chlorophyll concentration and freshwater discharge, and to three summer water quality variables: dissolved oxygen, salinity, and water temperature, in addition to a broad-scale climate indicator (Atlantic Multidecadal Oscillation). Our findings broadly demonstrate that bottom–up climate forcing acts to control at least some taxa at lower trophic levels in Chesapeake Bay. Ongoing phenological changes in regional climate and evidence of spatial dependence in responses of forage to environmental conditions underscore the likelihood that spatiotemporal foraging conditions for predator species will change under projected climate conditions. [ABSTRACT FROM AUTHOR]

Most present forecast systems for estuaries predict conditions for only a few days into the future. However, there are many reasons to expect that skillful estuarine forecasts are possible for longer time periods, including increasingly skillful extended atmospheric forecasts, the potential for lasting impacts of atmospheric forcing on estuarine conditions, and the predictability of tidal cycles. In this study, we test whether skillful estuarine forecasts are possible for up to 35 days into the future by combining an estuarine model of Chesapeake Bay with 35‐day atmospheric forecasts from an operational weather model. When compared with both a hindcast simulation from the same estuarine model and with observations, the estuarine forecasts for surface water temperature are skillful up to about 2 weeks into the future, and the forecasts for bottom temperature, surface and bottom salinity, and density stratification are skillful for all or the majority of the forecast period. Bottom oxygen forecasts are skillful when compared to the model hindcast, but not when compared with observations. We also find that skill for all variables in the estuary can be improved by taking the mean of multiple estuarine forecasts driven by an ensemble of atmospheric forecasts. Finally, we examine the forecasts in detail using two case studies of extreme events, and we discuss opportunities for improving the forecast skill. Plain Language Summary: This paper evaluates a suite of forecasts for Chesapeake Bay water temperature, salinity, and dissolved oxygen created using a numerical model. By comparing the model forecasts with observations, we show that the model forecasts for temperature and salinity are more accurate than reference forecasts of previously observed conditions or the long‐term mean; in other words, the forecasts are skillful. In general, the forecasts are skillful for at least 2 weeks into the future. Improvements to our forecasting system, such as predicting future river discharge into Chesapeake Bay, would likely improve the forecast skill even more. By showing that accurate, skillful forecasts are possible for a much longer time frame than previously considered, this paper takes an important step toward applying forecasts to improve water quality and fisheries management and to prepare for the impacts of extreme events like hurricanes and heat waves. Key Points: Skillful forecasts of estuarine temperature and salinity are possible beyond the few days typically consideredDissolved oxygen remains challenging to forecastTaking the mean of multiple forecasts driven by different atmospheric ensemble members improves the skill [ABSTRACT FROM AUTHOR]

A multimetric water quality standards (WQS) attainment indicator (MMI) was created to assess the habitat quality of the Chesapeake Bay and its tidal tributaries. The indicator uses metrics of dissolved oxygen concentrations, water clarity assessments, underwater bay grass acreages, and season-specific chlorophyll a distributions. This suite of metrics is recognized as symptomatic of eutrophication and responsive to nutrient and sediment management actions. Habitat criteria for these metrics were established by the US Environmental Protection Agency (USEPA) to protect the survival, growth, and reproduction of tidal bay living resources. The criteria were adopted into state WQS used to define outcome targets of the regulatory 2010 Chesapeake Bay Total Maximum Daily Load (TMDL). Direct accounting of monitoring results compared with state standards would be the most direct measure of water quality status related to achieving habitat health goal conditions under the TMDL. However, the Chesapeake Bay Program long-term water quality monitoring program that supports Clean Water Act 303d water quality impairment assessments does not provide sufficient measurement resolution across all temporal scales represented in these TMDL-related WQS. Recognizing data and analysis gaps, we developed the indicator to provide estimates of WQS attainment for the Chesapeake Bay. The structure of the indicator uses metrics that have been measured consistently with the historical Chesapeake Bay tidal water quality data collections since 1985. The ability to compute scores on contemporary and historical data will provide for bay-wide and spatially explicit long-term trend evaluations of habitat quality. [ABSTRACT FROM AUTHOR]

