The Impact of Gulf Stream Frontal Eddies on Ecology and Biogeochemistry near Cape Hatteras

Ocean physics and biology can interact in myriad and complex ways. Eddies, features found at many scales in the ocean, can drive substantial changes in physical and biogeochemical fields with major implications for marine ecosystems. Mesoscale eddies are challenging to model and difficult to observe synoptically at sea due to their fine-scale variability yet broad extent. In this work we observed a frontal eddy just north of Cape Hatteras via an intensive hydrographic, biogeochemical, and optical sampling campaign. Frontal eddies occur in western boundary currents around the globe and there are major gaps in our understanding of their ecosystem impacts. In the Gulf Stream, frontal eddies have been studied in the South Atlantic Bight, where they are generally assumed to shear apart passing Cape Hatteras. However, we found that the observed frontal eddy had different physical properties and phytoplankton community composition from adjacent water masses, in addition to continued cyclonic rotation. In this work we first synthesize the overall ecological impacts of frontal eddies in a simple conceptual model. This conceptual model led to the hypothesis that frontal eddies could be well timed to supply zooplankton to secondary consumers off Cape Hatteras where there is a notably high concentration and diversity of top predators. Towards testing this hypothesis and our conceptual model we report on the biogeochemical state of this particular eddy connecting physical and biological dynamics, analyze how it differs from Gulf Stream and shelf waters even in “death”, and refine our initial model with this new data. Key Points In-depth investigation of a frontal eddy in the Gulf Stream off Cape Hatteras, North Carolina Continued physical and biogeochemical differences are observed between the eddy and adjacent water masses even as it begins to shear apart We share a conceptual model of the ecological impact of frontal eddies with a hypothesis that they supply zooplankton to secondary consumers Plain Language Summary Frontal eddies are spinning masses of water (~30km in diameter) that move along western boundary currents like the Gulf Stream. When they form they carry productive coastal water into the Gulf Stream and drive upwelling within their cores. Together this leads to an increase in the amount of phytoplankton within them - much higher compared to surrounding nutrient-limited Gulf Stream water. On the east coast of the United States one common area of frontal eddy formation is just off Charleston, SC. Eddies then travel up the coast and dissipate near Cape Hatteras, NC. In this work we measured a wide range of physical and biological properties of a frontal eddy just north of Cape Hatteras. We compared these properties within the eddy to the coastal water on one side and the Gulf Stream water on the other, finding clear differences in phytoplankton community composition and other physical and chemical properties. Using the results of these observations together with previous studies we share a simple model for how frontal eddies may impact phytoplankton, zooplankton, and fish – hypothesizing that they may contribute to the high diversity and density of top predators off Cape Hatteras.

There is a long, though intermittent, history of studying frontal eddies in the Gulf Stream 125 (von Arx et al., 1955;Pillsbury, 1890;Webster, 1961). Extensive physical surveys describe these 126 features as cyclonic, cold-core eddies with substantial upwelling through isopycnal uplift (Bane 127 et al., 1981; Thomas N. Lee et al., 1981). In turn, phytoplankton respond intensely to this 128 nutrient upwelling, with observed levels of diatom blooms and chl-a 10-100 times greater than 129 those typically measured in Gulf Stream or outer shelf water (defined as depths from 40-200m 130 (T. N. Lee et al., 1991)). Menhaden and bluefish migrations to the South Atlantic Bight (SAB) 131 are suggested to be secondary to this response of primary producers as the high-levels of 132 phytoplankton provide a consistent food source for higher-level consumers (Yoder et al., 1981). 133 The Frontal Eddy Dynamics experiment in 1987 intensively surveyed a frontal eddy between 134 Cape Lookout and Cape Hatteras, providing some of the first evidence that these features 135 propagate northward and downstream beyond Cape Hatteras, with cross shore and along shore 136 temperature profiles demonstrating the extent of isotherm doming and continued upwelling 137 (Glenn & Ebbesmeyer, 1994b). This work suggested that not only did the feature move past 138 Cape Hatteras, but upwelling continued and a second warm filament formed beyond the cape. 139 The greatest temperature anomalies from upwelling were measured to occur around ~150m  streamer is clearly visible as is the cooler and higher chl-a eddy core. The chl-a image was 154 acquired 13 hours after the SST image so the eddy is slightly further downstream. Another 155 frontal eddy is visible downstream off of Cape Hatteras, though the warm streamer has been 156 pulled into the eddy core and mixed away but is still clearly evident from both the meander in the 157 stream, the beginning of the formation of a new warm streamer, and the positive chl-a anomaly.

