engleski

Recent microphytoplankton changes in the northern Adriatic

The microphytoplankton community of the northern Adriatic Sea is largely under the influence of the Po River nutrient contributions, particularly in spring (May and June), as a consequence of melting snow in the Alps, and in the autumn, frequently in October. In the winter the Po influence is restricted to the western part of the region, due to prevailing of currents towards southeast, along the eastern coast. Inflow of fresh nutrients to the northern aquatorium, which is generally oligotrophic, together with the usual factors influencing phytoplankton growth, influence the distribution of the microphytoplankton. Microphytoplankton crop and diversity were investigated in a three and a half year period, at three stations, chosen to represent the western, central and eastern part of the northern Adriatic. Samples were obtained from three depths (surface, 10m and 2m above the bottom), preserved with Lugol's solution and cell counts made using the Utermöhl settling technique. Horizontal, vertical and seasonal distributions were analyzed. Cell counts decreased from the eastern towards the western coast mostly in the interval from 2, 5ˇ106 cells L-1 to 0, 75ˇ106 cells L-1, but, with occasionally higher values measured at the western station, up to 1, 5ˇ107 cells L-1. Despite the shallowness of the region, vertical distribution of the microphytoplankton abundance is quite pronounced, ranging up to one order of magnitude in stratified conditions. Usual seasonal distribution, spring and autumn bloom, was somewhat changed in the analyzed period and of different intensities each of the analyzed years. Microphytoplankton in the northern Adriatic is mostly consisted of diatoms, among them the following being the most dominant: Asterionellopsis glacialis, Cerataulina pelagica and Skeletonema costatum, as well as species of the genus Nitzschia and Chaetoceros. S. costatum and A. glacialis are winter species, the first was dominant in the winters of 1999, 1999/2000 and 2000/2001, while the other one was dominant only in the winter 2001/2. C. pelagica is an early summer species, which was dominant in the samples of June and July of 2000, 2001 and 2002. Nitzschia and Chaetoceros species are constantly present during the entire year. However, in the 2001, Chaetoceros species were considerably less present than in the other years. Nitzschia species were more present in the lower layers of the aquatorium. In the eighties, significant elevation of the abundance and frequency of the appearance of Nitzschia delicatissima and N. longissima f. tenuirostris, as well as a decrease of N. seriata, was noticed ; e.g. N. seriata was present in high contribution only in the winter of 2001. Increase of abundance and frequency of Skeletonema costatum, a eurihaline and euriterm species, characteristic for eutrophic coastal zone, was also noticed in the last two decades. Blooms of Asterionelopsis glacialis and Cerataulina pelagica, which occurred in the analyzed period, were noticed only recently, they were in the period of late eighties and early nineties present only in small abundances. Changes in abundances and frequencies of the microphytoplankton community are most likely influenced by climatic changes (global warming and changes in the Po flow regime), which determine environmental conditions.

microphytoplankton; northern Adriatic

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