Seasonal Vertical Strategies in a High-Arctic Coastal Zooplankton Community

Vol. 555: 49–64, 2016 MARINE ECOLOGY PROGRESS SERIES Published August 18 doi: 10.3354/meps11831 Mar Ecol Prog Ser OPENPEN ACCESSCCESS Seasonal vertical strategies in a high-Arctic coastal zooplankton community Kanchana Bandara1,*, Øystein Varpe2,3, Janne E. Søreide2, Jago Wallenschus2, Jørgen Berge2,4, Ketil Eiane1 1Faculty of Biosciences and Aquaculture, Nord University, 8049 Bodø, Norway 2The University Centre in Svalbard (UNIS), 9171 Longyearbyen, Norway 3Akvaplan-niva, Fram Centre, 9296 Tromsø, Norway 4Faculty of Biosciences, Fisheries and Economics, UiT The Arctic University of Norway, 9037 Tromsø, Norway ABSTRACT: We studied the larger (>1000 µm) size fraction of zooplankton in an Arctic coastal water community in Billefjorden, Svalbard (78°40’ N), Norway, in order to describe seasonal vertical distributions of the dominant taxa in relation to environmental variability. Calanus spp. numerically dominated the herbivores; Aglantha digitale, Mertensia ovum, Beroë cucumis, and Parasagitta elegans were the dominant carnivores. Omnivores and detritivores were numerically less important. Descent to deeper regions of the water column (>100 m) between August and October, and ascent to the shallower region (<100 m) between November and May was the overall seasonal pattern in this zooplankton community. In contrast to other groups, P. elegans did not exhibit pronounced vertical migrations. Seasonal vertical distributions of most species showed statistical associations with the availability of their main food source. The vertical distribution of later developmental stages of Calanus spp. was inversely associated with fluorescence, indicating that they descended from the shallower region while it was still relatively productive, and ascended before the primary production had started to increase. Strong associations between the vertical distributions of secondary consumer M. ovum and Calanus spp., and tertiary consumer B. cucumis and M. ovum indicated that these carnivores seasonally followed their prey through the water column. We conclude that seasonal vertical migrations are a widespread trait in the high Arctic community studied, and predator−prey interactions seem particularly central in shaping the associations between the seasonal vertical strategies of adjacent trophic levels. KEY WORDS: Seasonal vertical migration · Food availability · Trophic interactions · Pelagic environments · Predator−prey interactions INTRODUCTION age (Lee et al. 2006, Varpe et al. 2009), diapause (Carlisle 1961, Hirche 1996), and seasonal vertical Pronounced seasonal oscillations in abiotic (e.g. migrations (Conover 1988). Zooplankton seasonal solar radiation, temperature, sea ice) and biotic (e.g. vertical migrations are understood as an adaptive be- food availability, predation pressure) environments havior that optimizes their position in the water col- offer challenges to zooplankton in high latitudes. In umn in response to seasonal variability in the envi- particular, seasonality in food availability is believed ronment (Werner & Gilliam 1984). We refer to this to be a significant challenge (Clarke & Peck 1991, behavior as their ‘seasonal vertical strategy’. Sea- Conover & Huntley 1991, Hagen 1999, Varpe 2012). sonal vertical strategies of some high-latitude herbi- Arctic zooplankton possess adaptations to counter a vorous zooplankton are well-documented (e.g. Con - seasonally variable food supply, such as energy stor- over 1988, Falk-Petersen et al. 2009), and their adap- © The authors 2016. Open Access under Creative Commons by *Corresponding author: [email protected] Attribution Licence. Use, distribution and reproduction are un - restricted. Authors and original publication must be credited. Publisher: Inter-Research · www.int-res.com 50 Mar Ecol Prog Ser 555: 49–64, 2016 tive value has also been analyzed in modeling studies (e.g. Fiksen 2000, Varpe et al. 2007). The underlying regulation of zooplankton seasonal vertical strategies has been a subject of interest since early 1900s (Russell 1927, Banse 1964). Seasonal variability in hydrography (Hirche 1991), photo - period (Sømme 1934, Miller et al. 1991), and visual predation (Kaartvedt 1996, Dale et al. 1999, Kaart - vedt 2000) are some external environmental cues that are thought to regulate seasonal vertical strategies. Internal (endogenous) regulation through seasonal changes in gonad development (Østvedt 1955), lipids and buoyancy (Visser & Jónasdóttir 1999), and long-term endogenous timers (Miller et al. 1991, Hirche 1996) have also been suggested. Many components of zooplankton life strategies are viewed as adaptations to seasonal variations in food supply (Ji et al. 2010, Varpe 2012), but the influence Fig. 1. Study area; sampling site is indicated