Life cycle and population dynamics of the calanoid copepod Pseudocalanus spp. in the Baltic Sea and North Sea

The seasonal and spatial distribution of Pseudocalanus acuspes in the Bornholm Basin (Central Baltic Sea) was studied on 16 cruises between March 2002 and May 2003 from stratified (10 m) multinet samples. The highest abundances were reached in May 2002 and April 2003 (618·10 and 869·10 ind. m , respectively). Ontogenetic vertical distribution was stage specific with differences of mean annual weighted mean depth >30 m between nauplii and males; it followed closely the hydrography which was characterized by a permanent halocline and a summer thermocline. The vertical distribution showed a positive correlation with salinity especially in the older developmental stages; the relationship to temperature was negative in the nauplii and copepodite stage I (CI). Most of the stages performed a seasonal migration. The consequences of the vertical distribution patterns in relation to the effects of climate and predation are discussed. A stage shift from nauplii in April/May to CIV and CV as overwintering stages indicated slow seasonal development. However, nauplii were observed all the year round, and the resulting stage structure did not allow to distinguish generations. Changes in the prosome length of females seemed to be related to the advection of water masses with different temperatures rather than to different generations. It could not be clarified whether the strong increase of nauplii and adults after an inflow event of cold, saline North Sea water in the beginning of 2003 was a result of advection or improvement in habitat conditions. Introduction Copepods of the genus Pseudocalanus often dominate the marine zooplankton communities of the northern hemisphere (Corkett and McLaren 1978; Conover and Siferd 1993; Kane 1993; Richter 1995). They are important grazers and prey for many commercially used fish (Mauchline 1998). In the zooplankton community of the Central Baltic Sea, Pseudocalanus acuspes is a key species. In earlier publications often called P. elongatus (e.g., Möllmann and Köster 1999; Walve and Larsson 1999; Möllmann et al. 2000, 2003; Dippner et al. 2001; Kornilovs et al. 2001; Viitasalo et al. 2001; Hinrichsen et al. 2002, 2003; Möllmann and Köster 2002), P. minutus (e.g., Dahmen 1995) or P. minutus elongatus (e.g., Hernroth 1985), (Bucklin et al. 2003; Grabbert et al., in preparation) identified mainly P. acuspes. Below we refer to earlier Baltic Sea references as Pseudocalanus sp. ; in our own data set we call the species P. acuspes. It serves as a major food organism not only for many larval fish (Hinrichsen et al. 2002; Möllmann et al. 2003), but also for adult pelagic planktivorous fish such as sprat and herring (Flinkman et al. 1992; Möllmann and Köster 1999, 2002). Particularly, cod larvae were found to actively select older developmental stages of Pseudocalanus sp. (Voss et al. 2003). Pseudocalanus acuspes is an inhabitant of the North Atlantic (Frost 1989), Norwegian fjords (Norrbin 1996), and Arctic and sub-Arctic seas (Siferd and Conover 1992; Runge and Ingram 1991). As in the North Sea the congener P. elongatus (Frost 1989; Bucklin et al. 2003) is the dominant species, there seems to be a gap between the North Atlantic and the Baltic Sea populations, suggesting it is part of the Baltic Sea glacial relict fauna together with several other species (e.g., Ojaveer et al. 1998; Kontula and Väinölä 2003). As in the brackish water of the Baltic Sea this marine species should live under marginal conditions, a high affinity to more saline waters is not surprising. Thus Hernroth and Ackefors (1979) and Hernroth (1985) observed especially older Communicated by O. Kinne, Oldendorf/Luhe J. Renz (&) Æ H. -J. Hirche Alfred Wegener Institute for Polar and Marine Research, PO Box 120161, 27515 Bremerhaven, Germany E-mail: jrenz@awi-bremerhaven.de Tel. : +49-471-48311042 Fax: +49-471-48311918 Marine Biology (2006) 148: 567–580 DOI 10.1007/s00227-005-0103-5 copepodite stages in deeper layers with higher salinity. In the Central Baltic Sea a permanent halocline restricts the water exchange between the bottom water and the surface layer (Launiainen et al. 1987). Hydrographic conditions are strongly dependent on the renewal of the bottom water through inflowing high saline and oxygenated water masses from the North Sea. This phenomenon occurs relatively rarely and is connected to the mean atmospheric circulation pattern (Matthäus and Franck 1992; Matthäus and Schinke 1994). Time series data for the biomass of Pseudocalanus sp. from the Central Baltic Sea showed large interannual variability with a positive relationship to salinity. Thus Behrends (1996), Ojaveer et al. (1998) and Möllmann et al. (2000) observed low abundances in years with low salinities. Vuorinen et al. (2003) compared biomass data and hydrographic parameters between the 1960s and 1992 and found also a relationship between abundance of Pseudocalanus sp. and salinity. More recently also top– down processes like predation by various species of fish larvae and fish were considered to be important controls for the Pseudocalanus sp. population (Möllmann and Köster 2002). As a result of the importance of this species in the pelagic food chain of the Central Baltic Sea and its sensitivity to climate, small hydrographic changes may strongly affect its predators (Möllmann and Köster 1999, 2002). The understanding of the relationship between physical forcing and the population dynamics of P. acuspes requires a detailed knowledge of its vertical distribution, physiological requirements and life cycle strategy. However, its life cycle has never been described in detail in respect to the hydrographic conditions in the Central Baltic Sea. Hernroth and Ackefors (1979) provided an overview, but vertical resolution was low (25– 50 m) and only one station was studied in the Bornholm Basin for 3–7 times per year. Here we used data from a high-resolution annual time series collected in the Bornholm Basin of the Central Baltic Sea from March 2002 to May 2003 during German GLOBEC (Global Ocean Ecosystem Dynamics) to describe the seasonal and spatial abundance, development and vertical distribution of P. acuspes in relation to hydrographic conditions. As our study covered the transition period from a long stagnation beginning in 1993 to several intrusions of North Sea water during the summer and fall and the inflow of cold, saline North Sea water in January 2003 (Feistel et al. 2004) we were also able to study the effect of changing hydrography on the vertical distribution of P. acuspes. Materials and methods