A diel method for estimating gross primary production (GPP) is applied to nearly continuous measurements of near‐surface dissolved oxygen collected at seven locations throughout the main stem of Chesapeake Bay. The data were collected through the Chesapeake Bay Interpretive Buoy System and span the period 2010–2016. At all locations, GPP exhibits pronounced seasonal variability consistent temperature‐dependent phytoplankton growth. At the Susquehanna Buoy, which is located within the estuarine turbidity maximum, rates of GPP are negatively correlated with uncalibrated turbidity data consistent with light limitation at this location. The highest rates of GPP are located immediately down Bay from the estuarine turbidity maximum and decrease moving seaward consistent with nutrient limitation. Rates of GPP at the mouth (First Landing Buoy) are roughly a factor of 3 lower than the rates in the upper Bay (Patapsco). At interannual time scales, the summer (June–July) rate of GPP averaged over all stations is positively correlated (r2 = 0.62) with the March Susquehanna River discharge and a multiple regression model that includes spring river discharge, and summer water temperature can explain most (r2 = 0.88) of the interannual variance in the observed rate of GPP. The correlation with river discharge is consistent with an increase in productivity fueled by increased nutrient loading. More generally, the spatial and temporal patterns inferred using this method are consistent with our current understanding of primary production in the Bay, demonstrating the potential this method has for making highly resolved measurements in less well studied estuarine systems. Plain Language Summary: A new method for estimating gross primary production from in situ dissolved oxygen data is applied to 7 years of data from Chesapeake Bay. Application of the method highlights pronounced spatial and temporal variability consistent with previous studies employing more labor intensive methods. The method shows significant promise for making high‐quality measurements of productivity with high temporal resolution in estuarine systems. Key Points: Application of a diel method for estimating gross primary production is applied to 7 years of observing system data from Chesapeake BayThe method demonstrates spatial variability with light limitation in the upper Bay transitioning to nutrient limitation in the lower BaySeasonal variability consistent with temperature‐dependent growth and interannual variability driven by inorganic nutrient inputs is shown [ABSTRACT FROM AUTHOR]

A method for estimating gross primary production (GPP) is presented and validated against a numerical model of Chesapeake Bay that includes realistic physical and biological forcing. The method statistically fits a photosynthesis‐irradiance response curve using the observed near‐surface time rate of change of dissolved oxygen and the incoming solar radiation, yielding estimates of the light‐saturated photosynthetic rate and the initial slope of the photosynthesis‐irradiance response curve. This allows estimation of GPP with 15‐day temporal resolution. The method is applied to the output from a numerical model that has high skill at reproducing both surface and near‐bottom dissolved oxygen variations observed in Chesapeake Bay in 2013. The rate of GPP predicted by the numerical model is known, as are the contributions from physical processes, allowing the proposed diel method to be rigorously assessed. At locations throughout the main stem of the Bay, the method accurately extracts the underlying rate of GPP, including pronounced seasonal variability and spatial variability. Errors associated with the method are primarily the result of contributions by the divergence in turbulent oxygen flux, which changes sign over the surface mixed layer. As a result, there is an optimal vertical location with minimal bias where application of the method is most accurate. Plain Language Summary: This paper presents and evaluates a method for estimating gross primary production from time variations of dissolved oxygen. The evaluation is performed on the output from a three‐dimensional circulation model of Chesapeake Bay that can simulate dissolved oxygen with high skill. Key Points: A new method for estimating gross primary production from in situ dissolved oxygen data is presentedApplication of the method to numerical model output demonstrates the method can accurately estimate gross primary productionVertical mixing contributes to diurnal oxygen variations, but there is an optimal vertical location where this effect is minimized [ABSTRACT FROM AUTHOR]

To protect the aquatic living resources of Chesapeake Bay, the Chesapeake Bay Program partnership has developed guidance for state water quality standards, which include ambient water quality criteria to protect designated uses (DUs), and associated assessment procedures for dissolved oxygen (DO), water clarity/underwater bay grasses, and chlorophyll-a. For measuring progress toward meeting the respective states' water quality standards, a multimetric attainment indicator approach was developed to estimate combined standards attainment. We applied this approach to three decades of monitoring data of DO, water clarity/underwater bay grasses, and chlorophyll-a data on annually updated moving 3-year periods to track the progress in all 92 management segments of tidal waters in Chesapeake Bay. In 2014–2016, 40% of tidal water segment-DU-criterion combinations in the Bay (n = 291) are estimated to meet thresholds for attainment of their water quality criteria. This index score marks the best 3-year status in the entire record. Since 1985–1987, the indicator has followed a nonlinear trajectory, consistent with impacts from extreme weather events and subsequent recoveries. Over the period of record (1985–2016), the indicator exhibited a positive and statistically significant trend ( p  < 0.05), indicating that the Bay has been recovering since 1985. Patterns of attainment of individual DUs are variable, but improvements in open water DO, deep channel DO, and water clarity/submerged aquatic vegetation have combined to drive the improvement in the Baywide indicator in 2014–2016 relative to its long-term median. Finally, the improvement in estimated Baywide attainment was statistically linked to the decline of total nitrogen, indicating responsiveness of attainment status to the reduction of nutrient load through various management actions since at least the 1980s. [ABSTRACT FROM AUTHOR]

