Some features of distribution and population structure of Pseudocalanus acuspes (Giesbrecht, 1881) (copepoda, crustacea) in the southeastern part of the Baltic Sea
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Abstract
Based on the plankton samples collected in the south-eastern part of the Baltic Sea (SEB) in the expeditions of Shirshov Institute of Oceanology RAS occurrence, spatial distribution and size and age structure of the key species of the copepod Pseudocalanus acuspes Giesbrecht, 1881 were studied. The current state of the P. acuspes population in SEB has not been fully described yet. Samples were taken with the WP-2 plankton net (Ø=56 cm, mesh size 100 microns) in different seasonsby vertical net hauls layer by layer. The size and age structure of the P. acuspes population was evaluated using microscopes, and the length of specimens was measured from the beginning of the cephalothorax to the tip of the furcal rami. Quantitative indicators, especially abundance, were ten times higher in the early spring period than in summer and autumn. There was a direct positive correlation between the abundance of the species and salinity of the SEB bottom water layer. The highest abundance and biomass of crustaceans were noted on the slope of Gdansk deep, and in the coastal zone (up to the depth of 30 m) this species was practically not found with the exception of a few nauplii. Vertical distribution showed that in the spring and autumn maximum abundance and biomass of P. acuspes was in the water layer below the upper boundary of the halocline, whereas in summer it was in the cold intermediate layer. Its population in different years and seasons was represented by all life stages: juvenile (nauplii and copepodites of different ages) and adult individuals, mainly females. The proportion of nauplial stages was the largest in early spring (on average, almost 70%), which indicated reproduction in this season at low water temperature of 4-7 ° C, preferred by the arctic species P. acuspes. The size of the adult Baltic P. acuspes individuals ranged from 1,03 to 1,63 mmand was smaller than in other areas of the World Ocean.