Disko Bay, West Greenland, the three Calanus species co-occur and can contribute .... difficult due to rough weather, and extremely low air temperatures made it ...
Marine Biology (2002) 140: 567–576 DOI 10.1007/s00227-001-0731-3
B. Niehoff Æ S.D. Madsen Æ B.W. Hansen Æ T.G. Nielsen
Reproductive cycles of three dominant Calanus species in Disko Bay, West Greenland
Received: 16 October 2000 / Accepted: 28 September 2001 / Published online: 1 February 2002 Springer-Verlag 2002
Abstract This study describes the annual reproductive cycles of the three dominant Calanus species, C. finmarchicus, C. glacialis and C. hyperboreus, in Disko Bay (West Greenland) in relation to seasonal phytoplankton development. Relative abundance of females, copepodite stage V (CV) and males, and the developmental stage of the female gonad were examined from plankton samples collected at weekly to monthly intervals from May 1996 to June 1997 with a WP2 net or a pump. During spring and summer, egg production rates were determined. Females of all three species were present year round. Maximum relative abundance was reached by C. hyperboreus females at the beginning of February, by C. glacialis in mid-February, and by C. finmarchicus in April. All three species reproduced successfully in Disko Bay. Their reproductive cycles were considerably different with respect to the timing of final gonad maturation and spawning, and hence in their relation to seasonal phytoplankton development. In all three species, early gonad development took place during winter, before living food became plentiful, suggesting that these processes were largely food independent. Final gonad maturation and spawning in C. finmarchicus was related to the phytoplankton concentration, reflecting that final gonad maturation processes are food dependent in this species. C. glacialis females matured and spawned prior to the spring bloom. Our results indicate that first internal lipid stores and later ice alga grazing supplied
Communicated by O. Kinne, Oldendorf/Luhe B. Niehoff (&) Alfred-Wegener Institute for Polar and Marine Research, Columbusstr., 27568 Bremerhaven, Germany E-mail: bniehoff@awi-bremerhaven.de S.D. Madsen Æ B.W. Hansen Roskilde University, Department of Life Sciences and Chemistry, 4000 Roskilde, Denmark T.G. Nielsen National Environmental Research Institute, Department of Marine Ecology, 4000 Roskilde, Denmark
final gonad maturation and egg production. Maximum egg production rates of C. glacialis were found in spring and summer, when the chlorophyll a concentration was high. Mature female C. hyperboreus were found from February until mid-April, when the chlorophyll a concentration was still low. In this species, reproductive activity was decoupled from phytoplankton development, and final maturation processes and spawning were solely fuelled by internal energy stores.
Introduction In northern pelagic ecosystems, the three Calanus species C. finmarchicus (Gunnerus), C. glacialis Jaschnov and C. hyperboreus Krøyer dominate the zooplankton communities. C. finmarchicus, as a boreal species, is associated with Atlantic Water and transported northward in the Arctic with the North Atlantic Current (Hirche et al. 1991; Mumm et al. 1998). C. glacialis inhabits the Polar Water of the Arctic Ocean and is transported south with the East Greenland Current (Grainger 1963; Hopkins 1969; Jaschnov 1970). C. hyperboreus reaches high abundances in the Greenland Sea and Baffin Bay, but is also found in the Arctic Ocean, the Norwegian Sea and the Gulf of Maine (Conover 1965, 1988; Hirche 1991; Hirche and Mumm 1992; Conover and Siferd 1993). In Disko Bay, West Greenland, the three Calanus species co-occur and can contribute together >90% to the zooplankton biomass during spring and early summer (Nielsen and Hansen 1995; Hansen et al. 1999; Madsen et al. 2001). The abiotic and biotic conditions in this ecosystem are highly variable. Though the water exchange with surrounding oceans is limited, due to a maximum sill depth of 300 m, Disko Bay is influenced by the West Greenland Current transporting Polar Water (originating from the East Greenland Current) and Atlantic Water (originating from the Irminger Current) northward along the coast of West Greenland (Buch 1990). During winter, the bay is ice covered and
