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Alan L. Shanks, Laura Brink
Upwelling, downwelling, and cross-shelf transport of bivalve larvae: test of a hypothesis
This study tests the commonly held belief that larvae of coastal organisms are swept out to sea during wind-driven upwelling, causing poor recruitment at the shore, whereas the reverse occurs during wind-driven downwelling. This hypothesis is based on the fact that the swimming speeds of most larvae (∼1 mm s–1) are much slower than ocean currents (cm's s–1), suggesting that the currents should sweep larvae about like seeds in the wind. Nevertheless, this has rarely been tested.
The hypothesis was refuted in our investigations: despite their slow swimming speed, the larvae were not swept along by the ocean currents. Some slow swimming bivalve larvae were swept onshore by upwelling and offshore by downwelling, and larvae of other bivalve taxa remained near the coast despite upwelling, downwelling and the transitions from one regime to another. We suggest that the larvae may act like balloonists –– by swimming in the vertical plane they can become trapped by convergent or divergent flows generated near the coastline by downwelling and upwelling regimes. We conclude that there is no reason to expect that any type of larva will be transported across the shelf under typical upwelling or downwelling conditions.
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