The Chesapeake Bay region is projected to experience changes in temperature, sea level, and precipitation as a result of climate change. This research uses an estuarine-watershed hydrodynamic-biogeochemical modeling system along with projected mid-21st-century changes in temperature, freshwater flow, and sea level rise to explore the impact climate change may have on future Chesapeake Bay dissolved-oxygen (DO) concentrations and the potential success of nutrient reductions in attaining mandated estuarine water quality improvements. Results indicate that warming bay waters will decrease oxygen solubility year-round, while also increasing oxygen utilization via respiration and remineralization, primarily impacting bottom oxygen in the spring. Rising sea level will increase estuarine circulation, reducing residence time in bottom waters and increasing stratification. As a result, oxygen concentrations in bottom waters are projected to increase, while oxygen concentrations at mid-depths (3 < DO < 5 mg L-1) will typically decrease. Changes in precipitation are projected to deliver higher winter and spring freshwater flow and nutrient loads, fueling increased primary production. Together, these multiple climate impacts will lower DO throughout the Chesapeake Bay and negatively impact progress towards meeting water quality standards associated with the Chesapeake Bay Total Maximum Daily Load. However, this research also shows that the potential impacts of climate change will be significantly smaller than improvements in DO expected in response to the required nutrient reductions, especially at the anoxic and hypoxic levels. Overall, increased temperature exhibits the strongest control on the change in future DO concentrations, primarily due to decreased solubility, while sea level rise is expected to exert a small positive impact and increased winter river flow is anticipated to exert a small negative impact. [ABSTRACT FROM AUTHOR]

A numerical circulation model with a very simple representation of dissolved oxygen dynamics is used to simulate hypoxia in Chesapeake Bay for the 30-yr period 1984-2013. The model assumes that the biological utilization of dissolved oxygen is constant in both time and space in an attempt to isolate the role that physical processes play in modulating oxygen dynamics. Despite the simplicity of the model it demonstrates skill in simulating the observed inter-annual variability of hypoxic volume, capturing 50% of the observed variability in hypoxic volume (<2 mg L−1) for the month of July and 58% of the observed variability for the month of August, over the 30-yr period. Model skill increases throughout the summer suggesting that physical processes play a more important role in modulating hypoxia later in the summer. Model skill is better for hypoxic volumes than for anoxic volumes. In fact, a simple regression based on the integrated January-June Susquehanna River nitrogen load can explain more of the variability in the observed anoxic volumes than the model presented here. Model results suggest that the mean summer (June-August) wind speed is the single-most important physical variable contributing to variations in hypoxic volumes. Previous studies have failed to document the importance of summer wind speed because they have relied on winds measured at Patuxent Naval Air Station, which does not capture the observed inter-annual variations in wind speed that are observed by stations that directly measure wind over the waters of Chesapeake Bay. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A coupled hydrodynamic-dissolved oxygen model is developed to simulate the seasonal cycle of dissolved oxygen (DO) in Chesapeake Bay and investigate processes regulating summer hypoxia in the estuary. A budget analysis of DO in the bottom water reveals a balance between physical transport and biological consumption. In addition to the vertical diffusive flux, the longitudinal and vertical advective fluxes are important suppliers of DO to the bottom water. The longitudinal advective flux is affected not only by gravitational circulation but also by wind-driven currents. The vertical advective flux is affected by wind-driven lateral circulation and shows a strong dependence on wind speed and direction. Up-estuary winds weaken the landward bottom flow and generate a clockwise lateral circulation that exchanges DO between the deep channel and adjacent shoals, thereby reducing the longitudinal advective flux and increasing the vertical advective flux. In contrast, down-estuary winds amplify the longitudinal flux and reduce the vertical flux. During the summer, water column respiration contributes to about 74 % of the total biological consumption and sediment oxygen demand accounts for about 26 %. Sensitivity analysis model runs are conducted to analyze how changing river flows and winds affect the hypoxia prediction and oxygen budget balance. Due to the compensating changes in longitudinal and vertical fluxes, the hypoxic volume is relatively insensitive to changes in the river flow. In contrast, the timing and size of hypoxic volume changes with wind speed. Sensitivity analysis also shows that plankton oxygen production, water column respiration, and sediment oxygen demand all affect the hypoxia prediction and bottom oxygen balance. [ABSTRACT FROM AUTHOR]

Eastern oyster abundance is at an all-time low, yet this species is a key component of many estuarine systems because it contributes to ecosystem function by providing habitat, improving water quality, stabilizing benthic and intertidal habitat, increasing landscape diversity and producing more oysters. Given the breadth of environmental benefits oysters provide, as well as their commercial and cultural importance, sustainable oyster production has become a priority in several regions, including the Chesapeake Bay. Current strategies include treating restored reefs as permanent sanctuaries, which provide long-term environmental benefits yet removes them from the fishery, or harvesting reefs on a rotational basis, which provides economic value yet decreases environmental benefits. The long term dynamics of these strategies is unknown. Oysters have a complex, biphasic life cycle (i.e., sessile adult and motile larval stages) and their viability is intimately tied to a suite of environmental factors including, but not limited to, flow regime, total suspended solids, temperature, salinity, and dissolved oxygen. In order to determine how different oyster management strategies affect oyster dynamics, we developed a multi-model approach that integrates a 2-D hydrodynamic model, a larval transport model, and a spatially-explicit, agent-based population dynamics model to simulate long term oyster dynamics. We applied our model to a ten reef system in the Great Wicomico River in the Chesapeake Bay, and simulated six different combinations of sanctuary and/or harvest management scenarios over an 8-year period. We evaluated the environmental and commercial benefits of each strategy. Our results indicated that sanctuary reefs are beneficial, and that the spatial position of sanctuary reefs strongly affected source-sink dynamics and must be considered before implementing a harvest regime. Simulations that did not consider the source/sink dynamics of the reefs yielded larger numbers of oysters for harvest in the short-term, yet resulted in a complete fishery collapse in the long term. Selective, rotational harvest, resulted in lower annual yield, but the fishery persisted throughout the eight year simulation. This integrated modeling approach helped reduce uncertainty within the study system and can help natural resource managers understand ecosystem-level processes leading to more informed decision making across spatial and temporal scales. [ABSTRACT FROM AUTHOR]

