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

The influence of thermal discharges on the phytoplankton community from a c oastal zone of the Gulf of Mexico was evaluated through their structure and photosynthetic behaviour focusing on responses to changes in light and tem perature. Biological and physicochemical parameters were measured over a pe riod of two years in an area with permanent hot water discharges from a the rmoelectric 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 r educed in the discharge area water column, due to turbulence. One hundred a nd 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 hig hest values of the phaeophytin:chlorophyll ratio found at the hottest stati on. The community structure did not show significative differences among sa mpling stations with respect to temperature variations. However, in the alg al assemblages influenced by thermal discharges, it was possible to observe alterations in the photosynthesis behaviour. Phytoplankton response to sho rt term photosynthesis experiments was segregated according to composition and origin of microalgal assemblages. Samples with larger heterogeneous com position had more consistent oxygen production responses. Algal communities exposed to hot effluent showed different degrees of photosynthesis rate re duction, higher light requirements (> 500 mu E m(2) s(-1)) and lower temper ature (25 degrees C) to achieve Pmax than algae sampled in sites without su ch exposure.