The Stellwagen Bank National Marine Sanctuary (SBNMS) is one of the most biologically diverse and productive areas in the Gulf of Maine (GoM) region, supporting numerous recreational and commercial activities including whale watching and fishing (Battista et al., 2006; ONMS, 2020). The emblematic feature of the Sanctuary is a shallow sandy bank that provides essential habitat for northern sand lance (Ammodytes dubius), a foundational species of the local food web (Office of National Marine Sanctuaries (ONMS), 2020; Suca et al., 2021). Sand lance are planktivorous forage fish that feed on the lipid rich copepod Calanus finmarchicus (Staudinger et al., 2020; Suca et al., 2021). They exhibit both a diel and seasonal cycle, emerging from the shelter of their coarse-sand habitat to feed during the late winter through early summer (Staudinger et al., 2020; Suca et al., 2021). After sand lance cease feeding, they utilize their lipid reserves for gonad development and spawn as temperatures decline in the late fall through early winter (Murray et al., 2019). In the broader Northwest Atlantic ecosystem sand lance are a common prey item for higher trophic level species, including herring and mackerel (Suca et al., 2018; Staudinger et al., 2020). They are also preferentially targeted by migratory species and due to their non-migratory nature have been shown to attract Atlantic cod (Richardson et al., 2014), great shearwaters (Powers et al., 2020), and humpback whales (Suca et al., 2021) to SBNMS. Thus, the Sanctuary along with its ecological and economic resources are highly dependent on sand lance which in turn is vulnerable to climate change directly and indirectly through its reliance on C. finmarchicus (Office of National Marine Sanctuaries (ONMS), 2020; Pershing et al., 2021; Suca et al., 2021).

The population of C. finmarchicus in the western GoM is among the most abundant across its biogeographic range despite its residence at the southern edge of its subarctic distribution (Melle et al., 2014). Throughout the GoM, C. finmarchicus serves as a critical link in the food web transferring lipid energy up to higher trophic levels, with most characteristic North Atlantic species feeding on them directly or indirectly (Pershing and Stamieszkin, 2020). While other copepod species—such as Centropages and Pseudocalanus spp.—along with krill and amphipods are commonly consumed by small pelagic fishes and other marine predators (Bowman, 2000; Suca et al., 2018), C. finmarchicus is the preferred prey for many, including sand lance (Suca et al., 2021) and North Atlantic right whales (Hudak et al., 2023). C. finmarchicus are targeted for their relatively large size and high nutritional value, derived from the lipids they accumulate as part of their life history strategy (DeLorenzo Costa et al., 2006; Record et al., 2018). They take advantage of the highly productive spring bloom, during which their populations grow significantly. The relatively large late-stage individuals then accumulate lipid reserves for sustenance through their dormancy period (Johnson et al., 2008). The overwintering, diapausing stages of this copepod are particularly abundant in the deep waters of Wilkinson Basin, where they dominate the mesozooplankton, comprising approximately 60-80% of the total biomass (Runge et al., in review; Runge et al., 2015). Because they accumulate and transfer large amounts of lipid energy in time and space, Calanus spp. play a vital role in structuring arctic and subarctic ecosystems (Record et al., 2018) to the extent that their distribution is a defining characteristic of the North Atlantic biome, including the GoM and SBNMS (Pershing and Stamieszkin, 2020). Warming of the GoM, at a rate nearly four times faster than the global average threatens the viability of C. finmarchicus populations in the region and all those species that depend on them, including sand lance (Grieve et al., 2017; Office of National Marine Sanctuaries (ONMS), 2020; Pershing and Stamieszkin, 2020; Pershing et al., 2021).

Starting after 2010, various plankton surveys recorded a transition in GoM C. finmarchicus populations to a decadal pattern of lower abundances, specifically there was a large decline in the fall and winter while spring abundances shifted higher (Sorochan et al., 2019; Ji et al., 2022; Pershing and Kemberling, 2023; Runge et al., in review). This transition coincided with rapid surface warming, and broader shifts in circulation patterns resulting in a greater influx of warm slope water into the GoM rather than cooler Labrador slope water (Meyer-Gutbrod et al., 2021; Pershing and Kemberling, 2023; Townsend et al., 2023). Shifts in the general oceanography and the flux of water masses entering the GoM have been hypothesized to impact late-stage abundance of C. finmarchicus in various ways. The higher temperatures likely impair their ability to overwinter (Maps et al., 2012; Runge et al., 2015), a shift in the timing of the spring bloom may lead to higher mortality rates (Honda et al., 2024), and the supply from sources within and upstream of the GoM may be diminished (Sorochan et al., 2019; Ji et al., 2022; Honda et al., 2023). At the same time, greater phytoplankton food availability in the late winter and early spring in the GoM enables increased early-stage C. finmarchicus abundance (Honda et al., 2024; Runge et al., in review). Moreover, increases in food availability in the fall, possibly due to decreased grazing pressure from older C. finmarchicus, has allowed the expansion of other mesozooplankton, particularly smaller copepods (Pershing and Kemberling, 2023).

