Myriam A. Barbeau

Movement patterns in the green sea urchin, Strongylocentrotus droebachiensis

Time-lapse video was used to record movement paths of Strongylocentrotus droebachiensis on a rocky bottom at 8 m depth, both at a grazing front and in recently formed barrens in the wake of the front. Urchins did not exhibit strong directionality in movement and we did not detect any differences in movement variables between the front and barrens. Density of conspecifics had a negative effect on the speed, move length (distance), and daily displacement of urchins, but did not significantly affect the proportion of time spent moving, the linearity index and the number of moves taken per day. The frequency distributions of turning angles between moves and steps were non-uniformly distributed, indicating directionality in individual paths. A correlated random walk model was used to predict the displacement of urchins through time and provided a good fit with observed data. Our results provide insight into the foraging behaviour of S. droebachiensis and are consistent with previous observations of small-scale movement in this species.

Effects of substrate on interactions between juvenile sea scallops (Placopecten magellanicus Gmelin) and predatory sea stars (Asterias vulgaris Verrill) and rock crabs (Cancer irroratus Say)

Abstract We investigated the effect of substrate (glass bottom, sand, granule, pebble) on predation of juvenile sea scallops ( Placopecten magellanicus ) by sea stars ( Asterias vulgaris ) and rock crabs ( Cancer irroratus ) at two prey sizes (11–15 mm and 24–28 mm shell height), and two prey densities (10 and 30 scallops per aquarium) in laboratory experiments. Specifically, we quantified predation rate and underlying behaviours (proportion of time a predator spent searching for and handling prey, encounter rate between predators and prey, and various outcomes of encounters). We detected a significant gradual effect of particle size of natural substrates on sea star predation: specifically, predation rate on and encounter rate with small scallops tended to decrease with increasing particle size (being highest for sand, intermediate for granule, and lowest for pebble). Substrate type did not significantly affect predation rates or behaviours of sea stars preying on large scallops or of rock crabs preying on either scallop size classes. Other factors, such as prey size and density, were important in the scallop-sea star and scallop-rock crab systems. For example, predation rate by sea stars and crabs and certain sea star behaviours (e.g. probability of consuming scallops upon capture) were significantly higher with small scallops than with large scallops. As well, in interactions between small scallops and sea stars, predation rate and encounter rate increased with prey density, and the proportion of time sea stars spent searching was higher at low prey density than high prey density. Thus, substrate type may be a minor factor determining predation risk of seeded scallops during enhancement operations; prey size and prey density may play a more important role. However, substrate type still needs to be considered when choosing a site for scallop enhancement, as it may affect other scallop behaviours (such as movement).

Spatial and Temporal Variation in the Population Dynamics of the Intertidal Amphipod Corophium Volutator in the Upper Bay of Fundy, Canada

Abstract The scale of variation in population densities and other demographic variables is an important consideration in the design and interpretation of experiments and sampling programs. Here, we studied spatial and temporal variation in populations of Corophium volutator, an intertidal amphipod that is the most abundant macro-invertebrate on mudflats in the upper Bay of Fundy, Canada. Variables that were quantified included density (peaked in mid-summer at 10,000s per m2), proportion juvenile (an index of population structure), sex ratio (female-biased throughout the year), proportion of females that were ovigerous (reproduction occurs between May and August), and fecundity (number of embryos per female). We studied populations at 4 different sites, i.e., separate mudflats, for 1–2 years, and estimated variance components at 3 spatial scales: metres (samples), 100s of metres (transects) and many kilometres (mudflats). Population density exhibited low variation between years (< 10% of the random variation), but showed high variation at our smallest and largest spatial scales (45% between samples, 2% between transects, and 40% between mudflats), i.e., the distribution of C. volutator within a mudflat was aggregated at the scale of metres, but not at that of 100s of metres. Thus, relatively few transects, but many samples per transect, are required for good representation of density on a mudflat. Fecundity was similar between mudflats, but proportion juvenile, sex ratio, and proportion of ovigerous females were strongly affected by site or the interaction between month and site (> 60% of the random variation in each case). The high variation observed between mudflats for most demographic variables demonstrates that several control sites are necessary for measuring natural variation, a critical consideration in studies of environmental impacts.

