David S. Wethey

Three decades of high-resolution coastal sea surface temperatures reveal more than warming

Understanding and forecasting current and future consequences of coastal warming require a fine-scale assessment of the near-shore temperature changes. Here we show that despite the fact that 71% of the worlds coastlines are significantly warming, rates of change have been highly heterogeneous both spatially and seasonally. We demonstrate that 46% of the coastlines have experienced a significant decrease in the frequency of extremely cold events, while extremely hot days are becoming more common in 38% of the area. Also, we show that the onset of the warm season is significantly advancing earlier in the year in 36% of the temperate coastal regions. More importantly, it is now possible to analyse local patterns within the global context, which is useful for a broad array of scientific fields, policy makers and general public.

Odor Plumes and Animal Navigation in Turbulent Water Flow: A Field Study

Turbulence causes chemical stimuli to be highly variable in time and space; hence the study of animal orientation in odor plumes presents a formidable challenge. Through combined chemical and physical measurements, we characterized the transport of attractant released by clam prey in a turbulent aquatic environment. Concurrently, we quantified the locomotory responses of predatory crabs successfully searching for sources of clam attractant. Our results demonstrate that both rheotaxis and chemotaxis are necessary for successful orientation. Perception of chemical cues causes crabs to move in the upstream direction, but feedback from attractant distributions directly regulates movement across-stream in the plume. Orientation mechanisms used by crabs difler from those employed by flying insects, the only other system in which navigation relative to odor plumes has been coupled with fluid dynamics. Insects respond to odors by moving upstream, but they do not use chemical distributions to determine across-stream direction, whereas crabs do. Turbulent eddy diffusivities in crab habitats are 100 to 1000 times lower than those of terrestrial grasslands and forests occupied by insects. Insects must respond to plumes consisting of highly dispersed, tiny filaments or parcels of odor. Crabs rely more heavily on spatial aspects of chemical stimulus distributions because their fluid dynamic environment creates a more stable plume structure, thus permitting chemotaxis.

Spatial pattern in barnacle settlement: day to day changes during the settlement season

Spatial pattern of settlement of cyprids of the barnacle Balanus balanoides (L.) was examined from photographs taken every low tide during the first 20 days of the settlement season. The locations of pits and cracks and the bases of detached adults were mapped in order to measure specificity of settlement in relation to surface contour and chemical cues associated with conspecifics. Cyprids were identified as to their date of settlement. Distances to nearest neighbours were computed for newly settling cyprids relative to each other, for newly settling cyprids relative to previously settled cyprids, and for newly settling cyprids relative to newly metamorphosed spat.

Linking Thermal Tolerances and Biogeography: Mytilus edulis (L.) at its Southern Limit on the East Coast of the United States

Temperature is a major factor contributing to the latitudinal distribution of species. In the Northern Hemisphere, a species is likely to be living very close to its upper thermal tolerance limits at the southern limit of its biogeographic range. With global warming, this southern limit is expected to shift poleward. Moreover, intertidal ecosystems are expected to be especially strongly affected, mostly due to their large daily and seasonal variations in temperature and exposure. Hence, these are model systems in which to conduct experiments examining the ecological effects of climate change. In this study we determined the upper lethal thermal limits, for both air and water, of the blue mussel Mytilus edulis via laboratory experiments. Tolerances vary seasonally, with a difference between media of 0.7 °C in June and 4.8 °C in November, as well as a decrease with multiple exposures. Measured lethal limits were then compared to field measurements of environmental temperature and concurrent measurements of mortality rates. Field results indicate that mortality in the intertidal occurs at rates expected from laboratory responses to elevated temperature. Hindcasts, retrospective analyses of historical data, indicate that high rates of mortality have shifted 51 and 42 days earlier in Beaufort, North Carolina, and Oregon Inlet, North Carolina, respectively, between 1956 and 2007. The combined data suggest that the historical southern limit of M. edulis near Cape Hatteras, North Carolina, is indeed the result of intolerance to high temperature, and that this range edge is shifting poleward in a manner indicative of global warming.