Declines in the C. finmarchicus population have significant implications for dependent higher trophic levels. The oceanographic changes in the GoM and subsequent decline in C. finmarchicus have been linked to shifts in the migratory patterns of the critically endangered North Atlantic right whale (NARW). The NARW, which selectively preys on lipid-rich C. finmarchicus (Hudak et al., 2023), responded by moving to foraging grounds in the Gulf of Saint Lawrence, leading to an increased risk of gear entanglements and ship strikes (Record et al., 2019; Meyer-Gutbrod et al., 2021, 2023). The NARW population, which had been recovering, is now declining and classified as undergoing an unusual mortality event. Recruitment of Atlantic herring, another C. finmarchicus predator, has also been low (Northeast Fisheries Science Center (NEFSC), 2018), and while this decline is consistent with other climate related changes, it has not been directly linked to changes in C. finmarchicus abundance (Pershing et al., 2021). Likewise, the emblematic groundfish Atlantic cod feeds on C. finmarchicus in its larval stages and later larger prey in SBNMS including herring and sand lance, among others (Richardson et al., 2014; Pershing et al., 2021). Cod stocks have been at historic lows in the GoM, and while population abundance has been linked to C. finmarchicus elsewhere, it is a challenge to separate the decline here from overfishing (Beaugrand and Kirby, 2010; Pershing et al., 2015). Meanwhile, lobster recruitment declines in the GoM have been linked to lower abundances of C. finmarchicus and shifts in phenology, resulting in a mismatch between larvae released in the water column and food availability (Carloni et al., 2018, 2024). At the scale of the entire Northeast Shelf, sand lance populations are significantly correlated with C. finmarchicus abundance in years prior to 2010 (Suca et al., 2021), but it is unclear whether this relationship still holds, especially when focused on Stellwagen Bank. NARW, Atlantic herring, Atlantic cod, lobster, and sand lance are all focal species of SBNMS and are all vulnerable to climate change via C. finmarchicus, which is a foundational species for the whole GoM, including SBNMS (Office of National Marine Sanctuaries (ONMS), 2020). The NARW case especially exemplifies how rapid climate-driven changes in C. finmarchicus abundances can have downstream effects on sanctuary focal species, including those which feed on sand lance like humpback whales and great shearwaters. However, the potential climate impact on sand lance is unique and presents an opportunity for investigating circulation-mediated vulnerability because sand lance are non-migratory and thus narrowly dependent on C. finmarchicus abundance advected over the bank.

The GoM is connected to the broader North Atlantic Ocean by a circulation pattern that brings a mix of different water masses into its northeastern sector. Water masses enter through the Northeast Channel at depth and are generally transported west to the deep basins of the GoM: Georges Basin, Jordan Basin, and Wilkinson Basin (Townsend et al., 2015). The surface currents enter from the western Scotian Shelf in Canadian waters, turning west and then follow the coastline in a counter-clockwise direction, eventually exiting the GoM through the Great South Channel and southern flank of Georges Bank ( Figure 1 , Pettigrew et al., 2005). The transport of water centered on the 100 m isobath along the coast is named the Maine Coastal Current (MCC). The MCC is strongly influenced by the influx of fresh cold waters from the Scotian Shelf as well as the many inputs of riverine water and contains rich nutrient concentrations that sustains high primary productivity through the spring and summer months (Churchill et al., 2005). The MCC enters Massachusetts Bay, where it diverges into two branches, the main portion of which passes over the eastern edge of Stellwagen Bank towards the outside of Cape Cod (Jiang et al., 2007). While this describes the general pattern of currents in the GoM they can vary depending on broader climate driven oceanographic changes in the GoM (Townsend et al., 2023), and variability in freshwater inflow (Churchill et al., 2005; Pettigrew et al., 2005) and wind forcing (Jiang et al., 2007; Burkholder et al., 2024).

