Απόκριση των πλαγκτονικών κοινοτήτων στην προσθήκη θρεπτικών: εποχιακή μελέτη στο πεδίο και εφαρμογή μαθηματικών μοντέλων

Abstract

Responses of water column plankton communities to nutrient enrichment through fishfarming were examined. The study was carried out at distances from fish farm cageslocated in the northern Aegean and in southern Cyprus, during three different seasons.Both areas were exposed with strong hydrodynamic conditions and samples weretaken upstream and downstream from fish cages. Current direction and velocity weretaken into account and station selection was made using in situ drifters. Physical andchemical variables were examined along with analysis of samples for all planktongroups from bacteria to mesozooplankton. At the same time, a three dimensionalbiogeochemical model was modified to local conditions and applied in order toidentify the fate of nutrients released from the fish farm and to evaluate if the changesthat the model predicted describe the simulated system adequately. Scenarios werealso 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 themodel was run again in an effort to better describe the simulated areas. Results fromCyprus showed that changes in the plankton community were mostly seasonal. Thecage presence affected the bacteria and diatom abundance which were higher(bacteria) and lower (diatoms) near the farm cages in July. The model simulation forCyprus was satisfactory as the comparison of field results with model outputs showedthat the model was within the range of values measured in the field. The changes, as aresponse to the farm presence, predicted by the model were limited. When farmproduction was doubled, which was one of the scenarios implemented, the modelpredicted increased bacterial biomass close to the farm cages in Cyprus. In Lesvos,significant changes were measured in the field in April and July. Despite differencesin current velocity and direction, most changes were measured at the same distancedownstream from the farm cages, at 100 m. In July results showed increased bacteria,nanoflagellate and mesozooplankton abundance, in conjunction with a decrease indiatom abundance and an increase in the abundance of large dinoflagellate and ciliatecells downstream from the cages. In April the sampling took place during a diatombloom in the wider area, bacteria abundance was significantly lower 100 mdownstream and diatom abundance peaked at the same distance. Dinoflagellateabundance increased downstream from the cages and so did the total microplanktonabundance. Predictions of the biogeochemical model in Lesvos were within the range observed in the field; however the simulation showed differences only for dissolvednutrients downstream from the cages, even though the resolution was quite high. Thesensitivity run of the model, where transport was set to zero, showed the effects of thefish farm clearly although the model predicted an increase in microzooplankton andmesozooplanton biomass, but no increase in microphytoplankton. The changesapplied to the food matrix greatly improved model performance and better simulationof the farm environment was achieved. In summary, the pattern of change was similaralbeit of different magnitude between areas and seasons. Current velocity anddirection were very important for the detection of changes downstream from the farmand plankton response to nutrient enrichment was maximized at intermediatedistances from the farm. The use of the biogeochemical model highlighted the groupsmost likely to be affected by the fish farm effluents and simulated the systemsatisfactorily. Apart from the importance of the results regarding the interactions offish farms with the environment, results can also assist evaluation of communityresonse to pertrubations in a dynamic environment.