Distribution of the crustacean zooplankton in a subtropical estuarine system: implications for the predator-prey interaction between the primary and secondary consumers.

The spring and summer zooplankton community in East Lagoon, Galveston Island, Texas was similar to the summer assemblages of many U.S. estuaries. Species diversity was low and the crustacean zooplankton were dominated by a single calanoid species, Acartia tonsa. Length regression studies led to the suggestion that the community was at times food resource limited. Furthermore the available resources appeared to be ecologically partitioned both inter- and intra-specifically among the community members. Inter- specific partitioning of prey resources was also observed for larval and postlarval fishes entering the estuary. Diet overlap between zooplanktivores was low. Prey resources appeared to be partitioned across prey size distributions, habitat separations, and temporal foraging patterns. Within their respective foraging strategies, predators were essentially generalists and diets were well varied between individuals and over time. It still remains unknown whether the juvenile fishes were food limited. Weekly zooplankton densities and spatial distributions were highly variable. Spatial/temporal distributions appeared to be influenced by the active response of the zooplankton to environmental stimuli. The larger copepodids were concentrated in the deeper areas of the lagoon in the midday hours, presumably taking refuge from visual predators. Under seemingly stressful conditions such as reduced surface salinities, population densities of the smaller copepodids were observed to decrease in the surface waters while densities simultaniously increased in the deeper less environmentaly variablle areas of the lagoon. Horizontal distribution characteristics were asymmetrical, particularly in the spring along the windward shoreline. At this time most copepodid instars were substantially represented in the shallow nearshore waters. Consequently larval and postlarval fishes within the estuary were able to sample concentrated prey resource in a lighted environment where protection from their predators and potentially higher growth rates were available. A wind driven circulation pattern resulting in a zone of divergence along the windward shoreline in the spring when the vertical temperature distribution within the lagoon was nonuniform was hypothesized to explain the zooplankton concentration. Prey density alone could not acount for observed predator selectivety within specific foraging zones. A variety of factors known to affect rates of predation in freshwater systems were confirmed for the estuarine zooplanktivores examined. These include prey density, prey size, prey/background contrast ratio, predator size (age), hunger level, and various behavioral characteristics such as social interactions, previous experience and temporal feeding responses. The perceptive abilities of the predators were significantly different. Juvenile menhaden and silversides were found to be visually obligate predators, whereas the feeding rates of juvenile spot were not correlated with incident illumination levels. It appears from the experimental laboratory results that within observed natural prey densities and under conditions appropriate for feeding to occure, the feeding rates of zooplanktivorous larval and postlarval fishes are approximately poportional to the densities of the potential prey. Therefore within the limits of natural prey densities, linear response predator-prey models may often be adequate to predict the affects of predation on known prey populations. The existing data supporting the final conclusion that the theories of predation have remained uncompromisingly similar between the freshwater and marine systems so far examined. Apparent dissimilarities between the systems have been due to inherent differences in the predator and zooplankton communities, not in the conceptual understanding of the predator-prey interaction.