| Disturbance creates spatial and temporal heterogeneity that influences the structure and function of ecosystems. Influences of disturbances on sessile species have received considerable attention, including examples of disturbances altering population persistence, species diversity, and patch dynamics. But we know less about impacts of disturbance on motile organisms. Through field surveys, manipulative experiments, and population modeling, I examined the effects of disturbance on an estuarine fish community. Specifically, I compared how the Neuse River estuarine fish community responded to frequent hypoxic disturbances and infrequent hurricane disturbances.; Hypoxic disturbances directly affected fish by altering their distributions. Monthly (May through October) trawl survey data from 1997–1999 demonstrated that fish avoid areas with dissolved oxygen concentrations <2.0mg/L. Analyses of spot and croaker responses also showed that dissolved oxygen avoidance thresholds differ depending on the spatial extent of hypoxia. When hypoxic zones were small, thresholds generally exceeded 3mg/L. Thus, the 2.0mg/L threshold is a minimum and probably used at some cost. Frequent disturbances due to hypoxia structured species assemblages at local scales, while regional factors such as recruitment and migration patterns drove community characteristics at larger scales. I demonstrated several mechanisms by which hypoxia indirectly affects demersal juvenile fish; it decreased habitat quality through crowding fish into areas with fewer resources and killed benthic prey species resulting in reduced growth rates of juvenile fishes.; Since large tropical storms and hurricanes are typically rare events, their effects are poorly understood. I predicted fish response to hurricanes based on their responses to frequent disturbances. My predictions were correct for the 1998 hurricane event, but failed more than 50% of the time for the fall 1999 hurricanes. Dissimilarities between predicted fish effects versus observed responses to infrequent disturbances could be explained by differences in the rate of change and seasonal timing of salinity changes associated with storms. Also, hurricanes resulted in simultaneous multiple stresses and my predictions derived from single stress experiences.; Population models including both disturbances suggested that infrequent hurricanes had smaller population level effects than chronic hypoxia. This study suggests that chronic disturbances resulting in habitat degradation may have a larger impact on demersal fish populations than large, natural storm disturbances that result in large pulses of direct mortality. |
