Early development of Calanus hyperboreus nauplii: Response to a changing ocean

To forecast effects of temperature changes on recruitment and population dynamics of the Arctic copepod Calanus hyperboreus, laboratory experiments investigating temperature and food effects on early development were performed in Disko Bay, western Greenland, in 2009, and ascent rates of C. hyperboreus eggs collected in east Greenland were measured in the laboratory. Ascent rates were highly variable both between and within clutches, ranging from 0.7 to 27.7 m d21, suggesting variability in the biochemical composition of the egg. Development of eggs were investigated between 0.8uC and 6.6uC, and hatching was fitted to a Belěhrádek temperature function (r2 . 0.99) with mean development time (MDT) of eggs ranging from 2.8 to 5.8 d. MDT of fed and starved nauplii was calculated for nauplii raised at 5uC. Fed nauplii developed through the first five nauplius stages (N1–N5) during 40 d of incubation, whereas development of starved nauplii ceased at N3. Nauplii were able to survive at least 30 d of starvation. Respiration rate was measured for N1 and N3 at 0uC, 5uC, and 10uC, and it increased with development stage and temperature from 0.05 6 0.01 to 0.29 6 0.08 nmol O2 nauplii21 h21 for N1 at 0uC and N3 at 10uC, respectively. A decrease in carbon and lipid content from egg to N3 indicates that nauplii are using stored lipids to cover their metabolic costs during the nonfeeding stages. Early stages of C. hyperboreus seem more affected by temperature than later stages, a vulnerability that might affect future recruitment. In Disko Bay, western Greenland, significant changes in hydrography and ice cover have occurred during the last decades. An inflow of deep Atlantic water occurred in 1997 (Hansen et al. 2012) causing a 1.5uC increase in bottom water temperature, and an acceleration of submarine melting of the Jacobhavns glacier has increased the meltwater input to the bay (Holland et al. 2008). Furthermore, from 1991 to 2004 a 50% decrease in sea ice cover and an earlier breakup of sea ice have been observed (Hansen et al. 2006). Such changes in magnitude and duration of sea ice cover affect both timing and duration of the phytoplankton spring bloom (Tremblay and Gagnon 2009). The spring bloom in Disko Bay drives the energy transfer through the marine food web, with the large Calanus copepods as key species during spring and early summer serving as an important link to higher trophic levels (Falk-Petersen et al. 2007). Three closely related Calanus species co-occur in Disko Bay: Calanus hyperboreus, Calanus glacialis, and Calanus finmarchicus. C. hyperboreus is a true Arctic species restricted to polar waters (Conover 1988) and the largest and most lipid rich of the three species (Lee et al. 2006; Swalethorp et al. 2011). C. hyperboreus has a 2–5 yr life cycle, depending on environmental conditions and food availability (Conover 1988; Falk-Petersen et al. 2007), and are believed to be multiannual-iteroparous (i.e., capable of spawning in successive years; Swalethorp et al. 2011; Hirche 2013). In Disko Bay the main spawning period of C. hyperboreus is from January to March (Niehoff et al. 2002; Henriksen et al. 2012), and molting into females, maturation of gonads, and production of eggs depend entirely on internal lipid reserves (Pasternak et al. 2001). The main lipid classes of zooplankton are wax esters (WEs), triacylglycerol (TAG), and phospholipids (PLs; Lee et al. 2006). In C. hyperboreus adults it is the energy-rich storage lipid WEs that dominate the lipid composition. The eggs of C. hyperboreus are spawned deep in the water column, and because they are positively buoyant, they float toward the surface. Because spawning takes place during winter, nauplii have to develop without food and survive on the lipids provided with the egg until the spring bloom. When the spring bloom initiates, they usually have reached the first feeding stage and are ready to exploit the abundance of food (Melle and Skjoldal 1998). The lipidfueled reproduction and the capability of the nauplii to survive for a period without food are advantageous in areas where the occurrence of the phytoplankton spring bloom is short and unpredictable. However, survival will probably be highly variable between different years, and changes in the timing of the spring bloom combined with increasing temperatures that increases the metabolism may have consequences for the survival of the nauplii and, thereby, the population dynamics. A number of studies have been conducted dealing with Calanus spp. biology and possible effects of global warming Limnology limn-58-06-19.3d 3/10/13 09:35:05 1 Cust # 13-053 * Corresponding author: [email protected] Limnol. Oceanogr., 58(6), 2013, 000–000 E 2013, by the Association for the Sciences of Limnology and Oceanography, Inc. doi:10.4319/lo.2013.58.6.0000