Larval distributions in inner‐shelf waters: The roles of wind‐driven cross‐shelf currents and diel vertical migrations

Abstract

To determine how physical processes and biological behaviors influence larval dispersal on the inner shelf, time series of larval concentrations were quantified during August 1994, on the Outer Banks of North Carolina, U.S.A. Zooplankton pumps, moored in 21 m of water at 3.2, 8.7, and 12.2 m above bottom, collected larvae every 3 h for 3 weeks. Physical variables and larvae were sampled at similar time and space scales. Larval concentrations were typically 10 2 -10 4 m Ϫ3 for polychaetes, bivalves, and gastropods and 10-10 2 m Ϫ3 for brachyuran crab zoeae. There were two dominant scales of variability, 3-6 h and 2-10 d. The high-frequency signal is partially explained by diel vertical migrations-nocturnal ascent and daytime descent. This pattern would allow larvae to feed in subthermocline waters while avoiding visual predators. Low-frequency variations tracked with water temperature. Highest concentrations of worm larvae occurred in cool (upwelled) water and of crab zoeae in warm (downwelled) water. At least two larval groups comprised the clam and snail time series, one with fairly high abundances in cool water and the other with peak concentrations in warm water. Wind-driven cross-shelf transport is the most plausible explanation for these low-frequency fluctuations. For example, dense patches of worm larvae overlying parental habitat (offshore muds) would be carried shoreward in cool, upwelling flows. In contrast, brachyuran zoeae in nearsurface waters would descend at the coast during downwelling and, together with larvae aloft nearshore sediments, be transported seaward below the thermocline. Thus advected by strong along-shore and weaker cross-shelf currents, larvae zigzag up and down the coast. Vertically traversing the water column while they feed and grow, larvae ultimately seek a suitable habitat in which to settle.The inner portion of the continental shelf (roughly 5-30 m depth) is a dispersal corridor between intertidal and offshore habitats. Replete with planktonic larvae, it also provides soft sediments for benthic adults. Physical processes, such as wind-driven upwelling and downwelling, thermal fronts, eddies, tides, internal waves, tidal bores, surface waves, and storms, operate over spatial scales of 1 m to 10 3 km and temporal scales of hours to weeks (Mann and Lazier 1991). In this high-energy environment, physics should play 1 Previously published as Cheryl Ann Butman. To whom correspondence should be addressed. Present address: Department of Biology, University of California, 621 Charles E. Young Drive South, Los Angeles, California, 90095-1606. AcknowledgmentsWe thank D. Simoneau, W. Ostrom, J. Bouthillette, L. Costello, C. Marquette, J. D. Sisson, V. Starczak, and the captains and crews of the R/V Endeavor and R/V Cape Hatteras for deploying and recovering the moorings. K. Doherty, K. Fairhurst, S. Longworth, and J.D. Sisson engineered and constructed the plankton pumps and moorings, for which we are grateful. We are indebted to numerous divers who battled the wicked Outer-Banks ...

Possible effects of downwelling on the recruitment of coral reef fishes to the Eilat (Red Sea) coral reefs

“…The effect of such currents on the larval supply to the reef depends on the location of the larvae in the water column (Shanks 1998;Garland et al 2002;Shanks and Brink 2005), and the local topography (McCulloch and Shanks 2003;Queiroga et al 2006). Upwelling and downwelling are characterized by flow to different (mostly opposite) directions near the bottom and at the surface.…”

Flocs, flows, and mechanisms decoupling larval supply from settlement

“…This diurnal pattern may result from active vertical migration within near-bottom waters. Larvae may move up (above pump depth) in the evening and down (to pump height) at midday, perhaps to avoid visual predators (Garland et al 2002). Horizontal transport potentially could redistribute larvae, but cross correlations between Sabellaria concentration and flow speed were not significant at any lag.…”

“…Measuring larval supply has been technology limited (Levin 1990). Collecting frequent, concurrent plankton samples at multiple locations is challenging, especially in high-energy coastal environments (Garland et al 2002). Methodological deficiencies can invalidate purported predictive relationships between supply and settlement.…”

Vertical distribution and migration of planktonic polychaete larvae in Onagawa Bay, north-eastern Japan

“…The phenomenon of DVM widely occurs in many marine zooplankton taxa (Rawlinson et al, 2004), including polychaete larvae (Garland et al, 2002). However, no difference was observed in the vertical distribution of polychaete larvae between the light and dark hours over the 21-h sampling period in Onagawa Bay (figs 4-6).…”