Nonlinear dynamics of the Black Sea ecosystem and its response to anthropogenic and climate variations
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The main objective of this research is to i) provide a quantitative understanding of the changes in the Black Sea ecosystem between 1960 – 1999, ii) to identify its food web dynamics including the infamous anchovy – Mnemiopsis shift in 1989, and iii) utilizing this understanding to explore future progressions of the Black Sea ecosystem under predicted future physical and biogeochemical changes. For this purpose, three different but complementary approaches were used all of which were detailed under three distinctive chapters in this thesis manuscript; i) a steady-state modelling approach utilizing mass-balance models of the Black Sea to quantitatively analyse and differentiate its ecosystem structure and functioning under four different regimes using ecological indicators, ii) time-dynamic modelling approach to explore the nonlinear food web dynamics over the course of its history between 1960 - 1999 to identify the shifts that led to transition between its four different periods explored by mass-balance models, and iii) coupled physical – biogeochemical and higher trophic level modelling approach to predict future changes in the Black Sea ecosystem under predicted climatological and physical conditions and explore management strategy options that is going to help the ecosystem recover to achieve its good environmental status (GES). All of the developed models were evaluated using historical time series data and results obtained from classical modelling approaches such as Virtual Population Analysis (VPA), which were carried out using data obtained from field sampling. The mass balance modelling results showed how the Black Sea ecosystem structure started to change after the 1960s as a result of a series of trophic transformations, which had occurred in the food web. These transformations were initiated by two main anthropogenic factors; i.e. fishing down the food web – gradually harvesting fish species in the ecosystem to the extent of extinction starting from higher-trophic-level species down to lower-trophic-level species - and nutrient enrichment, that led to increasing proliferation of opportunistic organisms in the ecosystem by removing predatory and competitive controls in the food web and in turn caused the transfer of large quantities of energy to these trophic dead-end opportunistic groups of organisms; i.e. jellyfish and heterotrophic dinoflagellates. Concurrently, an alternative short pathway for energy transfers was formed which converted significant amounts of system production back to detritus rather than transferred up to produce fish biomass by decreasing the transfer efficiency of energy flows from the primary producers to the higher trophic levels from 9% in the 1960s to 3% in the period from 1980-1987. The time-dynamic model results delineated that a break down in the ecosystem’s balance (homeostasis) sensu Odum (1985) in time happened with eutrophication, overfishing and establishment of trophic dead-end organisms. The sensitivity tests showed that interspecies competition and overfishing were the main drivers of changes within the ecosystem which were exacerbated by overpopulation of some r-selected organisms; i.e. Noctiluca and jellyfish species, in the food web and moderated by the changes in the primary production in the ecosystem. Incessant fisheries overexploitation since the beginning of 1980s caused the anchovy stock decline continuously and increasing resource competition between jellyfish and small pelagic fish due to their elevated proliferation enhanced by eutrophic conditions brought about the anchovy stock collapse in 1989. The predation exerted by Mnemiopsis on small pelagic fish eggs was found to be of secondary importance compared to the resource competition. However, all these stressors acted concomitantly in eroding the structure and functioning of the ecosystem by manipulating the food-web to reorganise itself by means of introduced and selectively removed organisms so that the average path lengths of recycled flows was shortened and the transfer efficiency of energy to higher trophic levels was further reduced to deprive the ecosystem of commercially important fish assemblages. The coupled model simulations showed that a decrease in the commercial fish stocks was predicted due to the predicted decrease in the basin-wide primary production that is caused by temperature-induced limitations on the phytoplankton growth. If current fishing intensity levels are kept status quo, some economically important small pelagic fish species of the Black Sea would also likely to disappear from the catches let alone the recovery of more valuable piscivorous fish stocks. In addition, maintaining the current exploitation levels of the fish stocks in the Black Sea was predicted to cause a further decrease in the proportion of large fish by weight in the whole fish community in the future. Although the predicted decrease in primary production in the future scenarios was found to confine the development of the fish stocks, fisheries were found to be the main driver in determining the future state of the stocks under changing environmental conditions. For management purposes, along with decreasing fishing mortality of the target stocks, monitoring and management of other fish species that are tightly coupled with the target species as a measure was found to be the most effective way of fisheries management and sustainable utilisation of fish stocks.