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Monthly surface currents at 2 km resolution near the mouths of three U.S. east coast bays were obtained from high-frequency radars (Coastal Ocean Dynamics Application Radar, CODAR) during 2012–2024. The currents near these bays, the Chesapeake Bay (CB), the Delaware Bay (DB) and the New York Bay (NB) were analyzed to infer similarity and differences, as well as potential common forcing from regional and basin-scale factors. The contribution to flow variability from local and remote forcing is evaluated by comparing surface currents with (a) river discharges into each bay, (b) with local and regional winds, and (c) with the North Atlantic Oscillation (NAO). The results show that surface flow variability near the mouth of the bays is linked with all three driving factors. The three bays often show similar flow patterns not only of the seasonal cycle, but also during extreme weather events. For example, increased surface flow into the bays from the Atlantic Ocean is seen when hurricanes are observed offshore in the fall, and increased surface flow from the bays is seen during winter storms. During positive NAO phases, eastward flow from all three bays increased due to intensified westerly winds, while during negative NAO phases flow decreased with weakening winds in the region. River discharges into the bays increased during 2012–2019 but decreased during 2019–2024. This change in river discharge trend was especially large in the CB, resulting in a change in trends of the surface currents. Monthly currents of each bay are only weakly correlated with the monthly river flow (R ~ 0.2–0.3; P < 0.05), while the seasonal cycles of rivers and currents have higher correlations (R ~ 0.6–0.7). Local winds show high correlations with the monthly currents (R ~ 0.75) with the current direction ~ 45° to the right of the wind, as expected from Ekman theory. However, contributions to current variability from regional and remote factors cannot be ignored. The results demonstrate the complex nature of the currents near the mouth of bays since multiple drivers, including estuarine, coastal and open ocean dynamics contribute to the observed variability. [ABSTRACT FROM AUTHOR]

The implementation of nature-based solutions (NbS), including living shorelines, to mitigate estuarine habitat loss is increasing at a pace exceeding the evaluation of their long-term success. Constructed oyster reefs (CORs) made of shell, concrete, stone, and other materials are one living shoreline tactic that is widely utilized, yet few studies have been conducted to understand the development of CORs within the context of both physical and ecological parameters over longer time scales (4 + years). A COR-based living shoreline project at the Gandy’s Beach Preserve (GBP) in Delaware Bay, NJ, USA, had dual goals of coastal protection and habitat provisioning, which prompted the development of a goal-driven monitoring framework to track project objectives. Methods were developed to quantify the following multi-disciplinary metrics over 7 years: elevations of CORs, waves (height, period, and direction), shoreline elevations, change in extent of vegetation patches, oyster density and size, nekton richness and community composition, and horseshoe crab impingement. The CORs met most of their habitat provisioning objectives as they were colonized by a multi-generational population of shellfish and created habitat for nekton, while posing negligible hazards to horseshoe crabs. However, none of the coastal protection objectives was fully achieved including material stability, wave attenuation, and sediment elevation increase. Results highlight the value of longer-term monitoring to understand performance and the need to match the scale and type of NbS tactic(s) with both the scale of the landscape and the site-specific hydrodynamic conditions to meet project goals. [ABSTRACT FROM AUTHOR]

Salinity is a major environmental factor that influences the population dynamics of fish and shellfish along coasts and estuaries, yet empirical methods for hindcasting salinity at specific sampling stations are not widely available. The specific aim of this research was to predict the salinity experienced by juvenile and adult oysters (Crassostrea virginica) collected at sampling stations in Delaware Bay. To do so, empirical relationships were created to predict salinity at five oyster bed stations using observing systems data. These relationships were then applied to construct indices of salinity exposure over an oyster's lifetime. Three independent salinity data sources were used in conjunction with observing systems data to construct and validate the predictive relationships. The root mean square error (RMSE) of the models ranged from 0.5 to 1.6 psu when model predictions were compared with the three independent data sets. Results demonstrated that data from an observing system near the head of Delaware Bay could be used to predict salinity within ± 2 psu at oyster bed stations as far down-estuary as 39 km. When these models were applied to estimate low salinity exposure of 2-year-old oysters via the metric of consecutive days below 5 psu, the indices suggested that there could be as much as a 42-day difference in low salinity exposure for oysters at stations just 31 km apart. The approach of using observing systems data to hindcast salinity could be applied to advance understanding of salt distribution and the effect of low salinity exposure on organisms in other estuaries, especially bottom-associated species. [ABSTRACT FROM AUTHOR]

Flooding is among the most common yet costly worldwide annual disasters. Previous studies have proven that synthetic aperture radar (SAR) is an effective tool for flooding observation due to its high-resolution and timely observations, and deep learning-based models can accurately extract water bodies from SAR imagery. However, many previous flood analyses do not account for influences of tides and permanent water bodies, and the comprehensive characteristics of coastal storm flooding are still not fully understood. This study therefore presents a novel approach for isolating storm-induced flood waters in coastal regions from SAR imagery through the identification and removal of permanent water bodies and tidal inundation. This methodology is applied to the Delaware Bay region, with ancillary geospatial data used to determine resulting landcover impacts. Results indicate that flooding primarily impacts agricultural and marsh regions, as well as urban areas like airports and road systems adjacent to rivers or large inland bays. The sensitivity impacts of tides on flood estimates reveals that estimates significantly increase if included in analysis, highlighting the importance of their removal prior to flood identification. Finally, exploration into intense coastal storm events in the Delaware Bay region reveal the importance of storm characteristics like high water levels, wind, and precipitation in generating extreme flooding conditions. The case study presented here has important implications for other coastal regions and provides an innovative and comprehensive approach to coastal storm flood identification and characterization which can benefit coastal managers, emergency responders, coastal communities, and researchers interested in coastal flood hazards.

Shark teeth are one of the most abundant vertebrate fossils, and because tooth size generally correlates with body size, their accumulations document the size structure of populations. Understanding how ecological and environmental processes influence size structure, and how this extends to influence these dental distributions, may offer a window into the ecological and environmental dynamics of past and present shark populations. Here, we examine the dental distributions of sand tigers, including extant Carcharias taurus and extinct Striatolamia macrota, to reconstruct the size structure for a contemporary locality and four Eocene localities. We compare empirical distributions against expectations from a population simulation to gain insight into potential governing ecological processes. Specifically, we investigate the influence of dispersal flexibility to and from protected nurseries. We show that changing the flexibility of initial dispersal of juveniles from the nursery and annual migration of adults to the nursery explains a large amount of dental distribution variability. Our framework predicts dispersal strategies of an extant sand tiger population, and supports nurseries as important components of sand tiger life history in both extant and Eocene populations. These results suggest nursery protection may be vital for shark conservation with increasing anthropogenic impacts and climate change.

