Speaker I – Iury Sousa, UMass Dartmouth
Title : Atmospheric cold pools in the Bay of Bengal
Abstract : Atmospheric cold pools, generated by evaporative downdrafts from precipitating clouds, are ubiquitous in the Bay of Bengal. We use data from three nearby moorings near 18°N to characterize a total of 465 cold pools (~150/yr). The cold pools are all dry, with a typical temperature drop of 2°C (max. 5°C) and specific humidity drop of 1 g/kg (max. 6 g/kg). Most cold pools last 1.5-3.5 hours (max. 14 hours). Cold pools occur almost every day in the North Bay from April to November, principally in the late morning, associated with intense precipitation that accounts for 80% of total rain. They increase the latent heat flux to the atmosphere by about 32 W/m2 (median), although the instantaneous enhancement of latent heat flux for individual cold pools reach 150 W/m2. During the rainiest month (July), the cold pools occur 21% of the time and contribute nearly 14% to the mean evaporation. A composite analysis of all cold pools shows that the temperature and specific humidity anomalies are responsible for ~90% of the enhancement of sensible and latent heat flux, while variation in wind speed is responsible for the remainder. Based on their phase speed, the estimated height of the cold pools primarily ranges from 375 to 2200 m, with taller fronts more likely to occur during the summer monsoon season (Jun-Sep). Our results indicate that the realistic representation of cold pools in climate models is likely to be important for improved simulation of air-sea fluxes and monsoon rainfall.
Speaker II – Grace Casselberry, UMass Amherst
Title : Quantifying depredation rates and spatial overlap between great hammerhead sharks (Sphyrna mokarran) and Atlantic tarpon (Megalops atlanticus) in a recreational fishing hotspot
Abstract : Studies of predator-prey interactions in marine ecosystems are difficult to conduct but essential for effective resource management. Human activities, like recreational angling, can alter the behavior of individuals within populations and facilitate predation events. The Atlantic tarpon (Megalops atlanticus) fishery in Florida is primarily catch-and-release, but fishing guides in the Florida Keys are increasingly reporting the loss of hooked tarpon to depredation, particularly by great hammerhead sharks (Sphyrna mokarran). This appears to be a serious issue, particularly in Bahia Honda, a putative tarpon prespawning aggregation and famous fishing hotspot. To better understand this issue, we used visual surveys to quantify depredation rates of tarpon by sharks in Bahia Honda, and acoustic telemetry to examine spatial overlap between predator and prey. Seventeen great hammerheads were tagged with acoustic transmitters and color-coded cattle tags to aid in visual identification of individuals, and 200 tarpon were implanted with acoustic transmitters. Results from standardized visual observations of fishing activity in spring, 2019 indicate nearly 15% of hooked tarpon suffer depredation mortality by great hammerhead sharks, and that water current direction and angling intensity contribute to depredation susceptibility. Moreover, analyses of movement patterns of both great hammerheads and tarpon indicate that hammerheads modify their space use in Bahia Honda channel with increasing tarpon presence. This, coupled with the visual survey data, will be the foundation for potential solutions to reduce depredation events and decrease the potential for angler-shark conflict.
Speaker III – Ting Wang, UMass Lowel
Title : Mercury Dynamics in a Salt Marsh Estuary in Massachusetts, USA
Abstract : Salt marsh estuaries serve as sources and sinks for nutrients and elements to and from estuarine water, which may enhance or alleviate watershed fluxes to the coastal ocean. Here, we quantified sources and sinks of mercury (Hg) in the intertidal Plum Island Sound estuary in Massachusetts. We first quantified pattern of dissolved (FHg) and total (THg) mercury concentrations in water across large tidal transects and found that concentrations were highest and strongly enhanced in upper marshes (1.31 ± 0.20 ng L−1 [mean±SD] and 6.56 ± 3.70 ng L−1, respectively), compared to riverine Hg concentrations (0.86 ± 0.17 ng L−1 and 0.88 ± 0.34 ng L−1, respectively). Hg concentrations declined from upper to lower marshes and were lowest in ocean water (0.38 ± 0.10 ng L−1 and 0.56 ± 0.25 ng L−1, respectively). Conservative mixing models using river and ocean water as endmembers indicate internal estuarine Hg sources which enhanced estuarine water Hg concentrations from riverine to the ocean (e.g., for FHg by +44%, for THg by +86%). We suggest that high Hg concentrations in salt marsh soils (average 200 ± 225 μg kg−1) lead to lateral Hg export to estuarine water re-mobilizing previous legacy sources accumulated in soils.
We also hypothesize that high Hg levels observed in salt marsh soils are derived in parts from atmospheric gaseous elemental Hg (GEM) uptake by salt marsh vegetation, similar to the dominant vegetation Hg source reported from upland ecosystems. We conducted monthly quantitative harvests of aboveground biomass and Hg concentration analyses throughout a full growing season, along with belowground biomass. Results showed that seasonally, Hg concentration and mass in current-season aboveground biomass near-linearly increased from June (3.9±0.2 µg kg-1 and 0.7±0.4 µg m-2, respectively) to November (16.2±2.0 µg kg-1 and 5.7±2.1 µg m-2, respectively) and resulted in substantial end-of-season pools sizes of Hg. In the next season, these Hg pools are senesced, and they decreased subsequently throughout the season due to slow transfer to soils and wrack export, suggesting an important role of plants as Hg sources to marshes. Ongoing stable mercury isotope analyses and continuous surface-atmosphere exchange fluxes are intended to further quantify various sources origins of Hg in these salt marshes.
Speaker IV – Ágata Piffer Braga, Umass Dartmouth
Title : Evolution of a mid-sized river plume front: from discharge to arrest
Abstract : Data collected in the Merrimack River plume front, ranging from the near-field until the beginning of the mid-field region (Rossby number approximately 0.6), were analyzed. The dataset enabled tracking of the evolution of frontal structure in a front-following reference frame, including the velocity, density, and turbulent kinetic energy (TKE) dissipation rate structure. Our measurements were mostly obtained from a collection of sensors mounted on an autonomous underwater vehicle (AUV) known as the UMassD T-REMUS, augmented by several surface drifters and video footage from an aerial drone. The sampling captured the first 6-8 hours of frontal propagation outside the mouth, characterized by an initial phase of radial propagation and rapid expansion, with the eventual arrest of the front approximately 4-6 hours later. At the time of frontal arrest, the motion of drifters tracking the front becomes dominated by along front, rather than cross front velocities, and frontal features become less well defined. The evolution of terms in the cross-front momentum balance as the front evolves suggests that the arrest of radial expansion and forward propagation may be driven by the increasing importance of Coriolis forcing to the momentum balance. The process by which the front stalls may have important implications for connectivity between the source water and the front. In turn, source water interaction with the arrested front may contribute to the observed weakening of frontal features, which may also be affected by the release of frontal energy via internal waves.