A unique combination of data collected from fixed instruments, spatial surveys, and a long‐term observing network in the Hudson River demonstrate the importance of spatial and temporal variations in atmospheric gas flux. The atmospheric exchanges of oxygen (O2) and carbon dioxide (CO2) exhibit variability at a range of time scales including pronounced modulation driven by spring‐neap variations in stratification and mixing. During weak neap tides, bottom waters become enriched in pCO2and depleted in dissolved oxygen because strong stratification limits vertical mixing and isolates sub‐pycnocline water from atmospheric exchange. Estuarine circulation also is enhanced during neap tides so that bottom waters, and their associated dissolved gases, are transported up‐estuary. Strong mixing during spring tides effectively ventilates bottom waters resulting in enhanced CO2evasion and O2invasion. The spring‐neap modulation in the estuarine portion of the Hudson River is enhanced because fortnightly variations in mixing have a strong influence on phytoplankton dynamics, allowing strong blooms to occur during weak neap tides. During blooms, periods of CO2invasion and O2evasion occur over much of the lower stratified estuary. The along‐estuary distribution of stratification, which decreases up‐estuary, favors enhanced gas exchange near the limit of salt, where vertical stratification is absent. This region, which we call the estuarine gas exchange maximum (EGM), results from the convergence in bottom transport and is analogous to the estuarine turbidity maximum (ETM). Much like the ETM, the EGM is likely to be a common feature in many partially mixed and stratified estuarine systems.
- Award ID(s):
- 2053240
- NSF-PAR ID:
- 10388335
- Date Published:
- Journal Name:
- Biogeosciences
- Volume:
- 19
- Issue:
- 14
- ISSN:
- 1726-4189
- Page Range / eLocation ID:
- 3523 to 3536
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
Abstract -
Leads play an important role in the exchange of heat, gases, vapour, and particles between seawater and the atmosphere in ice-covered polar oceans. In summer, these processes can be modified significantly by the formation of a meltwater layer at the surface, yet we know little about the dynamics of meltwater layer formation and persistence. During the drift campaign of the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC), we examined how variation in lead width, re-freezing, and mixing events affected the vertical structure of lead waters during late summer in the central Arctic. At the beginning of the 4-week survey period, a meltwater layer occupied the surface 0.8 m of the lead, and temperature and salinity showed strong vertical gradients. Stable oxygen isotopes indicate that the meltwater consisted mainly of sea ice meltwater rather than snow meltwater. During the first half of the survey period (before freezing), the meltwater layer thickness decreased rapidly as lead width increased and stretched the layer horizontally. During the latter half of the survey period (after freezing of the lead surface), stratification weakened and the meltwater layer became thinner before disappearing completely due to surface ice formation and mixing processes. Removal of meltwater during surface ice formation explained about 43% of the reduction in thickness of the meltwater layer. The remaining approximate 57% could be explained by mixing within the water column initiated by disturbance of the lower boundary of the meltwater layer through wind-induced ice floe drift. These results indicate that rapid, dynamic changes to lead water structure can have potentially significant effects on the exchange of physical and biogeochemical components throughout the atmosphere–lead–underlying seawater system.
-
Abstract Storms deepen the mixed layer, entrain nutrients from the pycnocline, and fuel phytoplankton blooms in midlatitude oceans. However, the effects of oceanic submesoscale (0.1–10 km horizontal scale) physical heterogeneity on the physical‐biogeochemical response to a storm are not well understood. Here, we explore these effects numerically in a Biogeochemical Large Eddy Simulation (BLES), where a four‐component biogeochemical model is coupled with a physical model that resolves some submesoscales and some smaller turbulent scales (2 km to 2 m) in an idealized storm forcing scenario. Results are obtained via comparisons to BLES in smaller domains that do not resolve submesoscales and to one‐dimensional column simulations with the same biogeochemical model, initial conditions, and boundary conditions but parameterized turbulence and submesoscales. These comparisons show different behaviors during and shortly after the storm. During the storm, resolved submesoscales double the vertical nutrient flux. The vertical diffusivity is increased by a factor of 10 near the mixed layer base, and the mixing‐induced increase in potential energy is double. Resolved submesoscales also enhance horizontal nutrient and phytoplankton variance by a factor of 10. After the storm, resolved submesoscales maintain higher nutrient and phytoplankton variance within the mixed layer. However, submesoscales reduce net vertical nutrient fluxes by 50% and nearly shut off the turbulent diffusivity. Over the whole scenario, resolved submesoscales double storm‐driven biological production. Current parameterizations of submesoscales and turbulence fail to capture both the enhanced nutrient flux during the storm and the enhanced biological production.
-
Sea surface height (SSH) is routinely measured from satellites and used to infer ocean currents, including eddies, that affect the distribution of organisms and substances in the ocean. SSH not only reflects the dynamics of the surface layer, but also is sensitive to the fluctuations of the main pycnocline; thus it is linked to events of nutrient upwelling. Beyond episodic upwelling events, it is not clear if and how SSH is linked to broader changes in the biogeochemical state of marine ecosystems. Our analysis of 23 years of satellite observations and biogeochemical measurements from the North Pacific Subtropical Gyre shows that SSH is associated with numerous biogeochemical changes in distinct layers of the water column. From the sea surface to the depth of the chlorophyll maximum, dissolved phosphorus and nitrogen enigmatically increase with SSH, enhancing the abundance of heterotrophic picoplankton. At the deep chlorophyll maximum, increases in SSH are associated with decreases in vertical gradients of inorganic nutrients, decreases in the abundance of eukaryotic phytoplankton, and increases in the abundance of prokaryotic phytoplankton. In waters below ∼100 m depth, increases in SSH are associated with increases in organic matter and decreases in inorganic nutrients, consistent with predicted consequences of the vertical displacement of isopycnal layers. Our analysis highlights how satellite measurements of SSH can be used to infer the ecological and biogeochemical state of open-ocean ecosystems.more » « less
-
Abstract High‐accuracy spectrophotometric pH measurements were taken during a summer cruise to study the pH dynamics and its controlling mechanisms in the northern Gulf of Mexico in hypoxia season. Using the recently available dissociation constants of the purified m‐cresol purple (Douglas & Byrne, 2017,
https://doi.org/10.1016/j.marchem.2017.10.001 ; Müller & Rehder, 2018,https://doi.org/10.3389/fmars.2018.00177 ), spectrophotometrically measured pH showed excellent agreement with pH calculated from dissolved inorganic carbon (DIC) and total alkalinity over a wide salinity range of 0 to 36.9 (0.005 ± 0.016,n = 550). The coupled changes in DIC, oxygen, and nutrients suggest that biological production of organic matter in surface water and the subsequent aerobic respiration in subsurface was the dominant factor regulating pH variability in the nGOM in summer. The highest pH values were observed, together with the maximal biological uptake of DIC and nutrients, at intermediate salinities in the Mississippi and Atchafalaya plumes where light and nutrient conditions were favorable for phytoplankton growth. The lowest pH values (down to 7.59) were observed along with the highest concentrations of DIC and apparent oxygen utilization in hypoxic bottom waters. The nonconservative pH changes in both surface and bottom waters correlated well with the biologically induced changes in DIC, that is, per 100‐μmol/kg biological removal/addition of DIC resulted in 0.21 unit increase/decrease in pH. Coastal bottom water with lower pH buffering capacity is more susceptible to acidification from anthropogenic CO2invasion but reduction in eutrophication may offset some of the increased susceptibility to acidification.