Objective: This paper applies a catch multiple survey analysis (CMSA) to Atlantic horseshoe crabs Limulus polyphemus in the Delaware Bay to generate robust population estimates for harvest management. Currently, horseshoe crabs along the U.S. Atlantic coast are harvested as bait for other fisheries and collected for their blood, which is used in a biomedical industry. The Delaware Bay is home to the largest population of horseshoe crabs and is a significant stopover for shorebirds to rebuild energy by consuming horseshoe crab eggs prior to completing their northward migration. To address this interrelationship, the Adaptive Resource Management (ARM) Framework has been used since 2013 to ensure that horseshoe crab harvest within the region takes into account the forage needs of migratory birds. Since its inception, the ARM Framework has used a single trawl survey's swept area‐based population estimates of horseshoe crab relative abundance and a theoretical population model developed primarily from literature‐derived values. With more data collected in the region in recent years and other sources of mortality that can now be quantified, a catch survey model can provide horseshoe crab population estimates going forward. Methods: A CMSA was used to estimate male and female horseshoe crab population size for 2003–2021 using all quantifiable sources of mortality and three fishery‐independent indices of abundance. Result: The CMSA results indicated that adult abundance of male and female horseshoe crabs was stable from 2003 to 2013 and then began to increase through 2017, a result that is consistent with stock rebuilding following a period of harvest restrictions as recommended by the ARM Framework. Population estimates were lower in recent years but remained above the levels estimated before implementation of the ARM Framework. In 2021, the CMSA estimated that there were over 6 million mature females and nearly 16 million mature male horseshoe crabs in the region. Conclusion: The CMSA provides the best and most comprehensive population estimates of horseshoe crabs in Delaware Bay and will improve modeling efforts within the ARM Framework going forward. Impact statementBeing able to estimate the size of the Atlantic horseshoe crab population in the Delaware Bay will help managers set appropriate harvest levels that will support its fisheries while also ensuring there are enough horseshoe crab eggs to feed migrating shorebirds. [ABSTRACT FROM AUTHOR]

Hydrodynamic models play a key role in simulating total water level (TWL), that is, a combination of river flow, tide, surge, wind and wave‐induced water level, and representing flood inundation dynamics in coastal areas. An appropriate selection of two‐dimensional (2D) models that integrate riverine and estuarine interactions with ocean dynamics is crucial to generate accurate TWL predictions and assist stakeholders and federal agencies in decision making and flood emergency responses. In this study, we compare the performance of two widely used hydrodynamic models (e.g., 2D HEC‐RAS and Delft3D‐Flexible Mesh [FM]) with respect to their ability of predicting TWL in Delaware Bay, United States. Based on a previously established model configuration, we simulate Hurricane Sandy and Isabel that affected the Bay and led to considerable damages and economic losses. We then evaluate model capabilities with tidal analysis, compare observed vs. simulated TWL and analyze spatiotemporal variations of TWL through scenario‐based simulations. Our results suggest that atmospheric forcing input in Delft3D‐FM significantly improves TWL predictions as compared to those of 2D HEC‐RAS. Furthermore, model simulations with Delft3D‐FM can be faster than 2D HEC‐RAS by a factor of 6–10. Despite these advantages, 2D HEC‐RAS (version 5.07) is a noncommercial software easier to implement and can be a simpler alternative for modeling extreme events when atmospheric forcing is not relevant in the model domain. [ABSTRACT FROM AUTHOR]

Low-lying coastal areas in the mid-Atlantic region are prone to compound flooding resulting from the co-occurrence of river floods and coastal storm surges. To better understand the contribution of non-linear tide-surge-river interactions to compound flooding, the unstructured-grid Finite Volume Community Ocean Model was applied to simulate coastal storm surge and flooding in the Delaware Bay Estuary in the United States. The model was validated with tide gauge data in the estuary for selected hurricane events. Non-linear interactions between tide-surge-river were investigated using a non-stationary tidal analysis method, which decomposes the interactions’ components at the frequency domain. Model results indicated that tide-river interactions damped semidiurnal tides, while the tide-surge interactions mainly influenced diurnal tides. Tide-river interactions suppressed the water level upstream while tide-surge interaction increased the water level downstream, which resulted in a transition zone of damping and enhancing effects where the tide-surge-river interaction was prominent. Evident compound flooding was observed as a result of non-linear tide-surge-river interactions. Furthermore, sensitivity analysis was carried out to evaluate the effect of river flooding on the non-linear interactions. The transition zone of damping and enhancing effects shifted downstream as the river flow rate increased.

Horseshoe crab population sizes and trends have been previously studied using surveys of spawning adults and counts of eggs from surface (top 5 cm) and deep (20 cm) sediment samples. The correlations between surface and deep eggs were studied at two locations, Delaware Bay and Jamaica Bay, USA, and the correlations between egg densities and spawning counts were examined in Jamaica Bay.There were significantly higher densities of eggs in deep sediments than in surface sediments. Only about 10% of the variability in surface egg density was explained by deep egg density. The numerical patterns between surface and deep eggs were similar between Delaware Bay and Jamaica Bay and across sampling dates.Nearly 20% of the deep samples in the combined data from Delaware Bay and Jamaica Bay with an egg density of ≥100,000 m−2 had zero surface eggs. Therefore, the use of surface eggs as an indicator of habitat suitability and spawning intensity may seriously underestimate the importance of a beach for spawning horseshoe crabs.When paired with nearest survey date, Jamaica Bay spawning indices did not predict deep or surface egg densities. This may be related to a temporal mismatch between survey methods, the extreme overdispersion (patchiness) of the eggs, and/or the dynamics of egg distribution after exhumation.Both egg density and spawning surveys can provide useful data on habitat suitability for horseshoe crabs and can offer excellent opportunities for student and citizen scientist engagement. More labour is required for egg surveys than spawning surveys because of the time required to sample, sort, and enumerate the eggs. [ABSTRACT FROM AUTHOR]

