Received: 2 November 2018 | Revised: 1 April 2019 | Accepted: 13 May 2019 DOI: 10.1111/maec.12546 ORIGINAL ARTICLE The intermingling of benthic macroinvertebrate communities during a period of shifting range: The “East of Nantucket” Atlantic Surfclam Survey and the existence of transient multiple stable states Eric N. Powell1 | Roger Mann2 | Kelsey M. Kuykendall1 | M. Chase Long2 | Jeremy R. Timbs1 1Gulf Coast Research Laboratory, University of Southern Mississippi, Ocean Springs, Abstract Mississippi A f survey o the region eastward of Nantucket provided an opportunity to examine 2 Virginia Institute of Marine Science, The the cold temperate–boreal boundary along the high-energy Great South Channel. College of William and Mary, Gloucester Point, Virginia Here described are the benthic macroinvertebrate community types encountered, with a focus on the influence of climate change on the range boundaries of the Correspondence Eric N. Powell, Gulf Coast Research benthic biomass dominants and the potential existence of transient multiple stable Laboratory, University of Southern states. The survey identified three primary community types. The shallowest sites Mississippi, 703 East Beach Drive, Ocean Springs, Mississippi 39564 were occupied by a surfclam-dominated community, comprising an abundance of Email: [email protected] large (≥150 mm) surfclams, and a few common attached epibiota primarily attached Funding information to exposed surfclam shell. Two communities exist at intermediate depths, one domi- National Science Foundation’s Industry/ nated by submarket and small market‐size surfclams (<150 mm) and the other, created University Cooperative Research Center SCeMFiS (Science Center for Marine by mussel mats and their attendant epibiota, crabs, sea urchins, and other mobile epi- Fisheries), Grant/Award Number: NSF fauna. Mussels are a foundational species, establishing a hard‐bottom terrain condu- #1266057 cive to these other denizens in soft‐bottom habitat. Cobbles were nearly ubiquitous, rocks were routinely recovered, and boulders were encountered occasionally. Slow growing attached epibionts were exceedingly rare and mobile epifauna were not ob- viously associated with these large sedimentary particles; nor were the surfclam or mussel communities. The frequency of barnacle scars suggests sediment scour under the high-flow regime characteristic of the surveyed region, which voids the habitat potential of these sedimentary particles. The abundance of surfclam shell indicates that surfclams have inhabited the shoaler depths for an extended time; limited shell at deeper sites supports the inference from the absence of large animals that these sites are relatively newly colonized and represent further evidence of an offshore shift in range brought on by increasing bottom water temperatures. The dichotomous nature of the two primary community types at mid-depths suggests that these two communities represent multiple stable states brought on by the interaction of an in- vading cold temperate species with the receding boreal fauna resulting in a transient Marine Ecology. 2019;00:e12546. wileyonlinelibrary.com/journal/maec © 2019 Blackwell Verlag GmbH | 1 of 17 https://doi.org/10.1111/maec.12546 2 of 17 | POWELL ET AL. intermingling of species, which, however, structure the habitat into exclusionary sta- ble states rather than overlapping in a co-occurrence ecotone. KEYWORDS climate change, cobble, community composition, mussel, range shift, Spisula 1 | INTRODUCTION in fact, might be characterized by the intermingling of community types as the dominant benthic sedentary and sessile species repo- As the world continues to warm, range shifts by benthic species sition in geographic space at varying rates. Herein, the community are increasingly documented (e.g., Southward, Hawkins, & Burrows, types encountered are described, with a focus on the benthic bio- 1995; Oviatt, 2004; Lucey & Nye, 2010; Wethey, Woodin, Berke, mass dominants, followed by a further inquiry as to the potential & Dubois, 2016). The northwest Atlantic is a particularly interest- existence of transient multiple stable states (for more on multiple ing case history due to the rapid rate of warming in comparison to stable states, see, e.g., Gray, 1977; Peterson, 1984; Coco, Thrush, other regions (Saba et al., 2016) and the existence of a long‐term Green, & Hewitt, 2006) as a characteristic of rapidly evolving range survey time series of two long‐lived benthic dominants, the Atlantic boundaries. surfclam, Spisula solidissima, and the ocean quahog, Arctica islandica (NEFSC, 2017a, 2017b). These two species occupy the cold temper- 2 | METHODS ate (Virginian Province—Hale, 2010) and boreal (Acadian Province— Hale, 2010) sides of the north-temperate advancing temperature 2.1 | Survey location and design boundary. As the surfclam responds more rapidly to warming tem- peratures with an across shelf shift in range (Hofmann et al., 2018; The survey domain is shown in Figure 1. Three regions are demar- Timbs, Powell, & Mann, 2018), the two species presently occupy