influx of freshwater species. The 

 myriad of backwater localities along 

 the tributaries allowed for many dif- 

 ferent species' populations to flour- 

 ish, and during the flood they were 

 washed down the rivers and into the 

 bay. Initially, more freshwater spe- 

 cies and taxa were added to the sam- 

 pling sites than estuarine species 

 were displaced or killed. Diversity 

 declined after this initial increase 

 probably because most of the fresh- 

 water zooplankton had already been 

 carried down the river, and because 

 flow rates declined so that fewer 

 remaining plankters were carried in- 

 to the bay. 



the May 1972 flood. Diversity ap- 

 peared to be regulated considerably 

 by the amounts and rates of river 

 inflow (Figure 3). During the first 

 four months the diversity trend fol- 

 lowed the river flow rate but was 

 one sampling delayed (time lag ef- 

 fect) . From the beginning of the 

 June flood through the October flood, 

 this relationship was no longer 

 found. In spite of the decrease in 

 river flow rate during the last of 

 the year, diversity in all zones 

 also decreased. Much of this de- 

 crease was due to the cold weather 

 when many meroplankters are no long- 

 er found in bay waters. 



It is evident that the zoo- 

 plankton community in the bay was 

 greatly changed by the flood and 

 that the changes occurred within 

 two weeks of the start of the flood, 

 and probably much sooner. Re-estab- 

 lishment of the typical estuarine 

 zooplankton community depends sub- 

 stantially on the reduction of riv- 

 er flow rates, and after flow rates 

 fall below freshet levels, it can 

 still take two months to re-estab- 

 lish the estuarine species in the 

 upper bay. Only about one month 

 was required to re-establish it in 

 the lower and middle bay areas. In 

 this specific case the east side of 

 the bay was first supplied with high- 

 er salinity water from Espiritu San- 

 to Bay which was rich in estuarine 

 zooplankton. Many tidal cycles and 

 their attendant circulation patterns 

 were required to re-establish the 

 estuarine zooplankton along the west 

 side of the bay. 



MULTIPLE FLOODS OF 1973 



The species composition of the 

 freshwater zooplankton that entered 

 the bay with the river inflow was 

 very much the same as found during 



The percentage of the diversity 

 of each zone, contributed by taxa 

 of freshwater origin, was greatly in- 

 creased by the June and October 

 floods (Figure 4) , and these percen- 

 tages were much higher for Zone 1 

 than for Zones 2 and 3. Percentages 

 contributed by freshwater taxa in 

 Zones 2 and 3 were similar and they 

 varied together more closely than 

 with that of Zone 1 during the en- 

 tire year. 



The total zooplankton density 

 decreased an order of magnitude from 

 the start of the year to the end, but 

 it also decreased much lower at times 

 between these end points (Figure 5). 

 Total density showed an inverse rela- 

 tionship to river flow rate. The 

 June and October floods each caused 

 a decline in total density of nearly 

 two orders of magnitude which was 

 never completely regained through the 

 rest of the year. The recovery time, 

 or time required for the density of 

 a zone to re-establish its preflood 

 level, appeared to be between two 

 weeks and a month, i.e. between one 

 and two sampling trips. The recov- 

 ery time depended on tides, circu- 

 lation patterns, spawning rates and 

 periods, and temperature. 



519 



