THE EFFECTS OF FLOODS ON THE ZOOPLANKTON ASSEMBLAGE OF SAN ANTONIO BAY, TEXAS 



DURING 1972 AND 1973 



Geoffrey A. Matthews 



National Marine Fisheries Service 

 Galvestion, TX 77550 



ABSTRACT 



Plankton tows and hydrographic 

 measurements were taken encompassing 

 a single flood in 1972, and three 

 floods in 1973 in San Antonio Bay. 

 The shallow bay was rapidly flushed 

 by influx of flood waters as was in- 

 dicated by reductions in salinity 

 and in the densities of the dominant 

 species, Acartia tonsa . Floods re- 

 placed the typical estuarine zoo- 

 plankter (Balanus sp . nauplii, Oi - 

 thona colcarva , Paracalanus cras - 

 sirostris , Oikopleura spp . , and the 

 cyphonautes larvae of Membranipora 

 sp.) with the freshwater ones (Diap- 

 tomus spp., Cyclops spp., Arcella 

 discoides , Moina sp., Diaphanosoma 

 sp. and other cladocerans) . During 

 the 1972 flood, total zooplankton 

 densities fell from 10,800/m before 

 the flood to 3,400/m after the 

 flood, but they increased rapidly 

 when the river flow returned to base 

 level. After the three floods in 

 1973, a cumulative decrease in total 

 density of over two orders of magni- 

 tude was found. There had been in- 

 sufficient time to reestablish pre- 

 flood densities between each flood. 

 The rapidity with which densities 

 were re-established and the areas in 

 which these increases were first 

 found indicates the majority of the 

 density changes were due to influx 

 of zooplankton-rich bay water from 



Espiritu Santo Bay, rather than from 

 population explosion by surviving re- 

 fuge populations. It is important to 

 note that the seasonal occurrence of 

 a flood may severely reduce the sur- 

 vival of a bay's annual recruitment 

 of economically important species 

 whose larval stages are members of 

 the zooplankton or which depend on 

 zooplankton as food. It is also 

 important to note the interdependency 

 of these estuaries as currents flow 

 carrying life from one into the next. 



INTRODUCTION 



Most estuarine plants and ani- 

 mals depend in some manner on fresh 

 water from rivers and streams for 

 their survival. The variability in 

 quality and quantity of the fresh- 

 water inflow during a year and 

 through several years can lead to 

 dramatic environmental changes in 

 an estuary, and thus in the organ- 

 isms living there. With the in- 

 creasing use of estuaries for var- 

 ious economic purposes it has be- 

 come essential to know what to ex- 

 pect when certain environmental 

 factors change. The objective of 

 this paper is to describe the effects 

 of floods on the zooplankton of a 

 shallow estuary, San Antonio Bay, 

 Texas . 



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