A red tide of Alexandrium fundyense in the Gulf of Maine

Birds of a feather eat plastic together: high levels of plastic ingestion in Great Shearwater adults and juveniles across their annual migratory cycle

Limited work to date has examined plastic ingestion in highly migratory seabirds like Great Shearwaters (Ardenna gravis) across the their entire migratory range, although this species is prone to ingest plastic as a wide-ranging procellariiform. We examined 217 Great Shearwaters obtained from 2008-2019 at multiple locations spanning their yearly migration cycle across the Northwest and South Atlantic to assess accumulation of ingested plastic as well as trends over time and between locations. A total of 2,328 plastic fragments were documented in the ventriculus portion of the gastrointestinal tract, with an average of 9 plastic fragments per bird. The mass, count, and frequency of plastic occurrence (FO) varied by location, with higher plastic burdens but lower FO in South Atlantic individuals from the breeding colonies. No fragments of the same size or morphology were found in the primary forage fish prey, the sand lance, (Ammodytes spp., n = 202) that supports Great Shearwaters in Massachusetts Bay, USA, suggesting the birds directly ingest the bulk of their plastic loads rather than accumulating via trophic transfer. Fourier-transform infrared spectroscopy indicated that low- and high-density polyethylene were the most common polymers ingested, within all years and locations. Individuals from the South Atlantic contained a higher proportion of larger plastic items and fragments compared to juveniles and non-breeding adults from the NW Atlantic, possibly due to increased use of remote, pelagic areas subject to reduced inputs of smaller, more diverse, and potentially less buoyant plastics found adjacent to coastal margins. Different signatures of polymer type, size, and category between similar life stages at different locations suggests rapid turnover of ingested plastics commensurate with migratory stage and location, though more empirical evidence is needed to ground-truth this hypothesis. This work is the first to comprehensively measure the accumulation of ingested plastics by Great Shearwaters over the last decade and across multiple locations spanning their yearly trans-equatorial migration cycle, and underscores their utility as sentinels of plastic pollution in Atlantic ecosystems.

Connections and clustering of Paralytic Shellfish Toxin events among coastal embayments in an archipelago partly mediated by advection

The potential for advection to influence harmful algal bloom (HAB) spread in adjacent embayments and islands has not been investigated in the Philippines as previous studies have focused on HAB dynamics within specific embayments. Advection of HABs may be relevant in the Samar-Leyte region where adjacent sites are subject to recurrent blooms of Pyrodinium bahamense. We used different analyses to extract the potential role of advection in blooms in the region. First, we used Bayesian and information theoretic approaches applied to historical data on shellfish bans to quantify spatial dependencies in HAB occurrences between sites. Then, to determine whether such dependencies are related to circulation patterns in the region, we analyzed connectivity using a hydrodynamic model coupled with a conservative tracer-based HAB model. The Bayesian network showed that in 7 out of 11 sites, the probability of a shellfish ban depended on the state of an adjacent site. Site pairs with direct dependence relations also shared relatively high similarity in HAB occurrences over time. In the modelled network, bans tend to occur sequentially, spreading from a few sites with relatively high probabilities for ban events. A subset of sites (sources) were found to be informative of future HAB event probabilities in other sites (destinations) over time lags that are generally longer the farther the destination. Modelled surface advection showed high connectivity strengths between sources and destinations associated with circulation features, e.g., an anticyclonic current in Leyte, wind-driven coastal current in western Samar, and tidally-driven flow in the shallow embayments in southwest Samar. High connectivities were correlated with direct dependence relations in the Bayesian network. Connectivity explained up to about 1/3 of the variance in statistical dependencies between ban signals. Our results show that Paralytic Shellfish Toxin events within this region can be due to advection of blooms originating from nearby areas rather than localized cyst dynamics within an embayment. Particular sites were also identified as potential indicator sites within spatial clusters for monitoring since they are typically where blooms originate and spread from.

