ECOLOGICAL EFFECTS OF COOLING WATER DISCHARGE ON HYDROLITTORAL EPILITHIC DIATOM COMMUNITIES IN THE NORTHERN BALTIC SEA

Abstract

The discharge of brackish cooling water from a nuclear power plant on the Swedish Baltic coast has major effects on the species composition, biomass and seasonal variation of epilithic diatom communities in the hydrolittoral zone. Greatest effects occur in winter and early spring, when enormous diatom blooms are caused by the higher water temperatures (6–10°C above natural level) and by the absence of an ice cover, which implies high light availability early in the season. A low light regime in mid-winter inhibits growth for a short period only. Canonical correspondence analysis (CCA) was used to display the relationship of community composition to sampling site and date. CCA reproduced the seasonal cycle, and placed each site in its correct relative position in terms of annual-mean temperature anomaly and flow rate. Individual species were placed according to their ecological optima with respect to the seasonal cycles of light, temperature and water level, and the site-dependent anomalies in temperature and flow rate. Seasonal variation exceeded variation between sites, and within-site variation was smaller than variation between sites. Seasonal variation can be attributed to the seasonal cycles of light, temperature and water level; variation between sites to temperature and flow rate anomalies, and the presence or absence of winter ice. Parallel studies (in press) of macroalgae, including macroscopic colonies of microalgae (blue-greens and diatoms), and macrofauna show weaker seasonal cycles, but all groups show some seasonal pattern and the patterns of variation between sites are similar for all three groups. Shannon-Weaver diversity is generally lower in the cold season than in the warm season; the cooling water discharge has no effect on diversity. The seasonal and spatial occurrences of the species can be related to growth forms and life-form strategies. Melosira spp. and Nitzschia filiformis are especially favoured by the cooling water discharge in winter. They respond opportunistically by growing fast in large upright colonies and do not need macroalgae as substrata to reach into the water column. Different morphologies and life histories affect the competititive balance among eurythermic species in different environments.