by the red dot. x of food availability on seasonal vertical strategies re- and y indicate coarse locations of the inner and outer sills of mains poorly understood, particularly for carni vorous Billefjorden respectively. Positions of the west Spitsbergen species. As the seasonal food supply is more pro- current (WSC) and the coastal current (CC) were adopted from Svendsen et al. (2002). s: Spitsbergen, n: Nordaust- nounced for Arctic herbivorous zooplankton (Conover landet, e: Edgeøya & Huntley 1991, Hagen 1999), whose energetic de- mands mainly depend on a short period of annual primary production (Falk-Petersen et al. 2009), it can be ships exist on seasonal timescales is not known, and argued that their seasonal vertical strategies are open for investigation. tightly coupled with food availability. Although verti- Investigating the seasonality of zooplankton strate- cal distributions of Calanus spp. appear to be associ- gies and interactions requires studying pelagic com- ated with food availability (chlorophyll a distrib - munities over the course of an annual cycle. Apart utions) in spring (Herman 1983, Søreide et al. 2008, from a few studies (e.g. Hop et al. 2006), year-round Basedow et al. 2010), it is less well-studied for the rest zooplankton community investigations are rare in the of the year. Compared to herbi vores, Arctic carnivo- Arctic. Here, we investigated seasonal vertical distri- rous and omnivorous zooplankton rely to a greater butions of the dominant herbivore and carnivore zoo- extent on a year-round food supply (Hagen 1999). plankton in a high-latitude coastal zooplankton com- Therefore, it has been suggested that their seasonal munity during a 10-month period in 2008 and 2009. adaptations are less pronounced compared to herbi- We studied the extent to which the seasonal vertical vores (Ji et al. 2010, Varpe 2012). As vertical distribu- distributions of the above zooplankton could be ex - tions vary seasonally in a number of carnivorous hy- plained by the seasonal dynamics of their primary dromedusae (e.g. Pertsova et al. 2006), ctenophores food source, or physical environmental variables (e.g. Siferd & Conover 1992), chaetognaths (e.g. such as temperature, salinity, and irradiance. Grigor et al. 2014), euphausiids (e.g. Lass et al. 2001), and copepods (e.g. Vestheim et al. 2005), it appears that seasonal vertical strategies of Arctic carnivo- MATERIALS AND METHODS rous zooplankton are more diverse than previously thought. Since many carnivores rely on herbivores as Study site their main food source, the potential influence of the vertical strategies of herbivorous zooplankton on Environmental variables and zooplankton samples their predators may be ecologically significant. This is were collected monthly between August 2008 and portrayed in the findings of Nelson et al. (1997) and May 2009 at a 189 m deep station (78° 39.72’ N, Sims et al. (2005), where a close resemblance be - 16°44.34’ E) within the inner basin of Billefjorden, tween the vertical behavior of planktivorous sharks located at the west coast of Spitsbergen, the largest and the diel vertical migration (DVM) of herbivorous island in the Svalbard archipelago (Fig. 1). The inner zooplankton were reported. Whether such relation- basin of Billefjorden remains ice-covered from ca. Bandara et al.: Zooplankton seasonal vertical strategies 51 December to June (Arnkværn et al. 2005). Two 50 to of the year. Therefore, raw voltage outputs of the 70 m deep sills located near the mouth of the fjord fluoro meter were presented as normalized values (Fig. 1) act as a topographical barrier that hinders the between 0 and 1 after removing some extreme advection of the Atlantic water masses into Billefjor- readings (sensor noise). This provided an approxi- den (Cottier et al. 2005, Nilsen et al. 2008). Because mate variation of the fluorescence during the study, of this, Arnkværn et al. (2005) argued that zooplank- because according to the calibration equation (Sea- ton population dynamics in Billefjorden are influ- point Sensors; data not shown), fluorescence is esti- enced more by internal processes than by ad vection. mated as a linear function of the voltage outputs. Raw voltage outputs (Oi) of the irradiance sensor were converted to PAR by applying a wet calibration Environmental variables factor (C = 5.05 × 1012), and a dark voltage of 0.0130 V (Biospherical Instruments) as: Temperature and salinity were profiled in situ PAR = C(10Oi –100.0130) (1) using either a CTD/STD model DS 204 (SAIV) or a Seabird™ CTD (Sea-Bird Electronics). Since no Temperature and salinity measurements were CTDs were deployed on 27 August and 07 Septem- visualized using the Spatial Analyst™ extension of ber 2008, and 23 March 2009 (Table 1), we ob - ArcGIS™ version

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