Modeling fish dynamics and effects of stress in a hydrologically pulsed ecosystem

Many wetlands undergo seasonal cycles in precipitation and water depth. This environmental seasonality is echoed in patterns of production of fish biomass, which, in turn, influence the phenology of other components of the food web, including wading birds. Human activities, such as drainage or other alterations of the hydrology, can exacerbate these natural cycles and result in detrimental stresses on fish production and the higher trophic levels dependent on this production. In this paper we model the seasonal pattern of fish production in a freshwater marsh, with special reference to the Everglades/Big Cypress region of southern Florida. The model illustrates the temporal pattern of production through the year, which can result in very high densities of fish at the end of a hydroperiod (period of flooding), as well as the importance of ponds and other deep depressions, both as refugia and sinks during dry periods. The model predicts that: (1) there is an effective threshold in the length of the hydroperiod that must be exceeded for high fish-population densities to be produced, (2) large, piscivorous fishes do not appear to have a major impact on smaller fishes in the marsh habitat, and (3) the recovery of small-fish populations in the marsh following a major drought may require up to a year. The last of these results is relevant to assessing anthropogenic impacts on marsh production, as these effects may increase the severity and frequency of droughts.