## 4 Discussion
Observed progressing delaying of the maximum of secondary production in relation to spring bloom of phytoplankton may lead to serious consequences for the whole organic production and higher trophic levels in the ecosystem. Zooplankton mortality estimates are still not a well-developed parameter in determining population dynamics. Therefore, selecting a proper methodology to describe this phenomenon can be challenging, and the main uncertainty results from inherent difficulties in measuring this process in the natural environment [48]. It is widely accepted that the daily copep mortality decreases with consecutive developmental stages or with an increase in size [49; 50]. Based on these assumptions we wanted to describe mortality rates of main copep taxa from the Gulf of Gdarsk at particular seasons of the year. The results obtained for _Acartia_ spp. and _T. longicornis_, however, show differences in mortality for particular stages from that described by the above-mentioned authors. The highest mortality rate for _Acartia_ spp. was observed for the oldest copepites (CV) with maximum values noted in spring: \(\sim\)0.3 day\({}^{-1}\). For _T. longicornis_, high rates were observed for CIII, CIV, and CV, within a range of 0.15 day\({}^{-1}\) to 0.28 day\({}^{-1}\). High variability in mortality estimates between copep species and developmental stages in both spatial and seasonal distribution were also observed by other authors [15; 51; 52]. In our research we observed cyclical changes in mortality, with the peak falling in the spring and summer season. [PERSON] et al. [53], on the other hand, recorded the highest mortality rates in summer and autumn, with the lowest--as in our research--in winter. Differences in seasonality of mortality in different regions may result from differences in the main cause of mortality. Mortality of copepods can be caused by predatory (consumptive) or physicochemical and biological factors as factors causing nonconsumptive mortality. Consumptive mortality may be associated with the abundance of predators [54], and is described in the literature as usually occurring in the autumn season, when the abundance of predators is the highest. This type of mortality described for _Calanus helgolandicus_ constituted an average of 89% of the total mortality for this species [53]. In the Baltic Sea, copepods are a valuable source of food for the commercially important fish species sprat and herring, but jellyfish can also have a significant predatory impact. The gelatinous zooplankton was the main cause of mortality variability observed in deep coastal sampling station located near the southwest of Plymouth, UK [55]. Nonconsumer mortality may result from death caused by age [56], diseases, and parasitism [57], exposure to environmental pollution [58], and physiological stress [59]. Field and laboratory studies show that nonconsumptive factors can account for 25-33% of the total death rate among adult copepods [60]. The species from genus Acartia had the highest abundance and biomass among the three investigated taxa, while _P. acuspes_ was far less abundant than the other two taxa. Such proportionsbetween these taxa are quite typical for the coastal region of the southern Baltic. _P. acuspes_ tends to dominate offshore areas of the Baltic Sea, is much less abundant in the coastal zones, and it is rarely present above the thermocline, especially during the warm season [61; 62]. Observed long-term biomass means for _Acartia_ spp. were in a similar range as those reported for this region by [PERSON] [30], while for _T. longicornis_, and especially _P. acuspes_, they were much lower. This was probably due to our sampling stations being located mostly in the inner, coastal part of the Gulf of Gdarsk. [PERSON] [30] also showed mostly negative anomalies for both _T. longicornis_ and _P. acuspes_ as well as the mostly neutral anomaly of _Acartia_ spp. during the late 1990s. This is consistent with our findings, which also showed strong positive anomalies for those taxa during the first decade of the 2000s.