ABSTRACT Viruses are ubiquitous and abundant in the oceans, and viral metagenomes (viromes) have been investigated extensively via several large-scale ocean sequencing projects. However, there have not been any systematic viromic studies in estuaries. Here, we investigated the viromes of the Delaware Bay and Chesapeake Bay, two Mid-Atlantic estuaries. Deep sequencing generated a total of 48,190 assembled viral sequences (>5 kb) and 26,487 viral populations (9,204 virus clusters and 17,845 singletons), including 319 circular viral contigs between 7.5 kb and 161.8 kb. Unknown viruses represented the vast majority of the dominant populations, while the composition of known viruses, such as pelagiphage and cyanophage, appeared to be relatively consistent across a wide range of salinity gradients and in different seasons. A difference between estuarine and ocean viromes was reflected by the proportions of Myoviridae, Podoviridae, Siphoviridae, Phycodnaviridae, and a few well-studied virus representatives. The difference in viral community between the Delaware Bay and Chesapeake Bay is significantly more pronounced than the difference caused by temperature or salinity, indicating strong local profiles caused by the unique ecology of each estuary. Interestingly, a viral contig similar to phages infecting Acinetobacter baumannii (“Iraqibacter”) was found to be highly abundant in the Delaware Bay but not in the Chesapeake Bay, the source of which is yet to be identified. Highly abundant viruses in both estuaries have close hits to viral sequences derived from the marine single-cell genomes or long-read single-molecule sequencing, suggesting that important viruses are still waiting to be discovered in the estuarine environment. IMPORTANCE This is the first systematic study about spatial and temporal variation of virioplankton communities in estuaries using deep metagenomics sequencing. It is among the highest-quality viromic data sets to date, showing remarkably consistent sequencing depth and quality across samples. Our results indicate that there exists a large pool of abundant and diverse viruses in estuaries that have not yet been cultivated, their genomes only available thanks to single-cell genomics or single-molecule sequencing, demonstrating the importance of these methods for viral discovery. The spatiotemporal pattern of these abundant uncultivated viruses is more variable than that of cultured viruses. Despite strong environmental gradients, season and location had surprisingly little impact on the viral community within an estuary, but we saw a significant distinction between the two estuaries and also between estuarine and open ocean viromes.

In the U.S., 525,000 horseshoe crabs (Limulus polyphemus) per year have been captured during 2013–2017, brought to biomedical facilities, and bled to produce Limulus amebocyte lysate (LAL), then mostly released to the area of capture. The Atlantic States Marine Fisheries Commission estimates short-term bleeding-induced mortality to be 15% (4% to 30%), resulting in mortality of approximately 78,750 horseshoe crabs annually in recent years comprising a minor portion (

Oyster and seawater samples were collected from five sites in the Chesapeake Bay, MD, and three sites in the Delaware Bay, DE, from May to October 2016 and 2017. Abundances and detection frequencies for total and pathogenic Vibrio parahaemolyticus and Vibrio vulnificus were compared using the standard most-probable-number–PCR (MPN-PCR) assay and a direct-plating (DP) method on CHROMagar Vibrio for total (tlh+) and pathogenic (tdh+ and trh+) V. parahaemolyticus genes and total (vvhA) and pathogenic (vcgC) V. vulnificus genes. The colony overlay procedure for peptidases (COPP) assay was evaluated for total Vibrionaceae. DP had high false-negative rates (14 to 77%) for most PCR targets and was deemed unsatisfactory. Logistic regression models of the COPP assay showed high concordances with MPN-PCR for tdh+ and trh+ V. parahaemolyticus and vvhA+ V. vulnificus in oysters (85.7 to 90.9%) and seawater (81.1 to 92.7%) when seawater temperature and salinity were factored into the model, suggesting that the COPP assay could potentially serve as a more rapid method to detect vibrios in oysters and seawater. Differences in total Vibrionaceae and pathogenic Vibrio abundances between state sampling sites over different collection years were contrasted for oysters and seawater by MPN-PCR. Abundances of tdh+ and trh+ V. parahaemolyticus were ~8-fold higher in Delaware oysters than in Maryland oysters, whereas abundances of vcgC+ V. vulnificus were nearly identical. For Delaware oysters, 93.5% were both tdh+ and trh+, compared to only 19.2% in Maryland. These results indicate that pathogenic V. parahaemolyticus was more prevalent in the Delaware Bay than in the Chesapeake Bay. [ABSTRACT FROM AUTHOR]

Compound flooding is usually induced by the concurrence of coastal storm surge and heavy precipitation induced river flooding, with the former involving oceanic processes and the latter involving hydrological processes. The simulation of these two types of processes is traditionally handled by two different types of models separately, i.e., hydrological models (e.g., NOAA's National Water Model (NWM)) and hydrodynamic models. This dichotomy leaves gaps in simulating the interrelated processes in a holistic fashion. In this paper, we present a creek-to-ocean 3D baroclinic model based on SCHISM (Semi-implicit Cross-scale Hydroscience Integrated System Model) that aims to unite traditional hydrologic and hydrodynamic models in a single modeling platform to simulate compound floods, by taking full advantage of the model polymorphism (i.e., a single model grid can seamlessly morph between full 3D, 2DV, 2DH, and quasi-1D modes). Using Hurricane Irene's impact on Delaware Bay as an example, a seamless 2D-3D model grid is implemented to include the entire US East Coast and Gulf of Mexico with a highly resolved Delaware Bay (down to 20-m resolution). The streamflow from NWM is injected into SCHISM grid at the intersections of NWM's segments and SCHISM's land boundary, and the pluvial and fluvial processes are directly handled by SCHISM. We demonstrate the model's accuracy, stability, and robustness with focus on the compound flooding events. The relative role of different physical processes in such events is examined by a series of sensitivity tests. Our results confirm the occurrence of backwater process into far upstream rivers and creeks and thus demonstrate the need for a dynamic two-way coupling between the hydrodynamic and hydrological models. [ABSTRACT FROM AUTHOR]

Several aquatic species use beach foreshores to lay their eggs. Understanding the dynamics of egg entrainment versus egg retention in foreshore sediment is important to delivering eggs to foraging species, and to predicting fecundity and population responses of beach nesting species to harvest and habitat management. Exhumation of these eggs is primarily by bioturbation and wave action. This study isolates effects of wave and swash processes on exhumation, entrainment, and transport of horseshoe crab eggs by conducting a 1-day field study designed to avoid effects of bioturbation by spawning and migratory shorebird predation. Dyed sand and horseshoe crab (Limulus polyphemus) eggs were injected into the foreshore of an estuarine beach in Delaware Bay, USA and sampled during 110 swash events using streamer traps. Significant wave heights ranged from 0.30 to 0.47 m. The quantity of eggs exhumed from the mid-foreshore by waves and swash was 3.8 times greater than from the upper foreshore where activation was by swash alone. Swash flows were skewed offshore, but egg transport was predominantly onshore from bore collapse. Egg release was greatest in early rising tide, making this the most productive time for shorebird foraging. Release of eggs from mid-foreshore may be dominated by wave action during high energy days with little spawning or by bioturbation by horseshoe crabs during intensive spawning. Activation depths in the swash were low, indicating that eggs buried on the upper foreshore can reach later stages of development after spawning ceases. [ABSTRACT FROM AUTHOR]