To analyze the correlations of summer anoxia/hypoxia in the Chesapeake Bay with watershed input and wind conditions, statistics were applied to nearly three decades of monitoring data. The results of Pearson correlation coefficients, multivariate regression analysis, and cluster analysis indicate that anoxia/hypoxia has a strong positive correlation with nutrient load and a moderate negative correlation with wind speed. Physical relationships among the relevant constituents were analyzed. Nutrient loads and the subsequent decay of organic matter are the primary factors that control the oxygen demand causing summer anoxia and hypoxia, while episodic wind can partially erode stratification and reduce anoxia/hypoxia. Although the extent of anoxia/hypoxia reduction differs with wind direction, higher wind speeds result in more destratification and anoxia/hypoxia reduction than lower wind speeds and are more important than the effect of wind direction. The influences of freshwater discharge, stratification, and temperature were also analyzed. Computer modeling results were used to obtain dissolved oxygen conditions at finer temporal and spatial scales to supplement the scattered and discrete observations from monitoring stations and for better understanding of anoxia/hypoxia development under episodic wind events. [ABSTRACT FROM AUTHOR]

• A water quality barometer is developed. • The water quality barometer is applied to dissolved oxygen data in Chesapeake Bay for the period 1985–2020. • The barometer can be aggregated to other spatial scales such as tidal basin and the entire bay. • The barometer can easily be extended to use with multiple water quality parameters. This paper develops a barometer that indexes water quality in the Chesapeake Bay and summarizes quality over spatial regions and temporal periods. The barometer has a basis in risk assessment and hydrology, and is a function of three different metrics of water quality relative to numerical criteria: relative frequency of criterion attainment; magnitude of deviation from a numerical criterion; and duration of criterion attainment. Metrics associated with these features are calculated at the station level, allowing flexibility for simultaneously evaluating multiple stressors, different designated uses, and physical characteristics of the water. The barometer score is then created as a geometric mean of the three metrics. The water quality barometer (WQB) station scores may be spatially aggregated to report habitat scores across a spectrum of spatial resolutions (e.g., management segment, tidal subsystem, or the whole tidal bay). Dissolved oxygen measurements in the Chesapeake Bay collected during summer seasons of 1985 to 2020 are used to evaluate water quality. The WQB score and its bootstrapped confidence interval are reported at the station, segment, tidal subsystem and whole tidal bay levels. Notably, water quality interpreted through application of the WQB with dissolved oxygen concentration data and averaged over the 29-year period of record is good (i.e. protects aquatic living resources) in tributaries such as the James River, Rappahannock River and others; but is not as good in areas such as the Upper Tributaries and the York River. Recent summaries indicate that while the water quality is improving in much of the bay and its tidal tributaries, however, there is an indication of decline in quality in the period 20182020, especially in the upper regions of the Bay. The barometer is designed around using the time series data produced by the Chesapeake Bay Programs annual monitoring strategy; the approach has application to other large water bodies with large scale monitoring programs with extended time series or for integrating information from environmental sensor systems. [ABSTRACT FROM AUTHOR]

Zoothamnium intermedium is an obligate epibiont ciliate and has been found in a diverse array of hosts and environments. Different studies have reported conflicting distribution patterns and host preferences, even though studies in Chesapeake Bay have suggested that the ciliate has a strong host specificity for two calanoid copepod species. We examined the life cycle, host preferences, and ecological conditions conducive to Z. intermedium presence on copepods in Chesapeake Bay, the largest estuary in North America. The York River tributary was sampled biweekly from fall 2014 through summer 2015 for plankton, peritrichs and bacteria in the water column. Bacterial abundance in the water column peaked in fall and late spring, coinciding with increased abundance and species richness of non-epibiont peritrichs. Among the plankton, only the calanoid copepods Acartia tonsa and Centropages hamatus were colonized by Z. intermedium. The peritrich epibiont displayed higher colonization rates on C. hamatus even when A. tonsa was far more abundant. Multivariate correlation analysis of infestation prevalence on A. tonsa showed a strong correlation with dissolved oxygen, salinity and water temperature. Such correlations, along with differences in host species biology, might be driving the seasonality of this epibiotic relationship. [ABSTRACT FROM AUTHOR]

