WP 3: Carrying capacity and energy flow through the food web

During cold periods, the phytoplankton blooms in Arctic waters are short, intense and closely connected to the melting of sea ice, whereas warmer periods bring slow and prolonged blooms of much higher production (Skjoldal et al 1987, Rey et al 1987, Skjoldal and Rey 1989). The seasonality and interannual fluctuations will be explored using a combination of observations, satellite images and models (NORWECOM.E2E model; Utne et al. 2012; Hjøllo et al. 2012). The flow of energy from primary production will ultimately limit upper trophic-level fishery yields. Hence, bottom-up processes are very important, as changes in climatic conditions (e.g. warming in the recent years) will influence the sea ice extent, timing and magnitude of blooming, and thus likely influence the primary productivity of the ocean. Remote sensing data at a finer temporal and spatial resolution will be explored in collaboration with Professor Kevin Arrigo, Department of Environmental Earth System Science, Stanford University, USA, who is an expert in this field and has agreed to be a part of this work package. Satellite data are important not only to explore spatial patterns in net primary production, but also to examine timing and magnitude of blooming and how it is affected by climate variability. The observations of chlorophyll and nutrients from the ecosystem cruises in the Barents Sea will be combined with satellite data for validation and comprehensive data analyses.

Bottom-up vs top-down control has been discussed in several papers (e.g. Yaragina and Dolgov 2009). Based on their observations of the North Sea, Kirby et al. (2009) point out the need of considering top down and bottom-up control together, to fully understand mechanisms of the marine ecosystems. In this work package we will also focus on the predation pressure from key fish species and zooplankton in addition to climate impact on new primary production (NPP), and how this will influence the zooplankton production and higher trophic levels. Production and links in warmer periods will be compared to those during cold periods.

With the exceptionally large stocks of demersal fish present in the Barents Sea at the time being, the carrying capacity of selected species is of particular interest, not only to researchers, but also to those involved in management and fisheries. The carrying capacity at different trophic levels can be tested in ecosystem models e.g. Atlantis model incorporated in WP1, by varying the biomass of the species of interest and study the effect on its growth and on the ecosystem as a whole.

In cold and warm periods, the links between phytoplankton and zooplankton may be quite different. The amount of energy transferred from phytoplankton production through the food chain depends on the number of levels it has to pass through, with shorter food chains having a much more efficient energy transfer. The food web topology developed in WP5, together with the diet analysis from WP2 is an important basis for this particular part of WP3. The information from WP3 will be fed into the available ecosystem models (Atlantis model in WP1) and they will be used to further explore differences between the cold and warm periods, with respect to recruitment, individual condition, distributions and energy transfer between different trophic levels.

The main aim of the WP3 will be to deliver answer to some of the focal questions:

- How does the plankton production vary between seasons and years?

- How do the different trophic levels respond to climate warming?

- What are the links between climate, new primary production (NPP), secondary production and fish biomass?

- What is the carrying capacity for key pelagic species and how variable is it?

- What is the carrying capacity at different trophic levels based on estimates of primary production?