Size structure and life cycle patterns of dominant pelagic amphipods collected as swimmers in sediment traps in the eastern Fram Strait

Abstract

Time series length-frequency data are presented for Themisto amphipods collected as swimmers by moored sediment traps since 2000 at the AWI deep-sea observatory HAUSGARTEN (79°N/4°E) in the eastern Fram Strait. Amphipod occurrences increased significantly from 2000 to 2009 at 200–300 m depth, and the North Atlantic species Themisto compressa was continuously present in the samples starting in 2004. We present year-round records of large adult Themisto amphipods, including the appearance of Themisto libellula with a total body length of up to 56.7 mm and juveniles starting from 4.0 mm. The length of Themisto abyssorum ranged from 4.2 to 25.6 mm, whereas it varied for Themisto compressa from 8.8 to 24.4 mm. Length-frequency analysis indicated a life span of 2 years for T. abyssorum and at least 3 years for T. libellula. The absence of juveniles for T. compressa suggested its reproduction in southern subarctic areas and its occasional northward migration with warmer Atlantic water into the eastern Fram Strait. The seasonal and long-term size structures of the three pelagic species were consistent over the course of the study, indicating no changes occurred in cohort development due to increasing abundances or warming water temperatures.

Introduction

Within the marine zooplankton community, pelagic amphipods are recognized as an important component of the Arctic food web. They represent a key link between herbivorous mesozooplankton (mainly copepods) and higher trophic levels including planktivorous fishes like polar cod (Boreogadus saida), capelin (Mallotus villosus) and herring (Clupea harengus), seabirds such as the little auk (Alle alle) as well as marine mammals including ringed- and harp seal (Phoca hispida and Phoca groenlandica) (Lampert, 1960; LeBrasseur, 1966; Lønne and Gulliksen, 1989, Lydersen et al., 1989, Dalpadado et al., 2000, Dalpadado et al., 2001, Dalpadado and Bogstad, 2004). The ecological importance of this group was described recently by Skjoldal et al. (2004) for the North Atlantic, and Bowman (1960) described pelagic amphipods in Arctic waters as an important component of the marine zooplankton community with a ‘respective status’ next to copepods and krill. Many studies followed Bowman's (1960) paper (Weigmann-Haass, 1997; Dalpadado et al., 1998; Wencki, 2000; Dalpadado, 2002, Weslawski and Legezynska, 2002), which describe the biology, distribution and appearances of pelagic Amphipoda in high Arctic ecosystems, especially in the Norwegian, Greenland and Barents Sea.

The most frequently encountered pelagic amphipods in the Arctic belong to the genus Themisto, with the species Themisto libellula and Themisto abyssorum (Hyperiidea) being dominant (Dunbar, 1957; Bowman, 1960, Schneppenheim and Weigmann-Haass, 1986; Koszteyn et al., 1995; Dalpadado et al., 2001; Dalpadado, 2002). Occasionally, a third Themisto species, T. compressa, is collected in the Arctic Ocean and its surrounding seas (Dunbar, 1964, Williams and Robins, 1981; Weigmann-Haass, 1997, Weslawski et al., 2006). Studies over the past decade included investigations on geographic and vertical distribution patterns, reproduction strategies, food sources, and abundance of Themisto (Koszteyn et al., 1995; Wencki, 2000; Dalpadado et al., 2001, Dalpadado et al., 2008; Auel et al., 2002, Dalpadado, 2002; Dale et al., 2006, Weslawski et al., 2006, Kraft et al., 2011). It was found that in the Barents Sea, the Greenland Sea and the eastern Fram Strait, T. libellula and T. abyssorum are the most abundant pelagic amphipod species in the epipelagic zone. At sampling sites influenced by Atlantic water, the subarctic species T. abyssorum was found to appear in higher abundances than its Arctic congener T. libellula (Dunbar, 1957; Koszteyn et al., 1995); Dalpadado, 2002, Dalpadado et al., 2008. Furthermore, the occurrence of the typical North Atlantic species T. compressa was restricted to warmer Atlantic water masses in the Barents Sea, Norwegian Sea and eastern Fram Strait (Weigmann-Haass, 1997); Dalpadado, 2002, Kraft et al., 2011 and has been shown to occur in large swarms in the northeast Atlantic (Lampitt et al., 1993, Angel and Pugh, 2000).

