Work Package 2: North Sea System Modelling

Work package number: 1
Work package coordination: 4, 3
Work package title: North Sea System Modelling
Participant id: 1, 2, 3, 5, 6

Objectives

A suite of existing models will be employed to hindcast both physical (hydodynamic) and trophodynamic relationships among lower (NPZD) and upper (e.g., MSVPA) within the North Sea. Biophysical relationships of this complex system will be retrospectively examined in this workpackage. To accomplish this, activities within this WP include to:

• Adapt hydrodynamic models to the project requirements including validation and calibration exercises.
• Employ coupled biophysical NPZD models to analyse lower trophic levels with a special focus on parameterisations for key functional groups.
• Utilize available models to derive multi-species relationships that represent historical changes in the upper trophic levels of the North Sea foodweb resulting from predator prey interactions.

Background

The impact of climate variability on ocean hydrodynamics (including water transport, temperature and stratification) is one of the major driving mechanisms of ecosystem change within the North Sea. Most marine species are planktonic during at least one or more life stages and, for these organisms, atmospherically forced (wind-driven) water currents are the primary transport mechanism of ingress to (and the dominant force influencing spatial distributions within and connectivity between) North Sea habitats.

Hydrodynamic properties also set ecophysiological limits on realized (exploitable) habitats, phenology within those habitats and, along with advection, influence shifts in species distributions. In order to understand and hope to predict changes in ecosystem structure and function, proper knowledge regarding hydrodynamic variability is essential. Hydrodynamics models available through ECODRIVE partners that provide highly resolved (3-D), time series of key physical features of the ecosystem (i.e., temperature, salinity, stratification, turbulence) including advective exchange with adjacent areas.

An ensemble of different hydrodynamic models (NORWECOM, HAMSOM and ECOSMO) are available for this purpose, each of which has been extensively validated (e.g., Janssen 2002, Janssen et al. 2001, Delhez et al. 2004, Pohlmann 1996). Biophysical models incorporating lower trophic level dynamics (NPZD models) allow the impact of drivers such as eutrophication (via increased river loadings of nutrients, carbon and carbonates) and climate variability on phytoplankton and zooplankton to be estimated. These model-based estimates can also be utilized as prey fields to incorporate climate-driven forcing on bottom-up processes potentially regulating the vital rates of key species. In terms of top down ecosystem processes, cohort based population models currently employed by ECODRIVE partners to assess commercial fish stocks for a large number of fish species.

Future predictions with these cohort based models rely entirely on the availability of future recruitment levels in combinations with scenarios on age specific fishing mortalities. Multi-species models (e.g. MSVPA, 4M and SMS) link the cohort models of a number of predators and prey allowing more realistic predictions than single species models, especially in situations where a major predator populations recover (Vinther 2001, Kempf et al. 2007, Floeter et al. 2005).

Description of work

Task 2.1 Hydrodynamic modeling

Hindcast simulations of the hydrodynamics within the North Sea and adjacent areas will be run for least 30 years. A common validation exercise will be used to compare long-term variability and trends in the North Sea hydrodynamics. Furthermore, direct and indirect output parameters including temperature, salinity, currents, turbulence and thermocline characteristics (depth, strength and extension), frontal zones and eddy activities (retention areas), flushing times, and heat flux-related indices will be examined. A multi decadal database of these ecosystem relevant parameters including monthly climatologies will be produced for the North Sea shelf area and regional sub-areas.

Task 2.2 Modelling Lower Trophic Levels

Simple as well as more advanced ecosystem models with different phytoplankton- (diatoms, flagellates, cocolithophores) and zooplankton groups (meso-, microzooplankton) will be applied in retrospective simulations. The outcome of the different models will be compared in order to judge to which extend even simple models are able to give satisfying results. Regime shifts and variations of selected plankton species can be depicted only by models with higher resolutions than the simple NPZD models.

Task 2.3 Modelling Upper Trophic Levels

Runs of the multi-species model SMS will be used to provide detailed age-specific population abundance estimates for most commercially-exploited fish species. Simulations will cover the period from1963 to the present and will be compared to observed recruitment data that are available for most fish species since 1981. These SMS model estimates will be compared with hydrodynamic and NPZD model outputs within WP3 to contribute to the understanding of trophic interactions and drivers of changes in ecosystem structure.