an cated. The first, located on the upper left, is the shoaler region ecotone extending along much of the mid‐Atlantic region (Powell, historically supporting significant surfclam landings. This region Ewing, & Kuykendall, 2019). The dynamics of this geographically falls outside of the survey stratum map (NEFSC, 2017a), and con- extensive ecotone remain unstudied, save for its documentation. sequently, the U.S. surfclam survey in the EEZ does not contain an Along the northern boundary of the cold temperate habitat in historical record of this region. Note that the western boundary ap- the northwestern Atlantic, surfclam habitat presently also abuts a proximates the inshore EEZ boundary and that the southern bound- higher energy boreal habitat characterized by mussel beds and as- ary abuts on the lower left the Nantucket Lightship Habitat Closure sociated biota. This interface remains unstudied due to the difficulty Area, an area closed to bottom‐tending gear. The larger region lo- of surveying the benthos in areas with high-velocity tidal currents, cated in the center and to the right falls within the proposed Great large-amplitude shifting sand waves, and boulder fields typical of South Channel Habitat Management Area. For ease of discussion, the region between Nantucket and Georges Bank across the Great these last two regions will be referred to in the results section and South Channel (Emery & Uchupi, 1965; Mann, Swift, & Perry, 1981). identified on all maps of faunal distribution. Over the last year, two initiatives independent of climate change A fixed grid design was chosen to insure that the region was focused attention on the absence of benthic survey data for a por- evenly and densely surveyed. Fixed grids are routinely used to tion of this high‐velocity region east of the island of Nantucket evaluate regions initially for later inclusion into a stratified random eastward to the Great South Channel, the cold temperate portion survey design (e.g., van der Meer, 1997; Morehead, Montagna, of which provides significant catch of Atlantic surfclams. The first & Kennicutt, 2008; HSRL, 2012; Powell, Mann, Ashton‐Alcox, is a redesign of the U.S. Atlantic surfclam stock survey (Jacobson & Kuykendall, & Long, 2017). A hub‐and‐spoke fixed grid design was Hennen, 2019). A survey design team identified this region east of implemented with spokes of 3‐nm length (Figure 1), a sampling den- Nantucket as the most important area supporting surfclams that fell sity approximating the densest sampling grid theoretically achiev- outside of the historical survey stratum map (NEFSC, 2017a) used able under the NEFSC (1988) survey protocol. Six of the 63 stations for the U.S. EEZ (exclusive economic zone) survey. The second was (9.5%) were repositioned under a standard NEFSC protocol permit- a newly created Habitat Management Area (HMA), within which the ting repositioning within 1 nm of the designated position. Stations use of bottom-tending gear such as hydraulic dredges would be lim- were moved for three reasons: (a) Some fell just inshore of the EEZ ited. Consequent of both reasons, a survey of the region eastward of inshore boundary and were moved offshore across the bound- Nantucket to the Great South Channel was undertaken, providing an ary line; (b) some stations fell just inside the Nantucket Lightship opportunity to examine the cold temperate–boreal boundary along Habitat Closure Area and were moved just north of that closure the high-energy Great South Channel. Survey results identified a line; and (c) some stations fell on untowable bottom, always loca- post‐2000 range shift offshore by Atlantic surfclams suggesting that tions too shallow for the vessel to safely tow, and were moved lat- the cold temperate–boreal boundary was in flux in this region and, erally into deeper water. POWELL ET AL. | 3 of 17 FIGURE 1 Above, the survey design. −70˚00' −69˚40' −69˚20' Gray lines outline (upper left) the sector −60 historically contributing most of the Atlantic surfclam landings in the surveyed region, (lower left) the northern portion of 41˚30' 41˚30' a1 c1 e1 the Nantucket Lightship Habitat Closure b1 d1 f1 Area, and (right) the upper half of the −20 Great South Channel HMA. Letters and a2 c2 e2 numbers are station designations for the b2 d2 f2 −40 a3 c3 e3 g1 I1 k1 63 stations in the survey. Depths are in 41˚20' 41˚20' b3 d3 f3 h1−20 j1 l1 meters. Below, the regional context with −20 the surveyed area outlined in bold in the c4 e4 g2 I2 k2 lower right insert d4 f4 h2 j2 l2 −20 −20 b4 c5 e5 g3 I3 k3 a5 b5 d5 f5 h3 j3 l3 41˚10' −20 41˚10' e6 g4 I4 k4 e7 f6 h4 j4 l4 g5 I5 k5 f7 −40h5 j5 l5 41˚00' 41˚00' −70˚00' −69˚40' −69˚20' −78˚ −76˚ −74˚ −72˚ −70˚ −68˚ −66˚ −64˚ 45˚ 45˚ Maine −80 44˚ 44˚ 43˚ 43˚ 40 Gulf of Maine New York Massachusetts− 42˚ 42˚ − − 4 8 0 0 41˚ Pennsylvania 41˚ ey s r 40 −80 40˚ Je − −70˚−69˚ 40˚ w 42˚00' Ne 39˚ 39˚ 41˚30' 38˚ 38˚ 37˚ 41˚00' 37˚ 36˚ 36˚ −78˚ −76˚ −74˚ −72˚ −70˚ −68˚ −66˚ −64˚ market‐size surfclams (≥120 mm) with selectivity steadily declining 2.2 | Survey towing and on‐deck at smaller sizes.