Diel and tidal studies are critical to understanding many aspects of fish behavior and habitat use. In this study, diel and tidal abundance, size composition and food habits of four common juvenile fish (Anchoa mitchilli, Cynoscion regalis, Leiostomus xanthurus, Micropogonias undulatus) present in Delaware Bay (USA) marsh creeks was assessed. Fish were collected with otter trawls near daytime and nighttime high tides and with seines near daytime and nighttime low tides at two mesohaline sites. Anchoa mitchilli and C. regalis were both significantly more abundant and significantly smaller in nighttime samples. Micropogonius undulatus was the most abundant species, while A. mitchilli was entirely absent, in low tide collections. An index of stomach fullness for A. mitchilli and L. xanthurus indicated feeding during periods of maximum flooding. Trophic interactions among the four species were also apparent. Both C. regalis and M. undulatus commonly consumed A. mitchilli only during nighttime high tides. Cynoscion regalis and M. undulatus both consumed large quantities of Fundulus heteroclitus (mummichog) during low tides during both the day and night. This important interaction demonstrates the trophic transfer of production in the marsh (F. heteroclitus) to the open ocean via the fall migration of piscivorous M. undulatus. These findings demonstrate the need for diel and tidal sampling programs if we are to adequately understand the dynamics of marsh ecosystems. [ABSTRACT FROM AUTHOR]

Delaware Bay, US is the only documented location where influenza A virus (IAV) is consistently detected in a shorebird species, the Ruddy Turnstone (RUTU; Arenaria interpres morinella). Although IAV in shorebirds has been well studied at this site for decades, the importance of other species in the avian community as potential sources for the IAVs that infect RUTUs each spring remains unclear. We determined the susceptibility of Mallards (Anas platyrhynchos) and Laughing Gulls (Leucophaeus atricilla), to IAVs isolated from RUTUs in order to gain insight into the potential host range of these viruses. Captive-reared gulls were challenged with RUTU-origin H6N1, H10N7, H11N9, H12N4, and H13N6 IAV, as well as Mallard-origin H6N1 and H11N9. We challenged captive-reared Mallards with the same viruses, except for H13N6. At a biologically plausible challenge dose (104 50% embryo infective doses/0.1 mL), one of five gulls challenged with both H6N1 IAVs shed virus. The remaining gulls were resistant to infection with all viruses. In contrast, all Mallards were infected and shed virus. The H12N4 Mallard challenge group was an exception with no birds infected. These results indicated that Mallards are permissive to infection with viruses originating from a shorebird host and that interspecies transmission could occur. In contrast, host adaptation of IAVs to RUTUs may compromise their ability to be transmitted back to gulls. [ABSTRACT FROM AUTHOR]

Each May for over three decades, avian influenza A viruses (IAVs) have been isolated from shorebirds and gulls (order Charadriiformes) at Delaware Bay (DE Bay), USA, which is a critical stopover site for shorebirds on their spring migration to arctic breeding grounds. At DE Bay, most isolates have been recovered from ruddy turnstones (Arenaria interpres), but it is unknown if this species is involved in either the maintenance or movement of these viruses outside of this site. We collected and tested fecal samples from 2823 ruddy turnstones in Florida and Georgia in the southeastern United States during four winter/spring sample periods—2010, 2011, 2012, and 2013—and during the winters of 2014/2015 and 2015/2016. Twenty-five low pathogenicity IAVs were recovered representing five subtypes (H3N4, H3N8, H5N9, H6N1, and H12N2). Many of these subtypes matched those recovered at DE Bay during the previous year or that year’s migratory cycle, suggesting that IAVs present on these southern wintering areas represent a source of virus introduction to DE Bay via migrating ruddy turnstones. Analyses of all IAV gene segments of H5N9 and H6N1 viruses recovered from ruddy turnstones at DE Bay during May 2012 and from the southeast during the spring of 2012 revealed a high level of genetic relatedness at the nucleotide level, suggesting that migrating ruddy turnstones move IAVs from wintering grounds to the DE Bay ecosystem.

Influenza A virus (IAV) infections in shorebirds at Delaware Bay, New Jersey, US, have historically included avian hemagglutinin (HA) subtypes H1–13 and H16. In a given year, infections are characterized by a limited number of HA and neuraminidase subtypes and a dominant HA subtype that often represents >50% of all isolates. Predominant HA subtypes shift between consecutive years. In addition, infection prevalence is consistently higher in Ruddy Turnstones (RUTU; Arenaria interpres morinella) compared to Red Knots (REKN; Calidris canutus rufa), despite comparable rates of exposure. To investigate a potential immunological basis for this phenomenon, a virus microneutralization assay was used to detect subtype-specific, neutralizing antibodies to H1–H12 in sera collected from RUTUs from 2012–16 and REKNs in 2012, 2013, and 2016. Neutralizing antibodies to one or more subtypes were detected in 36% (222/611) of RUTUs. Prevalence of antibodies to subtypes H6 and H11 remained high throughout the study, and these virus subtypes were isolated every year, suggesting a continual source of exposure. Antibody prevalence was intermediate for most IAV subtypes that were isolated in 2–3 of 5 yr (H1, H3, H5, H9, H10, and H12) but was low for H7 viruses, despite the isolation of this virus subtype in 3 of 5 yr. This suggests a reduced antigenicity of H7 IAVs compared to other subtypes. Antibody prevalence was low for H4 virus that was isolated once, and H2 and H8 viruses that were never isolated. Neutralizing antibodies were detected in 66% (169/257) of REKNs and subtype-specific antibody prevalences were higher in REKNs than RUTUs with few exceptions. The results suggest that population immunity influences which species is infected at Delaware Bay, indicate that IAV dynamics are subtype-dependent, and demonstrate the utility of the microneutralization assay as a supportive tool for field research. [ABSTRACT FROM AUTHOR]

