After each flood the zooplank- 

 ton did recover, but in each case 

 the recovery was incomplete. Densi- 

 ties were 10,000/m to 20,000/m be- 

 fore the April flood ^ decreased to 

 2,000/m to 6,000/m during the 

 flood and recovered to 4,000/m to 

 20,000/m afterwards. The June flood 

 arrived soon after this recovery, and 

 densities., declined again, this time 

 to 400/.m to 800/m ~ Recovery to 

 4,000/m to 12,000/m occurred be- 

 tween the two major flow periods of 

 this flood, and much of these densi- 

 ties were due to moderate populations 

 of freshwater zooplankters . Zoo- 

 plankton -density in Zone 1 fell to 

 only 64/m after this second pulse of 

 flood water. Equipment failure pre- 

 vented sampling the other zones. 

 After the flood, the densities re- 

 covered again to 1, 800/m to 11,000 

 /m , just slightly lower than the 

 preflood values. At the start of 

 the October -flood the densities were 

 about 850/m to 9,500/m , and they 

 declined to 70/m at the end of the 

 flood. Recovery after the flood was 

 delayed in Zone 1, but it was rapid 

 in Zones 2 and 3 with preflood densi- 

 ties being attained within a month. 



to Zone 1 sooner and for a longer 

 time than for the other zones which 

 is reasonable considering Zone 1 is 

 closest to the river mouth. 



The cumulative effects of the 

 floods during 1973 appear to be those 

 of temporarily increasing diversity 

 and decreasing density. Increased 

 diversity in the bay as a whole is 

 logical with the addition of fresh- 

 water taxa to those taxa already 

 existing in the bay. Much of the 

 decrease in density can be attrib- 

 utable to the relatively low densi- 

 ties of Balanus sp . nauplii in De- 

 cember 1973 versus the same time the 

 previous year. This is a result of 

 stressing or killing the adult bar- 

 nacles with the very low salinities 

 which existed in the bay for such 

 an extended period. Matthews et al. 

 (1975) noted relatively low standing 

 crops of phytoplankton from early 

 October through December 1973 as com- 

 pared with the other periods. This 

 paucity of food could have resulted 

 in the poor spawn among the surviving 

 barnacles, and thus the lower densi- 

 ties after the floods. 



Zooplankton of freshwater or- 

 igin contributed greatly to the to- 

 tal density of each zone during these 

 floods. Their contributions during 

 the April flood were relatively 

 minor, reaching only 33 percent of 

 the total density in Zone 1 and much 

 less for those in Zones 2 and 3 (Fig- 

 ure 6) . Their contributions during 

 the June flood were much greater, 

 reaching 97 percent for Zone 1 near 

 the middle of the flood, and 68 per- 

 cent and 35 percent for Zones 2 and 

 3 respectively. Similar levels of 

 contribution were found for each 

 zone during the October flood. Dur- 

 ing all three floods, the freshwater 

 taxa contributed a greater percentage 



CONCLUSIONS AND RECOMMENDATIONS 



Prolonged exposure of an es- 

 tuary to fresh water such as was 

 found during the floods in San Anto- 

 nio Bay in 1973 may be considered 

 damaging to the zooplankton and other 

 fauna of the area on a temporary ba- 

 sis. Typical estuarine fauna are 

 replaced by freshwater fauna and to- 

 tal zooplankton densities are usu- 

 ally greatly reduced during each 

 flood. Because the 1973 type of 

 flooding occurs once in 100 years or 

 less, and because its effects are 

 rapidly erased by influx of organisms 

 and zooplankton from neighboring 



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