Light energy as a cause of structural and functional variations in brackish water littoral mesocosms used in ecotoxicological research

The significance of two vital ecosystem regulating factors, i.e. light and temperature, was studied in littoral brackish water mesocosms during a four month experiment. Conspicuous light energy-related structural and functional responses were observed in the mesocosms. Functionally, the horizontal transport of organic carbon increased with lower temperature and light conditions. Systems with lower temperature and less light adapted by increasing phytoplanktic production instead of periphytic growth. All systems seemed to be nitrogen limited. Sedimentation was lower in colder than in warmer systems. Colder systems contained less organic carbon in the sediment but only insignificantly less carbon was bound in the macroscopic organisms in these systems as compared with warmer systems. This indicates adaptatory mechanisms on the structural level, as observed by invertebrate and fish studies. Small sized Jaera spp. was favoured by less efficient system carbon binding capacity. Typical littoral species such as Gammarus spp. and Theodoxus fluviatilis were favoured by efficient carbon binding environments and higher temperature. Lower temperature favoured mollusc species such as Macoma baltica and Mytilus edulis. Fish fry were favoured by higher temperature at the individual level but population mortality was negatively affected. The results obtained show that mesocosms are sensitive systems that rapidly respond to external perturbations, including pollution. However, in order to separate responses caused by natural variability from those caused by pollution, it is important that both structure and function are measured.