Shorebirds usually forage on intertidal flats that are exposed during low tide, and roost on higher areas when the tidal flats are covered with water. During spring migration on Delaware Bay (New Jersey) shorebirds mainly forage on horseshoe crab ( Limulus polyphemus ) eggs that are concentrated at the high tide line. However, they also use other habitats for foraging. We examined habitat use of 4 species of shorebirds (with declining populations) at five Delaware Bay beaches to determine their use of the intertidal habitat (2015, 2016). We observed birds in three sections at different distances from the mean high tideline (< 100 m, 101–200 m, and 201–300 m)ne. We examined the presence of red knots ( Calidris canutus rufa ), ruddy turnstones ( Arenaria interpres ), semipalmated sandpipers ( Calidris pusilla ), and sanderling ( Calidris alba ) as a function of date, tide cycle, section shorebirds foraged from the mean high tide line, and presence of other shorebird species. Understanding how these species use the intertidal flats is important because these habitats are at risk from coastal development, sea level rise, and decreases in intertidal space, including the possible expansion of intertidal oyster culture. Overall, knots were present in the intertidal on 67% of the surveys, turnstones were present on 86% of the surveys, semipalmated sandpipers were present on 77% of the surveys, and sanderling were present on 86% of the surveys. Use of the intertidal flats varied among beaches. Peak and mean numbers of shorebirds/ decreased in each census section, as distance to mean high tideline increased. In general, shorebirds foraged at the waters’ edge during high tide, and then moved out onto the intertidal flats. The strongest interspecific associations were between red knots and ruddy turnstones, and the lowest associations were between sanderling and semipalmated sandpipers. Variations in numbers of each species in 2016 were mainly explained by the number of other species, section (distance from the mean high tide line), location (one of 5 beaches), and date for all species (and minutes to low tide for sanderling). These data indicate that these 4 species use intertidal flats as they become available, and that the mean number in each newly exposed census section of the flats may be lower than in the previous one, partly as a result of some birds remaining in each previously-exposed section. We discuss the management and regulatory implications of shorebird use of the intertidal flats, which include protection of high quality intertidal for foraging by shorebirds. [ABSTRACT FROM AUTHOR]

We combined Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and mass spectrometric analysis of stable carbon isotopes, a traditional method for tracking DOM sources, to investigate the variation in the complexity of water samples collected along a salinity gradient in Delaware Bay in March and August 2014. Detailed analyses of optical properties (ultraviolet and visible (UV–Vis) absorbance and excitation-emission matrix (EEM) fluorescence spectroscopy) were carried out. Results from statistical parallel factor analysis (PARAFAC) of EEM data were also used to track and further constrain fluorescent DOM (FDOM). Optical analyses and FT-ICR-MS data showed large differences between late winter and summer conditions, and the stable carbon isotopic composition was more variable in summer than in late winter. Although terrestrial inputs of DOM are evident at low salinities in both seasons, our data suggest that a significant source of FDOM, from either in situ water column production or from benthic aquatic or semi-aquatic plant communities, contributes DOM to low and mid-salinity waters in Delaware Bay during summer but not in late winter. [ABSTRACT FROM AUTHOR]

Abstract: Understanding the interactions among biological and physical processes is essential to determining how the environment affects transport and survival of fishes. We examined vertical distribution in larval Atlantic menhaden (Brevoortia tyrannus) and Atlantic croaker (Micropogonias undulatus) using 126 depth stratified tows in Delaware Bay, USA, during two cruises, in December 2007 and February 2008. Menhaden larvae were 16.8–24.6 and 20.5–26.2 mm standard length in December and February. Corresponding lengths for croaker were 9.3–17.9 and 8.6–19.6 mm. Using empirical observations, and statistically derived models, we explored larval concentration for both species as a function of location, depth, diel period, tidal period, size, and pairwise interactions. Menhaden concentration was best modeled as a function of station, cruise, and interactions between depth and size as well as between station and cruise. No significant differences in larval menhaden concentration were present among tidal and diel periods. Croaker concentration was best modeled as a function of size and interactions between station and diel period, depth and size, cruise and size. Despite tidal period not emerging as a significant model parameter, we observed larger croaker larvae during nighttime flood tides. Our statistical models are consistent with processes of up‐estuary transport for both species, suggesting larvae are increasingly affected by behavioral responses as larvae grow, exhibiting stronger patterns in vertical distribution. The results refine our understanding of the potential importance of size‐related differences in vertical distribution for larval transport in these species. Future research should examine the interactions among size‐specific vertical migratory capabilities, vertical distribution, transport, and retention. [ABSTRACT FROM AUTHOR]

Fish diets play a critical role in our understanding of aquatic trophic dynamics and are an important component in developing ecosystem-based approaches to fisheries management. Although large nektonic fishes exert top-down predator effects on the food web and typically support viable commercial and recreational fisheries, little is known about the diet of this guild. We evaluated the diets (6327 stomachs) of four nektonic predatory fishes (Pomatomus saltatrix [78-395 mm], Cynoscion regalis [91-520 mm], Morone americana [156-361 mm], and Morone saxatilis [82-785 mm]) in Delaware Bay and in the adjacent ocean. To assess ontogenetic, geographic, and interspecific variation in diets, observations from individual fish stomachs were clustered into species-size class groups, and dietary overlap was estimated using multivariate analyses. A shift in diet composition, as well as diversity, occurred along the estuarine gradient and into the adjacent ocean. Some prey were shared by most predators, including some crustaceans (dominated by Callinectes sapidus, mysids, and Palaemonetes spp.), fundulids (dominated by Fundulus heteroclitus), engraulids (dominated by Anchoa mitchilli), and clupeids (dominated by Brevoortia tyrannus). However, inter- and intra-specific variation in diet was observed as well. In particular, M. americana consumed fewer engraulids and clupeids, and many more and diverse types of invertebrates, while P. saltatrix consumed more clupeids and less invertebrates. The lack of overlap in diet between the four predators evaluated, and between size groups for each predator, supports previous evidence that these groups feed in trophic guilds defined by species and by size within a species. The highly variable diets for these predators suggest high resolution spatial data are necessary in order to quantify their most important prey and their role in coastal ecosystems. [ABSTRACT FROM AUTHOR]

High microplastic concentrations in the Delaware Bay have prompted concern regarding harm to local species. We consider the extent to which the zooplankton is exposed to bay-derived microplastics, focusing on Atlantic blue crabs (Callinectes sapidus) during offshore larval migration. We simulate regional flow fields for a spawning season in the Delaware coastal system to advect passive Lagrangian microplastic and zooplankton tracers. Microplastic exposure levels are estimated from tracer distributions. Field sampling of zooplankton and microplastic concentrations for the Delaware Bay mouth and the adjacent shelf in August 2020 is utilized to appraise model performance. Three mechanisms elevating microplastics exposure are identified: zooplankton transport into microplastic-laden tidelines, displacement of microplastics into the buoyant outflow current, and aggregation in offshore plume fronts. Organization via the above mechanisms substantially enhance microplastic exposures over zooplankton migrations (by an average factor of at least 3.8). • Microplastic distribution is tied to surface winds and the offshore river plume front. • Tidelines, river outflow, and fronts amplify zooplankton exposure to microplastics. • Offshore plume fronts drive weaker exposure than less common transport pathways. • Mean microplastic exposures are comparable to source microplastic concentrations. [ABSTRACT FROM AUTHOR]

