Discerning effects of warming, sea level rise and nutrient management on long-term hypoxia trends in Chesapeake Bay

Community coalescence under variable hydrochemical conditions of the Chesapeake Bay shaped bacterial diversity and functional traits

Community coalescence related to bacterial mixing events regulates community characteristics and affects the health of estuary ecosystems. At present, bacterial coalescence and its driving factors are still unclear. The present study used a dataset from the Chesapeake Bay (2017) to address how bacterial community coalescence in response to variable hydrochemistry in estuarine ecosystems. We determined that variable hydrochemistry promoted the deterioration of water quality. Temperature, orthophosphate, dissolved oxygen, chlorophyll a, Secchi disk depth, and dissolved organic phosphorus were the key environmental factors driving community coalescence. Bacteria with high tolerance to environmental change were the primary taxa accumulated in community coalescence, and the significance of deterministic processes to communities was revealed. Community coalescence was significantly correlated with the pathways of metabolism and organismal systems, and promoted the co-occurrence of antibiotic resistance and virulence factor genes. Briefly, community coalescence under variable hydrochemical conditions shaped bacterial diversity and functional traits, to optimise strategies for energy acquisition and lay the foundation for alleviating environmental pressures. However, potential pathogenic bacteria in community coalescence may be harmful to human health and environmental safety. The present study provides a scientific reference for ecological management of estuaries.

The impact of extreme precipitation on physical and biogeochemical processes regarding with nutrient dynamics in a semi-closed bay

An extreme precipitation event in August 2012 changed the ecosystem of Jiaozhou Bay (JZB), China. Biochemical variables in the sea, river mouths, and rainwater were monitored simultaneously during the event. The impact of the following excessive riverine input and wet atmospheric deposition on nutrient dynamics were studied before. However, regulatory processes of nutrient dynamics were not quantified and analyzed. Therefore, a coupled physical-biological model (FVCOM-ERSEM) was used to study the physical and biochemical mechanisms of the variation of the dissolved inorganic nitrogen (DIN), phosphorus (DIP), and silicon (DISi), as well as chlorophyll-a (Chl-a). The results indicate that physical processes increase nutrients, while biological processes reduce them. The exchange with the Yellow Sea, as an important physical process, exports DIN to the Yellow Sea, but imports DIP and DISi to the JZB. Only 20 % of the excessive DIN due to extreme precipitation event was reduced by water exchange with the Yellow Sea. The rest (80 %) was reduced and changed into organic nitrogen through biological processes. This paper also examines the variation of the pelagic and benthic cycles of biochemical processes. In these cycles, phytoplankton take up and use nutrients in the bay, while zooplankton excretion in the pelagic cycle and benthic releases resupply them. Precipitation enriched the surface nutrients, which boosted primary production and organic matter transport to the bottom water.

What drove the nonlinear hypoxia response to nutrient loading in Chesapeake Bay during the 20th century?

Previous data analysis showed that the large expansion of hypoxia in Chesapeake Bay between 1950s and 1980s was correlated to the increased riverine nutrient loading, but the physical and biogeochemical processes driving this hypoxia response need to be better understood. Using a validated coupled hydrodynamic-biogeochemical model, we conducted a hindcast simulation of dissolved oxygen during the 40-year period (1950-1989) when the nutrient loading doubled. The model reproduced the observed decline in O₂ concentration at monitoring stations and the expansion of the hypoxic volume. The peak summer hypoxic volume expanded from ∼5 km³ during 1950-1969 to ∼10 km³ during 1970-1989. To discern how different physical and biochemical processes regulated dissolved O₂, we examined O₂ budget in a fixed control volume of the bottom water most susceptible to hypoxia. The increased water column respiration was found to be the dominant driver of the hypoxia expansion. Further analysis showed a nonlinear response to the nutrient loading. The accumulative hypoxia volume days per unit of nitrate load showed an abrupt (∼50 %) jump around 1968. The summer mean hypoxic volume increased with the winter-spring nutrient load, but it was 1.3 km³ (about 30 %) higher in 1968-1989 than in 1950-1967 at the same nutrient load. This upward shift in hypoxia was caused by the upward shift in the relationship between the water column respiration and winter-spring nutrient load. Hypoxia suppressed nitrification and denitrification processes in the sediment, amplifying nutrient recycling by 15 % and water column respiration by 12 %. Our modeling analysis demonstrated a feedback mechanism for driving the nonlinear hypoxia response to nutrient loading.

