First Nansen Legacy PhD successfully defended

In his PhD, Elliot Sivel has used Chance and Necessity modelling (CaN) to investigate the variability of the Barents Sea ecosystem and its responds to human-induced stressors. Chance and Necessity modelling (CaN) is a stochastic mass-balanced food-web modelling framework. Chance represents the indeterminacy of ecological processes, while Necessity corresponds to the physical and biological constraints of food-webs. Using CaN models, Sivel addressed three main questions in his thesis.

Biomass and trophic flows

First, what are the possible food-web configurations of the Barents Sea in terms of biomass and trophic flows. Sivel simulated biomass and trophic flow trajectories to define a reference for the stochastic variability of the Barents Sea food-web.

He also reconstructed past trajectories of the Barents Sea food-web to identify if the past variability of the Barents Sea food-web is representative of its possible variability, and he found that the Barents Sea food-web could be characterized by four food-web configurations and three trophic pathways corresponding to gradients of biomass and trophic flows, respectively. The results also showed that food-web configurations observed in recent decades corresponded only to a fraction of possible configurations.

Trophic controls in the Barents Sea and Norwegian Sea

Second, Sivel explored trophic control in the Barents Sea food-web and the Norwegian pelagic food-web. Wasp-waist trophic control (**) was previously described in the Barents Sea, where capelin plays an essential role in transferring energy from lower trophic levels to higher ones. Sivel found that fluctuating trophic control is to be expected in the Barents Sea food-web. Trophic control in reconstructed dynamics displayed fluctuations at inter-decadal timescale.

For the pelagic food-web of the Norwegian Sea, Sivel reconstructed past dynamics for planktivorous fish species and zooplankton groups. The results are consistent with earlier reported bottom-up trophic control of planktivorous fish species by zooplankton groups, but not with top-down control on copepods and krill,suggesting that previous conclusion may need to be reevaluated.

Effects of climate change and fisheries on ecosystem stability

Third, Sivel investigated possible combined effects of climate change and fisheries on the stability of the Barents Sea food-web. He used a scenario-based approach for four scenarios of climate change and fisheries (16 scenarios in total). Changes in stability displayed synergism between temperature and fisheries given that temperature affected only harvested species.

Species biomass was significantly affected by changes in temperature and fisheries while stability was weakly affected. Weak changes in stability were explained by the positive relationship between mean biomass and biomass variance (Taylor’s law). Given that the measure of stability corresponds to the inverse of the coefficient of variation, proportional increase of mean biomass and biomass variance resulted in constant stability values.

Sivel contributed to a development of a standardized protocol

Dynamical models have a central role in Sivel’s work, and their performance needs to be evaluated in relation to the specific objectives of this thesis. Unfortunately, an evaluation protocol allowing model evaluation in a consistent and transparent manner did not exist when Sivel started this study. Therefore, Sivel contributed to the development of a standardized protocol for reporting the evaluation of model applications.

This protocol was applied to a series of ecosystem models used in the framework of the Nansen Legacy project, among which the NDND model. This protocol aims at increasing the transparency and the reproducibility of the model evaluation process by developing a culture of reporting and describing such process.

This PhD work was supervised by Benjamin Planque (IMR), Ulf Lindstrøm (IMR) and Nigel Yoccoz (UiT). The thesis can be downloaded here.

(**) Wasp-waist trophic control: an intermediate trophic level is expected to control the abundance of predators through a bottom-up interaction and the abundance of prey through a top-down interaction. Previous studies suggest that the North Sea is mainly governed by bottom-up interactions driven by climate perturbations.