CARR and GIESEL: IMPACT OF THERMAL EFFLUENT 



the NGS but on each rising tide this effluent is 

 backed up by tidal action and much of it is retained 

 within the confines of this creek. At this time the 

 thermal effects extend to the uppermost reaches of 

 the creek. A second creek, Nichols Creek (see Fig- 

 ure 1), receives an injection of thermal effluent 

 during the initial stage of each rising tide. Nichols 

 Creek converges with San Carlos Creek just prior 

 to the juncture of both with the river. A major 

 branch of a third creek, Browns Creek, is situated 

 approximately 1 mile east of San Carlos Creek and 

 is completely beyond the zone of thermal influence 

 produced by the power plant. Browns Creek 

 served as a "control" creek that remained at am- 

 bient temperature. 



The three creeks are physically quite similar in 

 terms of their size, depth, and contiguous marsh- 

 land and upland areas. The substrate in each con- 

 sists primarily of soft black mud rich in organic 

 material. Scattered bars of a firmer sand- mud 

 composition are present. Each creek is lined on 

 either side with marsh grasses, primarily 

 Spartina alterniflora and J uncus roemarianus. 

 No submerged sea grass or other attached mac- 

 rophytes are present in the creek beds themselves. 

 The major variables affecting differentially the 

 habitats of the three creeks are the thermal 

 effluent, the chemical agents used in the cleaning 

 of condenser tubes, and the clearing and altera- 

 tion of the landscape necessary for the construc- 

 tion of the power plant. 



Sampling Stations 



Three sampling stations were established in 

 each of the three creeks (see Figure 1). One of the 

 stations in each creek was situated near the creek 

 mouth whereas the other two were situated at 

 appropriate distances upstream. The station sites 

 in each creek were selected such that two stations 

 were situated at the sites of juncture of small ad- 

 joining creeklets and the third at the edge of a bar. 

 During the hot season of 1973, samples were taken 

 from all nine stations during June and July and 

 from seven of the stations in September. 



Collecting Methods 



Fishes were collected at all stations with a bag 

 seine (50 x 6 ft) constructed of 3/8-inch stretch 

 mesh netting. The dimensions of the area seined 

 in each sample were measured with a steel tape at 

 the time the sample was taken. Areas sampled at 



the stations varied somewhat according to the 

 particular configuration of each seine haul; these 

 areas ranged from 102 to 403 m^ per station. Dur- 

 ing each sampling period, all seine hauls were 

 made during the day on consecutive days within 

 1.5 h of low tide. Specimens were preserved im- 

 mediately in 20% Formalin^-seawater and later 

 washed with tap water and stored in 75% iso- 

 propyl alcohol. Determinations of biomass are 

 based on weights of preserved specimens. Inver- 

 tebrates obtained in the samples were also re- 

 tained for future analysis. 



A Beckman electrodeless induction salinometer 

 was used to obtain measurements of temperature 

 and salinity. 



Quantitative core samples and plankton sam- 

 ples were also taken but their analyses are incom- 

 plete and they are not reported here. 



Presentation of Data 



To minimize the number of tables and figures 

 necessary for the presentation of data, analyses of 

 the samples taken from the three stations in each 

 creek have been pooled for each monthly collec- 

 tion. Although this method prohibits comparisons 

 of variations between individual stations within a 

 particular creek, this procedure provides a more 

 direct means of analyzing and comparing the 

 overall population structure within each creek. 



RESULTS 



Table 1 presents temperature and salinity 

 measurements taken from San Carlos Creek, 

 Nichols Creek, and Browns Creek during the 

 study period. The temperature data from Browns 

 Creek, which is beyond the range of thermal 

 influence produced by the power station, can be 

 used as a measure of the daytime ambient tem- 

 perature regime for a creek in this area. During 

 the June sampling period, the average recorded 

 temperature of water discharged by Units 1 and 2 

 of the NGS into San Carlos Creek was from 5.6° to 

 7.7°C above the average temperatures recorded at 

 the three stations in Browns Creek. During July, 

 San Carlos Creek received water from the power 

 station that averaged 8.0° to 8.9°C higher than the 

 averages recorded in Browns Creek. During Sep- 

 tember, this differential increased to 9.1° to 

 10.8°C. The highest temperature that we recorded 



^Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



69 