Biogeography, Competition, and Microclimate: The Barnacle Chthamalus fragilis in New England

Abstract Geographic limits of species are commonly associated with climatic or physical boundaries, but the mechanisms of exclusion at the limits of distribution are poorly understood. In some intertidal populations, the strengths of interactions with natural enemies are mediated by microclimate, and determine geographic limits. The northern limit of the barnacle Chthamalus fragilis in New England is the south side of Cape Cod, Massachusetts. South of the cape, Chthamalus has a refuge from competition in the high intertidal, which is too hot for survival of its superior competitor Semibalanus balanoides. North of the cape, the high intertidal is cooler, and Semibalanus survives, so Chthamalus has no refuge. Thus, geographic variation in the strength of competition may determine the geographic limit of Chthamalus. Intolerance of cold by Chthamalus cannot account for the geographic limit: transplants of Chthamalus 80 km beyond its northern limit survived up to 8 yr in the absence of competition with Semibalanus. At the geographic limit of Chthamalus in the Cape Cod Canal there are two bridges, 5 km apart. On the southern bridge, Chthamalus is abundant and occupies a refuge above Semibalanus. On the northern bridge in 2001, only 7 individual Chthamalus were present. Despite the proximity of the bridges, their microclimates are very different. The southern bridge, where Chthamalus is abundant, is up to 8°C hotter than the northern bridge. This higher temperature creates a refuge in the high intertidal for Chthamalus. On the cooler northern bridge, there is no refuge for Chthamalus. Because of the difference in temperatures of the water masses that meet in the canal, heat storage in the rock of the bridge piers causes the temperatures to differ between the bridges. Thus, geographic change in microclimate alters the strength of competition, and determines the geographic limit. “When we travel from south to north, or from a damp region to a dry, we invariably see some species gradually getting rarer and rarer, and finally disappearing; and the change in climate being conspicuous, we are tempted to attribute the whole effect to its direct action. But this is a very false view: we forget that each species, even where it most abounds, is constantly suffering enormous destruction at some period of its life, from enemies or from competitors for the same place and food; and if these enemies or competitors be in the least degree favoured by any slight change of climate, they will increase in numbers, and as each area is already fully stocked with inhabitants, the other species will decrease.”—Charles Darwin, On the Origin of Species, 1859, p. 69.

Chemical Induction of Larval Settlement Behavior in Flow

The ability of dissolved chemical cues to induce larval settlement from the water column has long been debated. Through computer-assisted video motion analysis, we quantified the movements of individual oyster (Crassostrea virginica) larvae in a small racetrack flume at free-stream flow speeds of 2.8, 6.2, and 10.4 cm/s. In response to waterborne chemical cues, but not to seawater (control), oyster larvae moved downward in the water column and swam in slow curved paths before attaching to the flume bottom. Effective stimuli were adult-oyster-conditioned seawater (OCW) and a synthetic peptide analog (glycyl-glycyl-L-arginine) for the natural cue. The chemically mediated behavioral responses of oyster larvae in flow were essentially identical to those responses previously reported in still water. Our experimental results therefore demonstrate the capacity of waterborne cues to evoke settlement behavior in oyster pediveligers under varying hydrodynamic conditions.

Ecological genetics in the North Atlantic: environmental gradients and adaptation at specific loci.

The North Atlantic intertidal community provides a rich set of organismal and environmental material for the study of ecological genetics. Clearly defined environmental gradients exist at multiple spatial scales: there are broad latitudinal trends in temperature, meso-scale changes in salinity along estuaries, and smaller scale gradients in desiccation and temperature spanning the intertidal range. The geology and geography of the American and European coasts provide natural replication of these gradients, allowing for population genetic analyses of parallel adaptation to environmental stress and heterogeneity. Statistical methods have been developed that provide genomic neutrality tests of population differentiation and aid in the process of candidate gene identification. In this paper, we review studies of marine organisms that illustrate associations between an environmental gradient and specific genetic markers. Such highly differentiated markers become candidate genes for adaptation to the environmental factors in question, but the functional significance of genetic variants must be comprehensively evaluated. We present a set of predictions about locus-specific selection across latitudinal, estuarine, and intertidal gradients that are likely to exist in the North Atlantic. We further present new data and analyses that support and contradict these simple selection models. Some taxa show pronounced clinal variation at certain loci against a background of mild clinal variation at many loci. These cases illustrate the procedures necessary for distinguishing selection driven by internal genomic vs. external environmental factors. We suggest that the North Atlantic intertidal community provides a model system for identifying genes that matter in ecology due to the clarity of the environmental stresses and an extensive experimental literature on ecological function. While these organisms are typically poor genetic and genomic models, advances in comparative genomics have provided access to molecular tools that can now be applied to taxa with well-defined ecologies. As many of the organisms we discuss have tight physiological limits driven by climatic factors, this synthesis of molecular population genetics with marine ecology could provide a sensitive means of assessing evolutionary responses to climate change.