Living shorelines gain increasing attention because they stabilize shorelines and reduce erosion. This study leverages physics-based models and bagged regression tree (BRT) machine learning algorithm to simulate wave dynamics at a living shoreline composed of constructed oyster reefs (CORs) in upper Delaware Bay. The physics-based models consist of coupled Delft3D-FLOW and SWAN in four-level nested domains. The model accuracy converges with increasing mesh resolution. The simulated wave-induced current circulation substantiates the effectiveness of CORs in trapping sediments. The simulated yearly-averaged wave power correlates qualitatively with historical shoreline retreat rates. BRT is adopted to improve the model accuracy, identify key processes responsible for simulation errors in wave height (H s) and wave period (T p), and quantify their importance. In the CORs sheltered area, BRT reveals that simulation errors of wind seas mainly arise from wind forcing, wave breaking and wave triad interactions. Wave breaking is seven times more important than wind forcing for simulating H s , while wind forcing and triad interactions are of equal importance for simulating T p. Simulation errors of swells mostly stem from bottom friction and offshore wave boundary conditions. Results from this study can help the assessment and adaptive management of CORs-based living shoreline restoration projects under climate change. • Delft3D and SWAN are coupled for long-term wave simulations at a living shoreline. • Bagged regression tree (BRT) improves accuracy of physics-based hydrodynamic models. • BRT identifies key processes that cause errors in physics-based hydrodynamic models. • Modeled wave power is qualitatively correlated with measured shoreline retreat rate. [ABSTRACT FROM AUTHOR]

The spatial extent and vegetation characteristics of tidal wetlands and their change are among the biggest unknowns and largest sources of uncertainty in modeling ecosystem processes and services at the land-ocean interface. Using a combination of moderate-high spatial resolution (≤30 meters) optical and synthetic aperture radar (SAR) satellite imagery, we evaluated several approaches for mapping and characterization of wetlands of the Chesapeake and Delaware Bays. Sentinel-1A, Phased Array type L-band Synthetic Aperture Radar (PALSAR), PALSAR-2, Sentinel-2A, and Landsat 8 imagery were used to map wetlands, with an emphasis on mapping tidal marshes, inundation extents, and functional vegetation classes (persistent vs. non-persistent). We performed initial characterizations at three target wetlands study sites with distinct geomorphologies, hydrologic characteristics, and vegetation communities. We used findings from these target wetlands study sites to inform the selection of timeseries satellite imagery for a regional scale random forest-based classification of wetlands in the Chesapeake and Delaware Bays. Acquisition of satellite imagery, raster manipulations, and timeseries analyses were performed using Google Earth Engine. Random forest classifications were performed using the R programming language. In our regional scale classification, estuarine emergent wetlands were mapped with a producer’s accuracy greater than 88% and a user’s accuracy greater than 83%. Within target wetland sites, functional classes of vegetation were mapped with over 90% user’s and producer’s accuracy for all classes, and greater than 95% accuracy overall. The use of multitemporal SAR and multitemporal optical imagery discussed here provides a straightforward yet powerful approach for accurately mapping tidal freshwater wetlands through identification of non-persistent vegetation, as well as for mapping estuarine emergent wetlands, with direct applications to the improved management of coastal wetlands.

The American horseshoe crab, Limulus polyphemus, is found along the Atlantic and Gulf of Mexico coasts in genetically isolated populations. Eggs are laid in shoreline beaches that expose developing embryos to combinations of environmental stressors. Whether populations of L. polyphemus differ in multi-stressor tolerance had never been tested. We assessed the multi-stressor tolerance of L. polyphemus embryos from a population in Delaware Bay (DE) and determined whether these differed from the multi-stressor tolerance of embryos from a more southerly Florida Gulf Coast (FGC) population. We monitored the field sediment temperatures and determined multi-stressor tolerance of DE embryos, then compared these to published data for FGC embryos. For multi-stressor tolerance, we assessed development success of embryos in 2-week exposures to 36 full-factorial combinations of temperature (20, 25, 30, 35 °C), salinity (5, 15, and 34 ppt), and ambient O (5, 13, and 21% O), followed by 2 weeks in recovery conditions. Sediment temperatures in the DE site ranged from 9.5 to 46 °C, with extended periods exceeding 35 °C. Development success was similar between the DE and FGC populations in 14 of 26 multi-stressor combinations. The DE embryos were generally more successful in conditions that included high temperature or moderate hyposalinity, whereas the FGC embryos were generally more successful in conditions that included extreme hyposalinity. This suggests that although multi-stressor tolerances are generally similar between the two populations, specific differences exist that correlate more with differences in nest microenvironment than latitude. [ABSTRACT FROM AUTHOR]

Sea level rise and increasing human activities have decreased intertidal habitat in many places in the world. The expansion of aquaculture in intertidal areas may impact birds and other organisms using these habitats, leading to questions of sustainability of both aquaculture and functioning estuarine ecosystems. Understanding the effect of oyster culture on shorebird activity, particularly on Red Knots ( Calidris canutus rufa ), a species on the U.S. Threatened List, is important for adaptive management and the expansion of oyster culture. In May 2013 we experimentally compared Red Knot and shorebird use of a beach section with racks and a control, and in 2016 we compared the use of sections with artificial reefs, oyster racks, and control on Delaware Bay, New Jersey (USA). The data included only times when no workers or other people were present. Censuses, conducted every 30 min throughout the day (279 censuses in 2013, 231 censuses in 2016), included the number of Red Knots and other shorebirds in each treatment section. In 2013, the total number of shorebirds was significantly higher in the rack section than in the control section, except for Red Knots and Ruddy Turnstones ( Arenaria interpres ) that occurred in higher numbers in the control than in the rack section. In 2016 Red Knot numbers were also significantly lower in the rack section. In 2013, the mean number of Red Knots/census was 13 for racks vs 59 for the control (P < 0.0002). In 2016, the mean number of Red Knots/census was 13 for racks and over 68 for other treatments (P < 0.0001). Treatment, date, and number of non-Knot shorebirds explained 60% (2013) and 69% (2016) of the variation in Red Knot numbers. Red Knots avoided the sections with racks while both foraging and roosting, suggesting that caution should be used before placing oyster racks in areas used for foraging by Red Knots. [ABSTRACT FROM AUTHOR]

Delaware (DE) Bay, in the northeastern USA, has long been recognized as a hotspot for avian influenza A virus (IAV); every spring, this coastal region serves as a brief stopover site for thousands of long-distance migrating shorebirds, en route to breeding grounds in the Arctic. During these stopovers, IAV has been consistently recovered from ruddy turnstones (Arenaria interpres) that are likely to become infected as they feed by probing sand and cobble in search of food. In May 2010–2012, we successfully isolated 19 IAV from environmental samples (sand, n = 18; horseshoe crab eggs, n = 1) obtained from DE Bay sites. Two of these viruses were subjected to laboratory conditions similar to those in the DE Bay spring-time environment, and remained infectious for 7 days. Here, through the recovery of IAV from environmental samples, temperature monitoring at and below the sand surface and simulated laboratory trials, we provide evidence that the beach environment may enable localized transmission and short-term maintenance of IAV in this unique ecosystem.

