Extended Abstract

The increase of eutrophication during the last four decades has been noticed in many estuarine and coastal areas, particularly those affected by riverine freshwater or domestic and industrial wastewater. One of these areas is also Kaštela Bay, semi-enclosed coastal bay in the Middle Adriatic, which receives a great amount of untreated wastewater, both domestic and industrial. The narrow coastal strip of the Bay is a highly developed area with various uses (habitable areas, industries, harbour, tourism, etc.). The towns of Split and Trogir, as well as several villages are located there with more than 350.000 inhabitants. The population of this area has increased sevenfold over the last 50 years. Because intensive urbanization and industrialization have not been followed up by the development of the necessary urban structure, large quantities of urban and industrial wastewater are released untreated into the Bay. The trend of eutrophication of the Kaštela Bay has been paralleled by an increase in urbanization and industrialization of this region.

Since the early fifties regular monthly observations of basic hydrographic, chemical and biological parameters have been carried out in the Bay. The measuring of primary production on a monthly basis started in 1962. Changes in the Bay, such as a gradual increase in primary production and changes in the regular seasonal cycle of phytoplankton fluctuations were first observed at the end of the seventies. The structure of the phytoplankton community has also changed and dinoflagellate species have been dominant rather than diatoms. After the analysis of the diatom/dinoflagellate relationship to primary production, it was established that the positive peaks of primary production coincided with the negative peaks of diatom/dinoflagellate ratio that is a higher presence of dinoflagellates. By the end of summer 1980, the occurrence of «red tide» in the Bay was observed for the first time. Since then «red-tide» blooms have occurred regularly every summer, always provoked by the dinoflagellate species Lyngulodinium polyedra Stein. Occasionally, the «red tide» bloom has caused mortalities to shellfish and demersal fish (1980, 1985, 1987, 1989 and 1990). All these changes were accompanied by a steady increase in percent oxygen saturation in the surface layers and reduction in bottom layers.

During the usual summer blooms a number of potentially toxic and suspect toxic phytoplankton species, such as Dinophysis fortii, D. acuminata, D. caudata, D. sacculus, D. acuta, Alexandrium minutum, Lyngulodinium polyedra, Prorocentrum micans, P. lima were observed. The results of a systematic survey of the phytoplankton composition in the Kaštela Bay, available since the early thirties, provided enough evidence to verify an autochthonous origin of most of the organisms found in the Bay; however their abundance and frequency of occurrence have apparently increased.

There was a continuous tendency of higher eutrophication in the Bay until the mid nineties when significant changes occurred once again. By the mid nineties there occurred a decrease in the primary production lasting up to the present. The values of the primary production, especially during summer time, are considerably lower than in the eighties when they were enormously high. Diatoms once again became the dominatant group of organisms in the phytoplankton community.

Regularly, each summer during eighties the Lyngulodinium polyedra caused high intensity red tide blooms (>107 cells dm3). However, very often in recent years it is not present in the phytoplankton community in the Bay or it appears in negligible numbers. The regular seasonal cycle of phytoplankton has been re-established, that is besides spring and autumn maximum, there appears the summer minimum of phytoplankton. That is to say by the middle seventies in the Kaštela Bay the usual summer minimum disappeared and instead, beyond the spring and autumn maximum appeared a third maximum, even more noticeable than those of spring and autumn. All these positive changes can be related to the closure of many industrial plants situated on the Bay coast during the last war in Croatia.
At the beginning of the nineties many plants stopped production, i.e., breweries, dairy plants, slaughter houses, plastic products plants, etc. For the same reason tourist activities have been suspended and consequently the level of wastewater was lower than in the same period previous to the nineties.

Since the same changes have been observed in the Adriatic open waters, the question rises spontaneously: is eutrophication in the Kaštela Bay caused exclusively by anthropogenic influence or is it possibly the consequence of both negative anthropogenic impact and global climate changes? That is a question very difficult to clarify since the period of less intensive wastewater output coincided with the period characterized by unusual climate conditions.