Reducing Hypoxia in an Urban Estuary Despite Climate Warming

Driving factors of colored dissolved organic matter dynamics across a complex urbanized estuary

The role of estuaries in sourcing and transforming dissolved organic matter - the largest reservoir of organic carbon in the ocean - still presents many unknowns for coastal biogeochemical cycles, and is further complicated by increasing human pressures and a changing climate. Here, we examined the major drivers of colored dissolved organic matter (CDOM) dynamics in Long Island Sound (LIS), a heavily urbanized estuary of National Significance with a storied water quality past. A comprehensive new optical dataset, including measurements of CDOM absorption and fluorescence signatures, was integrated with biological and hydrological measurements to capture the spatiotemporal heterogeneities of LIS, including its urban-to-rural gradient, dynamic river mouths, and blue carbon ecosystems across seasons, following episodic storm events, and over five years. Results reveal longitudinal gradients in both DOM amount and quality. While carbon-rich and humic terrigenous DOM was dominant in the heavily riverine-influenced Central to Eastern LIS, an uncoupling between CDOM absorption (aCDOM) and dissolved organic carbon (DOC) concentration in Western LIS, and a stronger correlation with Chlorophyll-a, indicated increased autochthonous CDOM production. Closer to the New York City urban core, aCDOM was highly correlated to turbidity, consistent with increased wastewater influences. Fluorescence PARAFAC analysis provided strong evidence for seasonal processing of CDOM in LIS, related to increased summertime photochemical degradation of humic-like components and shoulder-season microbial processing. Riverine CDOM export was influenced by discharge amount, residence time, and coastal wetlands acting as additional sources of strongly humic and aromatic organic matter. These measurements allowed us to assess how hydrologic, biological, and anthropogenic processes impact DOM dynamics and, subsequently, biogeochemical variability and trophic status in this complex urbanized estuary, with implications for water quality management and policy. Results discussed here are applicable beyond LIS, as urbanized estuaries globally face similar hydrological and anthropogenic forcings.

Phytoplankton assemblage responses to nitrogen following COVID-19 stay-in-place orders in western Long Island Sound (New York/Connecticut)

This study evaluated water quality, nitrogen (N), and phytoplankton assemblage linkages along the western Long Island Sound (USA) shoreline (Nov. 2020-Dec. 2021) following COVID-19 stay-in-place (SIP) orders through monthly surveys and N-addition bioassays. Ammonia-N (AmN; NH3+NH4+) negatively correlated with total chlorophyll-a (chl-a) at all sites; this was significant at Alley Creek, adjacent to urban wastewater inputs, and at Calf Pasture, by the Norwalk River (Spearman rank correlation, p < 0.01 and 0.02). Diatoms were abundant throughout the study, though dinoflagellates (Heterocapsa, Prorocentrum), euglenoids/cryptophytes, and both nano- and picoplankton biomass increased during summer. In field and experimental assessments, high nitrite + nitrate (N + N) and low AmN increased diatom abundances while AmN was positively linked to cryptophyte concentrations. Likely N + N decreases with presumably minimal changes in AmN and organic N during COVID-19 SIP resulted in phytoplankton assemblage shifts (decreased diatoms, increased euglenoids/cryptophytes), highlighting the ecological impacts of N-form delivered by wastewater to urban estuaries.

The concentration of CH4, N2O and CO2 in the Pearl River estuary increased significantly due to the sediment particle resuspension and the interaction of hypoxia

Hypoxia and sediment particle resuspension (SPR) alter the biogeochemical cycle of estuarine and coastal seas, which in turn affects the production and emission of methane (CH4), nitrous oxide (N2O) and carbon dioxide (CO2) greenhouse gases (GHGs) in estuaries. Despite the importance of CH4, N2O and CO2 in estuarine ecosystems, little is known about their magnitude and spatiotemporal variation under the combined influence of hypoxia and SPR. This study utilized continuous mooring observations to investigate the temporal and spatial variations of GHGs before and after hypoxia in the Pearl River Estuary (PRE). The results showed that the concentration of GHGs in the water column increased significantly following hypoxia as compared to its absence. The synergistic effect of SPR and hypoxia significantly enhances GHGs production and accumulation in bottom water. Anaerobic mineralization of organic matter (OM) in an environment with severely low dissolved oxygen (DO) is the primary determinant for increased CH4 concentration, while OM and CH4 oxidation are the main drivers for maintaining high CO2 concentration in subsurface water. Hypoxic development enhanced denitrification N2O production in the water column. The presence of SPR enhanced oxygen-consuming coupled hypoxia significantly stimulated the increase of CH4, N2O and CO2 concentrations in the water column. Hypoxic development results in an increased water-air GHGs flux, but this effect may be masked by runoff plumes with high GHGs concentrations in the regions near the river outlets. This study highlights that hypoxia leads to significant increases in anaerobic GHGs production and subsequent emissions from estuarine water columns.

