Circadian rhythms of Arctic zooplankton from polar twilight to polar night – patterns, processes, and ecosystem implications (CircA)
Unit leader: Dr Tove Gabrielsen (UNIS)
This unit is an interdisciplinary research unit that will examine processes occurring during the polar night such as DVM (Berge et al 2009) and bioluminescence (Berge et al 2011) of zooplankton, and determine the relationship and interaction between individuals and their environment. The Unit will work primarily with pelagic and sympagic organisms, and will aim at detecting and describing diurnal patterns and activity levels characteristic for important components of high Arctic marine food webs. This includes examining the direct relationship between patterns of active migration and bioluminescence potential with the prevailing light climate, including irradiance of lunar, solar and auroral origin. The work will implement and combine moored auxiliary observation platforms (Berge et al 2009, Wallace et al 2010) with autonomous underwater vehicles (Berge et al 2011, Schofield et al 2010), and will include an important lab component to describe metabolic activity rates (e.g. respiration, fecal pellet production, and enzymatic rates). The Unit will also search for patterns and processes hitherto not described, in order to achieve a more comprehensive overview of relevant biological processes in the polar night.
This unit has been financed as an own project, see “CircA” link.
Unit 2: Benthic communities, biodiversity and food web structure
Unit leader: Professor Paul Renaud (APN and UNIS)
The main objective of this Unit is to determine the structure and function of Arctic seafloor communities during polar night. Main research questions include how benthic communities during winter differ from those during summer months, and how do differences in community structure and food supply (quantity/quality) contribute to differences in trophic structure of the Arctic benthos during polar night? In order to optimize relevance for Units 1 and 3, we will focus on hyperbenthic organisms.
Background: Recent studies have demonstrated unexpected activity in the pelagic zone during the polar night. These include diurnal vertical migration without apparent light cues (Berge et al 2009), diverse patterns of bioluminescence by pelagic organisms (Berge et al 2011), and foraging by organisms previously assumed to be primarily visual predators (Themisto sp., sea birds: Kraft et al 2012, Berge et al 2011). Benthic communities, however, have rarely been studied during the polar night (but see Weslawski et al 1991, Legezynska et al 2012), and hyperbenthic communities, those organisms living in the bottom several meters of the water column, have been even less investigated. This latter group includes overwintering copepods, and a variety of other taxa collected in both benthic and pelagic sampling (chaetognaths, mysids, amphipod crustaceans, krill, and demersal fish: Hirche et al 2006). Densities of hyperbenthos are poorly known, but these organisms feed on surface sediments or benthic fauna, have high lipid stores (e.g. Connelly et al 2012), and are often abundant in diets of fish and mammals (e.g. Grønvik and Klemetsen 1987). As such, they can play an important but understudied ecosystem role linking benthic and pelagic habitats, and enhancing biogeochemical cycling. Several studies have recently been conducted employing stable isotopes (SI) to identify trophic position and food source of Arctic benthos during summer in Svalbard fjords (Renaud et al 2011, Kedra et al 2012, Berge and Renaud in prep). We will compare these to SI-derived food webs determined for the Polar night period. In addition we will use new genetic techniques (developed in Unit 1 projects) to study the diets of hyperbenthic organisms and benthic taxa that macerate their food beyond recognition.
Polar night is an excellent time to study these communities, as predator populations are low (and/or light required by visual predators is absent). Although many benthic and hyperbenthic organisms are assumed to feed on sinking phytodetritus during summer (Connelly et al 2012), we know virtually nothing about their trophic position during winter. Furthermore, some elements of the benthos are nearly absent during summer when regular sampling is conducted, but are very abundant in winter, and are likely prey for fish and invertebrate predators as light becomes available (e.g. caprellid amphipods inconspicuous in summer but very abundant in winter – J. Berge pers obs). Another very common amphipod species, Pontoporeia femorata is virtually only known from females and immature males, mature males believed to occur during winter months are yet to be described (Brandt and Berge 2007, Bousfield in prep). These are only two out of many examples, and the organisms represent a 'hidden biodiversity' component of the community with unexamined ecosystem roles. How many taxa are rare or absent from our records because they are not present during 'normal' (spring-autumn) sampling times, but instead are either eaten or migrate when spring arrives? We will use planned cruises for UNIS graduate courses to sample during summer/autumn for comparison to winter benthic/hyperbenthic sampling.