The ecological role and life cycles of Themisto species at high latitudes have not been fully investigated: during few studies has the pelagic amphipod community been sampled year-round in seasonally ice-covered Arctic regions (Dalpadado et al., 2008, Makabe et al., 2010). Regular sampling with plankton nets in the Barents Sea suggested interspecific differences between the live cycles of T. abyssorum and T. libellula; for example a 1-year life span was hypothesized for the boreal-Atlantic species T. abyssorum, while its Arctic congener T. libellula seemed to show a 2-year life-cycle (Dalpadado, 2002). Other studies addressing the size structure for the genus Themisto from the Barents, Greenland and Norwegian Seas as well as the central Arctic Ocean indicated that the life-spans of the species seemed to increase with increasing latitude, reaching up to 2 years for T. abyssorum (Bogorov, 1940, Bowman, 1960, Hoffer, 1972, Koszteyn et al., 1995, Vinogradov et al., 1996) and 3 or more years for T. libellua (Koszteyn et al., 1995; Auel and Werner, 2003, Dale et al., 2006, Weslawski et al., 2006). However, sampling for those studies mostly took place during a short period in summer. In addition, data on the life cycle patterns of T. compressa are scarce and restricted to sampling regions in the North Atlantic and North Sea, but these data indicate that the species breeds multiple times per year (Sheader, 1977, Sheader, 1981). For Arctic waters, a few studies did mention the presence of T. compressa in the present day, but the respective authors did not give information on life cycle features of this species (Brandt, 1997, Weigmann-Haass, 1997; Dalpadado et al., 2001; Dalpadado, 2002).

One useful tool to extend our knowledge on the seasonal appearances and size structures of the pelagic amphipod community now seems to be the use of long-term datasets, which are obtained by time-series sediment traps deployed at different positions and depths in the open water column. This sampling method has been shown to successfully collect samples throughout the year in various Arctic regions including the open waters of the eastern Fram Strait, the Beaufort Sea and Kongsfjorden, West Spitsbergen (Willis et al., 2006, Willis et al., 2008; Bauerfeind et al., 2009, Makabe et al., 2010; Kraft et al., 2011). While predominantly used for assessing sinking organic matter in the water column, sediment traps are expected to provide an improved understanding of pelagic processes in times of climate change (Bauerfeind et al., 2009). Zooplankton collected in sediment traps, termed ‘swimmers’, have been analyzed in previous studies in order to improve our knowledge on zooplankton patterns, e.g. in the Greenland Sea (Seiler and Brandt, 1997), in the eastern Fram Strait (Kraft et al., 2011) and in the southeastern Beaufort Sea (Makabe et al., 2010). Zooplankton which are collected in moored sediment traps first enter the trap actively; the organisms then die instantly when they come into contact with the poison or preservative in the collector cups (Knauer et al., 1979). As swimmers do not belong to the sinking particles (e.g. zooplankton fecal pellets and dead planktonic organisms), most protocols require the separation of these swimmers from the samples prior to the analysis of the sediment matter (e.g. Michaels et al., 1990; Buesseler et al., 2007). Removing and sorting these swimmer zooplankton groups may provide the opportunity of new insight into zooplankton composition and the important ability to study year-round datasets.

To our knowledge, none of the past studies on the genus Themisto in the Eurasian Arctic addressed a continuous multi-year analysis of length-frequency data, as all the data were collected during short time frames of less than one year or during repeated summer sampling covering several years. Therefore, the present study was conducted to investigate the year-round and long-term size structure development of the dominant Themisto species in the eastern Fram Strait. Based on the results, we follow the growth of cohorts throughout the year and highlight similarities and differences between T. abyssorum, T. compressa and T. libellula, respectively, using time series samples of sediment traps during the years 2000 to 2009.