Search for: All records

Antarctic coastal polynyas are hotspots of biological production with intensive springtime phytoplankton blooms that strongly depend on meltwater‐induced restratification in the upper part of the water column. However, the fundamental physics that determine spatial inhomogeneity of the spring restratification remain unclear. Here, we investigate how different meltwaters affect springtime restratification and thus phytoplankton bloom in Antarctic coastal polynyas. A high‐resolution coupled ice‐shelf/sea‐ice/ocean model is used to simulate an idealized coastal polynya similar to the Terra Nova Bay Polynya, Ross Sea, Antarctica. To evaluate the contribution of various meltwater sources, we conduct sensitivity simulations altering physical factors such as alongshore winds, ice shelf basal melt, and surface freshwater runoff. Our findings indicate that sea ice meltwater from offshore is the primary buoyancy source of polynya near‐surface restratification, particularly in the outer‐polynya region where chlorophyll concentration tends to be high. Downwelling‐favorable alongshore winds can direct offshore sea ice away and prevent sea ice meltwater from entering the polynya region. Although the ice shelf basal meltwater can ascend to the polynya surface, much of it is mixed vertically over the water column and confined horizontally to a narrow coastal region, and thus does not contribute significantly to the polynya near‐surface restratification. Surface runoff from ice shelf surface melt could contribute greatly to the polynya near‐surface restratification. Nearby ice tongues and headlands strongly influence the restratification through modifying polynya circulation and meltwater transport pathways. Results of this study can help explain observed spatiotemporal variability in restratification and associated biological productivity in Antarctic coastal polynyas.

 

This study examines the process of water-column stratification breakdown in Antarctic coastal polynyas adjacent to an ice shelf with a cavity underneath. This first part of a two-part sequence seeks to quantify the influence of offshore katabatic winds, alongshore winds, air temperature, and initial ambient stratification on the time scales of polynya destratification through combining process-oriented numerical simulations and analytical scaling. In particular, the often-neglected influence of wind-driven circulation on the lateral transport of the water formed at the polynya surface—which we call Polynya Source Water (PSW)—is systematically examined here. First, an ice shelf–sea ice–ocean coupled numerical model is adapted to simulate the process of PSW formation in polynyas of various configurations. The simulations highlight that (i) before reaching the bottom, majority of the PSW is actually carried away from the polynya by katabatic wind–induced offshore outflow, diminishing water-column mixing in the polynya and intrusion of the PSW into the neighboring ice shelf cavity, and (ii) alongshore coastal easterly winds, through inducing onshore Ekman transport, reduce offshore loss of the PSW and enhance polynya mixing and PSW intrusion into the cavity. Second, an analytical scaling of the destratification time scale is derived based on fundamental physical principles to quantitatively synthesize the influence of the physical factors, which is then verified by independent numerical sensitivity simulations. This work provides insights into the mechanisms that drive temporal and cross-polynya variations in stratification and PSW formation in Antarctic coastal polynyas, and establishes a framework for studying differences among the polynyas in the ocean.

 

This is Part II of a study examining wintertime destratification in Antarctic coastal polynyas, focusing on providing a qualitative description of the influence of ice tongues and headlands, both common geometric features neighboring the polynyas. The model of a coastal polynya used in Part I is modified to include an ice tongue and a headland to investigate their impacts on the dispersal of water formed at the polynya surface, which is referred to as Polynya Source Water (PSW) here. The model configuration qualitatively represents the settings of some coastal polynyas, such as the Terra Nova Bay Polynya. The simulations highlight that an ice tongue next to a polynya tends to break the alongshore symmetry in the lateral return flows toward the polynya, creating a stagnant region in the corner between the ice tongue and polynya where outflow of the PSW in the water column is suppressed. This enhances sinking of the PSW and accelerates destratification of the polynya water column. Adding a headland to the other side of the polynya tends to restore the alongshore symmetry in the lateral return flows, which increases the offshore PSW transport and slows down destratification in the polynya. This work stresses the importance of resolving small-scale geometric features in simulating vertical mixing in the polynya. It provides a framework to explain spatial and temporal variability in rates of destratification and Dense Shelf Water formation across Antarctic coastal polynyas, and helps understand why some polynyas are sources of Antarctic Bottom Water while others are not.

