GAS SUPERSATURATION IN SURFACE WATERS OF AQUACULTURE PONDS

During the 1988 and 1989 growing seasons, dissolved gas pressures were measured in 0.04 ha to 0.06 ha earthen ponds (n = 17) used to produce channel catfish or bluegill sunfish, both with and without supplement al aertion. Surface waters in early morning hours (0630 to 0830 h) wer e typically saturated with nitrogen (N-2) and argon (Ar), undersaturat ed with dissolved oxygen (DO), and supersaturated with carbon dioxide. Total gas pressures (Delta P) were low, averaging -40 mmHg (range -12 7 to 62 mmHg). During afternoon hours (1300 to 1500 h) Delta P increas ed to a mean of 111 mmHg (range -46 to 334 mmHg); 34% of afternoon Del ta P values were above that known to cause mortality in channel catfis h during continuous exposure bioassays (115 mmHg), yet no excessive mo rtality was observed. High afternoon Delta P values resulted from DO s upersaturation caused by phytoplankton photosynthesis and, to a lesser extent, N-2 and Ar supersaturation. The N-2 and Ar supersaturation ap parently resulted from cooling and saturation of surface waters with a ir during evening hours, and the subsequent increase in water temperat ure during daylight hours (daily increase (X) over bar = 3.4 degrees C ; range 1.1 to 7.9 degrees C) without sufficient gas release.