PHYSICAL CONSTRAINTS ON ECOLOGICAL PROCESSES: A FIELD TEST OF ODOR-MEDIATED FORAGING

The physicochemical environment can strongly constrain the outcome of ecological interactions such as predation, mating, and competition. This is especially true of processes mediated by the sense of olfaction, because wind and water currents control the dispersal of odor signals and act as ancillary cues during odor plume following. In the field, we examined how variations in the physical and chemical properties of odor plumes would alter the foraging behavior of the blue crab Callinectes sapidus, a common predator/ scavenger in tidal marsh creeks in the southeastern United States. We video-recorded re- sponses of naturally foraging crabs to odor plumes of varying composition and odor release rate (characteristic of clams of differing size). During each trial we presented crabs with an experimental plume that was a mixture of fluorescein-dyed seawater and clam mantle fluid, oyster mantle fluid, or a suite of amino acids, and a control plume which consisted of dyed seawater only. In addition to manipulating the chemical composition and odor release rate of the plume, we allowed flow speed to vary naturally with the tide. We tested for effects of odor composition, odor release rate, and flow speed on the success (i.e., finding the target) and efficiency (i.e., search path direction) of blue crab foraging. Mantle fluid solutions and wounded prey items elicited active search and upstream walking, while control and amino acid solutions had no effect on crab behavior. Odors released at a low rate (either low volume flow or low concentration) elicited fewer responses from crabs, and the resulting search was less efficient and less successful than responses to odors released at higher rates. Ambient current speed also affected both search success and efficiency. There was a decline in search success when current speed in the tidal channel was below 1 cm/s; search success remained constantly high, however, when current speed was above this threshold. Search efficiency was directly proportional to ambient current speeds. Such relationships between hydrodynamic and chemical properties of the environ- ment and foraging success and efficiency suggest that variation in the physicochemical environment can influence the detectability of prey and strategies employed by foragers. These results extend beyond the foraging of marine crustaceans into other olfactory- mediated interactions and habitats.

GEOGRAPHIC LIMITS AND LOCAL ZONATION: THE BARNACLES SEMIBALANUS (BALANUS) AND CHTHAMALUS IN NEW ENGLAND

The interactions between the intertidal barnacles Semiba!anus (Ba/anus) ba! anoides and Chthama!usfragilis were examined in order to determine whether the factors which influence local zonation in the intertidal also contribute to the estab lishment of geographic limits. Both physical and biotic factors influence intertidal zonation at the northern limit of Chthamalus in New England. On sloping surfaces Semibalanus died at all shore levels higher than mid tide level, apparently as a result ofdesiccation associated with high summer temperatures. Chthamalus settlement occurred at all shore levels above mean tide level, and postsettlement mortality apparently restricts Chthama!us to high shore locations where Semibalanus growth and survival is inhibited. North of the northern limit of Chthamalus, Semibalanus does not suffer summer heat death, so it occupies the zone where Chthamalus would have a refuge from corn petition further south. The northern limit of Chthamalus is set not by factors directly related to cold acting on Chthamalus. Rather the northern limit appears to be set by cold which allows the dominant competitor to exclude Chthamalus from its refuge zone. South of the northern limit the competitor, Semiba!anus, is excluded from the high shore by high summer temperatures.

SUN AND SHADE MEDIATE COMPETITION IN THE BARNACLES CHTHAMALUSAND SEMIBALANUS: A FIELD EXPERIMENT

The barnacles Chthamalus fragilis and Semibalanus balanoides compete for space in the high intertidal zone in southern New England. Chthamalus settles throughout the intertidal and persists in the absence of competition with Semibalanus. Semibalanus also settles throughout the intertidal but is usually eliminated from the high intertidal zone by heat and/or desiccation. In a field experiment in the high intertidal zone, Semibalanus survived the high summer temperatures and overgrew Chthamalus under an opaque roof. Under a transparent roof and in control areas with no roof, Semibalanus died in mid summer, and Chthamalus persisted. Hence the intensity of interspecific competition is mediated by physical stress which primarily affects the dominant competitor.