Special Report: It’s all about timing!

The research project “Climate effects on planktonic food quality and trophic transfer in Arctic Marginal Ice Zones II” (CLEOPATRA II) has, after three years of hard field and laboratory work combined with modelling work, achieved new knowledge on the complexity in high-Arctic sea ice ecosystems, and the importance of timing of reproduction and growth in such strong seasonal environments.

In strong seasonal environments, marine organisms must carefully consider when to invest in growth and reproduction, and when to stay inactive to save energy during unfavourable conditions. The high-Arctic light climate is extreme with up to four months of continuous darkness, and in seasonal ice-covered seas this can be up to nine months of pitch darkness if sea ice and snow conditions are severe. Depending on the prevailing sea ice and snow conditions, the onset of the algal spring bloom may start as early as April or as late as August.

First, small microscopic algae specialised to photosynthesise under extreme low light intensities, start to grow within and at the bottom of the sea ice. Then, when the sea ice starts to melt and break-up, small microscopic algae in the water column gets sufficient light for photosynthesis and the phytoplankton bloom to start. These algal blooms, and not the calendar, set the spring date, fuelling the entire system with fresh food rich in omega-3 and omega-6 fatty acids that are essential for successful reproduction, growth and development of all organisms in marine ecosystems, as well as for human health.

Algae grazed upon by zooplankton are the fundament for the rich marine life in the Arctic. Among Arctic zooplankton, the relatively large copepod Calanus glacialis (4-5mm), which is strictly Arctic, comprise up to 80% of the zooplankton biomass in seasonal ice-covered shelf seas. Due to its ability to convert low-energy carbohydrates and proteins in algae into high-energy wax ester lipids, it is extremely lipid-rich (50-70% lipids of its dry weight) and thus a very important food item for higher trophic levels, such as fish, birds and mammals. The fate of this nutritious copepod in a warmer Arctic is thus of vital importance to predict the vulnerability of Arctic marine ecosystems to climate change.

 Winter dormancy at depth and timing of ascent

 Calanus glacialis is a seasonal migrator, with older developmental stages staying in deeper waters in dormancy during the dark, food-poor winter. It is assumed to migrate up to surface waters again in spring when algal food become plentiful, but we revealed during our monthly field campaign throughout an entire year that it actually starts to migrate up much earlier, in midwinter, three to four months prior to the spring bloom.

Seasonal changes in the activity of key enzymes of specific metabolic and catabolic pathways can be used as proxies to assess the overall activity of an organism. In our study, C. glacialis reduced its metabolic enzyme activity to half when residing at the overwintering depth. True diapause is defined as arrested development and reproduction, and reduced metabolic activity in a torpid state. For one year we measured different metabolic parameters in monthly intervals and found a clear seasonal pattern with lower oxygen consumption and low digestive enzyme activity (proteinase and lipase/esterase) in winter compared to summer for C. glacialis. The diapause intensity of C. glacialis seems to be less and with a greater potential for flexible physiological adjustments compared to the profound metabolic adjustments found in copepods from the deep open ocean. Maybe the term ‘active diapause’, which implies arrested development and reproduction, but no profound reduction in activity, is more appropriate for this shelf copepod. In future studies, we will use genetic approaches to better understand intra and interpopulational differences in Calanus diapause patterns.

Reproduction

The degree to which the reproduction in Calanus glacialis relies on stored energy reserves (capital breeding) and/or intake of fresh algal food (income breeder) was one key question we studied extensively in field, laboratory and by modelling exercises. We found that females can utilise stored energy and produce viable eggs prior to the spring bloom (i.e. perform capital breeding), but that the egg production rates of these starved females was an order of magnitude lower than that of females actively feeding. In the field, high egg production rates were first observed when food became plentiful. Despite actively feeding, females almost depleted their internal lipid resources, suggesting that both stored and fresh input of energy is important for successful reproduction in C. glacialis.

Interestingly, the amount of lipids invested per egg varied with food availability, suggesting a reproduction strategy not previously described for C. glacialis: when food is absent or low, females invest in fewer but more lipid-rich eggs; when food is favourable, females produce many, but rather lipid-poor eggs. This strategy increases the likelihood of offspring surviving until more favourable food conditions appear, and maximises offspring production when algal food conditions are good.

So far our results show that C. glacialis has evolved very flexible life history strategies. Especially our studies during the dark Arctic winter have given us new important knowledge of the life strategy of Calanus glacialis in Arctic shelf seas. Food is important for successful reproduction and recruitment, but strategies are in place to cope with the unpredictability and high variability in the timing of the Arctic spring bloom. Many metabolic pathways during winter are more active than previously assumed, and activated long before the spring algal bloom commences. This may prove favourable for C. glacialis in a rapid changing Arctic, but competition for resources and changed predation pressure due to invasion of temperate species into the Arctic marine ecosystems yet remains to be investigated.

See www.mare-incognitum for publications and papers in preparations.

Fact box:

CLEOPATRA II is funded by the Norwegian Research Council, project no.: 216537, 2012-2015, and Helmholtz Graduate School for Polar and Marine Research (POLMAR) at the Alfred Wegener Institute (AWI), Germany. It is headed by the University Centre in Svalbard (UNIS) and is one of several projects organised under the Mare Incognitum project umbrella (www.mare-incognitum.no).

CLEOPATRA II’s international partners are Germany (AWI), Poland (Institute of Oceanology, Polish Academy of Science, IOPAS) and Russia (Institute of Oceanology, Russian Academy of Sciences, RAS), whereas national partners include the Norwegian Polar Institute (NPI), Akvaplan-niva (APN), and UiT The Arctic University of Norway.

 

Associate Professor Janne E Søreide

Dr Scient, Marine Biology

The University Centre in Svalbard (UNIS)

Department of Arctic Biology

 

tel: +47 79023300

 

www.mare-incognitum.no