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the surface water temperature is below –1.5C (Nielsen, unpublished data). After the ice melt, the spring bloom starts, usually in April–May, and lasts for about 2 weeks (Andersen 1981). However, duration and extent of the ice cover are related to climatic conditions, and, consequently, onset and development of the spring bloom vary from year to year (Buch 1990; Andersen 1981). After the spring bloom, only minor blooms develop, caused by intrusions of nutrients due to temporal mixing of the surface water (Anderson 1981; Nielsen and Hansen 1999). The Calanus species have evolved different reproductive strategies to cope with seasonally different feeding conditions. In C. finmarchicus, final gonad development and reproduction are dependent on food supply (e.g. Runge 1985; Diel and Tande 1992; Plourde and Runge 1993; Hirche et al. 1997; Niehoff et al. 1999). C. glacialis is able to reproduce prior to the spring bloom without feeding, relying on internal lipid reserves as shown by Hirche and Kattner (1993). High reproductive activity, however, occurs only when food is available (Smith 1990; Hirche and Kattner 1993; Kosobokova 1999). Gonad maturation and egg production in C. hyperboreus take place in winter, when no food is available, and are fuelled by stored wax esters only (Conover 1962, 1967; Lee 1974; Conover and Siferd 1993; Hirche and Niehoff 1996). This knowledge had been mainly gained during cruises and laboratory experiments. Time series studies on reproduction are rare and usually cover only the spring and summer period, due to problematic climatic and logistic conditions (e.g. Dawson 1978; Diel and Tande 1992; Plourde and Runge 1993; Kosobokova 1999; Niehoff et al. 1999). As part of an investigation of the annual succession and production of the plankton community in Disko Bay, the present study therefore aims to describe the annual cycles of gonad development and spring spawning of the three dominant Calanus species in Disko Bay, and to compare their reproductive strategies.
Materials and methods Sampling The time series was conducted from 20 May 1996 to 19 June 1997, at a permanent station (250 m deep), approximately 1 nautical mile off Qeqertarsuaq (Godhavn) (69¢15N; 53¢33W) in Disko Bay, West Greenland (Fig. 1). Sampling was performed aboard the R.V. ‘‘Porsild’’ (Arctic Station, University of Copenhagen, Denmark) or from the sea ice. Copepods were sampled with a WP2 net (200 lm mesh size, vertical hauls) or with a submersible pump (3000 l min–1) equipped with a conical net (50 lm mesh size). Casts from 50 m to the surface were carried out weekly or whenever weather conditions allowed. Casts from the whole water column were conducted monthly. On board, the samples were gently concentrated and immediately preserved in 4% formalin buffered with sodium borate for determination of abundance and gonad development stage. From preserved samples, copepodite stage V (CV) males and females were sorted. Details on sampling and sorting criteria are described in Madsen et al. (2001). In addition, vertical profiles of water temperature, salinity, density and fluorescence of the whole column were measured on every cruise. To determine the chlorophyll a concentration, water samples were taken at 5 or 6 depths in
the upper 30–60 m. In monthly intervals, additional samples were taken at 50, 100, 150 and 200 m depth. Chlorophyll samples from the upper 60 m were size fractioned (11 lm). Amounts of 3–15 l of seawater were filtered onto GF/F filters, the chlorophyll a was extracted in 96% ethanol over night (Jespersen and Christoffersen 1987), and the concentration was determined spectrophotometrically (Strickland and Parsons 1972). Mean chlorophyll a concentration was calculated for 2 m and fluorescence maximum depths. Integrated chlorophyll a concentration was calculated for 200 m depth to the surface, based on fluorescence data calibrated against measured chlorophyll a concentration. Egg production For egg production experiments, oblique casts (WP2 net) from 50 m (Calanus finmarchicus and C. glacialis) or 200 m (C. hyperboreus) to the surface were conducted. The live samples were gently diluted in surface seawater, decanted into large dark containers (25 l), and transported to the land-based laboratory at the Arctic Station (University of Copenhagen). Individual live copepod females were sorted within 11 lm size fraction [mainly Phaeocystis spp., Thalassiosira sp., Detonula confervaceae, and pennate diatoms (Levinsen et al. 2000b)] dominated the phytoplankton community (Fig. 2). Surface water temperature at that time did not exceed 0C. During summer, the water column remained stratified due to low salinity (33&) and relatively warm surface water (up to 6.8C in August, see Fig. 5) in the upper 30 m layer. The chlorophyll a concentration ranged from 0.3 to 3.7 lg l–1 (63–205 mg m–2, see Fig. 5). The size composition of the phytoplankton
shifted to cells 11 lm and