A three-dimensional mechanistic model of Prorocentrum minimum blooms in eutrophic Chesapeake Bay

Planktonic Prorocentrum, common harmful dinoflagellate, are increasing in frequency, duration, and magnitude globally, as exemplified by the number of blooms of P. minimum in Chesapeake Bay that have nearly doubled over the past 3 decades. Although the dynamics of transport and seasonal occurrence of this species have been previously described, it has been challenging to predict the timing and location of P. minimum blooms in Chesapeake Bay. We developed a new three-dimensional mechanistic model of this species that integrates physics, nutrient cycling and plankton physiology and embedded it within a coupled hydrodynamic-biogeochemical model originally developed for simulating water quality in eutrophic estuarine and coastal waters. Hindcast simulations reproduced the observed time series and spatial distribution of cell density, in particular capturing well its peak in May in the mid-to-upper part of the estuary. Timing and duration of the blooms were mostly determined by the temperature-dependent growth function, while mortality due to grazing and respiration played a minor role. The model also reproduced the pattern of overwintering populations, which are located in bottom waters of the lower Bay, and are transported upstream in spring by estuarine flow. Blooms develop in the mid-upper parts of the estuary when these transported cells encounter high nutrient concentrations from the Susquehanna River and favorable light conditions. Diagnostic analysis and model-sensitivity experiments of nutrient conditions showed that high nitrogen:phosphorus conditions favor bloom development. The model also captured the observed interannual variations in the magnitude and spatial distribution of P. minimum blooms.

The oil spill transport across the shelf-estuary interface

Oil spills caused by ship collisions and offshore oil wells are an ongoing risk for estuaries in the northern Gulf of Mexico. The fate and transport of the oil spill across the interface between a bar-built estuary and the adjacent coast are influenced by multi-scale forcing mechanisms and their corresponding interactions. Of primary interest are the alongshore currents on the shelf encountering strong tidal flows at the estuary entrance. A new cross-scale model was developed for Galveston Bay to reproduce the multi-scale flows. The model was employed in regionally-distributed numerical Lagrangian experiments to investigate the oil spill transport across the shelf-estuary interface. The influence of the multi-scale flows on the oil spill transport was characterized in terms of Lagrangian connectivity and Lagrangian flushing. The new Galveston Bay model was also used to evaluate the Texas City "Y" spill and resulted in a reasonable agreement with the NOAA observations. This research enhances our understanding of the oil transport across the threshold between two contiguous water systems and highlights the importance of resolving the multi-scale flows for the purpose of oil spill predictions.

Spatiotemporal genetic structure of regional-scale Alexandrium catenella dinoflagellate blooms explained by extensive dispersal and environmental selection

Paralytic Shellfish Poisoning (PSP) caused by the dinoflagellate Alexandrium catenella is a well-known global syndrome that negatively impacts human health and fishery economies. Understanding the population dynamics and ecology of this species is thus important for identifying determinants of blooms and associated PSP toxicity. Given reports of extensive genetic heterogeneity in the toxicity and physiology of Alexandrium species, knowledge of genetic population structure in harmful algal species such as A. catenella can also facilitate the understanding of toxic bloom development and ecological adaptation. In this study we employed microsatellite markers to analyze multiple A. catenella strains isolated from several sub-regions in the Gulf of Maine (GoM) during summer blooms, to gain insights into the sources and dynamics of this economically important phytoplankton species. At least three genetically distinct clusters of A. catenella were identified in the GoM. Each cluster contained representatives from different sub-regions, highlighting the extent of connectivity and dispersal throughout the region. This shared diversity could result from cyst beds created by previous coastal blooms, thereby preserving the overall diversity of the regional A. catenella population. Rapid spatiotemporal genetic differentiation of A. catenella populations was observed in local blooms, likely driven by natural selection through environmental conditions such as silicate and nitrate/nitrite concentrations, emphasizing the role of short-term water mass intrusions and biotic processes in determining the diversity and dynamics of marine phytoplankton populations. Given the wide-spread intraspecific diversity of A. catenella in GoM and potentially elsewhere, harmful algal blooms will likely persist in many regions despite global warming and changing environmental conditions in the future. Selection of different genetic lineages through variable hydrological conditions might impact toxin production and profiles of future blooms, challenging HAB control and prediction of PSP risk in the future.