Deciphering the water quality impacts of COVID-19 human mobility shifts in estuaries surrounding New York City

The COVID-19 pandemic altered human mobility, particularly in large metropolitan areas. In New York City (NYC), stay-at-home orders and social distancing led to significant decreases in commuting, tourism, and a surge of outward migration. Such changes could result in decreased anthropogenic pressure on local environments. Several studies have linked COVID-19 shutdowns with improvements in water quality. However, the bulk of these studies primarily focused on short-term impacts during shutdown periods, without assessing longer-term impacts as restrictions eased. Here, we examine both concurrent lockdown and societal reopening impacts on water quality, using pre-pandemic baseline conditions, in two highly urbanized estuaries surrounding NYC, the New-York Harbor estuary and Long Island Sound (LIS). We compiled datasets from 2017 to 2021 of mass-transit ridership, work-from-home trends, and municipal wastewater effluent to assess changes in human mobility and anthropogenic pressure during multiple waves of the pandemic in 2020 and 2021. These were linked to changes in water quality assessed using high spatiotemporal ocean color remote sensing, which provides near-daily observations across the estuary study regions. To distinguish anthropogenic impacts from natural environmental variability, we examined meteorological/hydrological conditions, primarily precipitation and wind. Our results show that nitrogen loading into the New York Harbor declined significantly in the spring of 2020 and remained below pre-pandemic values through 2021. In contrast, nitrogen loading into LIS remained closer to the pre-pandemic average. In response, water clarity in New-York Harbor significantly improved, with less of a change in LIS. We further show that changes in nitrogen loading had higher impact on water quality than meteorological conditions. Our study demonstrates the value of remote sensing observations in assessing water quality changes when field-based monitoring is hindered and highlights the complex nature of urban estuaries and their heterogeneous response to changes in extreme events and human behavior.

High-frequency dissolved oxygen dynamics in an urban estuary, the Long Island Sound

The seasonal occurrence of deep-water hypoxia in western Long Island Sound (LIS) has been documented for decades by water quality cruise surveys and fixed mooring buoys. While previous studies have focused on factors modulating bottom dissolved oxygen (DO) at subtidal timescales, here we analyze continuous timeseries data from a moored buoy during summers 2021 and 2022 to examine factors controlling high-frequency fluctuations in surface and bottom DO at diurnal and semidiurnal timescales. Fluctuations in surface DO at diurnal timescales are associated with biological production, while fluctuations in bottom DO near semidiurnal timescales are associated with horizontal advection of DO by tides from the upper East River tidal strait into western LIS. Results from timeseries analysis are supported by weekly cruise surveys that resolve horizontal and vertical DO gradients in the western narrows. However, inferences regarding the duration of hypoxia during a given summer vary across datasets in part because weekly survey data do not resolve dominant timescales of variability within a particular summer. While prior studies have illustrated the importance of nutrient loading, stratification, and wind in controlling the development of hypoxia, the results presented here demonstrate the role of tidal advection in modulating hypoxia in far western LIS. Despite stronger stratification in 2021, the duration of hypoxia was 11.1 days shorter compared to 2022 in part due to greater advection of DO by tidal currents that intermittently increased bottom DO near the buoy. Furthermore, five-year averaged hypoxic area in the western narrows has increased since 2017, which highlights the spatially variable response of DO to nutrient load reductions. Future analysis of hypoxia in LIS should focus on leveraging high-frequency information contained in continuous datasets to improve estimates of hypoxia based on less temporally resolved water quality surveys.

