An official website of the United States government
Abstract Dissolved organic matter (DOM) is the foundation of the microbial loop and plays an important role in estuarine water quality and ecosystem metabolism. Because estuaries are influenced by DOM with different sources and composition, changing hydrologic regimes, and diverse microbial community assemblages, the biological fate of DOM (i.e., microbial degradation) differs across spatiotemporal scales and between DOM pools. To better understand controls on DOM degradation, we characterized the biogeochemical and physical conditions of the York River Estuary (YRE), a sub-estuary of the Chesapeake Bay in southeast Virginia (USA), during October 2018 and February, April, and July 2019. We then evaluated how these conditions influenced the degradation of dissolved organic carbon (DOC) and nitrogen (DON) and chromophoric dissolved organic matter (CDOM) by conducting parallel dark incubations of surface water collected along the YRE. Compared to other sampling dates, DOC reactivity (ΔDOC (%)) was over two-fold higher in October when freshwater discharge was lower, temperatures were warmer, and autochthonous, aquatic sources of DOC dominated. ΔDOC (%) was near zero when allochthonous, terrestrial sources of DOC were more abundant and when temperatures were cooler during higher discharge periods in February when precipitation in the Chesapeake Bay region was anomalously high. DON was up to six times less reactive than DOC and was sometimes produced during the incubations whereas ΔCDOM (%) was highly variable between sampling periods. Like ΔDOC (%), spatiotemporal patterns in ΔDON (%) were controlled primarily by hydrology and DOM source and composition. Our results show that higher freshwater discharge associated with prolonged wet periods decreased estuarine flushing time and increased the delivery of allochthonous DOM derived from terrestrial sources into coastal waters, resulting in lower rates of DOM degradation especially under cool conditions. While these findings provide evidence for seasonal variation in DOM degradation, shifting environmental conditions (e.g., increasing temperatures and precipitation) due to climate change may also have interactive effects on the magnitude and composition of DOM exported to estuaries and its subsequent reactivity.
more »
« less
Importance of Estuary–Ocean Exchange on Hypoxia in Mid-Lower Chesapeake Bay
Abstract In previous water quality modeling studies in Chesapeake Bay, the severity of summer hypoxia tended to be underestimated in the mid-lower Bay area. The underlying reason has not been well understood. In this study, we test a new hypothesis with respect to the estuary–ocean exchange. This hypothesis was motivated by observed high levels of dissolved organic carbon (DOC) near the Bay mouth that, if transported into the Bay, could potentially draw down the dissolved oxygen (DO) in the mid-lower Bay through the microbial respiration of DOC. Using a high-resolution 3D water quality model that includes both the Bay and a portion of the coastal ocean, we provide evidence that supports this hypothesis and demonstrates the important connection between the Bay and the coastal ocean that entrains DOC into the mid-lower Bay during the summer hypoxic season, which leads to a more realistic representation of DO dynamics there. We also outline remaining research questions about the DOC reactivity and sources in the Chesapeake Bay that warrant further study.
more »
« less
- Award ID(s):
- 2048902
- PAR ID:
- 10670084
- Publisher / Repository:
- Springer Nature
- Date Published:
- Journal Name:
- Estuaries and Coasts
- Volume:
- 49
- Issue:
- 1
- ISSN:
- 1559-2723
- Page Range / eLocation ID:
- 1-17
- Subject(s) / Keyword(s):
- Hypoxia, 3D water quality modeling, Estuary–ocean exchange, Dissolved organic carbon
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Shallow coastal ecosystems are threatened by marine heatwaves, but few long-term records exist to quantify these heatwaves. Here, 40-year records of measured water temperature were constructed for a site in a system of shallow bays with documented heatwave impacts and a nearby ocean site; available gridded sea-surface temperature datasets in the region were also examined. Water temperatures at both sites increased significantly though bay temperatures were consistently 3-4°C hotter in summer and colder in winter and were more variable overall, differences not captured in high-resolution gridded sea-surface temperature datasets. There was considerable overlap in heatwave events at the coastal bay and ocean sites. Annual heatwave exposure was similar and significantly increased at both sites while annual heatwave intensity was significantly higher at the bay site owing to the high variance of the daily temperature anomaly there. Event frequency at both sites increased at a rate of about 1 event/decade. Future simulations indicate all heatwave metrics increase, as do days above 28°C, a heat stress threshold for seagrass. Ocean temperatures on the U.S. mid-Atlantic margin have rarely exceeded this threshold, while summer bay temperatures commonly do, allowing ocean exchange with coastal bays to provide thermal relief to bay ecosystems. This will have changed by 2100, creating a thermal environment that threatens seagrass communities in these systems. Documenting such change requires development of long-term water temperature records in more shallow coastal systems.more » « less
