1). Five of the stations (So1-5 m, So2-10 m, So3-20 m, So4-30 m, J23-40 m) were located on a depth gradient transect and one station (M2–10 m depth) was located in Puck Bay. The zooplankton material was collected using a closing-type Copenhagen net of 0.50 m inlet diameter and 100 μm mesh size, equipped with a flowmeter.
Qualitative and quantitative laboratory analyses were performed in accordance with the HELCOM guidelines included in the Combine manual Annex C-7 (www.helcom.fi), except for the nauplii, which were identified to species level. Adults of the genus Acartia were identified only to genus level, owing to the similarity between the three Acartia species, these are referred to as Acartia spp. Biomass was calculated from abundance with weight standards CH5424802 nmr after Hernroth (1985); afterwards, obtained values were integrated over the whole depth layer. Finally, seasonal (Winter December–March, Spring
April–June, Summer July–September, Autumn October–December) biomass Y-27632 cost values were derived by averaging corresponding months (Table 1). Carbon was calculated as 5% of wet weight after Mullin (1969); this conversion rate is usually used for Baltic copepods although as showed by Tanskanen (1994) it may lead to underestimation of zooplankton biomass. With assumption of non-limiting food conditions, the production of the investigated species’ copepodite stages was calculated using Edmondson and Winberg’s equation (Edmondson and Winberg, 1971): equation(1) PCi=Ni×ΔWiDiwhere PCi represents daily potential production of stage i (wet weight), Ni is the abundance of the corresponding development stage i, Di is the development time of stage i (day−1) and ΔWi is the difference in wet weight of stage i. Di of developmental stages were computed using Belehrádek’s function ( Belehrádek, 1957): equation(2) Di=a(T−α)−bDi=a(T−α)−bwhere
a is 1288, 1466, 3044, and α is −10.5, −10.4, −13.9 for Acartia spp., T. longicornis and Pseudocalanus sp. copepodite stages, respectively, and b value is 2.05, all after McLaren (1978) and McLaren et al. (1989). T was the ambient temperature (°C) and was determined for each stage based on its WMD ( Dzierzbicka-Głowacka et ADP ribosylation factor al., 2013). Estimates of zooplankton mortality were computed with the method described by Aksnes and Ohman (1996). We initially assumed that recruitment rate pi (ind. day−1) to stage i was constant over a time period corresponding to the duration of the stage αi (days). Furthermore duration of each stage was constant for every individual, and the mortality for the period αi can be expressed by a constant θi (true mortality rate of the stage i) (day−1). While estimating mortality we assumed that rate of stage i and i + 1(θ) was considered for a period equal to the corresponding duration of two consecutive stages (αi + αi+1).