Key questions to be answered: Which benthic and hyperbenthic taxa represent the 'hidden biodiversity' of the Arctic polar night? How do hyperbenthic communities during winter differ from those during summer months? How do differences in community structure and food supply (quantity/quality) contribute to differences in trophic structure of the Arctic benthos/hyperbenthos during polar night compared to twilight or midnight sun periods?
Methods: Epibenthos and hyperbenthos will be collected during summer/autumn and the Polar night in Atlantic- and Arctic- influenced fjords in Svalbard using an epibenthic sledge fitted with 2 stacked 30cm-high 1mm mesh nets. These will collect fauna from the sediment surface and up to 60 cm above the sea floor. In addition, benthic trawls will be used in both seasons to collect larger taxa (e.g. fish, crabs), and a broader component of the benthic community. Catches of the epibenthic sledge will be fixed and later, in the lab, identified and quantified (abundance and biomass). Species lists will be compared to the register of species recorded from Svalbard (Palerud et al 2004), and any new species records will be added to the database. Since this habitat has been rarely sampled in Svalbard, or the Arctic in general, we expect a large number of new taxa for the region. Subsamples of the fauna (whole organisms or muscle tissue, depending on size of the organism) will be frozen (-20 C) for stable isotope (C and N) analysis using standard methods (e.g. Hobson et al 1995, Renaud et al 2011, Kedra et al 2012). Trawl catches will be processed by counting and weighing all taxa caught during a 15 min trawl. As for the sledge samples, representatives from each taxon will be frozen (whole or muscle) for later stable isotope analysis. Individuals from both trawl and sledge samples will be sampled for gut-content analysis. Larger animals/specimens will have guts removed and contents will be identified. Both count- and mass- based proportions of each prey item will be calculated (e.g. Renaud et al 2012). Small animals (macroplankton, large polychaetes), and those with guts predominantly containing highly macerated material, will have their guts removed and contents determined to broad taxonomic group (class, order) by genetic analyses (link to activities ongoing in Unit 1).
Potential carbon baseline end members for the food web will be sampled. Macroalgae will be collected by dredging or SCUBA diving, and suspended particulate material will be filtered (ashed GF/F filters) from both 50 m and near-bottom in the water column. The 50 m sample will be collected using a CTD rosette, whereas the near-bottom sample (50 cm above bottom) will be collected with a bottom-triggered Nisken bottle (Connelly et al 2012). Surface sediments will also be collected by grab sampling. Filters will be frozen and analysed for stable isotope signature using standard methods (e.g. Hobson et al 1995, Renaud et al 2011). Hyperbenthic community structure in different fjords and from different seasons will be compared using univariate (ANOVA on total biomass and density) and multivariate (correspondence analysis) methods. Diets from gut analysis will be compared by multivariate methods, and dietary overlap among predators will be assessed using the Schoener Index (e.g. Renaud et al 2012). Food webs will be constructed from stable isotope signatures assuming a 3.4 per mil fractionation in Delta15N between trophic levels (Renaud et al. 2011). Carbon source will be evaluated by comparing the Delta13C of fauna with that of potential carbon sources, and assuming a fractionation of approximately 1 per mil per trophic level. The importance of benthic-pelagic transfer of energy by hyperbenthos will be evaluated by a combination of the carbon source of their diets and their dietary role in pelagic/demersal predators.
Unit 3: Case studies focusing on climate change and winter ecology
Unit leader: Professor Jørgen Berge (UiT and UNIS)
This Unit will aim at transcribing the results from unit 1 and 2 into a new and more comprehensive understanding of Arctic marine ecosystems during the polar night across benthic, sympagic and pelagic habitats. At present, there are several recent publications documenting hitherto unknown patterns and processes (Berge et al 2009, 2011, 2012) that open up new perspectives regarding our understanding of ecosystem resilience and resistance towards both anthropogenic and climatic changes. Unit 3 will use these and the knowledge generated through Units 1 & 2 to elucidate the significance of winter processes in a climate change perspective such that new findings in the first units can feedback to help direct further process studies in Unit 3. At the present state three central case studies are fully described herein, representing known unknowns. However, new knowledge will be generated through the course of the project, and more unknowns are likely to be detected that will be followed up in this Unit. Also, a main aim of Unit 3 will be to relate the findings to IPCC climate scenarios, in order to provide advice on management and potential ecosystem implications towards policy makers. Case studies include dedicated parts on bivalves (blue mussels and Icelandic scallops), ice associated fauna and the role of visual versus tactile predators, and are all designed to both focus on effects of climate change as well as providing basic knowledge relevant in an Earth system science perspective.