Diel and Semi-lunar Cycles in the Swimming Activity of the Intertidal, Benthic Amphipod Corophium volutator in the Upper Bay of Fundy, Canada

Abstract Although movement of individuals has important consequences on population dynamics and various ecological interactions, it is often difficult to quantify fully. We investigated the temporal variation in the number of the amphipod Corophium volutator swimming in the water column during periods of immersion over an intertidal mudflat in the Bay of Fundy, Canada, in spring-summer 2006. Swimming is an important mode of dispersal, since the number of swimming amphipods can peak at over 30,000 individuals within a 20-cm-diameter, stationary plankton net over a period of immersion of ∼4 h. Amphipods swim throughout spring-summer, but abundance in the water column is less in May than in the other months. As well, amphipods swim during the day and night, but the number swimming shows periodicity in relation to diel time of high tide, with peaks when high tides occur around 1:45 am. Finally, the number of amphipods swimming shows periodicity in relation to lunar cycles, with peaks around the time of new moon and full moon. We developed a statistical model describing the swimming activity of C. volutator based on month, diel time of high tide, and day of the lunar calendar. The model accurately predicts the timing of peaks, but does not predict well the amplitude of the highest peaks. Overall, the model gives a very good approximation of the number of swimmers (61% of the variation is explained) and provides a strong basis for future modeling of spatial population dynamics of C. volutator.

FORMATION AND PROPAGATION OF FEEDING FRONTS IN BENTHIC MARINE INVERTEBRATES: A MODELING APPROACH

Feeding fronts are a striking pattern of spatial distribution observed in both marine and terrestrial ecological systems. These fronts not only determine the abundance and distribution of prey populations, but on a broader scale they may also affect the structure and dynamics of entire communities. Several mechanisms leading to the formation of feeding fronts have been proposed, and chemotaxis has been suggested as an important component. Here we develop two mathematical models that show front formation can occur with simple kinesis (and without chemotaxis) in two marine invertebrates with different feeding habits: a microphagous sea star (Oreaster reticulatus) that feeds on sediments and an herbivorous sea urchin (Strongylocentrotus droebachiensis) that grazes kelp beds. We utilize a large body of detailed empirical information on movement pattern, foraging behavior, and front dynamics for each species to develop, parameterize, and evaluate our models. We found that our model predictions for the rate of advance of a front and its relationship to the density of consumers at the front were in close agreement with independently collected, empirical observations in both systems. This work shows that simple local interactions between mobile consumers and a stationary resource can result in large-scale heterogeneous patterns of abundance of both species.

Diet reconstruction using next-generation sequencing increases the known ecosystem usage by a shorebird

ABSTRACT Molecular scatology and next-generation sequencing identified previously unknown linkages among ecosystems in the diet of Semipalmated Sandpipers (Calidris pusilla) in the Bay of Fundy, Canada. During their annual migratory stopover, the birds consumed a wider range of prey items than previously reported, which suggests that they are not selecting for the amphipod Corophium volutator and are acting as generalist foragers. Our analysis identified several novel prey items—arachnids, crabs, bivalves, several terrestrial and freshwater insect species, ctenophores, cnidarians, and fish (likely eggs or juveniles)—indicating that Semipalmated Sandpipers consume prey from marine, freshwater, and terrestrial ecosystems. Connections between Semipalmated Sandpipers and freshwater and terrestrial ecosystems were previously unknown in the Bay of Fundy. Current conservation efforts for this species are focused on beach and intertidal habitats; however, we may also need to consider the surrounding freshwater and terrestrial habitat.

