temperatures increased 4.4° C. per month in 

 the spring and decreased 4.1° C. per month in 

 the fall, the range between the warm and cool 

 months was 21.9° C. 



SALINITY 



Salinities in Clear Lake ranged from 0.1 to 

 23.7 p.p.t. during the study (table 3). I com- 

 pared the 1958 data by using analysis of vari- 

 ance in the same manner as for temperature. 

 Differences between stations were significant 

 (F = 5.68, F.05 for 7 and 376 d.f. =2.04). 

 Duncan's multiple range test (Steel and Torrie, 

 1960) was used to determine the salinity zones 

 shown in figure 4. 



Average monthly salinities were nearly 

 always lower in zone IV than in zone I (fig. 5), 

 and were usually intermediate in zones II and 

 III. Because collections were made only in 

 zones II and III for the entire 9 years, data 

 from these two zones were used to evaluate 

 fluctuations over the 9-year period. 



Salinity trends for the 9 years are shown in 

 figure 6. Yearly averages were relatively low 

 from 1958 through 1961 (grand average, about 

 8 p.p.t.), and generally high in 1962 through 

 1965 (grand average, about 13 p.p.t.). Average 

 salinity in the lake dropped below 10 p.p.t. in 

 1966. The annual average salinity was highest 

 (15.9 p.p.t.) in 1963 and lowest (4.8 p.p.t.) in 

 1961. 



Average monthly stream flow in Clear Creek 

 also is compared with salinity of Clear Lake 

 in figure 6. In general, stream flow was related 

 inversely to salinity during the period of com- 

 parison. The correlation coefficient (r) was 

 -0.57. 



The correlation between monthly average 

 salinity and local precipitation is not as ap- 



KILOMETERS 



STATUTE MILES 



ZONE is: 



Figure 4. — Clear Lake, Tex. divided into zones 

 of different salinities. 



parent (r = -0.41) as when the data are grouped 

 by 4-month intervals. The use of 4-month 

 average salinities and total rainfall during 

 the same periods reduces variation in the 

 data, as indicated by a correlation coefficient 

 of -0.57. When these values were used, trends 

 in the data became apparent (fig. 7). For 

 example, in 1959-61, when rainfall was 

 considerably higher than the 9-year average, 

 salinities m the lake were low; in 1962-65, 

 when rainfall was below the 9-year average, 

 salinities in the lake were correspondingly 

 high. Variations in rainfall from year to 

 year coincided with fluctuations in stream 

 flow and these, in turn, were reflected by 

 variations in the salinity of Clear Lake. 



DISSOLVED ORGANIC NITROGEN 



Dissolved organic nitrogen ranged from 23.5 

 to 171.5 fig.ai./l. (table 4). Monthly averages 

 were computed by combining data from all 

 stations; the lowest and highest average values 

 (33.8 and 171.5 jug.at./l.) were recorded in 

 1964. Monthly mean values in table 4showthat 

 nitrogen fluctuated erratically and had no ob- 

 vious seasonal pattern. Neither river discharge 

 (r = 0.08) nor rainfall (r = -0.05) was corre- 

 lated with nitrogen. 



Dissolved organic nitrogen averaged 66.4 

 jUg.at./l. over the 3 years. 



TOTAL PHOSPHORUS 



The concentration of total phosphorus ranged 

 from 1.7 to 26.3 ^g.at./l. (table 5). Monthly 

 mean values fluctuated between 4.7 and 12.1 

 /ig.at./l. from 1964 through 1965 and reached 

 a minimum of 3.7 /ig.at./l. in March 1966. A 

 gradual increase from April through June 1966 

 was followed by a marked increase in July. 

 Values remained high for the rest of the year, 

 and the maximum mean was 18.8 ^g. at. /l. in 

 November. Stream flow (r = - 0.24) and rainfall 

 (r =-0,07) were not correlated with phosphorus. 



The high phosphorus recorded in Clear Lake 

 in the summer and fall of 1966 probably re- 

 sulted from intrusion of water from upper 

 Galveston Bay, where the phosphorus varied 

 from 10.9 to 25.0 ug. at. /I. at that time (Pullen, 

 unpublished data). Dye studies made in a model 

 of the Galveston Bay system show a marked 

 interchange of water between upper Galveston 

 Bay and Clear Lake (U.S. Corps of Engineers, 

 personal communication). 



Total phosphorus in Clear Lake averaged 8.2 

 fig. at. /l. for the 3 years. 



OXYGEN 



Dissolved oxygen ranged from 3.8 to 19.3 

 p. p.m. during this study (table 6). Meanmonthly 

 values for 1965 and 1966 indicate a seasonal 



