Table 2.— Comparisons of annual mean bottom water 

 temperatures between habitats within each bay area 

 in the Galveston Bay system, 1963-66. 



temperatures were usually highest in the per- 

 ipheral habitat and similar in open-water and 

 channel habitats. The differences in average 

 temperatures between habitats were, however, 

 usually less than 1° C. Although the differences 

 were small, they were statistically significant in 

 6 of 20 comparisons. The differences were 

 highly significant in mid-Galveston Bay in 3 of 

 the 4 years; the temperatures in the peripheral 

 habitat were distinctly higher than in the other 

 two habitats. 



We had expected larger temperature differ- 

 ences between the shallower open-water habi- 

 tat and the deeper ship-channel habitat than 

 were observed. The lack of large differences 

 could not be attributed to the method used to 

 combine our data because similar results were 

 apparent throughout each year when the tem- 

 peratures were plotted by date. It is likely that 

 large ocean-going vessels passing through the 

 channel caused substantial mixing of the sur- 

 face and bottom water, thus causing water 

 temperatures in the channel to remain similar 

 to those of the adjacent open water. 



Comparison Between Years 



Differences in temperature between years in 

 the whole system were compared by combining 

 habitat area data and plotting the average and 

 range by date and year (Figure 3). Average 

 temperature varied as much as 7° C between 

 years in the winter months, and only about 3° 

 C in the summer months. Year-to-year varia- 

 tions occurred in the seasonal cycles of water 

 temperature. For example, if an arbitrary value 

 of 20° C is selected, temperatures averaged 

 above 20° C for about 7.5 months in 1963 and 

 only about 5.5 months in 1966. 



Figure 3. Average water temperature by date and year 

 and the monthly mean, standard deviation, and range 

 of temperatures in the Galveston Bay system for all 

 years combined, 1963-66. 



Four- Year Average 



All temperature observations taken during 

 the 4-year period were averaged by month to 

 show the average seasonal trend (Figure 3). 

 Bottom water temperatures were lowest in 

 January with a mean of 11° C and a range of 

 18° C. Average monthly temperatures 

 increased from January to July with the most 

 rapid increase- from March to May. Mean tem- 

 perature in July was 30° C with a monthly 

 temperature range of only 7° C. Values 

 decreased rapidly from September to October, 

 were about the same in October and 

 November, and then decreased sharply again 



