MODELING SOME EFFECTS OF BEHAVIOR ON LARVAL SETTLEMENT IN A TURBULENT BOUNDARY-LAYER

A one-dimensional model of larval concentration and settlement flux in a turbulent boundary layer was used to consider how some aspects of larval behavior in the plankton or on the bottom might affect settlement rates. The focus was on behavioral modifications of two terms in the model-the vertical (fall) velocity (w(f)), a composite measure of swimming and gravitational sinking, and the probability of settlement per unit time (p) for larvae interacting with the bottom. Depth-independent changes in vertical speed can increase the settlement rate by up to an order of magnitude. Such changes might be produced by negative phototactic responses that induce passive sinking of larvae throughout the water column during daylight, by photonegative swimming responses that may occur in relatively shallow or clear water (where swimming responses are fairly uniform throughout the water column), or by photonegative swimming responses that are of similar magnitude over a broad range of light intensities. In addition to increasing the long-term average settlement rate, negative phototactic responses should increase temporal variability in settlement at time scales of less-than-or-equal-to 24 h, as long as larvae are responding to changes in light intensity at depth. This prediction, coupled with results from GROSS et al. [(1992) Journal of Marine Research, 50, 611-642], suggests that for larvae that respond to light there may be two sources of substantial (order-of-magnitude) temporal variability in settlement that operate at time scales of 24 h or less-diel periodicity in light intensity and semi-diurnal or diurnal tidal periodicity in boundary shear stress and turbulence intensity. The additional effect of phase shift between tidal and light cycles introduces substantially less variability to the average (>24 h) settlement rate than do individual effects of the two forcings. Behavioral responses that produce a depth dependence in vertical speed can affect the settlement rate only if w(f) varies roughly by a factor of 2 or more very close to the bottom (i.e. within a few percent of total boundary layer thickness for tidal boundary layers). Phototactic or barokinetic responses of larvae are not likely to produce depth gradients in vertical speed strong enough to affect settlement rates. However, such gradients might be produced by a recent contact with a potential settlement site, by effects of velocity gradients near the bottom on larval orientation, or by larval responses to chemical cues associated with the bottom. Behavioral responses to fluid forces exerted on larvae on the bottom can alter the settlement rate by an order of magnitude or more. However, the settlement rate is predicted to be most sensitive to effects of weak, rather than strong, flow on the settlement probability. Thus, understanding larval responses to weak flows may be especially important to predicting settlement rates.