Texas and Louisiana coastal vulnerability and shelf connectivity

A numerical study of connectivity between the continental shelf and coast in the northwestern Gulf of Mexico using a circulation model and surface-limited numerical drifters shows that despite seasonal changes in winds, the overall connectivity of the shelf with the coastline is similar in the winter and summer, though it extends more offshore in Texas in summer. However, there is a spatial pattern to the connectivity: more of the inner shelf is connected with the coast in Texas as compared with Louisiana. Subsets of the coast do have seasonal variability: the coast near both Galveston and Port Aransas has more connectivity from upcoast in the winter and from offshore and downcoast in the summer. In both seasons, we find drifters reach the Port Aransas coast most frequently, with a stronger trend in the summer. These results are important for assessing likely pathways for spilled oil and other potentially hazardous material.

Variational Data Assimilative Modeling of the Gulf of Maine Circulation in Spring and Summer 2010

A data assimilative ocean circulation model is used to hindcast the Gulf of Maine (GOM) circulation in spring and summer 2010. Using the recently developed incremental strong constraint 4D Variational data assimilation algorithm, the model assimilates satellite sea surface temperature and in situ temperature and salinity profiles measured by expendable bathythermograph, Argo floats, and shipboard CTD casts. Validation against independent observations shows that the model skill is significantly improved after data assimilation. The data-assimilative model hindcast reproduces the temporal and spatial evolution of the ocean state, showing that a sea level depression southwest of the Scotian Shelf played a critical role in shaping the gulf-wide circulation. Heat budget analysis further demonstrates that both advection and surface heat flux contribute to temperature variability. The estimated time scale for coastal water to travel from the Scotian Shelf to the Jordan Basin is around 60 days, which is consistent with previous estimates based on in situ observations. Our study highlights the importance of resolving upstream and offshore forcing conditions in predicting the coastal circulation in the GOM.

Seasonal and Interannual Variability in Gulf of Maine Hydrodynamics: 2002-2011

In situ observations including long-term moored meteorological and oceanographic measurements and multi-year gulf-wide ship survey data are used to quantify interannual variability of surface wind, river runoff, and hydrographic conditions in the Gulf of Maine during summers 2002-2011. The cumulative upwelling index shows that upwelling (downwelling)-favorable wind conditions were most persistent in 2010 (2005) over the 10-year study period. River discharge was highest in 2005; peak runoff occurred in early April in 2010 as opposed to late April to middle May in other years. Moored time series show that coastal water temperature was 0.5-2 °C warmer than average in summer 2010, and about 2 °C colder than average in 2004. Coastal salinity in April 2010 was the lowest in the 10-year study period. Both moored Acoustic Doppler Current Profiler (ADCP) current measurements and dynamic height/geostrophic velocity calculations based on gulf-wide ship survey data show May-June 2010 had one of the weakest alongshore transports in the western Gulf of Maine during the 10-year study period, likely associated with intrusions of warm slope water and fresher-than-usual Scotian Shelf water. Comparisons of coastal currents to the Paralytic Shellfish Poisoning (PSP) closure maps resulting from A. fundyense blooms suggest a linkage between alongshore transport and the downstream extent of toxicity.

Near-bottom circulation and dispersion of sediment containing Alexandrium fundyense cysts in the Gulf of Maine during 2010-2011

The life cycle of Alexandrium fundyense in the Gulf of Maine includes a dormant cyst stage that spends the winter predominantly in the bottom sediment. Wave-current bottom stress caused by storms and tides induces resuspension of cyst-containing sediment during winter and spring. Resuspended sediment could be transported by water flow to different locations in the Gulf and the redistribution of sediment containing A. fundyense cysts could alter the spatial and temporal manifestation of its spring bloom. The present study evaluates model near-bottom flow during storms, when sediment resuspension and redistribution are most likely to occur, between October and May when A. fundyense cells are predominantly in cyst form. Simulated water column sediment (mud) concentrations from representative locations of the Gulf are used to initialize particle tracking simulations for the period October 2010-May 2011. Particles are tracked in full three-dimensional model solutions including a sinking velocity characteristic of cyst and aggregated mud settling (0.1 mm s^-1). Although most of the material was redeposited near the source areas, small percentages of total resuspended sediment from some locations in the western (~4%) and eastern (2%) Maine shelf and the Bay of Fundy (1%) traveled distances longer than 100 km before resettling. The redistribution changed seasonally and was sensitive to the prescribed sinking rate. Estimates of the amount of cysts redistributed with the sediment are small compared to the inventory of cysts in the upper few centimeters of sediment.