Relative Importance of Biotic and Abiotic Forces on the Composition and Dynamics of a Soft-Sediment Intertidal Community.

Top-down, bottom-up, middle-out and abiotic factors are usually viewed as main forces structuring biological communities, although assessment of their relative importance, in a single study, is rarely done. We quantified, using multivariate methods, associations between abiotic and biotic (top-down, bottom-up and middle-out) variables and infaunal population/community variation on intertidal mudflats in the Bay of Fundy, Canada, over two years. Our analysis indicated that spatial structural factors like site and plot accounted for most of the community and population variation. Although we observed a significant relationship between the community/populations and the biotic and abiotic variables, most were of minor importance relative to the structural factors. We suggest that community and population structure were relatively uncoupled from the structuring influences of biotic and abiotic factors in this system because of high concentrations of resources that sustain high densities of infauna and limit exploitative competition. Furthermore, we hypothesize that the infaunal community primarily reflects stochastic spatial events, namely a “first come, first served” process.

Apparent redox potential discontinuity (aRPD) depth as a relative measure of sediment oxygen content and habitat quality

As hypoxic conditions spread in our oceans, indices that quickly and efficiently assess oxygen content in sediment pore water, and habitat quality are increasingly becoming desirable. Depth to the apparent redox potential discontinuity (aRPD) in sediment is one such index; however, the relationship between aRPD depth and oxygen content is still unclear. We quantified oxygen content in sediment pore water with a dissolved oxygen (DO) probe, utilizing luminescent technology, to determine how DO concentration varies with depth in the sediment, and with position relative to the aRPD (above versus below) on an intertidal mudflat in the Bay of Fundy, Canada. Based on generalized linear regression models, we found that depth in the sediment was a poor predictor of pore water DO concentration, while position in the sediment relative to the aRPD was a good predictor. Further, sediment above the aRPD had higher DO content than sediment below. We maintain that aRPD depth is a good measure of relative oxygen content in sediment pore water, as well as habitat quality. However, we recommend caution as the factors that influence aRPD depth are numerous and variable; therefore, the relationship between aRPD depth and DO content should be experimentally confirmed in any system of interest before aRPD depth is used as an indicator.

Dispersal of marine benthic invertebrates through ice rafting

Knowledge of dispersal vectors used by organisms is essential to the understanding of population and community dynamics. We report on ice rafting, a vector by which intertidal benthic invertebrates can be transported well outside their normal dispersal range during winter in temperate climates. We found multiple invertebrate taxa in sediment-laden ice blocks sampled in the intertidal zone. A large proportion of individuals were alive and active when freed from the ice. Using radio tracking, we found that ice blocks can travel over 20 km within a few days. Given the abundance of highly mobile ice blocks carrying viable invertebrates, we conclude that ice-rafting is likely an important dispersal vector, contributing to spatial community dynamics in intertidal systems. This mechanism helps explain observed genetic structure of populations, but it also raises concerns about potential negative impacts of climate change on connectivity between populations.

IMMERSION IN NEUTRAL RED SOLUTION AS A MASS-MARKING TECHNIQUE TO STUDY THE MOVEMENT OF THE AMPHIPOD COROPHIUM VOLUTATOR

Abstract We evaluated the usefulness of immersion in neutral red solution as a marking technique for studies of movement of the amphipod Corophium volutator. Amphipods were immersed for 20 minutes in solutions of neutral red at different concentrations (25, 50 and 100 mg L−1). Amphipods incorporated dye at all concentrations, and the hepatopancreas remained distinctly stained red for at least 7 days. No increase in mortality was detected at any concentration when compared to a control treatment (0 mg L−1). In a separate field experiment, staining did not significantly affect tendency of C. volutator to move. Overall, immersion in neutral red seems to be an appropriate technique to rapidly mark large numbers of C. volutator and is suitable for short-term mark-recapture experiments. This technique may also apply to other small-sized crustacean species with relatively transparent exoskeletons.