A study of the zooplankton assemblage of San Antonio Bay, Texas, and of the effects of river inflow on the composition and the persistence of this assemblage

Abstract

The composition and abundance of the zooplankton community in San Antonio Bay were studied. Particular attention was paid to the occurrences and densities of selected taxa and to the effects of floods on this zooplankton community. Samples were collected twice per month over a 29-month period from March 1972 through July 1974. Each sample consisted of a one-minute oblique tow using a #10 mesh conical nylon net having a half-meter diameter mouth fitted with a flowmeter. A total of 279 taxa belonging to groups such as protozoans, rotifers, cladocerans, copepods, decapod zoeae, insect larvae, and fish larvae were identified from the 568 samples analyzed. This zooplankton community contained species and taxa which were typical of most Texas estuaries and which represented an indicator group. These were: Acartia tonsa, Pseudodiaptomus coronatus, Paracalanus crassirostris, Oithona colcarva, Copepod nauplii, Balanus spp. nauplii and cyprids, Gastropod veligers, Bivalve veligers, Cyphonautes larvae, and Polychaete larvae. During floods several freshwater species and taxa became abundant in the bay. These were: Arcella discoides, Brachionus calyciflorus and B. quadridentatus, Moina micrura, Diaphanosoma brachyurum, Diaptomus spp., Cyclops spp., Eucyclops agilis, and several insect larvae. The seasonal abundance of zooplankton (all-inclusive) varied each year of the study. It was highest during the first 12 months. Maxima occurred in late winter or early spring when Balanus spp. nauplii rose to 100,000/m^3 at several sites. A smaller peak occurred during the summer due to Acartia tonsa populations which rose to 35,000/m^3 at several sites during the summer of 1972. Floods caused total zooplankton densities to decrease between one and two orders of magnitude in all zones. Zooplankton densities were unable to regain initial values after the first flood in 1973, and they decreased more and more with each successive flood. Only when river flow rates returned to less than 100 m^3/sec could the typical estuarine indicator group return to dominance. The recovery period was from two weeks to two months for the lower bay and the upper bay respectively. The return to dominance of the estuarine indicators was predominantly due to importation of large populations of these taxa with influxes of saline water from Espiritu Santo Bay...