It is hypothesized that the high abundances of C. finmarchicus found in Wilkinson Basin result from the combination of high primary productivity in the GoM and a favorable circulation pattern. This mechanism, known as CAST (Coastal Amplification and Supply and Transport), describes how abundant spring-through-summer food availability in the Maine Coastal Current (MCC) leads to rapid C. finmarchicus population growth, which is then advected to Wilkinson Basin, where the copepods enter diapause and accumulate at depth (Ji et al., 2017). Recent analyses of fixed-station time series data in the MCC and Wilkinson Basin in the western GoM have revealed that the population decline of C. finmarchicus over the past 15 years has been restricted to the fall and winter, while summer and spring abundances have remained the same or higher (Runge et al., in review). This finding supports the CAST mechanism and may also result from an increase in late winter/early spring phytoplankton in Wilkinson Basin (Runge et al., in review; Honda et al., 2024; Ji et al., 2017). In this study, we explore whether the CAST mechanism may also be responsible for supplying Stellwagen Bank with C. finmarchicus, thus linking SBNMS productivity and the foraging habitat of sand lance to upstream sources of C. finmarchicus. Since both C. finmarchicus abundance and GoM circulation patterns are impacted by climate change, this linkage also presents a possible indicator for monitoring SBNMS climate vulnerability. Our investigation employs ocean simulation modeling with Lagrangian particle tracking to conduct numerical experiments. Our specific objectives are to identify the upstream sources and to investigate seasonal and interannual variability in connectivity. Furthermore, we evaluate the connectivity between Stellwagen Bank and time series stations, from which observations of C. finmarchicus could serve as useful indicators of upstream productivity.

The particle tracking numerical experiments align with our conceptual understanding of the mean circulation pattern in the GoM and highlight its seasonality and interannual variability in a novel manner. Particle tracking models are frequently used to demonstrate retention and connectivity, but previous studies focused on limited years (e.g. Huret et al., 2007; Ji et al., 2012), specific days of the year (e.g. Boucher et al., 2013), mean values (e.g. Suca et al., 2022), or a specific process of interest (e.g., Churchill et al., 2011). By quantifying the spring-to-summer connectivity over 39 years, our results illustrate long-term patterns of connectivity in the GoM, further support the CAST hypothesis (Ji et al., 2017), and extend its application to the supply of C. finmarchicus to Stellwagen Bank and beyond. Although several upstream strata, including those that host the MBON time series stations, are connected to Stellwagen Bank, we identified stratum 36 to the immediate north as the primary source. Thus, C. finmarchicus entrained in the MCC or aggregating in Wilkinson Basin are first advected to stratum 36 and may then be transported over the bank. Notably, we observe that the greatest connectivity between Stellwagen Bank and CMTS occurs in late spring when lipid-rich stages of C. finmarchicus at the time series station are abundant and sand lance on the bank are actively feeding. We further find the greatest connectivity between WBTS and strata 36 in late winter when females are emerging from diapause and seeding the next generation of C. finmarchicus (Runge et al., in review). Although C. finmarchicus are observed to be seasonally present at the time series stations in the area immediately surrounding Stellwagen Bank, relating the abundance there to the rest of the GoM is challenging due to differing sampling protocols (Costa et al., 2023). Still, these coastal zones encompassed by stratum 35 may be an important and more consistent source of C. finmarchicus supply to the sandy bank.

Population spatial structure and habitat connectivity are well-recognized considerations for managing fisheries stocks and MPAs, but usually in the context of fish reproduction and larval dispersal (Churchill et al., 2011, 2017; Kritzer and Liu, 2014; Suca et al., 2022). Here, we considered the role of C. finmarchicus and connectivity in the productivity of the SBNMS. Although we focus on the shallow sandy bank as foraging habitat for sand lance, C. finmarchicus play a vital ecological role for the whole Sanctuary as a grazer and prey for many species, including larval fish like cod and the NARW (Office of National Marine Sanctuaries (ONMS), 2020). Secondary production does occur over the bank, but the limited area and rapid exchange of water mean that advected supply will dominate the foraging habitat for sand lance and other planktivores (this study; Suca et al., 2022). Shoaling over the bank may also act as a mechanism for aggregating C. finmarchicus populations as they are transported from deeper waters to the shallow bank and become concentrated (see Sorochan et al., 2021). However, C. finmarchicus are pelagic species that migrate to depth to escape predation; the shallow bank prevents both diel vertical migration and diapause, exposing them to higher mortality (Baumgartner et al., 2011). When Calanus are unable to escape to depth, visual predators like sand lance can readily feed on the relatively large copepods, particularly the late stages, leading to a pattern of lower abundance over shallow shelf habitats (Baumgartner et al., 2011; Melle et al., 2014). Meanwhile, in the relatively shallow waters of the GoM, Wilkinson Basin provides an important refuge for Calanus populations despite predation at depth by non-visual predators, including chaetognaths, ctenophores, and siphonophores (Wiebe et al., 2022).