The effects of climate warming on microbe-mediated mechanisms of sediment carbon emission

Due to significant differences in biotic and abiotic properties of soils compared to those of sediments, the predicted underlying microbe-mediated mechanisms of soil carbon emissions in response to warming may not be applicable for estimating similar emissions from inland water sediments. We addressed this issue by incubating different types of sediments, (including lake, small river, and pond sediments) collected from 36 sites across the Yangtze River basin, under short-term experimental warming to explore the effects of climate warming on sediment carbon emission and the underlying microbe-mediated mechanisms. Our results indicated that under climate warming CO₂ emissions were affected more than CH₄ emissions, and that pond sediments may yield a greater relative contribution of CO₂ to total carbon emissions than lake and river sediments. Warming-induced CO₂ and CH₄ increases involve different microbe-mediated mechanisms; Warming-induced sediment CO₂ emissions were predicted to be directly positively driven by microbial community network modularity, which was significantly negatively affected by the quality and quantity of organic carbon and warming-induced variations in dissolved oxygen, Conversely, warming-induced sediment CH₄ emissions were predicted to be directly positively driven by microbial community network complexity, which was significantly negatively affected by warming-induced variations in pH. Our findings suggest that biotic and abiotic drivers for sediment CO₂ and CH₄ emissions in response to climate warming should be considered separately when predicting sediment organic carbon decomposition dynamics resulting from climate change.

Distribution of nitrogen (N) and phosphorus (P) in seasonal low-oxygen marine ranching in northern Yellow Sea, China

Seasonal low-oxygen in marine ranching in the northern Yellow Sea has been one of the major environmental problems in coastal waters in recent years. Nitrogen (N) and phosphorus (P) are important nutrients, which are susceptible to the concentration of dissolved oxygen (DO). This article studied the effects of low-oxygen on nutrients represented by N and P fractions in marine ranching in the northern Yellow Sea. The results showed that there were significant layer differences in temperature and salinity during the low-oxygen period. In the seawater, the nutrient distribution in the death disaster zone of sea cucumbers and the non-disaster zone was similar, and DO had a strong positive correlation with dissolved inorganic nitrogen (DIN). In the sediment, significant regional differences existed in nutrient concentration, and the concentration of total phosphorus (TP) decreased significantly with the increase in DO content. The results showed that the sources and sinks of nitrogen and phosphorus nutrients were inconsistent in this zone, and migration and transformation of the existing form of nitrogen with the seasonal changes in the water environment was a main factor for N distribution. This study extended the understanding of the effects of seasonal low-oxygen on N and P.

Transitions in nitrogen and organic matter form and concentration correspond to bacterial population dynamics in a hypoxic urban estuary

Nitrogen (N) inputs to developed coastlines are linked with multiple ecosystem and socio-economic impacts worldwide such as algal blooms, habitat/resource deterioration, and hypoxia. This study investigated the microbial and biogeochemical processes associated with recurrent, seasonal bottom-water hypoxia in an urban estuary, western Long Island Sound (WLIS), that receives high N inputs. A 2-year (2020-2021) field study spanned two hypoxia events and entailed surface and bottom depth water sampling for dissolved nutrients as inorganic N (DIN; ammonia-N and nitrite + nitrate (N + N)), organic N, orthophosphate, organic carbon (DOC), as well as chlorophyll a and bacterial abundances. Physical water quality data were obtained from concurrent conductivity, temperature, and depth casts. Results showed that dissolved organic matter was highest at the most-hypoxic locations, DOC was negatively and significantly correlated with bottom-water dissolved oxygen (Pearson's r = -0.53, p = 0.05), and ammonia-N was the dominant DIN form pre-hypoxia before declining throughout hypoxia. N + N concentrations showed the reverse, being minimal pre-hypoxia then increasing during and following hypoxia, indicating that ammonia oxidation likely contributed to the switch in dominant DIN forms and is a key pathway in WLIS water column nitrification. Similarly, at the most hypoxic sampling site, bottom depth bacteria concentrations ranged ~ 1.8 × 10⁴-1.1 × 10⁵ cells ml^-1 pre-hypoxia, declined throughout hypoxia, and were positively and significantly correlated (Pearson's r = 0.57; p = 0.03) with ammonia-N, confirming that hypoxia influences N-cycling within LIS. These findings provide novel insight to feedbacks between major biogeochemical (N and C) cycles and hypoxia in urban estuaries.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s10533-023-01021-2.