Water quality changes and shift in mechanisms controlling hypoxia in response to pollutant load reductions: A case study for Shiziyang Bay, Southern China

Shiziyang Bay, located in the upstream of the Pearl River Estuary, has frequently suffered from hypoxia since 2000, which has persisted in recent years despite effective controls on anthropogenic pollutant loads. To explore the underlying causes, changes in dissolved oxygen (DO), nutrients, chemical oxygen demand (COD), and chlorophyll a (Chl a) along the bay in response to altered pollutant inputs were investigated using observations collected in summers of 2015-2019 and historical data during 2000-2008. In addition, DO sources and sinks were calculated based on data from August 2020 and laboratory incubations for water column respiration (WCR) and sediment oxygen uptake, and were compared with their equivalents in August 2008 to elucidate changes in primary processes controlling hypoxia. The results showed that ammonia has decreased significantly with pollutant control, while other parameters responded in different trends, especially for Chl a (with a substantial increase over time). The intensified eutrophication contributed to high COD levels, leading to a strong WCR (as dominant oxygen depletion) close to that in the 2000s and thereby maintaining low-oxygen conditions despite reduced effluent discharges. The shifted primary oxygen-consuming substances from allochthonous inputs to in-situ phytoplankton production were also evidenced by significant correlation between oxygen consumption rate and Chl a in recent data. Simultaneously, the enhanced algal blooms could also modulate oxygen supply, resulting in higher photosynthetic oxygen production and lower air-sea reaeration compared with the past. Furthermore, the impact of major environmental changes on exacerbated eutrophication was explored and it was speculated that notable declined sediment loads would be important by improving light conditions to promote phytoplankton proliferation in the bay. Collectively, substantial control on eutrophication as well as tracking DO source-to-sink processes is of great importance to mitigate hypoxia in Shiziyang bay.

Nitrogen reductions have decreased hypoxia in the Chesapeake Bay: Evidence from empirical and numerical modeling

Seasonal hypoxia is a characteristic feature of the Chesapeake Bay due to anthropogenic nutrient input from agriculture and urbanization throughout the watershed. Although coordinated management efforts since 1985 have reduced nutrient inputs to the Bay, oxygen concentrations at depth in the summer still frequently fail to meet water quality standards that have been set to protect critical estuarine living resources. To quantify the impact of watershed nitrogen reductions on Bay hypoxia during a recent period including both average discharge and extremely wet years (2016-2019), this study employed both statistical and three-dimensional (3-D) numerical modeling analyses. Numerical model results suggest that if the nitrogen reductions since 1985 had not occurred, annual hypoxic volumes (O₂ < 3 mg L^-1) would have been ~50-120% greater during the average discharge years of 2016-2017 and ~20-50% greater during the wet years of 2018-2019. The effect was even greater for O₂ < 1 mg L^-1, where annual volumes would have been ~80-280% greater in 2016-2017 and ~30-100% greater in 2018-2019. These results were supported by statistical analysis of empirical data, though the magnitude of improvement due to nitrogen reductions was greater in the numerical modeling results than in the statistical analysis. This discrepancy is largely accounted for by warming in the Bay that has exacerbated hypoxia and offset roughly 6-34% of the improvement from nitrogen reductions. Although these results may reassure policymakers and stakeholders that their efforts to reduce hypoxia have improved ecosystem health in the Bay, they also indicate that greater reductions are needed to counteract the ever-increasing impacts of climate change.

A three-dimensional mixotrophic model of Karlodinium veneficum blooms for a eutrophic estuary

Blooms of dinoflagellate Karlodinium veneficum are widely distributed in estuarine and coastal waters and have been found to cause fish kills worldwide. K. veneficum has a mixed nutritional mode and relies on both photosynthesis and phagotrophy for growth; it is a mixotroph. Here, a model of mixotrophic growth of K. veneficum (MIXO) was developed, calibrated with previously-reported laboratory physiological data, and subsequently embedded in a 3D-coupled hydrodynamic (ROMS)-biogeochemical (RCA) model of eutrophic Chesapeake Bay, USA. The resulting ROMS-RCA-MIXO model was applied in hindcast mode to investigate seasonal and spatial distributions. Simulations showed that K. veneficum blooms occurred during June-August and were confined to the upper and middle Bay, consistent with long-term field observations. Autotrophic growth dominated in spring but heterotrophic growth dominated during the summer. The number of prey ingested by K. veneficum varied from 0.1 to 0.6 day^-1 and the food vacuole content reached up to 50% of the core mixotroph biomass. The ingestion rate increased with prey density and also when P:N ratio fell below ∼0.03 (N:P ∼ 33), indicating that K. veneficum only switched to mixotrophic feeding in P-deficient waters when sufficient prey were available; this occurred during the summer months. The digestion rate increased with both the food vacuole content and temperature. The modeling analysis affirms K. veneficum as a phagotrophic 'alga' which is primarily photosynthetic but switches to mixotrophic feeding under nutrient deficient conditions.

