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

A one-dimensional model of larval concentration and settlement flux in a turbulent boundary layer was used to consider how some aspects of l arval behavior in the plankton or on the bottom might affect settlemen t rates. The focus was on behavioral modifications of two terms in the model-the vertical (fall) velocity (w(f)), a composite measure of swi mming and gravitational sinking, and the probability of settlement per unit time (p) for larvae interacting with the bottom. Depth-independe nt changes in vertical speed can increase the settlement rate by up to an order of magnitude. Such changes might be produced by negative pho totactic responses that induce passive sinking of larvae throughout th e water column during daylight, by photonegative swimming responses th at may occur in relatively shallow or clear water (where swimming resp onses are fairly uniform throughout the water column), or by photonega tive swimming responses that are of similar magnitude over a broad ran ge of light intensities. In addition to increasing the long-term avera ge settlement rate, negative phototactic responses should increase tem poral variability in settlement at time scales of less-than-or-equal-t o 24 h, as long as larvae are responding to changes in light intensity at depth. This prediction, coupled with results from GROSS et al. [(1 992) Journal of Marine Research, 50, 611-642], suggests that for larva e 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-d iurnal or diurnal tidal periodicity in boundary shear stress and turbu lence intensity. The additional effect of phase shift between tidal an d light cycles introduces substantially less variability to the averag e (>24 h) settlement rate than do individual effects of the two forcin gs. Behavioral responses that produce a depth dependence in vertical s peed can affect the settlement rate only if w(f) varies roughly by a f actor of 2 or more very close to the bottom (i.e. within a few percent of total boundary layer thickness for tidal boundary layers). Photota ctic or barokinetic responses of larvae are not likely to produce dept h 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 exert ed on larvae on the bottom can alter the settlement rate by an order o f magnitude or more. However, the settlement rate is predicted to be m ost sensitive to effects of weak, rather than strong, flow on the sett lement probability. Thus, understanding larval responses to weak flows may be especially important to predicting settlement rates.