Case study 1: Physiology and growth of bivalves – Chlamys islandica and Mylitus edulis.
This case study will focus on an abundant and ecologically characteristic species, C. islandica that is common throughout the Barents Sea and Svalbard waters. For the second target species, M. edulis, Svalbard represents its very edge and northernmost limit of its distribution, and has only been regularly observed since 2005 (Berge et al 2005, pers obs) especially in Kongsfjorden. Both species are of high commercial value internationally, and one (C. islandica) has since 2006 been included in in situ experimental set-ups on our observatory in Kongsfjorden, where their ecology, growth and reproduction are being studied (Ambrose et al 2012). Through this case study we aim at including also M. edulis into this programme, while at the same time expanding the range of in situ experiments to both Rijfjorden and Tromsø. The main questions involved include how environmental conditions, especially during the polar night (light, temperature, food) limit the range of these important species, and if a continued warming of the Arctic may facilitate a further expansion of their distribution.
Objective and hypothesis: The main objective is to characterize growth and reproduction patterns of mussels and scallops over an entire year while taking into consideration the seasonal variation in environmental factors and studied on a regional scale between Tromsø (Barents Sea) and Svalbard (Greenland Sea). By contrasting one species at its very edge of its distributional range (M. edulis) with one within the core of its distribution (C. islandica), we aim at gaining vital information as to how spatial and temporal differences in environmental conditions (light, temperature, salinity and oxygen) will affect the biology and physiology (energy storage, growth, reproduction etc.) of the bivalves. Importantly, through both field and lab experiments, we will also study how individuals in different states, particularly size, age and maturity level, will respond to changes in environmental factors within ecologically relevant ranges. Mapping these differences will improve our current understanding and ability to use the selected species for monitoring purposes and will provide information on how the species respond to changes in the climate.
Approach: Natural populations of mussels (coastal) and scallops (shallow shelf areas) will be studied in both specified regions. The organisms will be sampled monthly during one year and the environmental conditions continuously recorded using air and underwater loggers. Phytoplankton biomass will also be followed as an indication of food availability using a standard SeaBird CTD fitted with chl a sensors. Both species will also be incorporated into the Svalbard marine observatories currently being operated by the PI and partners. Caged individuals will be deployed in both Kongsfjorden and Rijpfjorden, the former representing an Atlantic influenced area in which both species occur naturally and the latter a high Arctic location that currently is above the northern distributional limit for blue mussels. However, fossil records clearly show that even Rijpfjorden has held a natural population of blue mussels during the Holocene climate optimum, and does thus represent a very realistic and relevant location for climate research on the selected species. Lab experiments will be conducted in Tromsø and Svalbard that supplement the in situ experiments, and will focus on identifying key physical drivers that regulate the growth, reproduction and distribution of the selected species, and how these may change according to IPCC predictions. These lab experiments, as well as the in situ experiments on Svalbard, will be conducted in close collaboration with ongoing projects such as Polarisation (PI Dr Nahrgang) and EcoTab.
Case study 2: The future of ice associated fauna.
Several reports have emerged recently concerning the extent of the Arctic summer sea ice, documenting that we are now facing a historical and dramatic decline in the total abundance of summer sea ice in the Arctic (National Snow and Ice Data Center). A reduction and potential disappearance of Arctic sea ice in the summer poses challenges for the ecosystems in the high north not known from historical records. However, based on the newly discovered pelagic phase of the life cycle for the until-now assumed obligate ice associated species Apherusa glacialis (Berge et al 2012), the implications may not be as obvious and detrimental as assumed until now. This discovery forms the basis of a new conceptual hypothesis on how species, until now considered totally dependent on sea ice, have a highly advanced lifecycle that include both extensive vertical migrations in the water column as well as utilizing deep ocean currents to retain their position within the Arctic Ocean (see Berge et al 2012).