The Chesapeake Bay Program Modeling System: Overview and Recommendations for Future Development

The Chesapeake Bay is the largest, most productive, and most biologically diverse estuary in the continental United States providing crucial habitat and natural resources for culturally and economically important species. Pressures from human population growth and associated development and agricultural intensification have led to excessive nutrient and sediment inputs entering the Bay, negatively affecting the health of the Bay ecosystem and the economic services it provides. The Chesapeake Bay Program (CBP) is a unique program formally created in 1983 as a multi-stakeholder partnership to guide and foster restoration of the Chesapeake Bay and its watershed. Since its inception, the CBP Partnership has been developing, updating, and applying a complex linked modeling system of watershed, airshed, and estuary models as a planning tool to inform strategic management decisions and Bay restoration efforts. This paper provides a description of the 2017 CBP Modeling System and the higher trophic level models developed by the NOAA Chesapeake Bay Office, along with specific recommendations that emerged from a 2018 workshop designed to inform future model development. Recommendations highlight the need for simulation of watershed inputs, conditions, processes, and practices at higher resolution to provide improved information to guide local nutrient and sediment management plans. More explicit and extensive modeling of connectivity between watershed landforms and estuary sub-areas, estuarine hydrodynamics, watershed and estuarine water quality, the estuarine-watershed socioecological system, and living resources will be important to broaden and improve characterization of responses to targeted nutrient and sediment load reductions. Finally, the value and importance of maintaining effective collaborations among jurisdictional managers, scientists, modelers, support staff, and stakeholder communities is emphasized. An open collaborative and transparent process has been a key element of successes to date and is vitally important as the CBP Partnership moves forward with modeling system improvements that help stakeholders evolve new knowledge, improve management strategies, and better communicate outcomes.

A three-dimensional mechanistic model of Prorocentrum minimum blooms in eutrophic Chesapeake Bay

Planktonic Prorocentrum, common harmful dinoflagellate, are increasing in frequency, duration, and magnitude globally, as exemplified by the number of blooms of P. minimum in Chesapeake Bay that have nearly doubled over the past 3 decades. Although the dynamics of transport and seasonal occurrence of this species have been previously described, it has been challenging to predict the timing and location of P. minimum blooms in Chesapeake Bay. We developed a new three-dimensional mechanistic model of this species that integrates physics, nutrient cycling and plankton physiology and embedded it within a coupled hydrodynamic-biogeochemical model originally developed for simulating water quality in eutrophic estuarine and coastal waters. Hindcast simulations reproduced the observed time series and spatial distribution of cell density, in particular capturing well its peak in May in the mid-to-upper part of the estuary. Timing and duration of the blooms were mostly determined by the temperature-dependent growth function, while mortality due to grazing and respiration played a minor role. The model also reproduced the pattern of overwintering populations, which are located in bottom waters of the lower Bay, and are transported upstream in spring by estuarine flow. Blooms develop in the mid-upper parts of the estuary when these transported cells encounter high nutrient concentrations from the Susquehanna River and favorable light conditions. Diagnostic analysis and model-sensitivity experiments of nutrient conditions showed that high nitrogen:phosphorus conditions favor bloom development. The model also captured the observed interannual variations in the magnitude and spatial distribution of P. minimum blooms.

Reducing Hypoxia in an Urban Estuary Despite Climate Warming

Seasonal hypoxia is a serious threat to coastal ecosystems. This study on hypoxia in Long Island Sound (LIS), a large urbanized estuary, focuses on responses to managed nitrogen load reductions and climate change. At the analyzed station in western LIS, warming in bottom waters (0.8 °C per decade) favors hypoxia. Total nitrogen concentrations have decreased (0.06 mg L^-1 per decade) with load reductions, but no linear temporal trend in chlorophyll is discernible. Bottom dissolved oxygen has increased (0.48 mg L^-1 per decade), despite warming-induced solubility decreases (0.13 mg L^-1 per decade). Decreasing trends in hypoxic area and volume (100 km² and 1 km³ per decade) reflect improved conditions and are coincident with reducing loads. Regressions link hypoxic extent to nitrogen loads, chlorophyll, salinity, and winds. Though mitigation has reduced hypoxia, these improvements will not be sustained in the warming climate without continued intervention. The warming-induced oxygen solubility decrease forecasted for 2099 (0.4 mg L^-1) would erode 35% of the observed oxygen gains. Implementing a nitrogen load reduction of 1.2 × 10⁶ kg year^-1 before the century's end would offset the oxygen solubility decline. This overall approach is applicable to areas experiencing warming and continued development that complicate efforts to reign in hypoxia.