Physical disturbance can be an important force at the individual, population, and community levels of organization. The effects of disturbance may differ for mobile and sessile organisms, however, because of differences in the potential for escape and postdisturbance recolonization by survivors. I used field sampling and laboratory experiments to examine how episodic movement of severely oxygen-depleted (hypoxic) bottom water into nearshore habitat in the Chesapeake Bay affects population density, recruitment, and reproduction of a mobile species--the naked goby (Gobiosoma bosc), a benthic oyster bed fish. Oxygen depletion is a common physical disturbance in freshwater, estuarine, and coastal aquatic systems. In this study, episodic hypoxia influenced mortality, size structure of the population, reproductive behavior, and spatial distribution. Intrusion of severely hypoxic water occurred in late July and early August during the 2-yr study. These intrusions coincided temporally with peak periods of recruitment, and caused the most severe drops in dissolved oxygen concentrations in deep and mid-depth areas of the oyster reef, where recruitment was highest. Laboratory experiments suggested that newly settled recruits require higher oxygen concentrations for survival than do older individuals. Field samples also indicated that these new recruits are less able to escape to more highly oxygenated shallow water refuges when an intrusion occurs. Thus, the spatial and temporal patterns of recruitment and disturbance, and physiological requirements, combine to result in extremely high mortality of new recruits during severe intrusions. In contrast to effects on new recruits, some large juveniles and adults successfully migrate inshore when oxygen levels decline. In both field samples and laboratory experiments, adult males continued to guard eggs and shelters until dissolved oxygen closely approached lethal levels. Calculations based on size-specific physiological tolerances and swimming speeds suggest that the occurrence of lethal conditions in the fluctuating environment may be more predictable to larger individuals than to new recruits. This predictability may increase the possibility of an appropriate response to low oxygen disturbances by large juveniles and adults. After the disturbance abates, surviving individuals recolonize abandoned areas. This ability of mobile animals to recolonize a disturbed area as adults or juveniles, rather than solely through reproduction, may lead to differences in postdisturbance ecological interactions and differences in selection for colonizing ability between mobile and sessile species.
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