MEPS 552:223-240 (2016)  -  DOI: https://doi.org/10.3354/meps11787

ABSTRACT: The paucity of robust empirical demonstrations of wave-induced shifts in the spatial dynamics of functionally important consumers limits the ability to formulate accurate predictions about the frequency and magnitude of changes in marine communities resulting from ongoing shifts in sea state. We used 2 experiments in an oscillatory wave tank and observations over 6 mo at 2 barrens sites in Newfoundland to examine effects of the wave environment and population density on the spatial dynamics of the green sea urchin Strongylocentrotus droebachiensis in food-depleted habitats. The 2 experiments mimicked barrens conditions to identify wave velocities and urchin densities triggering shifts in displacement, microhabitat use, distribution, and aggregation. Field observations tested the generality of results from the laboratory experiments by examining variation in wave height and associated changes in microhabitat use and distribution. Results demonstrated that as wave velocity increases, S. droebachiensis: (1) proportionately reduces displacement; (2) progressively abandons flat, horizontal surfaces and avoids vertical ones in favor of microhabitats that facilitate anchorage; and (3) increasingly forms 2-dimensional aggregations, whose physiognomy varies with velocity and urchin density. We established that shifts in wave velocity and population density in the order of 0.1 m s^–1 and a few tens of individuals m^-2, respectively, can elicit important changes in the way urchins disperse, cluster, and use seabed topography. The strong inclination of S. droebachiensis to reduce displacement, favor microhabitats that facilitate anchorage, and increasingly form 2-dimensional aggregations as wave velocity increases in the absence of food, is most likely a behavioral adaptation to mitigate hydrodynamic forces while reinforcing attachment to minimize dislodgement risk.

KEY WORDS: Spatial dynamics · Urchin barrens · Wave action · Displacement · Aggregation · Habitat use · Population density · Microcosm experiments