The Seasonal And Inter-Annual Variability Of Ichthyoplankton Biodiversity In The Gdansk Deep Of The Baltic Sea During Last Decades

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The assemblage of pelagophilous fishes, which reproduce in the Baltic Sea deeps, has formed under the influence of environmental conditions that extremely limited its species richness. The composition of ichthyoplankton from the Gdansk Deep usually includes eggs of 4 fish species only (cod, sprat, flounder, rockling), co-occurrence of which is observed in spring season. Extremeness of reproduction conditions (low salinity and recurring oxygen deficiency) is one of the main reasons of species composition poorness, which becomes even more pronounced in some seasons with a decrease in frequency and intensity of the North Sea water inflows. Based on AtlantNIRO investigations for 1992 – 2006, and the analysis of literature data published in previous decades, significant seasonable and decade-to-decade changeability of Shannon’s index of general diversity and Simpson’s domination index were revealed under the invariable species richness. Over the period greater than 50 years (beginning from late 40s), the index of general diversity exhibited a stable downward tendency, whereas the domination index permanently increased. It was structural reorganization of the pelagophilous fish community, which reproduced in the deeps, that resulted in transition from the polydominant to the monodominant ichthyoplankton assemblage. The influence of abiotic environment on spatial variability of Shannon’s index is also shown. The mechanisms that compensate low species saturation, and augmenting other kinds of the ichthyoplankton assemblage diversity are considered. At present the assemblage structural elements remained as well as a possibility of recurrence to a former state in case of adequate climatic changes.