ECOLOGY OF WESTERN GULF ESTUARIES 



A wide range of activities was included in 

 this project during this fiscalyear. Hydrologi- 

 cal and biological sampling continued through- 

 out the Galveston estuary. Once a month from 

 July through December, we occupied 65 stations 

 within the bays, lagoons, and bayous and 

 8 stations in the bordering marshes. In Janu- 

 ary, we began sampling weekly so we could 

 study intensively the postlarval and juvenile 

 brown shrimp when they abound in the estuary. 

 Considerable effort has been devoted, in the 

 past year, to evaluating data collected on 

 postlarval and juvenile brown shrimp during 

 1963 and 1964, We have analyzed sediment 

 samples from the estuary and have pre- 

 pared a map depicting the major sediment 

 types. We have also studied the possible 

 relation between the concentration of organic 

 matter in the sedinnents and the abundance 

 of postlarval and juvenile shrimp. A primary 

 productivity study spanning 1 yr. (July 1964 

 through June 1965), has been completed; the 

 resulting data are now being analyzed. 



Hydrology 



Following the normal seasonal pattern, 

 monthly mean water temperature was highest 

 in August 1964 and lowest in January 1965. 

 During 1964-65 winter, waters in the estuary 

 were considerably warmer than in 1962-63 

 and 1963-64 winters. The lowest value reported 

 this winter was 10.0° C. (50.0° F.) inFebruary 

 and March 1965 as contrasted to a low of 

 3.5° C. (38.3° F.) in January 1964 and 2.7° C, 

 (36.9° F.) in January 1963. These exceptionally 

 warm conditions were apparently responsible 

 for more animals overwintering in the estuary 

 than have been observed in previous years. 



From Novennber 1964 to the end of this 

 fiscal year, salinity has been somewhat lower 

 in the Galveston estuary than during the same 

 period in 1962-63 and 1963-64, and since 

 April, the waters of upper Trinity Bay have 

 been essentially fresh. Biological data from 

 these waters should give us a good picture 

 of how extremely low salinities affect the 

 density and abundance of the animal popula- 

 tions. 



Throughout the bay system, the annual 

 seasonal highs for dissolved oxygen were ob- 

 served during winter and the lows during 

 sunnmer. Monthly mean dissolved oxygen at 

 the bottom (daytime) varied between 16.7 p. p.m. 

 (parts per million) near the mouth of the 

 San Jacinto River in October 1964 during a 

 plankton bloom and 4,9 p,p,m, in the Gulf 

 during July 1964. 



During the year, total nitrogen and phosphate 

 levels were consistently highest in the upper 

 estuary and declined Gulfward. Total nitrogen 

 varied from a monthly mean low in July 1964 



of 19.1 Ag.at./l. (microgram atoms per liter) 

 in the near- shore Gulf of Mexico to a high of 

 122.2 //g. at./ 1 , near the mouth of the San 

 Jacinto River in December. In July, the highest 

 level of total phosphate ( 1 3,36 »< g,at./l.) was 

 recorded near the mouth of the San Jacinto 

 River, and the lowest in the near-shore Gulf 

 of Mexico. The area near the nnouth of the 

 San Jacinto River receives large amounts of 

 industrial waste and domestic sewage. This 

 pollution load in the waters of the lower 

 estuary and the near-shore Gulf of Mexico is 

 diluted considerably through tidal exchange 

 with the open sea (fig. 27). 



Primary Productivity 



Although the analysis of the data is not yet 

 complete, preliminary findings indicate that 

 the greatest prinnary productivity' was con- 

 sistently in the surface water. As light penetra- 

 tion, because of high turbidity, diminished 

 rapidly with depth in the water column, pro- 

 ductivity diminished correspondingly. Highest 

 seasonal levels of production in the surface 

 waters were in the summer and winter, 

 whereas the seasonal lows occurred in the 

 fall and spring. 



Production throughout the year was highest 

 in the waters of upper Galveston Bay and 

 lowest in lower Galveston Bay. Discharge 

 of sewage and industrial and municipal waste 

 is heaviest in the upper Galveston Bay area, 

 and these waters consistently carry the 

 greatest nutrient load; thus suggesting that 

 nutrients supplied from these sources (pollut- 

 ants) account in part for the high productivity. 



Figure 27. --Map of the Galveston Bay estuarine system. 



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