The pattern of lower C. finmarchicus abundance over shallow areas is most apparent during the overwintering period. It is well-documented that abundances of C. finmarchicus on Georges Bank reach an annual high during the spring when the population is growing, but as temperatures increase and the copepods develop to the diapausing stage, the population declines throughout the summer and fall (Meise and O’Reilly, 1996; Kane, 2007). Likewise, on Jeffreys Ledge, a narrow bank with a depth <50 m to the north of Stellwagen Bank, C. finmarchicus population abundances follow a similar pattern, with the lipid-rich CV stages observed at concentrations as high as 10 individuals per liter in late spring, then declining to non-existent by late fall, presumably due to visual predation (Runge and Jones, 2012). Limited observations have been made of C. finmarchicus populations on Stellwagen Bank, but data provided by Suca et al. (2021) indicate that, like other shallow areas, abundances are highest in the spring and decline toward fall. MWRA observations averaged over Massachusetts Bay also follow this seasonal pattern (Turner et al., 2006). Perhaps because the area of Stellwagen Bank is small and the water flux is high, the standing stock of C. finmarchicus is quickly replenished by advected supply from upstream sources. However, the relative importance of this connectivity to abundance is not clear when considering all the factors that control the population variability of C. finmarchicus and continues to be a point of discussion (Wiebe et al., 2022).

Recent studies have highlighted the predominant role of internal growth processes in shaping C. finmarchicus abundance in the GoM’s deep basins. Specifically, Ji et al. (2022) used scaling and sensitivity analyses to show that internal growth, rather than external supply, primarily drives seasonal fluctuations in population levels, with dynamics being most sensitive to mortality during the winter-spring period. Honda et al. (2023) corroborated these findings through synchrony analysis, demonstrating that the spring abundances of C. finmarchicus in the deep basins are largely influenced by heterogeneous internal dynamics. When these internal dynamics are similar across spatially separated strata, the Moran effect (1953) may produce synchrony between their abundances. Similarly, the abundance of C. finmarchicus in strata supplying the Stellwagen Bank foraging habitat may reflect distant areas due to their internal similarities, as observed across the entire western GoM on an interannual basis (Honda et al., 2023, unpublished data). While C. finmarchicus population abundances in any given region are influenced by both heterogeneous internal dynamics and advective supply, each subpopulation depends on these factors to different degrees. At one extreme, a seed supply of adult C. finmarchicus is advected into a region, but subsequent generation abundances are primarily determined by local reproduction, growth and mortality rates. At the other extreme, there is minimal local production, and the observed abundance is largely due to advection from upstream.

Based on the results here and our understanding of C. finmarchicus ecology in the GoM, we reason that the population over Stellwagen Bank is generally more influenced by external exchange than local population growth, with supply from the adjacent strata 35 and 36 supporting abundance over Stellwagen Bank. The populations of the supplying strata are then driven to a greater extent by internal dynamics, following Honda et al. (2023), but advective supply following the CAST hypothesis (Ji et al., 2017) is nonetheless important. This describes the general balance of processes as we understand it, but for any given time period, a “powershift” among these processes could occur (Ji et al., 2022), as suggested by the interannual variability in connectivity. In most years, the absolute abundance of C. finmarchicus in the western GoM may be primarily influenced by internal dynamics, but the external supply of a seed stock may still pose a potential bottleneck. In the winter months, the abundance of C. finmarchicus in the western MCC can be below detectable levels at the relatively shallow CMTS station (see Runge et al., in review) and likewise at the MWRA stations (MWRA, unpublished data). For a new generation to establish in the absence of a resident population, some individuals must be transported in from upstream, and that number will influence abundance after a given time. Potentially, the local abundance of C. finmarchicus on Stellwagen Bank could be stifled when population growth is low and its connectivity to seeding areas –such as the upstream MCC or the deep overwintering basins— is reduced, limiting the absolute supply of the copepod prey.