We will carry out experiments to elucidate the fate of taxa generally associated with sea ice under an ice-free scenario. By taking advantage of the ongoing Arctic observatories in Rijpfjoren and Kongsfjorden, we will use these as experimental platforms to investigate potential survival and the ecophysiological state of organisms known to be important in the drift-ice food chain; Apherusa glacialis, Gammarus wilkitzkii, Onisimus nanseni, O. glacialis and Boreogadus saida. Cages / enclosures with these species will be deployed at the two observatories (during winter months), with potential information including survival, reproductive output and ecophysiological state. With the exception of the Onisimus spp for which the reproductive biology is only rudimentary known, these species are known to reproduce during winter (e.g. Weslawski & Legenskaya 2002, Berge et al 2012, Nahrgang et al in prep). They may hence be particularly vulnerable towards changes in the climate at this time of year, with a reduced sea ice extent as one of the most critical factors. Although a reduction of the Arctic sea ice is expected to be particularly severe during summer and autumn, sea ice cover and characteristics are likely to be affected also during the winter.
Main research question: To investigate the physiological and reproductive biology of key elements of the high-Arctic food chain in order to assess their susceptibility towards a reduced ice cover.
Approach: Through in situ experiments in Kongsfjorden and Rijpfjorden (see also case study 1), two fjords under contrasting climatic regimes (Wallace et al 2010), in which enclosures will be deployed and maintained throughout winter months. Growth, reproductive and physiological status will be examined and compared with natural populations (for the ice associated amphipods only during spring and autumn due to logistical challenges).
Case study 3: The role of visual versus non-visual (e.g tactile) predators during the polar night.
Predators in the pelagic generally use two main feeding modes; they either search for prey using vision (visual predators, e.g. many fish, birds and large zooplankton) or they search for prey by sensing vibrations and movements (tactile predators, many zooplankton). As a result, the prey encounter of visually searching predators is tightly bound to the light regime and prey encounter will be a function of day and night, time of the year and latitude. The extreme seasonality of high latitudes, including the polar night, creates a unique research laboratory for our endeavours to understand the relative roles of different prey encounter modes and for the functioning and constraints of visual predators in the north.
In a recent paper, Kraft et al (2012) documented the presence of Calanus prey in the stomachs of Themisto libellula and T. abyssorum collected within the Arctic during the darkest part of the polar night (80°N). These predators have been assumed to depend on visual detection of their prey items (Land 2000, Bowman 1960), which therefore raises important questions as to how they are able to detect their prey. Furthermore, Themisto are often found to be the dominating macrozooplankton component in many areas of the Arctic (Kraft et al 2012). New evidence of clearance rates and predation efficiency of the tactile predator Mertensia ovum on Calanus spp. have revealed that it may remove as much as one third of its prey population within one day, hence enforcing a very strong predation pressure (Majaneva et al in press). Clearly non-visual predators can exert significant pressure on zooplankton prey. Kaartvedt (2006) hypothesized that it is the light regime, and not climatic conditions, that restrict the northward distribution of planktivorous fish that otherwise dominate sub-Arctic seas, but are more or less absent from high Arctic seas. Hence, it follows that a reduced ice cover and increased temperatures will not automatically lead to a northward expansion of the distribution range of e.g. herring and capelin. It is therefore a need to study the mechanisms of prey detection from the dominating predators during the polar night (including benthic organisms studies in Unit 2), in order to gain insight into how a continued warming of the Arctic may lead to introductions of new and more boreal predators (e.g. Renaud et al 2011).
Main research questions: What is the main mode of predation during the polar night? How sensitive are key visually searching predators (such as the polar cod) to seasonal and diurnal changes in the light regime? Does bioluminescence play a role as light source (e.g. Haddock et al 2010)?
Approach: Lab experiments in contrasting light climates (ranging from absolute darkness to light levels characterising midwinter values within the core distribution range of herring and capelin) using a range of visual and tactile key predators (Themisto spp, Thysanoessa spp, Boreogadus saida, M. ovum and Parasagitta elegans) and their calanoid prey (methods outlined in Majaneva et al in press). Also, evidence of in situ predation will be gathered from stomach analyses (Kraft et al 2012 for methods). Bioluminescence of pelagic organisms will be measured and characterised using a AUV fitted with a bathyphotometer (Berge et al 2012), and will be related to both observed predation (through e.g. stomach analyses) and distribution of prey and predators. Models of prey encounter for visually searching fish predators (e.g. Varpe and Fiksen 2010) will be developed in order to predict prey encounter at different latitudes for given concentrations of prey and for given ice thickness and snow cover (central factors in modifying the light regime of the pelagic in the Arctic). These models can then inform us about likely shifts in species distributions, and for migrating species, how seasonality and species distributions interact.