remained unchanged in Zone 3. Fresh- 

 water taxa accounted for only 15 per- 

 cent of the diversity in Zone 1, 16 

 percent in Zone 2, and 3 percent in 

 Zone 3. The zooplankton throughout 

 the bay was returning to its estua- 

 rine dominants with few exceptions. 



Salinities were slightly higher 

 in all three zones during the sam- 

 pling on July 20, but river flow rate 

 had not decreased from the previous 

 sampling date. Zooplankton density 

 had increased substantially in Zone 

 1, but had decreased by half in 

 both Zones 2 and 3. A few fresh- 

 water taxa contributed to the zoo- 

 plankton only in Zone 1. Diversi- 

 ty had increased only in Zone 1, 

 and had fallen slightly in Zones 2 

 and 3. Arcella discoides was the 

 only freshwater species to reach 

 the dominance list, and it was in 

 Zone 1. The ctenophore, Mnemiopsis 

 mccradyi, reached the dominance list 

 in both Zones 2 and 3, and should 

 be considered as a possible cause 

 for the decrease in zooplankton den- 

 sities in these two zones. 



The species and taxa which most 

 characterize the estuarine zooplank- 

 ton community were also most often 

 found in the dominance tables be- 

 cause they contributed greatly to 

 the densities in each zone and par- 

 ticularly to those in Zones 2 and 3. 

 These species and taxa forming the 

 estuarine zooplankton community in 

 San Antonio Bay are Acartia tonsa , 

 Balanus sp . nauplii, Oithona colcar - 

 va , Pseudodiaptomus coronatus , Para - 

 calanus crassirostris , cyphonautes 

 larvae of Membranipora sp., spionid 

 larvae, polychaete larvae, and gas- 

 tropod veligers. Acartia tonsa was 

 usually very abundant, and is known 

 to tolerate very low salinities 

 (Conover 1956) . Even the flood could 

 not displace it from being the domi- 

 nant zooplankter. Only during the 



winter and spring with salinities 

 above 20 parts per thousand was A. 

 tonsa often replaced as the dominant 

 taxon by Balanus sp. nauplii. 



Many of the typically estuarine 

 species were replaced by species and 

 taxa of freshwater origin during the 

 flood (Table 3) . The most character- 

 istic of these freshwater taxa were 

 the freshwater calanoids Eurytemora 

 affinis and several species of Dia - 

 ptomus ; the freshwater cyclopoids 

 Cyclops sp . , Eucyclops sp . , Apocy - 

 clops panamensis and Microcyclops 

 sp.; the cladocerans Moina micrura 

 and Diaphanosoma brachyurum ; the 

 rotifers Brachionus quadridentatus , 

 B. Calycif lorus and Platyias quad - 

 ricornis; and the protozoan Arcella 

 discoides . There were many other 

 taxa of freshwater rotifers, clado- 

 cerans, copepods , and insect larvae 

 that entered the bay with floods 

 and freshets, but most were found in 

 low densities and frequencies. 



Most of these freshwater spe- 

 cies are characteristic of backwater 

 areas (Ward and Whipple 1959; Cooper 

 1967) rather than the open river it- 

 self. These freshwater species' pop- 

 ulations may have been dense locally, 

 but when they were washed into the 

 bay by the floods their densities 

 were considerably lower than those 

 of the estuarine zooplankters in- 

 habiting the bay. The dilution and 

 displacement of bay water by the 

 fresh water of a flood creates a nat- 

 ural dilution of the estuarine zoo- 

 plankton, and when the diluting wa- 

 ter has relatively few zooplankters 

 the result is a reduction in the to- 

 tal zooplankton density in the bay. 

 This is what happened during the May 

 flood. 



Diversity, however, was in- 

 creased in the bay because of the 



518 