Given the substantial variability observed in our experiments and the potential climate impact on the foraging habitat of Stellwagen Bank, monitoring upstream C. finmarchicus abundance and modeling its connectivity is warranted. Suca et al. (2021) found that over the whole Northeast Shelf, the abundance of age 2–3 year-old sand lance was correlated to the population of C. finmarchicus 3–4 years earlier, when the parental generation of those observed sand lance would have been feeding prior to fall spawning. Thus, FVCOM hindcast ocean simulations with particle tracking, combined with time series observations and other environmental variables, could be used to build a forecast of habitat suitability and climate vulnerability. Alternatively, or in addition to the Lagrangian measure of connectivity, we may rely on current speeds at buoys to provide a Eulerian measure. Since these long-term observations of currents have been in place, there has been a significant decline in current speed, particularly in spring, attributed to an increase in southwesterly winds (Burkholder et al., 2024). As demonstrated here, reduced current speed diminishes the connectivity between the MCC and Stellwagen Bank, which, if the trend continues, may manifest in decreased abundances of C. finmarchicus on Stellwagen Bank. The model suggested here would provide both a useful forecast of C. finmarchicus abundance over Stellwagen Bank or other areas and would elucidate the variable influences of advected supply and internal production. Notably, long-term data sets of abundance and connectivity are vital for building indices used in the model and emphasizes the importance of continued monitoring. Fortunately, the MWRA and MBON time series stations have been monitored for 17-25 years and continue to be active. These stations may also inform secondary production in other regions downstream. Specifically, the experiments showed high connectivity between WBTS and southern New England, where NARW have been observed to be feeding in increasing numbers in recent years (National Academies of Sciences, Engineering, and Medicine, 2024).

In practice, it may be challenging to accurately forecast the abundance of C. finmarchicus in downstream areas due to the numerous factors involved, including the connectivity between a time series station and Stellwagen Bank—or any other area. However, monitoring and modeling would provide valuable insight into the confidence with which time series data can be used as an indicator for the habitat suitability and climate vulnerability of the downstream area. In a highly connected year, we would increase our confidence that the indicator would likely reflect similar conditions in both locales, versus a low-connectivity year where abundance may be higher at the time series location but not at Stellwagen Bank. This Bayesian approach can also be applied when examining data within the framework of synchrony, where the Moran effect is responsible for similarity between distinct spatial areas experiencing the same environmental forcing. When environmental indicators from two locations diverge, we can lower our confidence that a measure of C. finmarchicus abundance in one area will be representative of the other. Building on the findings presented here, the next steps toward developing a decision support tool for the SBNMS involve applying such an approach through statistical analysis. The statistical model could relate sand lance habitat to connectivity, C. finmarchicus abundance at various time series stations, and environmental factors like the influx of Warm Slope Water into the GoM. Demonstrated significant relationships would then support the use of those variables in forecasts of foraging habitat or as standalone indicators. Such information will be useful for stakeholders making data-driven decisions and will ultimately contribute to the resilience of the SBNMS in the face of ongoing climatic changes. For example, forecasts of favorable sand lance foraging habitat could signal a likely increase in humpback whale presence, aiding whale-watching operations in planning their season while simultaneously alerting other stakeholders to exercise caution during vessel transit to mitigate ship strikes. More generally, tracking the upstream abundance of C. finmarchicus and its connectivity to a downstream location could provide a valuable indicator of climate vulnerability because of the foundational role of C. finmarchicus in North Atlantic food webs. Here we focused specifically on Stellwagen Bank with broader implications for the whole Sanctuary, but this approach could readily be applied to any area or species of interest.

This study underscores the critical role of habitat connectivity in the Gulf of Maine for the ecological health of the Stellwagen Bank National Marine Sanctuary. Through Lagrangian particle tracking experiments, we demonstrated that the supply of Calanus finmarchicus to Stellwagen Bank is connected to upstream sources, particularly the Maine Coastal Current. Notably, we revealed previously undescribed seasonal patterns and large interannual variability in this connectivity—a finding that enhances our understanding of the region’s ecological dynamics. Understanding these dynamic connectivity patterns enhances our ability to predict ecological conditions at Stellwagen Bank based on upstream indicators. This insight is crucial for enabling stakeholders to make informed, data-driven decisions. Ultimately, accounting for the dynamics of habitat connectivity—including its seasonal and interannual variability—will contribute to the resilience of the Stellwagen Bank National Marine Sanctuary in the face of ongoing climatic impacts.