Work Package 1: North Sea System Description

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


The objectives of this workpackage are to identify possible mechanisms behind documented ecosystem changes in North Sea biota by both reviewing past and recent literature and revisiting existing time series. This workpackage will:

  • Combine available data sets on biotic (plankton, zooplankton, fish) and abiotic (NAO, AMO) variables and asses strength of relationships;
  • Evaluate responses of the different life-history stages of key species and species groups to abiotic factors thereby creating the ecophysiological underpinning necessary to identify optimal habitats for members of the North Sea foodweb;
  • Assess the responses of key species, species groups and species life stages to biotic factors to understand key species trophic interactions within the North Sea foodweb.


Current understanding of the drivers of changes in North Sea ecosystem structure and function lacks a sufficient knowledge on historical (+50 years) ecosystem states in relation to climate, eutrophication and fisheries.  It is clear that spurulous conclusions regarding drivers of trophodynamic may result from merely inspecting time series of <30 years.  For example, sardine and anchovy are not novel species in the North Sea merely extending their habitat due to global warming.  These zooplanktivores were previously established in the system (i.e., 70 years ago) at a time when the AMO was in the same phase as currently observed.  Furthermore, it is now clear that categorizing the shorter-term (decadal-scale) atmospheric index (NAOI) into two phases is an over-simplification since novel analyses have yielded a third and fourth (i.e., “Blocking” and an “Atlantic Ridge”) regime (Cassou et al. 2004).

Although consideration of both the NAOI (four phases) and AMO (two phases) opens up a number of new avenues for understanding and interpreting past observed (climate driven) long-term changes in the North Sea ecosystem, causal explanations of ecosystem change will only be possible after understanding the relative roles of key abiotic (ecophysiological) and biotic (trophodynamic) factors on changes key species.  In terms of abiotic factors, Fry’s metabolic scope, scope for growth, and oxygen limited thermal tolerance are important concepts that have been used to define optimal habitats.  Measurements of organismal-level bioenergetics parameters offers a powerful route to predicting the impacts of abiotic environmental factors on the vital rates of organisms. Key parameters include energy gains via rates of growth and food consumption and energy losses via rates of respiration.

In terms of trophodynamic processes, complex relationships are possible. For example, the abundance of anchovy and sardine in some systems tend to negatively correlated and fluctuate based upon changes in the structure of lower trophic levels (e.g, sardine prefer larger zooplankton that feed on diatoms in intermittently mixed cold waters where anchovy prefer smaller zooplankton that feeding on flagellates occurring primarily in warmer, more stable water columns).  The large magnitude of documented changes in the relative abundance of species in lower- (e.g., copepods) and upper (large invertebrates and fish) trophic levels in the North Sea demands that key predator-prey relationships be identified.

Description of work

Task 1.1 Time series analysis of climate-related long-term ecosystem changes

Compilation and analysis of relevant biotic and abiotic long-term time series data using uni- and multivariate statistics. Relevant time-series include those on plankton from the CPR-survey and Helgoland Roads as well fishery statistics (IBTS survey) and benthos, etc. By comparing these series to climatic indices (NAO, AMO, EAP), temporal and spatial

patterns will emerge, such as regular cyclic changes and/or regime shifts. Particular emphasis will be placed on the relative importance of advection processes regulating fluxes of organisms into the North Sea through the Channel versus the northern boundary.

Task 1.2 Responses of (key) species to abiotic factors (Ecophysiology)

Compilation and analysis of datasets on the response of key trophic players including phytoplankton (e.g., diatoms, dinoflagellates, coccolithophores), zooplankton such as copepods (e.g., Calanus, Temora, Pseudocalanus, Acartia, Oithona, etc.) and fish species including both small pelagic “wasp-waist” (sprat, sandeel, herring, sardine, anchovy) and larger piscivorous top predators to abiotic factors (e.g., temperature, oxygen, salinity, turbulence, light). For key species, life history information will be compiled, since the response of species to abiotic factors is frequently stage-specific. Response surfaces will be generated to define optimal abiotic habitat characteristics.  In the absence of detailed (laboratory-based) physiological data, a statistical approach will be applied to derive preference and limits based upon realized (occupied) habitats from spatially- and temporally resolved field data (CPR, IBTS) and hydrographic measurements.

Task 1.3 Responses of (key) species to biotic factors (Trophic Coupling)

Diet data on planktivorous fish species and life stages will be extracted from the literature with the aim of identifying the degree to which life stages of individual species rely on lower trophic level resources and identify links between changes lower and upper trophic levels. Since information on the size-specific feeding of sardine and anchovy is completely absent from the North Sea, a limited investigation quantifying diet selectivity of these zooplanktivores species and their potential competitors (herring, sprat, sandeel, mackerel and horse mackerel) is intended. Samples will be taken on standard cruises (ICES hydro-acoustic survey, IBTS, University Hamburg surveys). Time series of predator abundances and species or group specific fish and plankton predation will be available from the results of the BECAUSE project (EU F6).