MATERIALS AND METHODS 



STUDY AREA 



San Antonio Bay covers an area 

 of about 305 km and is located in 

 the middle of the Texas coastline 

 at latitude 28°20' North and longi- 

 tude 96°45' West. It is a shallow 

 bar-built estuary with an average 

 natural depth of 1.5 m and contains 

 many shallower oyster reefs and few 

 places as deep as 3 m, however, re- 

 cent shell dredging in the middle 

 bay area has increased the depth in 

 about 20 percent of this section to 

 4 m. Matagorda Island isolates San 

 Antonio Bay from the Gulf of Mexico, 

 and most salt water must flow into 

 Matagorda Bay and through Expiritu 

 Santo Bay before reaching San Antonio 

 Bay. Fresh water from the combined 

 flows of the San Antonio and Guada- 

 lupe Bay flow into upper San Antonio 

 Bay (Figure 1). Annual evaporation 

 slightly exceeds annual rainfall in 

 normal years. 



SAMPLING REGIME 



Eleven sites were selected to 

 represent the bay (Figure 2). To 

 facilitate biological analyses with 

 respect to salinity, these sites 

 were partitioned into: Zone 1 = 

 the upper bay, Zone 2 = the middle 

 bay, and Zone 3 = the lower bay. 

 Zooplankton was collected at each 

 site twice per month by making a 

 one-minute oblique tow with a #10 

 mesh (150 micron pore width) conical 

 Nitex net which had a mouth diameter 

 of 0.5 m and a length of 1.3 m. A 

 flowmeter mounted in the net mouth 

 measured the amount of water fil- 

 tered on each tow. After each tow, 

 the net was washed and the bucket's 



contents were preserved in 5 to 10 



percent Formalin. Water temperature 



and salinity were taken immediately 

 following the tow. 



DATA COLLECTION 



River flow rates were obtained 

 for the rivers and creek from the 

 U.S. Geological Survey annual re- 

 cords. Ten-day average river flow 

 rates were calculated for each sam- 

 pling time. Each average was based 

 on the sum of the daily flow rates 

 of each of the three tributaries for 

 the day of sampling plus the nine 

 previous days, i.e. the summation of 

 30 values divided by 10. 



SAMPLE ANALYSIS 



Methods similar to those used 

 by Hopkins (1966) were used to ana- 

 lyze each zooplankton sample. A sub- 

 sample taken with a Hensen-Stemple 

 pipet and containing between 200 and 

 1,000 organisms was examined from 

 each tow. Each organism was identi- 

 fied to the lowest taxon possible-- 

 usually to genus or species. Counts 

 from the subsample were converted to 

 numbers per cubic meter of bay water. 



RESULTS AND DISCUSSION 



THE SINGLE FLOOD OF 1972 



Collections on May 4, before the 

 flood, showed fairly high densities 

 of zooplankton in Zone 1 and moderate 

 levels in Zones 2 and 3 (Table 1). 

 The composition of the zooplankton 

 was typically estuarine for all zones 

 at this time. Just before the flood 

 there was a freshet which introduced 

 sufficient fresh water to reduce the 



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