 

The marine coastal region makes up just 10% of the total area of the global ocean but contributes nearly 20% of its total primary production and over 80% of fisheries landings. Unicellular phytoplankton dominate primary production. Climate variability has had impacts on various marine ecosystems, but most sites are just approaching the age at which ecological responses to longer term, unidirectional climate trends might be distinguished. All five marine pelagic sites in the US Long Term Ecological Research (LTER) network are experiencing warming trends in surface air temperature. The marine physical system is responding at all sites with increasing mixed layer temperatures and decreasing depth and with declining sea ice cover at the two polar sites. Their ecological responses are more varied. Some sites show multiple population or ecosystem changes, whereas, at others, changes have not been detected, either because more time is needed or because they are not being measured.

 

Abstract The Northeast US shelf ecosystem is undergoing unprecedented changes due to long-term warming trends and shifts in regional hydrography leading to changes in community composition. However, it remains uncertain how shelf occupancy by the region's dominant, offshore small pelagic fishes, also known as forage fishes, has changed throughout the late 20th and early 21st centuries. Here, we use species distribution models to estimate the change in shelf occupancy, mean weighted latitude, and mean weighted depth of six forage fishes on the Northeast US shelf, and whether those trends were linked to coincident hydrographic conditions. Our results suggest that observed shelf occupancy is increasing or unchanging for most species in both spring and fall, linked both to gear shifts and increasing bottom temperature and salinity. Exceptions include decreases to observed shelf occupancy by sand lance and decreases to Atlantic herring's inferred habitat suitability in the fall. Our work shows that changes in shelf occupancy and inferred habitat suitability have varying coherence, indicating complex mechanisms behind observed shelf occupancy for many species. Future work and management can use these results to better isolate the aspects of forage fish life histories that are important for determining their occupancy of the Northeast US shelf. 

This dataset includes hatch and larval period for sand lance collected in 2019 and results from particle tracking runs of simulated sand lance larvae throughout the Northeast U.S. Shelf as part of Long-Term Ecological Research (NES-LTER). Release dates vary by region, corresponding to hatch and settlement dates of settling sand lance collected in 2019. Particles were depth-keeping throughout the upper 40 m to best replicate our understanding of the vertical distribution of sand lance larvae. Data were used to determine the average particle transport pathways from these sand lance habitats, including connectivity among the three hotspots, and spatial variability of connectivity within each hotspot. Further information can be found within the manuscript: Suca, J. J., Ji, R., Baumann, H., Pham, K., Silva, T. L., Wiley, D. N., Feng, Z., & Llopiz, J. K. (2022). Larval transport pathways from three prominent sand lance habitats in the Gulf of Maine. Fisheries Oceanography, 31( 3), 333-352. https://doi.org/10.1111/fog.12580 

Abstract The lipid-rich calanoid copepod, Calanus finmarchicus, plays a critical role in the Gulf of Maine pelagic food web. Despite numerous studies over the last several decades, a clear picture of variability patterns and links with key environmental drivers remains elusive. This study applies model-based scaling and sensitivity analyses to a regional plankton dataset collected over the last four decades (1977–2017). The focus is to describe the gulf-wide spatio-temporal patterns across three major basins, and to assess the relative roles of internal population dynamics and external exchanges. For the spring stock, there is strong synchrony of interannual variability among three basins. This variability is largely driven by internal population dynamics rather than external exchanges, and the internal population dynamics are more sensitive to the change of top-down mortality regime than the bottom-up forcings. For the fall stock, the synchrony among basins weakens, and the variability is influenced by both internal mortality and external dilution loss. There appears to be no direct connection between the spring stock with either the preceding or subsequent fall stock, suggesting seasonal or sub-seasonal scales of population variability and associated drivers. The results highlight seasonally varying drivers responsible for population variability, including previously less recognized top-down control. 

Abstract The signal of phytoplankton responses to climate-related forcing can be obscured by the heterogeneity of shelf seascapes, making them difficult to detect from fragmented observations. In this study, a physical–biological model was applied to the Northwest Atlantic Shelf to capture the seasonality of phytoplankton. The difference in phytoplankton seasonality between the Mid-Atlantic Bight (MAB) and the Gulf of Maine (GoM) is a result of the interplay between nutrients and temperature: In the MAB, relatively high temperature in the cold season and longer oligotrophic environment in the warm season contribute to an earlier winter bloom and a later fall bloom; in the GoM, low temperature and strong mixing limit phytoplankton growth from late fall to early spring, resulting in a later spring bloom and an earlier fall bloom. Although the temperature difference between the GoM and the MAB might decrease in the future, stratification and surface nutrient regimes in these two regions will remain different owing to distinct thermohaline structures and deep-water intrusion. The spatial heterogeneity of phytoplankton dynamics affects pelagic and benthic production through connections with zooplankton and benthic–pelagic coupling.