A hydrogen sulfide odor was always 

 noticed whenever pumped samples from 

 the monimolimnion were collected. 



Total Kjedahl nitrogen (TKN) 

 values (Figure 7) were low over the 

 study period in both the mixolimnion 

 and chemocline ( x - 0.96 mg/ 1 and x = 

 0.79 mg/1, respectively). Higher 

 amounts of TKN were, however, always 

 present in the monimolimnion ( x = 

 3.13 mg/1). Ammonia-nitrogen concen- 

 trations in this layer (x= 0.69 mg/ 

 1) made up as much as 22 percent of 

 the TKN concentration. Data from the 

 monimolimnion also indicated that 

 there may have been an association 

 between rainfall and TKN levels in 

 this layer. 



Total organic carbon was sampled 

 in profile on 13 March 1979, and 

 values ranged from a low of 6 mg/1 in 

 the mixolimnion to a high of 13.5 mg/ 

 1 at the bottom of the chemocline 

 (3.0 m). 



Salinity profiles in the lake 

 indicated that a slight freshening of 

 the mixolimnion occurred during the 

 periods of January-March and July- 

 August. Salinities in the mixolim- 

 nion ranged from a low of 1.44°/oo in 

 September to a high of 7.3°/oo in 

 December. On the average, salinities 

 increased from 4°/oo to 40°/oo with 

 an increase of depth from 2 m to 3 m. 

 Stable hypersaline conditions pre- 

 vailed throughout the year in the 

 monimolimnion reaching an average of 

 43.0°/oo at the 6.1 m depth. 



Total phosphorus (Figure 7) 

 ranged from 0.001 mg/1 to 0.9 mg/1 

 with highest concentrations occurring 

 in the monimolimnion. There was no 

 seasonal trend evident for this 

 nutrient although there was a peak 

 in the October sample. Total phos- 

 phorus was also highest in the moni- 

 molimnion, with a trend toward higher 

 concentrations in the rainy season 

 (June - September) apparent. 



Ammonia-nitrogen averaged 0.545 

 mg/1 ( s.d. = 0.481 ) over an annual 

 cycle in the monimolimnion but was 

 found in very low concentrations (x = 

 0.028 mg/1; s.d. = 0.035) in the mix- 

 olimnion and chemocline. Nitrate- 

 nitrogen formed the second most abun- 

 dant form of nitrogen but like am- 

 monia, was found in greater concen- 

 trations with increasing depth (Mixo- 

 limnion x" = 0.001 mg/1: chemocline 

 x = 0.016 mg/1; and monimolimnion x = 

 0.018 mg/1). The same was true for 

 NO -N which increased from an average 

 of 0.001 mg/1 in the mixolimnion to 

 0.008 mg/1 in the monimolimnion. 



DISCUSSION 



In Lake Marco Shores the chemo- 

 cline is formed at the depth deter- 

 mined by the relative rates of sur- 

 face and ground water net inputs, the 

 density difference between the mixo- 

 limnion and the monimolimnion, and 

 the amount of energy available for 

 mixing. Water is not retained above 

 a critical elevation at Lake Marco 

 Shores, and it is discharged as sur- 

 face and ground water flow to the SR 

 951 Canal. Hence, the chemocline is 

 not driven further downward as the 

 rainy season progresses. 



The vertical physical structure 

 of Lake Marco Shores is similar to 

 that of other meromictic lakes. Heat 

 retention in the monimolimnion was 

 observed by Anderson (1958) for Hot 

 Lake, Washington, where the green 

 house effect allowed heating of the 

 monimolimnion to 50°C. He noted that 

 this heat was retained through the 

 winter. The relatively high specific 

 heat capacity of such dense, saline 

 lower layers has been found to be so 

 effective at retaining heat that the 

 construction of ponds of similar 

 structure has received much recent 

 theoretical and practical attention 

 for use in space heating in cold 

 climates (Rabl and Nielson 1975; 



227 



