Components of Larval Transport: Larval Behavior and Variability in Physical Transport

Woods Hole Oceanographic Institution, Woods Hole MA

Investigators

Abstract

A major question in marine ecology is the source of recruits in shallow-water open populations, a question that involves identification of the larval transport process. This project will work on three components of larval transport: larval behavior, larval accumulation in transporting fronts, and the effects of low-frequency modulation on larval transport. It will focus on internal tidal bores, which first produce outcropping of the thermocline and the offshore migration of near-shore surface water. Then, a few hours later, the flow reverses, the dense cold water migrates off-shore, and warm water returns to the shore-lead by a warm bore front. For resolving the be-havioral and accumulation questions, an adaptive sampling scheme will be used, where sampling is initiated when internal tidal bores are remotely detected with a telemetry system. The behavioral hypothesis that accu-mulation in fronts with convergent currents result from larvae swimming actively against the down-welling currents will be tested. The hypothesis predicts that lar-vae that accumulate in transporting fronts with convergent and downwelling currents should ori-ent against the downwelling flow. This will be tested by observing horizontal convergent and downwelling currents in fronts with moored ar-rays of temperature loggers and Doppler current meters, and by observing in situ larval orientation in convergent fronts and outside the fronts. Larval orientation will be observed with a high-resolution Video Plankton Recorder, the VPR. For the frontal accumulation process, the idea that the origin of frontal larvae is offshore of the propagating internal bore warm fronts (e.g. as op-posed to onshore origin) will be tested. This hypothesis predicts that larval concentration offshore of the fronts is much larger than the larval concentration onshore of the fronts, and that the current speed behind the front u is faster than the rate of advance (propaga-tion speed) of the front c. This will be tested by sampling larval concentration across the fronts, by observing the front's phase speed and circulation, and by conducting experiments re-leasing drifters around the fronts. For the low- frequency forcing, two related ideas will be investigated. First, the intensity of larval transport by inter-nal tidal bores depends on the raising and out-cropping of the thermocline by remote or local wind driven upwelling, will be tested with daily barnacle settlement measurements, time se-ries of temperature at three depths, currents ob-served with a Doppler current meter, and intense larval sampling during upwelling events. The sec-ond hypothesis, low-frequency variabil-ity in larval transport and temperature field at La Jolla, California is related to northward propa-gating events from the Baja California coast, will be tested with daily barnacle settlement moni-toring in La Jolla, AVHRR sea-surface temperature satellite data, and time series of wind, tempera-ture, and pressure in La Jolla and three locations South of La Jolla.