Seasonal And Year-To-Year Variability Of Ichthyoplankton Assemblage Diversity Indices In The Coastal Zone Of The Southeastern Baltic Under The Influence Of Spatial-Temporal Environmental Dynamics

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The seasonal and year-to-year changes in ichthyoplankton assemblage abundance and species composition are demonstrated based on investigations carried out in the coastal zone of the SE Baltic in 1992-2006. It was shown that depth gradient, bottom substratum variety and seasonal changeability of environment within the shallow-water zone provided higher ichthyoplankton diversity as compared with the open sea. The larvae of 15 fish species, the reproduction of which takes place by spawning of bottom eggs or carrying them in brood pouch, were the principal ichthyoplankton component. Pelagic eggs and fish larvae, from main spawning taking place in deepwater basins also occurred periodically. On average, the peak species richness and abundance were observed in July. The Shannon’s indices of general diversity, Simpson’s domination indices and Pielou’s equitability indices were calculated to characterize the ichthyoplankton assemblage. It was shown that their values underwent significant inter-annual variability depending on the abundance of dominant species Pomatoschistus minutus. Three types of hydrographic situations that determined patterns of larval P. minutus spatial distribution and abundance were revealed: 1) prolonged upwelling and cold water penetration from intermediate layer to 10 - 15m depths, 2) alternation of upwelling and down-welling events, 3) prolonged development of down-welling at lack of cold water intrusions. Accordingly, phenomena observed were as: 1) low abundance, minimal species richness, and average values of diversity and domination indices, 2) average levels of abundance and species richness, high diversity index and low domination index, 3) outburst of P. minutus abundance, minimal diversity index and high domination index.