To study horseshoe crab Limulus polyphemus spawning behavior and migration over a large-spatial extent (>100 km), we arrayed fixed station radio receivers throughout Delaware Bay and deployed radio transmitters and archival tags on adult horseshoe crabs prior to their spawning season. We tagged and released 160 females and 60 males in 2004 and 217 females in 2005. The array covered approximately 140 km of shoreline. Recapture rates were >70% with multi-year recaptures. We categorized adult age by carapace wear. Older females tended to spawn earlier in the season and more frequently than young females, but those tendencies were more apparent in 2004 when spawning overall occurred earlier than in 2005 when spawning was delayed possibly due to decreased water temperatures. Timing of initial spawning within a year was correlated with water temperature. After adjusting for day of first spring tide, the day of first spawning was 4 days earlier for every 1 degree (°C) rise in mean daily water temperature in May. Seventy nine % of spawning occurred during nighttime high tides. Fifty five % of spawning occurred within 3 d of a spring tide, which was slightly higher than the 47% expected if spawning was uniformly distributed regardless of tidal cycle. Within the same spawning season, males and females were observed spawning or intertidally resting at more than one beach separated by >5 km. Between years, most (77%) did not return to spawn at the same beach. Probability of stranding was strongly age dependent for males and females with older adults experiencing higher stranding rates. Horseshoe crabs staging in the shallow waters east of the channel spawned exclusively along the eastern (NJ) shoreline, but those staging west of the channel spawned throughout the bay. Overall, several insights emerged from the use of radio telemetry, which advances our understanding of horseshoe crab ecology and will be useful in conserving the Delaware Bay horseshoe crab population and habitats[Current Zoology 56 (5): 563–574, 2010].

Delaware Bay oyster (Crassostrea virginica) populations are influenced by two lethal parasites that cause Dermo and MSX diseases. As part of the US National Science Foundation Ecology of Infectious Diseases initiative, a program developed for Delaware Bay focuses on understanding how oyster population genetics and population dynamics interact with the environment and these parasites to structure the host populations, and how these interactions might be modified by climate change. Laboratory and field studies undertaken during this program include identifying genes related to MSX and Dermo disease resistance, potential regions for refugia and the mechanisms that allow them to exist, phenotypic and genotypic differences in oysters from putative refugia and high-disease areas, and spatial and temporal variability in the effective size of the spawning populations. Resulting data provide inputs to oyster genetics, population dynamics, and larval growth models that interface with a three-dimensional circulation model developed for Delaware Bay. Reconstruction of Lagrangian particle tracks is used to infer transport pathways of oyster larvae and MSX and Dermo disease pathogens. Results emerging from laboratory, field, and modeling studies are providing understanding of long-term changes in Delaware Bay oyster populations that occur as the oyster population responds to climate, environmental, and biological variability.

The annual Natural Oyster Ground Survey is the primary tool used in the management of the oyster fishery in the Delaware portion of Delaware Bay, USA. This survey monitors relative abundance, annual mortality, and recruitment of oysters from commercially important beds and tracks relative changes in the overall stock. Studies have shown that the commercial dredge gear used for this and similar surveys does not collect live oysters, boxes, and cultch with equal efficiency. We compared catch rates of live oysters, boxes, and cultch using this gear to adjacent diver-collected samples. Dredge efficiency varied by bed with the lowest efficiency on the bed with the least fishing pressure, possibly due to a higher degree of reef consolidation. Dredge efficiencies between size classes (20–65 mm, 65–75 mm, >75 mm) were highly variable, although the dredge may be biased towards the capture of live oysters and larger (>65 mm) individuals. Oyster abundance (not including spat) estimated from annual surveys and corrected for dredge efficiency on the commercially important Delaware oyster beds increased from approximately 240 million to 491 million individuals between 2007 and 2008. A comparison of three estimation techniques (bed-specific efficiencies, mean efficiencies, and bootstrapping) is discussed in the broader context of estimating population abundance, size structure, recruitment, and mortality, when corrected and uncorrected for dredge efficiency. [ABSTRACT FROM AUTHOR]

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ABSTRACT Salt marsh birds in eastern North America are high conservation priorities due to their restricted ranges, threats from sea-level rise, and an overall lack of information related to their population status or trends. We calculated estimates of detection probabilities for clapper rail ( Rallus crepitans), willet ( Tringa semipalmata), seaside sparrow ( Ammodramus maritimus), and saltmarsh sparrow ( A. caudacutus) breeding in Delaware Bay, USA. Our objectives were to model the effects of survey covariates on detection probabilities for occupancy and abundance estimates, determine how survey frequency (no. visits to a site) and survey timing (when in the season sites are sampled) affected the estimates, and how varying the number and timing of visits affected the estimates compared to the recommendations based on the North American Marsh Bird Monitoring Protocol. To make these comparisons, we defined 4 survey frequency (2, 3, 4, and 8 visits) and 3 survey timing (early, early-middle-late, and late season) scenarios. Our results are based on 480 surveys (i.e., call broadcast point counts) to 30 sampling locations located at Bombay and Prime Hook National Wildlife Refuges during the 2008 and 2009 breeding seasons. The scenario of 3 visits early in the season provided the lowest error estimates of occupancy and abundance. Distributing the survey effort across the season, our early-middle-late scenario, provided high variance estimates and should be avoided when designing monitoring programs for these tidal marsh obligate species. © 2015 The Wildlife Society. [ABSTRACT FROM AUTHOR]