-
Small crustaceans, such as the mysid Neomysis americana (S.I. Smith 1873), are a central component of coastal food webs and, while generally tolerant of a wide-range of environmental conditions, can be negatively affected by poor water quality. In this study, daily growth rates (GRD) and clutch size metrics of N. americana collected during the early and late summer of 2018–2019 were evaluated for the Choptank and Patuxent rivers, major tributaries of Chesapeake Bay known to exhibit different oxygenation regimes. Genetic variation in the mitochondrial CO1 locus was assessed to evaluate the potential intraspecific genetic structure within Chesapeake Bay. CO1 haplotype network analysis, phylogenetic analysis, and analysis of molecular variance revealed no genetic differences between Choptank and Patuxent river populations, with all Chesapeake Bay individuals belonging to a single genetic lineage (lineage C), of the N. americana cryptic species complex. Total and size-specific clutch size were approximately 18% and 53% higher, respectively, in the normoxic Choptank River during the early summer. Embryos within the marsupium, corrected for clutch size and female length, were consistently larger in the Choptank River during later larval development stages. Size-specific clutch size showed correlations with bottom water dissolved oxygen concentration (positive) and water temperature (negative). GRD did not differ between rivers or seasonally but juveniles grew twice as fast as adults. Given that all individuals genotyped from both rivers belonged to lineage C of the N. americana cryptic species complex, it is hypothesized that bottom water hypoxia (rather than genetic differentiation) is responsible for reduced clutch size in the Patuxent River. Our findings build on other recent work by providing evidence of a direct, negative relationship between hypoxia and local population dynamics of N. americana, a key ecological component of Chesapeake Bay’s food web.more » « less
-
Abstract River networks serve as conduits for dissolved organic matter (DOM) and carbon (DOC) from inland to coastal waters. Human activities and climate change are altering DOM sources, causing hydrological and biogeochemical shifts that impact DOC concentrations and changing the transport and transformation of DOM and DOC. Here, we synthesize current knowledge of changing DOM sources, DOC concentrations, and the associated hydrological and biogeochemical changes during transport along inland‐to‐coastal gradients, focusing on impacts to coastal and estuarine DOM and DOC. We project that continued land‐use changes, hydrological management, and sea‐level rise will result in more microbial and labile DOM, higher DOC concentrations, and an overall decoupling of DOC quantity and DOM quality. Understanding how these changes vary among river networks is essential to forecast coastal and estuarine water quality, ecosystem health, and global carbon cycling.more » « less
-
The size and distribution of Phytoplankton populations are indicators of the ecological status of a water body. The chlorophyll-a (Chl-a) concentration is estimated as a proxy for the distribution of phytoplankton biomass. Remote sensing is the only practical method for the synoptic assessment of Chl-a at large spatial and temporal scales. Long-term records of ocean color data from the MODIS Aqua Sensor have proven inadequate to assess Chl-a due to the lack of a robust ocean color algorithm. Chl-a estimation in shallow and coastal water bodies has been a challenge and existing operational algorithms are only suitable for deeper water bodies. In this study, the Ocean Color 3M (OC3M) derived Chl-a concentrations were compared with observed data to assess the performance of the OC3M algorithm. Subsequently, a regression analysis between in situ Chl-a and remote sensing reflectance was performed to obtain a green-red band algorithm for coastal (case 2) water. The OC3M algorithm yielded an accurate estimate of Chl-a for deep ocean (case 1) water (RMSE = 0.007, r2 = 0.518, p < 0.001), but failed to perform well in the coastal (case 2) water of Chesapeake Bay (RMSE = 23.217, r2 = 0.009, p = 0.356). The algorithm developed in this study predicted Chl-a more accurately in Chesapeake Bay (RMSE = 4.924, r2 = 0.444, p < 0.001) than the OC3M algorithm. The study indicates a maximum band ratio formulation using green and red bands could improve the satellite estimation of Chl-a in coastal waters.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.1007/s12237-025-01622-0