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Frontal eddies form where energy is transferred from the mean flow of the current to 159 eddy kinetic energy due to instability processes, often influenced by the local bathymetry. In the 160 northern SAB (Charleston to Cape Lookout, where the eddy in this study formed), this energy   Based on the literature we developed a simple qualitative conceptual model as a 217 framework for our study (Figure 3). In this model we expect to see a bloom of phytoplankton 218 soon after eddy formation, followed by a zooplankton bloom around a week after the 219 phytoplankton peak, and increased secondary consumers soon after the zooplankton peak. We 220 expect the biology to lag the physics such that phytoplankton growth above the baseline follows 221 upwelling by a day or two and terminates soon after upwelling ends. Our study investigates a  Importantly phytoplankton growth is still likely elevated due to upwelling of nutrients even as 233 standing biomass returns to baseline levels or even below due to grazing. Additionally while 234 phytoplankton and zooplankton biomass is grown in-place, secondary consumer biomass is not  fisheries for snappers, groupers, tunas, and mackerels, and recreational fishing due to the high 248 density of major sport fish such as tunas and other billfish. 249 We surveyed a frontal eddy just northeast of Cape Hatteras (Figures 1 and 2      These inherent optical properties (i.e., absorption, attenuation, and backscattering) and 312 products calculated from them were used as proxies for a range of particulate properties. γ, 313 which is estimated as the spectral slope of cp, is a strong proxy for particle size distribution with 314 a higher γ indicating smaller average particle sizes (Emmanuel Boss et al., 2001). Scattering is 315 driven primarily by particle concentrations, and backscattering is sensitive to both particle     is similar to Gulf Stream salinity, but is slightly cooler in temperature and thus denser. After 392 classifying water masses based on T and S more nuanced patterns are visible (Figure 6). At first 393 glance, the eddy appears to be on the continuum between slope and Gulf Stream water or, at 394 times, indistinguishable from the Gulf Stream. This is true for most of the optical proxies.

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Measurements of cp are similar in the shelf, slope, eddy, and Gulf Stream waters ( Figure S4). cp surprisingly γ (negatively correlated with the particle size distirbution) decreases as we move 403 offshore, indicating larger particles offshore compared to inshore, in contrast to HH_G50 trends.

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A few properties however reveal the eddy as distinct from the Gulf Stream, with depleted 405 nitrate and silicate compared to the Gulf Stream water, enhanced non-phycoerythrin containing 406 picocyanobacteria compared to all other water masses, the highest bacteria to chl-a ratio, and 407 lowest volumetric NCP (Figures 5 and 6). In the eddy cp(460), and bbp(440), and γ are marginally 408 lower compared to Gulf Stream (Figures 5 and 6).

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Profiling data shows a MLD of ~10m for shelf and slope water with eddy or Gulf Stream 410 water underlying that shallow shelf/slope water and a second thermocline around ~30-50m. The

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MLD for the eddy ranges from approximately 30-50m and Gulf Stream is the same range from 412 35-55m. Calculating MLD at the front at these scales is often problematic given the intense 413 mixing and turbulence across the front, so these exact depths should be interpreted with caution. 414 We observe a fairly consistent cold and fresh intrusion ( Figure S8) moving from the shelf to the  Figure S1.

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The VMP data reveals the physical structure across all transects, with a general trend of    this may indicate mixing from depth in these regions or higher nutrient uptake rates, not reflected 565 in free nutrient concentrations ( Figure 6).

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Acoustic backscatter shows substantial structure in all water masses ( Figure 8). As   (driving cp to be flatter and lower γ), but actually higher overall, indicating higher particle 600 concentrations.

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So more particles, larger average particle sizes, and particles that are similarly or 602 relatively more organic than those inshore. This leads us to the conclusion that it is either due to  timeline of this increase in zooplankton is variable across taxa, but in this work, the authors 618 found zooplankton increased dramatically from five days to two weeks after peak chl-a 619 concentration, and peak chl-a concentration was approximately a week after eddy formation.

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Similar work has shown that copepods, which are slower to respond, may persist for a few 621 weeks, and doliolids which respond within days, were also found to decrease faster, persisting 622 for 7-9 days (Deibel, 1985). This work was done on the southern half of the SAB (with eddy 623 generation around Florida and dissipation just upstream of the Charleston Gyre) and may have 624 different characteristics than the northern section of our study, but if we assume similar 625 responses, zooplankton abundance should peak right when the eddy is passing Cape Hatteras.

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Our acoustic data does not support an enhancement of zooplankton and small fish 627 biomass in the eddy compared to Gulf Stream water at the time of our measurements (Figure 9).

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The variability of some transects adds substantial uncertainty to these comparisons and we don't In much of the previous work on frontal eddies the assumption was that these eddies 641 decayed onto the outer shelf of the SAB, and an ongoing question was that if there is such a 642 substantial input of nutrients due to upwelling why is the outer shelf of the northern SAB not 643 more productive in higher trophic levels? We suggest based on our work and previous studies 644 that a large amount of this new production could be moving into grazer biomass, and while some    Even more generally, anywhere a western boundary current follows the continental shelf and the 711 system is nutrient limited frontal eddies may be a reliable mechanism for increasing primary 712 production and transfer efficiency to higher trophic levels.