Human development has degraded Chesapeake Bay's health, resulting in an increase in the extent and severity of hypoxia (≤2 mg O l). The Bay's hypoxic zones have an adverse effect on both community structure and secondary production of macrobenthos. From 1996 to 2004, the effect of hypoxia on macrobenthic production was assessed in Chesapeake Bay and its three main tributaries (Potomac, Rappahannock, and York Rivers). Each year, in the summer (late July − early September), 25 random samples of the benthic macrofauna were collected from each system, and macrobenthic production in the polyhaline and mesohaline regions was estimated using Edgar's allometric equation. Fluctuations in macrobenthic production were significantly correlated with dissolved oxygen. Macrobenthic production was 90 % lower during hypoxia relative to normoxia. As a result, there was a biomass loss of ~7,320-13,200 metric tons C over an area of 7,720 km, which is estimated to equate to a 20 % to 35 % displacement of the Bay's macrobenthic productivity during the summer. While higher consumers may benefit from easy access to stressed prey in some areas, the large spatial and temporal extent of seasonal hypoxia limits higher trophic level transfer, via the inhibition of macrobenthic production. Such a massive loss of macrobenthic production would be detrimental to the overall health of the Bay, as it comes at a time when epibenthic and demersal predators have high-energy demands. [ABSTRACT FROM AUTHOR]

Seasonal shifts of dissolved inorganic nitrogen (DIN) and the dynamics of microbial communities for nitrogen transformation were investigated in the water column of Chesapeake Bay. The relative abundance of nitrogen over phosphorus (N*) showed a strong seasonal and spatial pattern: gradually decreased from upstream to downstream; high in winter and low in summer. Because the phosphorus concentration remained relatively stable, the spatiotemporal pattern of N* implied that a substantial fraction of DIN was removed in the bay, especially in summer. Correlation analyses indicated the functional microbial communities and environmental variables, such as temperature, dissolved oxygen, salinity, played important roles for connecting the seasonal variation of N*. Among them, temperature was the trigger factor. High temperature in the summer induced the growth of functional microbes, which subsequently consumed a large portion of DIN inputted from the tributaries and reduced the N*. The current study provided the relative importance of microbial communities and environmental variables in driving the DIN loss in the bay. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Achieving and maintaining the water quality conditions necessary to protect the aquatic living resources of the Chesapeake Bay and its tidal tributaries has required a foundation of quantifiable water quality criteria. Quantitative criteria serve as a critical basis for assessing the attainment of designated uses and measuring progress toward meeting water quality goals of the Chesapeake Bay Program partnership. In 1987, the Chesapeake Bay Program partnership committed to defining the water quality conditions necessary to protect aquatic living resources. Under section 303(c) of the Clean Water Act, States and authorized tribes have the primary responsibility for adopting water quality standards into law or regulation. The Chesapeake Bay Program partnership worked with U.S. Environmental Protection Agency to develop and publish a guidance framework of ambient water quality criteria with designated uses and assessment procedures for dissolved oxygen, water clarity, and chlorophyll a for Chesapeake Bay and its tidal tributaries in 2003. This article reviews the derivation of the water quality criteria, criteria assessment protocols, designated use boundaries, and their refinements published in six addendum documents since 2003 and successfully adopted into each jurisdiction's water quality standards used in developing the Chesapeake Bay Total Maximum Daily Load. [ABSTRACT FROM AUTHOR]

Abstract: The Chesapeake Bay Ecological Prediction System (CBEPS) automatically generates daily nowcasts and three-day forecasts of several environmental variables, such as sea-surface temperature and salinity, the concentrations of chlorophyll, nitrate, and dissolved oxygen, and the likelihood of encountering several noxious species, including harmful algal blooms and water-borne pathogens, for the purpose of monitoring the Bay's ecosystem. While the physical and biogeochemical variables are forecast mechanistically using the Regional Ocean Modeling System configured for the Chesapeake Bay, the species predictions are generated using a novel mechanistic–empirical approach, whereby real-time output from the coupled physical–biogeochemical model drives multivariate empirical habitat models of the target species. The predictions, in the form of digital images, are available via the World Wide Web to interested groups to guide recreational, management, and research activities. Though full validation of the integrated forecasts for all species is still a work in progress, we argue that the mechanistic–empirical approach can be used to generate a wide variety of short-term ecological forecasts, and that it can be applied in any marine system where sufficient data exist to develop empirical habitat models. This paper provides an overview of this system, its predictions, and the approach taken. [Copyright &y& Elsevier]

