Scott R. Marion

Multiple stressors threaten the imperiled coastal foundation species eelgrass (Zostera marina) in Chesapeake Bay, USA

Abstract Interactions among global change stressors and their effects at large scales are often proposed, but seldom evaluated. This situation is primarily due to lack of comprehensive, sufficiently long‐term, and spatially extensive datasets. Seagrasses, which provide nursery habitat, improve water quality, and constitute a globally important carbon sink, are among the most vulnerable habitats on the planet. Here, we unite 31 years of high‐resolution aerial monitoring and water quality data to elucidate the patterns and drivers of eelgrass (Zostera marina) abundance in Chesapeake Bay, USA, one of the largest and most valuable estuaries in the world, with an unparalleled history of regulatory efforts. We show that eelgrass area has declined 29% in total since 1991, with wide‐ranging and severe ecological and economic consequences. We go on to identify an interaction between decreasing water clarity and warming temperatures as the primary drivers of this trend. Declining clarity has gradually reduced eelgrass cover the past two decades, primarily in deeper beds where light is already limiting. In shallow beds, however, reduced visibility exacerbates the physiological stress of acute warming, leading to recent instances of decline approaching 80%. While degraded water quality has long been known to influence underwater grasses worldwide, we demonstrate a clear and rapidly emerging interaction with climate change. We highlight the urgent need to integrate a broader perspective into local water quality management, in the Chesapeake Bay and in the many other coastal systems facing similar stressors.

Faunal Communities Are Invariant to Fragmentation in Experimental Seagrass Landscapes

Human-driven habitat fragmentation is cited as one of the most pressing threats facing many coastal ecosystems today. Many experiments have explored the consequences of fragmentation on fauna in one foundational habitat, seagrass beds, but have either surveyed along a gradient of existing patchiness, used artificial materials to mimic a natural bed, or sampled over short timescales. Here, we describe faunal responses to constructed fragmented landscapes varying from 4–400 m2 in two transplant garden experiments incorporating live eelgrass (Zostera marina L.). In experiments replicated within two subestuaries of the Chesapeake Bay, USA across multiple seasons and non-consecutive years, we comprehensively censused mesopredators and epifaunal communities using complementary quantitative methods. We found that community properties, including abundance, species richness, Simpson and functional diversity, and composition were generally unaffected by the number of patches and the size of the landscape, or the intensity of sampling. Additionally, an index of competition based on species co-occurrences revealed no trends with increasing patch size, contrary to theoretical predictions. We extend conclusions concerning the invariance of animal communities to habitat fragmentation from small-scale observational surveys and artificial experiments to experiments conducted with actual living plants and at more realistic scales. Our findings are likely a consequence of the rapid life histories and high mobility of the organisms common to eelgrass beds, and have implications for both conservation and restoration, suggesting that even small patches can rapidly promote abundant and diverse faunal communities.

Massive crab recruitment events to the shallow subtidal zone

Highdensity and temporally pulsed propagule recruitment strategies are common in many plants (e.g., masting, Kelly 1994), and both terrestrial (e.g., cicada emergence; Williams and Simon 1995) and marine invertebrates (reviewed in Caley et al. 1996). The extent and effects of recruitment pulses on population dynamics of the species involved and their subsidy to local predator populations are broadly appreciated (Karban 1982, Byström 2006). While well documented for many sessile marine invertebrates, such as barnacles (e.g., Strathmann and Branscomb 1979), curiously, such massrecruitment events to the marine benthos have not, to our knowledge, been described for mobile consumer species like crabs. Large, episodic influxes of new settlers could have very strong, yet underappreciated, impacts on the communities in which they land, both as a source of prey and as predators on local organisms. During a SCUBA dive at Port Orford, Oregon, USA on 19 April 2016, Galloway observed a remarkably dense benthic aggregation of recently settled Cancer (Metacarcinus) magister megalopae, the final larval stage of decapod crustaceans, as well as new recruits (firstinstar settlers). Between 7 and 13 m depth (the deepest descent) and throughout the 45min dive, every surface of the rocky reef, including sessile invertebrates, algae, vertical rock walls, and cobbles, in an estimated 100m2 area, was completely covered with new recruits. The recruits were stacked upon each other, several individuals deep. Without a camera during this dive, Galloway collected a handful of crabs and photographed them on the shore to document their size (Appendix S1). Based on the carapace width of the recruits (mean = 7.1 mm, SD = 0.69, n = 12), we estimated a density of ~22,000 crabs/m2 (one layer of crabs), and densities may have been as high as ~65,000 crabs/m2 (three layers of crabs). There were no recruits observed at this site on an earlier dive on 4 April 2016. Observations by an Oregon Department of Fish and Wildlife (ODFW) biologist (J. Watson, personal communication) were consistent with ours. Two days before, Watson was sampling demersal rockfish using hook and line near Depoe Bay, Oregon (250 km to the north), and reported that an usually large proportion of rockfish caught, particularly black rockfish (Sebastes melanops), were regurgitating newly settled Dungeness crabs. On a SCUBA dive at the original dive site (Graveyard Point; 42.73837° N, 124.49938° W) 23 d later (13 May 2016), there were still dense aggregations of new recruits, with densities of ~5,000–11,000 crabs/m2 (Fig. 1a), but the recruits were now located at the interface between the rocky reef and the cobble/sandy bottom. On this same day, ODFW biologists were conducting remote operated vehicle (ROV) surveys near the Redfish Rocks Marine Reserve, ~7 km south of the Port Orford dive site, and also observed dense aggregations of new recruits (~3,000– 7,000 crabs/m2; Video S1). On nine subsequent dive surveys at rocky reefs in the Port Orford area (sites within a 20km2 area), and as late as 31 August 2016, juvenile age 0+ crabs (~6–13 mm carapace width) were observed in groups in all habitat types, including vertical walls, at densities ranging from Ecology, 0(0), 2017, pp. 1–4