A combination of larval behavior and physical factors influence the spatial patterns of settlement of marine organisms. Of particular importance to the settlement process is the blend of passive transport and active responses to water flow near the settlement habitat. Field experiments with the naked goby Gobiosoma bosc, a benthic oyster reef fish, indicated that larvae aggregate in low-flow areas on the downcurrent sides of rocks, and shift position with changing flow directions. Larger aggregations of larvae were found in downcurrent positions at rocks that created larger low-flow zones, and during parts of the tidal cycle with higher ambient flow velocities. Settlement occurred in a highly aggregated pattern that reflected larval distributions. Most settlement measured in a field experiment was adjacent to downcurrent sides of rocks rather than in other positions near rocks, or away from structures that would decrease downcurrent flow velocities. These results suggest that the active response of fish larvae to either direct or indirect effects of flow on reefs may be important to fine-scale spatial patterns of settlement. Because zooplankton densities downcurrent of rocks were sinular to, or lower than, densities upcurrent and lateral to rocks, spatial distnbutions of prey are unlikely to explain larval distributions. Instead, active preference for low-flow areas may enable fish larvae to maintain their position on oyster reefs, the preferred settlement habitat.

Low dissolved oxygen concentrations, caused by density stratification of the water column and excess nutrient inputs, occur in many aquatic habitats. Laboratory experiments we conducted indicated that low dissolved oxygen has the potential to strongly alter the absolute and relative importance of a suite of estuarine predators of fish larvae. At dissolved oxygen concentrations 22 mg l-', predation on naked goby Gobiosolna bosc larvae by an important invertebrate predator of plankton in Chesapeake Bay [the sea nettle scyphoinedusa Chrysaora quinqueclrrha) increased. In contrast, at the same oxygen concentrations, predation by 2 vertebrate predators, juvenile striped bass Morone saxatllis and adult naked goby, decreased. Changes in consumption of larvae most likely resulted from impaired ability of larvae to escape the scyphornedusa, and decreased attack rates by adult and juvenile fishes. Fish predators increased gill ventilation rates even at oxygen levels higher than those leading to decreased predation. However, we could detect no comparable change in behavior of the sea nettle even at 1 mg 1-', the lowest oxygen concentration tested The observed changes in trophic interactions occurred at dissolved oxygen concentrations that are not lethal during short exposures, and that commonly occur in the Chesapeake Bay and other eutrophic estuaries during summer. Thus, low oxygen has the potential to cause significant changes in the importance of alternate trophic pathways in estuarine systems.

S~m p l e models and an example from the published literature suggest how consumer mobility can alter the relative importance to producers of consumption (predation and herbivory) and competition follow~ng a disturbance that opens habitat for colonization. Highly mobile consumer species often can colonize disturbed habitat at juvenile and adult life stages with high per capita consumption rates. In contrast, consumers with limited mobility, as well as sessile or sedentary prey, can b e restricted to colonization of disturbed habitat by reproductive propagules, and initially have low per capita consumption, and low biomass and per capita production, respectively. Following a disturbance, the relationship between potential consumption and production can differ in important ways from that prior to the disturbance if consumer and prey species differ markedly in mobility, but may be similar to that prior to the disturbance if consumers and prey are both l~mited to colonization by reproductive propagules. Functional responses of consumers, and growth rates of consumers relat~ve to producers, can determine how strongly consumer mobility will influence post-disturbance interactions. In California (USA) kelp forests, sea urchins may be able to colonize disturbed patches as adults or can b e restricted to colonization by larval settlement depending on habitat characteristics and how their foraging behavior affects sea urchin susceptibility to winter storms. On Naples Reef, near Santa Barbara, California, recovery of macroalgae including giant kelp was inhibited under grazing by adult urchins but was rapid when sea urchins colonized through larval settlement.