We examined the processes influencing summer hypoxia in the mainstem portion of Chesapeake Bay. The analysis was based on the Chesapeake Bay Monitoring Program data collected between 1985 and 2007. Self-organizing map (SOM) analysis indicates that bottom water dissolved oxygen (DO) starts to be depleted in the upper mesohaline area during late spring, and hypoxia expands down-estuary by early summer. The seasonal hypoxia in the bay appears to be related to multiple variables, (e.g., river discharge, nutrient loading, stratification, phytoplankton biomass, and wind condition), but most of them are intercorrelated. The winter-spring Susquehanna River flow contributes to not only spring-summer buoyancy effects on estuarine circulation dynamics but also nutrient loading from the land-promoting phytoplankton growth. In addition, we found that summer hypoxia is significantly correlated with the late winter-spring (February-April) northeasterly-southwesterly (NE-SW) wind. Based on winter-spring (January-May) conditions, a predictive tool was developed to forecast summer (June-August) hypoxia using river discharge and NE-SW wind. We hypothesized that the late winter-spring wind pattern may affect the transport of spring bloom biomass to the western shoal or the deep channel of the bay that either alleviates or increases the summer hypoxic volume in the midbay region, respectively. To examine this hypothesis, residual flow fields were analyzed using a hydrodynamic ocean model (Regional Ocean Modeling System; ROMS) between 2000 and 2003, two hydrologically similar years but years with different wind conditions during the spring bloom period. Simulation model results suggest that relatively larger amounts of organic matter could be transported into the deep channel in 2003 (severe hypoxia; frequent northeasterly wind) than 2000 (moderate hypoxia; frequent southwesterly wind). [ABSTRACT FROM AUTHOR]

Abstract: Understanding the changes of hydrodynamics in estuaries with respect to magnitudes of sea-level rise is important to understanding the changes of biogeochemical processes that are coupled tightly with the physical processes. Based on the 21st century sea-level rise scenarios projected by the U.S. Climate Change Science Program (), the Chesapeake Bay was chosen as a prototype to study the responses of the estuary to potential future sea-level rise. The numerical model results show that the average salt content, salt intrusion length, and stratification will increase as sea level rises. The changes of these parameters have obvious seasonal and inter-annual variations. Both the salt content and stratification show more increase in spring (following the high-flow periods) and wet years than in autumn (following the low-flow periods) and dry years. The salt intrusion length has larger increase and greater standard deviation in autumn than in spring. The transport time scales are used to illustrate the variations of transport processes as sea level rises, and results indicate that (1) the exchange flow would be strengthened but the downstream transport of fresh water would be slower; (2) the residence time of the Bay would increase due to the increased volume and change of circulation; (3) the vertical transport time (reference to water surface) has more pronounced increase and the volume of water mass with different age groups increases with different rates. As a result, the retention time of dissolved substances in the Bay would increase. Although the increased tidal currents would strengthen the vertical mixing, the increased stratification would weaken the vertical exchange. The increase of vertical transport time is due to the impact of stratification changes, which overwhelms the impact of tidal changes. As the bottom dissolved oxygen (DO) supply is predominated by the vertical exchanges in the Chesapeake Bay, the increased upstream transport time has a weak impact on hypoxia conditions in the middle and upper portions of the Bay. The weakened vertical exchange would result in less DO supply from the surface to the bottom layer. [Copyright &y& Elsevier]

This study describes the influence of submersed plant beds on spatial distributions of key water quality variables. An on-board flow-through water sampling system was used to investigate patterns in turbidity, chlorophyll- a, temperature, dissolved oxygen, and pH across a robust stand of the submersed plant Stuckenia pectinata. Spatially interpolated maps show that water quality conditions were significantly altered within this plant bed, especially during months of peak biomass, and that reduction of suspended particles focused at the bed's edge. Comparison with a suite of submersed plant beds indicated that patterns were related to canopy height, shoot density, and cross-shore bed width. Wide and dense stands with tall canopies showed reduced turbidity and increased light penetration, while smaller sparser beds often showed elevated within-bed turbidity. These results suggest that bed effects on water quality conditions vary seasonally with plant canopy architecture and bed size, providing tentative guidelines for restoring self-sustaining beds. [ABSTRACT FROM AUTHOR]

Eutrophic depletion of dissolved oxygen (DO) and its consequences for ecosystem dynamics have been a central theme of research, assessment and management policies for several decades in the Chesapeake Bay. Ongoing forecast efforts predict the extent of the summer hypoxic/anoxic area due to nutrient loads from the watershed. However, these models neither predict DO levels nor address the intricate interactions among various ecological processes. The prediction of spatially explicit DO levels in the Chesapeake Bay can eventually lead to a reliable depiction of the comprehensive ecological structure and functioning, and can also allow the quantification of the role of nutrient reduction strategies in water quality management. In this paper, we describe a three dimensional empirical model to predict DO levels in the Chesapeake Bay as a function of water temperature, salinity and dissolved nutrient concentrations (TDN and TDP). The residual analysis shows that predicted DO values compare well with observations. Nash-Sutcliffe efficiency (NSE) and root mean square error-observations standard deviation ratio (RSR) are used to evaluate the performance of the empirical model; the scores demonstrate the usability of model predictions (NSE, surface layer = 0.82-0.86; middle layer = 0.65-0.82; bottom layer = 0.70-0.82; RSR surface layer = 0.37-0.44; middle layer = 0.43-0.58 and bottom layer = 0.43-0.54). The predicted DO values and other physical outputs from downscaling of regional weather and climate predictions, or forecasts from hydrodynamic models, can be used to forecast various ecological components. Such forecasts would be useful for both recreational and commercial users of the Chesapeake Bay. [ABSTRACT FROM AUTHOR]

