## 4 Discussion  
The aim of our study was a description of seasonal and interannual patterns of secondary production and mortality rates for the main southern Baltic copepods. The main factors determining zooplankton production are temperature and food availability [29]. Therefore, we decided to investigate seasonal and interannual fluctuations of secondary production of three copep taxa: _Acartia_ spp., _T. longicornis_, and _P. acuspes_ in the southern Baltic. This is even more important in the context of increasing water temperatures observed in the Baltic related to global warming [30]. Higher water temperature leads to shorter generation time and smaller body size of copepods, also causing individuals to reach reproductive age quicker, and causing rapid increases in density [31]. However, different copep species have their individual temperature optimums, at which their development is most optimal [10]. Therefore, when estimating the rate of secondary production of these copepods, we used a _Di_ function that takes into account the individual temperature optimums. We are also aware that accurate estimates, most approximate to the natural state of the secondary production based on mathematical expressions, require a method that combines a variety of factors. Because of that our estimations do not fully reflect actual, real as in the natural environment, secondary production values. However, they allow for an approximation of these values and enable recognition of trends or anomalies occurring in the studied ecosystem. Results obtained in our research showed clear correlation between seasonal production fluctuations in the Gulf of Gdansk and the hydrological conditions, mainly water temperature (Figure 21). The highest correlation was recorded during summer, mainly for the young copepite stages of _Acartia_ spp. as well as nauplii and copepites of _T. longicornis_. This is consistent with research carried out by [PERSON] et al. [32] in the North Sea, which also indicates that the production coefficient is significantly positively correlated with the average water temperature. However, research from the Western Scheldt Estuary [33], showed that neither the biomass nor the secondary copep production was associated with chlorophyll concentration, and the temperature seemed to have a significant impact only on the predominance of certain copepods. In contrary to those two taxa, _P. acuspes_ showed mostly negative correlation of secondary production with temperature. This species in the central Baltic is associated with the deeper, more saline, colder water layer [34; 35; 36]. This was visible in high mean values of secondary production of nauplii and older copepites (CIV and CV) noted during winter seasons (Figure 8). We can therefore clearly state that, similarly to other water basins, water temperature is one of the main factors controlling not only biomass and abundance [17; 37] but also secondary production of main copep taxa in the Gulf of Gdansk. Temperature was responsible for 11.8% of variability observed in RDA. In addition to temperature, variability of secondary production was also to some extent explained by the concentration of dissolved oxygen (5%) and chlorophyll \(a\) (4.5%) (Table 6). Our results indicate that the water salinity does not have a statistically significant effect on abundance of copepite developmental stages (Table 6). There was also no correlation for both production and mortality rates. These results differed from literature data from other brackish [38] or low salinity water bodies [36; 39]. For example, in the Lake Waihola both elevated water temperature and salinity affected population dynamics of _Boeckella hamata_. Additionally, higher temperature and salinity, which were related to the increase of egg production, were offset by the higher mortality rates of reproductive females [38]. [PERSON], on the other hand, described that the combination of temperature and salinity had a greater impact on mortality of _Eurptemor affinis_ than each of these factors separately [40].