| Abstract |
Responses of water column plankton communities to nutrient enrichment through fish farming were examined. The study was carried out at distances from fish farm cages located in the northern Aegean and in southern Cyprus, during three different seasons. Both areas were exposed with strong hydrodynamic conditions and samples were taken upstream and downstream from fish cages. Current direction and velocity were taken into account and station selection was made using in situ drifters. Physical and chemical variables were examined along with analysis of samples for all plankton groups from bacteria to mesozooplankton. At the same time, a three dimensional biogeochemical model was modified to local conditions and applied in order to identify the fate of nutrients released from the fish farm and to evaluate if the changes that the model predicted describe the simulated system adequately. Scenarios were also applied using the model in order to evaluate impacts under future conditions and, after sample analysis was complete, the model food matrix was modified and the model was run again in an effort to better describe the simulated areas. Results from Cyprus showed that changes in the plankton community were mostly seasonal. The cage presence affected the bacteria and diatom abundance which were higher (bacteria) and lower (diatoms) near the farm cages in July. The model simulation for Cyprus was satisfactory as the comparison of field results with model outputs showed that the model was within the range of values measured in the field. The changes, as a response to the farm presence, predicted by the model were limited. When farm production was doubled, which was one of the scenarios implemented, the model predicted increased bacterial biomass close to the farm cages in Cyprus. In Lesvos, significant changes were measured in the field in April and July. Despite differences in current velocity and direction, most changes were measured at the same distance downstream from the farm cages, at 100 m. In July results showed increased bacteria, nanoflagellate and mesozooplankton abundance, in conjunction with a decrease in diatom abundance and an increase in the abundance of large dinoflagellate and ciliate cells downstream from the cages. In April the sampling took place during a diatom bloom in the wider area, bacteria abundance was significantly lower 100 m downstream and diatom abundance peaked at the same distance. Dinoflagellate abundance increased downstream from the cages and so did the total microplankton abundance. Predictions of the biogeochemical model in Lesvos were within the range
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observed in the field; however the simulation showed differences only for dissolved nutrients downstream from the cages, even though the resolution was quite high. The sensitivity run of the model, where transport was set to zero, showed the effects of the fish farm clearly although the model predicted an increase in microzooplankton and mesozooplanton biomass, but no increase in microphytoplankton. The changes applied to the food matrix greatly improved model performance and better simulation of the farm environment was achieved. In summary, the pattern of change was similar albeit of different magnitude between areas and seasons. Current velocity and direction were very important for the detection of changes downstream from the farm and plankton response to nutrient enrichment was maximized at intermediate distances from the farm. The use of the biogeochemical model highlighted the groups most likely to be affected by the fish farm effluents and simulated the system satisfactorily. Apart from the importance of the results regarding the interactions of fish farms with the environment, results can also assist evaluation of community resonse to pertrubations in a dynamic environment.
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Απόκριση των πλαγκτονικών κοινοτήτων στην προσθήκη θρεπτικών : εποχιακή μελέτη στο πεδίο και εφαρμογή μαθηματικών μοντέλων / Τατιάνα Μ. Τσαγκαράκη; τριμλής συμβουλευτική επιτροπή, Καρακάσης Ιωάννης, Πήττα Παρασκευή, Λύκα Κωνσταντία.