Changes in the physical and chemical structure of estuaries control the aerobic scope for activity of coastal fishes and thereby influence the quality and extent of nursery habitats. We evaluated the effects of temperature, dissolved oxygen, and salinity on the ecophysiology of a species that completes its life cycle in estuaries: white perch ( Morone americana), which were reared at treatment levels that emulated nursery conditions in the Chesapeake Bay. Salinity influenced only consumption rate and energy density, which were diminished at the highest salinity level (16). In hypoxic environments (≤40% saturation), routine metabolic rates increased as much as fourfold while growth rate decreased threefold and consumption rate decreased twofold. Experimental growth rates were within the range of growth rates observed in the field. Results indicate that hypoxia substantially reduces potential nursery production for a dominant estuarine species, through its influence on diminished aerobic capacity for growth and activity. [ABSTRACT FROM AUTHOR]

Spatial interpolation methods are frequently used to estimate values of physical or chemical constituents in locations where they are not measured. Very little research has been conducted, however, to investigate the relative performance of different interpolation methods in surface waters. The study reported here uses archived water quality data from the Chesapeake Bay to compare three spatial interpolation methods: inverse distance weighting, ordinary kriging, and a universal kriging method that incorporates output from a process-based water quality model. Interpolations were performed on salinity, water temperature, and dissolved oxygen “snap shots” (cruise-based data sets) taken between 1985 and 1994 at 21 different depths for multiple locations in the mainstem Bay, using data compiled by the prototypical Chesapeake Bay Environmental Observatory. The kriging methods generally outperform inverse distance weighting for all parameters and depths. Incorporating output from the water quality model through universal kriging appears to improve some of the interpolations by specifically accounting for some physical and biogeochemical features of the estuary. Such integration of process-based information with statistical interpolation warrants further study. [ABSTRACT FROM AUTHOR]

To meet the requirements of the Clean Water Act, the States of Maryland and Virginia are using benthic biological criteria for identifying impaired waters in Chesapeake Bay and reporting their overall condition. The Chesapeake Bay benthic index of biotic integrity (B-IBI) is the basis for these biological criteria. Working together with the states and the US Environmental Protection Agency, we developed a method for impairment decisions based on the B-IBI. The impaired waters decision approach combines multiple benthic habitat-dependent indices in a Bay segment (equivalent to water bodies in the European Water Framework Directive) with a statistical test of impairment. The method takes into consideration uncertainty in reference conditions, sampling variability, multiple habitats, and sample size. We applied this method to 1430 probability-based benthic samples in 85 Chesapeake Bay segments. Twenty-two segments were considered impaired for benthic community condition. The final decision for each segment considers benthic condition in combination with key stressors such as dissolved oxygen and toxic contaminants. [Copyright &y& Elsevier]

In an effort to better portray changing health conditions in Chesapeake Bay and support restoration efforts, a Bay Health Index (BHI) was developed to assess the ecological effects of nutrient and sediment loading on 15 regions of the estuary. Three water quality and three biological measures were combined to formulate the BHI. Water quality measures of chlorophyll-a, dissolved oxygen, and Secchi depth were averaged to create the Water Quality Index (WQI), and biological measures of the phytoplankton and benthic indices of biotic integrity (P-IBI and B-IBI, respectively) and the area of submerged aquatic vegetation (SAV) were averaged to create the Biotic Index (BI). The WQI and BI were subsequently averaged to give a BHI value representing ecological conditions over the growing season (i.e., March–October). Lower chlorophyll-a concentrations, higher dissolved oxygen concentrations, deeper Secchi depths, higher phytoplankton and benthic indices relative to ecological health-based thresholds, and more extensive SAV area relative to restoration goal areas, characterized the least-impaired regions. The WQI, P-IBI and BHI were significantly correlated with (1) regional river flow (r =−0.64, −0.57 and −0.49, respectively; p <0.01), (2) nitrogen (N), phosphorus (P) and sediment loads (all positively correlated with flow), and (3) the sum of developed and agricultural land use (highest annual r 2 =0.86, 0.71 and 0.68, respectively) in most reporting regions, indicating that the BHI is strongly regulated by nutrient and sediment loads from these land uses. The BHI uses ecological health-based thresholds that give an accurate representation of the health conditions in Chesapeake Bay and was the basis for an annual, publicly released environmental report card that debuted in 2007. [Copyright &y& Elsevier]

