average salinity in Zone 1 to about 

 7 parts per thousand. Total zooplank- 

 ton density in Zone 1 increased 

 slightly over its value at the pre- 

 vious sampling (19 April), but it de- 

 creased in both Zones 2 and 3. A few 

 common freshwater zooplankters such 

 as Cyclops sp . , Diaptomus sp . , and 

 cladocerans were introduced into 

 Zone 1, but no freshwater-related 

 changes in diversities were found in 

 the zooplankton of Zones 2 and 3 at 

 this time (4 May) . 



The flood began on May 8, peak- 

 ed on May 16, and had decreased to a 

 freshet level by the sampling trip on 

 May 23. Salinities in all zones had 

 fallen to between 1 and 4 parts per 

 thousand. Several changes had oc- 

 curred in the zooplankton, and total 

 zooplankton densities in Zones 1-3 

 had decreased to 20, 73, and 26 per- 

 cent, respectively, of what they had 

 been 19 days earlier. The percent 

 of the total density contributed by 

 taxa of freshwater origin had in- 

 creased from 1.3 to 54.5 percent in 

 Zone 1, and from to 17 percent and 

 43 percent for Zones 2 and 3, and 

 most of the dominant taxa in all 

 three zones were of freshwater origin 

 (Table 2). 



By the collection time of June 

 7, the river flow rate had decreased 

 to only 5 percent of the maximum 

 flood flow rate, but the river rate 

 was still slightly elevated above 

 base flow rate. Salinity remained 

 depressed in Zone 1 and increased 

 only very slightly in Zones 2 and 3. 

 Zooplankton densities were now even 

 lower in Zone 1 , but they had dou- 

 bled in Zone 2 and had quadrupled 

 in Zone 3. Contributions by taxa 

 of freshwater origin to these den- 

 sities were down to 7.4, 0.4, and 



0.9 percent for Zones 1 to 3, respec- 

 tively, and only in Zone 1 were they 

 representing about half of the domi- 

 nant taxa (Table 3) . Diversities in 

 all zones were lower than during the 

 previous sampling. The percent of 

 the diversity contributed by fresh- 

 water taxa was also lower, but it 

 was still between 44 and 18 percent. 



Freshwater inflow increased to 

 freshet levels again on June 19, just 

 a few days prior to sampling. Sali- 

 nity remained at about 1.5 parts per 

 thousand in Zone 1, but slight in- 

 creases in salinities to about 6 and 

 10 parts per thousand were found in 

 Zones 2 and 3 respectively. Densi- 

 ties reached a low in Zone 1 at 584/ 

 m , but increased in Zones 2 and 3 to 

 above 20,000/m . Freshwater taxa ac- 

 counted for virtually nothing in 

 Zones 2 and 3. Diversity increased 

 in Zones 1 and 2 but not in Zone 3. 

 Freshwater taxa accounted for 54 per- 

 cent and 12 percent of the diversity 

 in Zones 1 and 2 respectively; none 

 was found in Zone 3. All of the 

 dominants in Zone 1 were of freshwa- 

 ter origin except Acartia tonsa . 



River flow rate continued to 

 decrease after the spike in late 

 June, and salinity in Zone 1 final- 

 ly increased to 2.5 parts per thou- 

 sand at the time of the sampling on 

 July 6, but it remained unchanged 

 in Zones 2 and 3. Zooplankton den- 

 sities increased an order of magni- 

 tude in Zone 1 and also increased 

 again in Zones 2 and 3. Contribu- 

 tions by freshwater taxa to the den- 

 sity in Zone 1 decreased to about 3 

 percent and they increased in Zone 

 2 to 0.5 percent. Diversity de- 

 creased by almost half in Zone 1, 

 just slightly in Zone 2, and 



514 



