On the influence of hot-water discharges on phytoplankton communities from a coastal zone of the Gulf of Mexico

The influence of thermal discharges on the phytoplankton community from a coastal zone of the Gulf of Mexico was evaluated through their structure and photosynthetic behaviour focusing on responses to changes in light and temperature. Biological and physicochemical parameters were measured over a period of two years in an area with permanent hot water discharges from a thermoelectric plant. The temperature in the sampling area ranged from 23.5 to 36 degrees C with differences between the coldest and the hottest station from 5.3 to 9.2 degrees C. Photosynthetically active radiation (PAR) were reduced in the discharge area water column, due to turbulence. One hundred and one different taxa were identified with a strong predominance of Diatoms. The chlorophyll a concentration ranged from 0.3 to 6.1 mu g L-1, with highest values of the phaeophytin:chlorophyll ratio found at the hottest station. The community structure did not show significative differences among sampling stations with respect to temperature variations. However, in the algal assemblages influenced by thermal discharges, it was possible to observe alterations in the photosynthesis behaviour. Phytoplankton response to short term photosynthesis experiments was segregated according to composition and origin of microalgal assemblages. Samples with larger heterogeneous composition had more consistent oxygen production responses. Algal communities exposed to hot effluent showed different degrees of photosynthesis rate reduction, higher light requirements (> 500 mu E m(2) s(-1)) and lower temperature (25 degrees C) to achieve Pmax than algae sampled in sites without such exposure.