Excessive nutrient loads to the Chesapeake Bay cause violations of the new dissolved oxygen water quality standard established to protect the Bay’s living resources. Reducing nitrogen and phosphorus loads is necessary to achieve the dissolved oxygen standard. Based on a set of water quality model runs, a response surface method to establish a function of dissolved oxygen (DO) versus total nitrogen (TN) and total phosphorus (TP) loads was used, which plots as a three-dimensional surface. For a specific criterion for DO, i.e., achievement of the DO standard, a curve of DO versus TN and TP loads that meets the DO criterion can be isolated. Each of the paired TN and TP loads on this trade-off curve results in an equivalent level of DO, but usually at different nutrient reduction costs. This paper explores cost-effective alternatives in nutrient reduction to achieve the DO water quality standard in the deep water designated use of Segment CB4, which is the last and most difficult region for achievement of DO standards in the Chesapeake. This paper analyzes DO response surface plots and nitrogen–phosphorus trade-off curves. The effects of nutrient limitation on algal growth, water clarity, and DO concentrations in two different nitrogen and phosphorus load scenarios are examined to understand the responses of water quality to nitrogen and phosphorus trades. [ABSTRACT FROM AUTHOR]

Abstract: Quantifying parameter and prediction uncertainty in a rigorous framework can be an important component of model skill assessment. Generally, models with lower uncertainty will be more useful for prediction and inference than models with higher uncertainty. Ensemble estimation, an idea with deep roots in the Bayesian literature, can be useful to reduce model uncertainty. It is based on the idea that simultaneously estimating common or similar parameters among models can result in more precise estimates. We demonstrate this approach using the Streeter–Phelps dissolved oxygen sag model fit to 29 years of data from Chesapeake Bay. Chesapeake Bay has a long history of bottom water hypoxia and several models are being used to assist management decision-making in this system. The Bayesian framework is particularly useful in a decision context because it can combine both expert-judgment and rigorous parameter estimation to yield model forecasts and a probabilistic estimate of the forecast uncertainty. [Copyright &y& Elsevier]

Two distinct hypoxic patterns were revealed from high-frequency dissolved oxygen (DO) data collected from North Branch of Onancock Creek, a shallow coastal estuary of the Chesapeake Bay, from July to October 2004. Diurnal hypoxia developed associated with large DO swings during fair weather and hypoxialanoxia developed for prolonged 2-5-day periods following rainfall events. A simplified diagnostic DO-algae model was used to investigate DO dynamics in the creek. The model results show that the modeling approach enables important features of the DO dynamics in the creek to be captured and analyzed. Large anthropogenic inputs of nutrients to the creek stimulated macroalgae blooms in the embayment. High DO production resulted in supersaturated DO in daytime, whereas DO was depleted at night as the high respiration overwhelmed the DO supply, leading to hypoxia. Unlike deep-water environments, in this shallow-water system, biological processes dominate DO variations. High macroalgae biomass interacting with low light and high temperature trigger the development of prolonged hypoxic/anoxic postrainfall events. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Nitrous oxide (N2O) is a greenhouse gas and an ozone depletion agent. One of the major uncertainties in the global N2O budget is the contribution of the coastal region, including estuaries, which can be sites of intense N2O efflux. Incubation experiments with nitrogen stable isotope tracer (15N) enabled the investigation of the environmental controls of N2O production in the water column of Chesapeake Bay, the largest estuary in North America. The highest potential rates of N2O production (7.5 ± 1.2 nmol-N L−1 hr−1) were detected during summer anoxia, during which oxidized nitrogen species (nitrate and nitrite) were absent from the water column. At the top of the anoxic layer, N2O production from denitrification was stimulated by addition of nitrate and nitrite. The relative contribution of nitrate and nitrite to N2O production was positively correlated with the ratio of nitrate to nitrite concentrations. Increased oxygen availability, up to 7 µM oxygen inhibited both N2O production and the reduction of nitrate to nitrite. Therefore, reducing the nitrogen input into the Chesapeake Bay has two potential impacts on the N2O efflux: In the short-term, N2O emission will be mitigated due to nitrogen deficiency. In the long-run, eutrophication will be alleviated and subsequent re-oxygenation of the bay will further inhibit N2O production. [ABSTRACT FROM AUTHOR]

Abstract: The Chesapeake 2000 Agreement committed its state and federal signatories to “define the water quality conditions necessary to protect aquatic living resources” in the Chesapeake Bay (USA) and its tidal tributaries. Hypoxia is one of the key water quality issues addressed as a result of the above Agreement. This paper summarizes the protection goals and specific criteria intended to achieve those goals for addressing hypoxia. The criteria take into account the variety of Bay habitats and the tendency towards low dissolved oxygen in some areas of the Bay. Stressful dissolved oxygen conditions were characterized for a diverse array of living resources of the Chesapeake Bay by different aquatic habitats: migratory fish spawning and nursery, shallow-water, open-water, deep-water, and deep-channel. The dissolved oxygen criteria derived for each of these habitats are intended to protect against adverse effects on survival, growth, reproduction and behavior. The criteria accommodate both spatial and temporal aspects of low oxygen events, and have been adopted into the Chesapeake Bay states – Maryland, Virginia, and Delaware – and the District of Columbia''s water quality standards regulations. These criteria, now in the form of state regulatory standards, are driving an array of land-based and wastewater pollution reduction actions across the six-watershed. [Copyright &y& Elsevier]