Detection of prey DNA from faecal samples is a non-invasive method being successfully applied to the dietary analysis of birds as well as other vertebrates and invertebrates. We have employed this technique to examine the significance of Horseshoe Crab eggs (Limulus polyphemus) for Dunlins (Calidris alpina), Semipalmated Sandpipers (Calidris pusilla), Least Sandpipers (Calidris minutilla) and Short-billed Dowitchers (Limnodromus griseus) during northbound migration at their major stopover area in Delaware Bay. This was accomplished through polymerase chain reaction amplification of Limulus DNA from faecal samples using new Horseshoe Crab-specific primers. We detected the consumption of crab eggs in all study species, although results suggest that eggs may be a less important food source for Least Sandpipers than for the other species. Consumption of eggs increases as the migration season progresses, implying a strong preference of birds for this resource at the end of May, when eggs are readily accessible. The results confirm the value of Horseshoe Crab eggs for migrating shorebirds in this crucial stopover area. The Limulus-specific primers we designed can be effectively used for detection of semidigested Limulus DNA from faecal samples and thus they can be employed in the dietary analysis of other predators that seasonally capitalise on Horseshoe Crabs or their eggs. [ABSTRACT FROM AUTHOR]

Construction of bulkheads is a common response to erosion of estuarine shorelines. Bulkheads are usually built incrementally, resulting in wider sandy beaches remaining as enclaves between bulkhead segments. This paper measures the characteristics of bulkheads and enclaves and evaluates (1) whether horseshoe crabs ( Limulus polyphemus) utilize enclaves for spawning when they provide partial sheltering during periods of high wave energies; and (2) whether eggs become trapped in the enclaves and are available to shorebirds when the beaches in front of bulkheads are inundated. The characteristics of bulkheads and beach enclaves were identified in five developed reaches in Delaware Bay, USA. Counts of horseshoe crabs were made in enclaves in two of these reaches during times of high wave energies and compared to counts in nearby unarmored segments. Egg tracer and trapping experiments were conducted at one of the enclaves to assess egg movement. Results indicate that the percent of bulkheads intersecting the beach below mid-foreshore varies from 10 to 50 %. Spawning densities were greater in enclaves than on longer unarmored segments on some days. Enclaves serve as a sink for eggs moving along the base of the bulkheads. Most birds feeding on horseshoe crab eggs preferred sites outside the enclaves and bulkhead segments. Any advantage of bulkheads creating enclaves and sinks for eggs moving alongshore is likely overridden by their disruption to natural process and habitats, but having unarmored enclaves between bulkhead segments may be preferable to one continuous bulkhead, based on environmental benefits. [ABSTRACT FROM AUTHOR]

A small, turbid and nutrient-rich tributary, the Murderkill Estuary, and a large estuarine ecosystem, the Delaware Bay, are tightly linked and form an efficient, tidally-coupled biogeochemical reactor during the summer. Nitrate loading from the Murderkill Estuary generates an instantaneous increase in biological oxygen production in the adjacent Delaware Bay. We are able to capture this primary production response with continuous hourly measurements of dissolved oxygen, chlorophyll, and nitrate. The nitrate influxes from the Murderkill support primary production rates in the Delaware Bay margins that are twice as high as the average production rates measured in the central Bay regions. This elevates chlorophyll in the Bay margins in the summer and fuels metabolism. Tidal transport of the newly produced autochthonous chlorophyll particles from the Bay into the Estuary could also provide a source of labile material to the marshes surrounding the Murderkill, thus perhaps fueling marsh respiration. As a consequence of the tidal coupling between Delaware Bay and the Murderkill Estuary, ecosystem productivity and metabolism in the Bay and Estuary are linked, generating an ecosystem feedback mechanism. Storms modulate this tidally-coupled biogeochemical reactor, by generating significant nitrate and salinity changes. Depending on their magnitude and duration, storms induce large phytoplankton blooms in the Delaware Bay. Such large phytoplankton blooms may occur more often with climate change, since century-long discharge records document an increase in storm frequency. [ABSTRACT FROM AUTHOR]

Structured decision making (SDM) is an increasingly utilized approach and set of tools for addressing complex decisions in environmental management. SDM is a value-focused thinking approach that places paramount importance on first establishing clear management objectives that reflect core values of stakeholders. To be useful for management, objectives must be transparently stated in unambiguous and measurable terms. We used these concepts to develop consensus objectives for the multiple stakeholders of horseshoe crab harvest in Delaware Bay. Participating stakeholders first agreed on a qualitative statement of fundamental objectives, and then worked to convert those objectives to specific and measurable quantities, so that management decisions could be assessed. We used a constraint-based approach where the conservation objectives for Red Knots, a species of migratory shorebird that relies on horseshoe crab eggs as a food resource during migration, constrained the utility of crab harvest. Developing utility functions to effectively reflect the management objectives allowed us to incorporate stakeholder risk aversion even though different stakeholder groups were averse to different or competing risks. While measurable objectives and quantitative utility functions seem scientific, developing these objectives was fundamentally driven by the values of the participating stakeholders. [ABSTRACT FROM AUTHOR]

Plastics are ubiquitous in the global ocean and present a potential hazard to marine organisms. One challenge in assessing the impact of plastics on marine organisms is calculating their exposure to the background plastics field. Guided by observational data of marine organisms and microplastics in the Delaware Bay, this study applies a rational approach for determining exposure times based on a hydrodynamic model of the Delaware Bay paired with particle tracking. We consider two different marine organism behaviors, organisms that stay at the surface and those that exhibit diel vertical migration (DVM), as well as different release locations in the bay in order to understand the spatial variability in microplastics exposure. Microplastics are either assumed to be neutrally-buoyant or surface-trapped, so that they either move with the full three-dimensional estuarine circulation or only with the surface currents. Total exposure time is then determined from the modeled time-integrated background microplastics concentrations for each simulated marine organism path. Exposure was found to be greatest for surface-trapped organisms exposed to surface-trapped plastics as they both tend to aggregate in tidelines. DVM limits exposure to plastics by removing organisms from the surface-trapped plastics field. For neutrally-buoyant plastics, surface-trapped organismal exposure time greatly depends on the organism release location. Exposure is modified by two competing DVM influences: Lesser plastic concentrations at depth decrease the exposure, whereas limiting motion toward the mouth of the bay where the plastic concentrations are smaller increases the exposure. Marine organism exposure to microplastics in a dynamic estuarine environment greatly depends on plastic buoyancy and an organism's vertical position. Both of these factors need to be taken into account for comprehensive microplastic impact studies. [Display omitted] • Hydrodynamic model paired with particle tracking used to calculate exposure. • Consider effects due to organism behavior, microplastics buoyancy, and location. • Exposure is not constant, but varies over time for both microplastic buoyancies. • Plastic's buoyancy and organism's vertical position greatly influence exposure. • Diel vertical migration results in competing effects on exposure. [ABSTRACT FROM AUTHOR]