Abstract. The ecosystem model 3-D CEMBS connected with the population model, described in this paper, was used to determine the temporal distributions of T. longicornis in the Gdańsk Basin (the southern Baltic Sea) divided into the coastal zone P2 (the Gulf of Gdańsk) and the open sea P1 (Gdańsk Deep). The population model for T. longicornis consists of twelve equations for twelve states of variables, six for the mass W_i and six for the abundance Z_i, i.e. two states of variables W_i and Z_i, for each of the six model stages of the development; the stages were grouped as follows: eggs – Egg, stages not taking food – NI–NII, subsequent stages of nauplii – NIII–NVI, two copepodid stages – CI–CIII and CIV–CV and the last stage of adult organisms – CVI. Seasonal dynamics of T. longicornis is described by average changes in the total biomass as a sum of biomass of the examined ontogenesis stages, which are the sum of the products of the mass W_i and the abundance Z_i of individual organisms at a given stage.

The empirical verification of the population model based on in situ data obtained from the analysis of biological material collected in 2010–2011 in the region of Gdańsk Deep (P1) and in the western part of Gdańsk Bay (P2), and in 2006–2007 – only in Gdańsk Bay (P2). The highest values of the modelled T. longicornis biomass occurred in the period of high temperatures, i.e. in summer, in June 2010 and July 2011 in the Bay of Gdańsk – at station P2, and between late June and early July, and for almost the whole summer in Gdańsk Deep – at station P1, and amounted to respectively ca. 5200 mg_w.w. m^–2 and 6300 mg_w.w. m^–2 at station P2 and 24 500 mg_w.w. m^–2 and 27 800 mg_w.w. m^–2 at station P1. In 2006 and 2007 at station P2 the highest numerical values were recorded between late July and early August, exactly at the same time as environmental data, and amounted to 4300 mg_w.w. m^–2 and 5800 mg_w.w. m^–2, respectively. The results determined from the model are 0.25–2 times higher compared to in situ data. The most similar values were obtained for 2007.