MEPS 265:45-56 (2003)  -  doi:10.3354/meps265045

Current and wave dynamics in the shallow subtidal: implications to the ecology of understory and surface-canopy kelps

James E. Eckman1,*, David O. Duggins2, Christopher E. Siddon3,4

1Office of Naval Research, Code 322, 800 North Quincy Street, Arlington, Virginia 22217, USA
2Friday Harbor Laboratories, University of Washington, 620 University Road, Friday Harbor, Washington 98250, USA
3Department of Ecology and Evolutionary Biology, Brown University, Providence, Rhode Island 02912, USA
4Present address: Juneau Center, School of Fisheries and Ocean Sciences; University of Alaska Fairbanks, 11120 Glacier Highway, Juneau, Alaska 99801-8677, USA

ABSTRACT: Current and wave properties were studied in the semi-protected waters of the San Juan Archipelago, Washington, at 6 shallow subtidal sites chosen to include a wide range of variability in exposure to both tidal currents and waves. Within each site, 4 to 6 plots measuring roughly 50 to 100 m2 each were established on nearly horizontal rock platforms at mean depths ranging from 6.1 to 11.0 m, with most plots at mean depths of 7.0 to 9.2 m. Plots were established nominally to minimize within-site variability in hydrodynamics, and were locations of several species of kelp (reported in a companion paper). For more than a year replicate measurements of flow and pressure (mean depth and wave signals) were collected at all sites in 2 Hz bursts 25 cm above the substratum, a height relative to understory kelps, providing a unique, detailed spatial and temporal characterization of a shallow subtidal hydrodynamic regime pertinent to these plants. Despite the intended similarity of plots within sites, local-flow microhabitat remained substantial at scales relative to understory plants, and largely ameliorated differences in tidal signals among most sites. Greater than 50% of the spatial variability in maximum tidal current speed, and 31 to 44% of the variance in the duration of periods of calm flow (speeds consistently <20 cm s-1), occurred within sites at scales of meters, and not at the larger (km) scales that separated sites. In contrast, wave effects at 7 to 11 m depth were predictable spatially. Significant wave impacts were recorded only at 2 sites characterized by a large, open fetch. Wave-dominated flows were recorded by replicate sensors multiple times within both of these sites, and the strongest instantaneous wave speeds approached 150 to 200 cm s-1. Maximum wave-generated speeds recorded just above the substratum at these 2 sites were ~2 to 3× maximum tidal currents. These results help to explain patterns noted in concurrent studies of the population dynamics and morphology of several species of kelp.

KEY WORDS: Hydrodynamics · Kelp · Wave · Tidal current

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