As the ocean’s major primary producers, phytoplankton are the base of the marine food web, with changes to their abundance, biomass, and community composition resonating upwards through zooplankton to fish, seabirds, and cetaceans. Our understanding of the direct relationships between different levels of the food web is often challenged by a lack of biological data, the confounding effects of multiple pressures (including climate change), and spatial variability in marine environments. From a policy perspective, however, knowing how changes in one part of the food web impacts other parts of the food web is critically important to successfully managing marine biodiversity and fisheries.

As a shelf sea, the North Sea is highly productive, with commercial fisheries a key cultural and economic sector for most countries in the region. In European countries fishing is managed through the EU’s Common Fisheries Policy (CFP). The CFP ensures regional countries are allocated fishing quotas for commercial fish stocks. Though many European fish stocks continue to be overfished, some of recently recovered, due to improved fisheries management (Engelhard, Lynam, Garcıa-Carreras, Dolder, & Mackinson, Environ Cons, 2015). Fishing, however, is not the only regulator of commercial fish stocks. The health of fish stocks is also dependent on the health of lower trophic (food web) levels, such as plankton and forage fish, which support commercial stocks as prey. This ‘bottom-up’ regulation of fish stocks is influenced by climate change and natural ecological variability as well as direct human pressures such as eutrophication, which decreases water clarity and increases nutrients resulting in changes in plankton communities.

Interannual variation in annual primary production (PP), mean abundance of small copepods, and large copepods, and a standardized index of fish stock recruitment (including sandeel, sprat, herring, Norway pout, cod, haddock and whiting), in the North Sea

Using long-term biological time-series, our new paper found that during the past 25 years, primary productivity has significantly decreased throughout the North Sea due to warming sea surface temperatures and decreasing anthropogenic nutrient inputs (the decreasing nutrients are a positive response to improved sewage treatment and farming practices in the EU). A corresponding decrease was also found in the abundance of small copepods, which graze on phytoplankton, and the recruitment of key commercial fish species such as sandeel, sprat, herring, Norway pout, cod, haddock, and whiting. These changes suggest bottom-up control of fish stock productivity in the North Sea – from climate and nutrients to primary production (phytoplankton), and from primary production to zooplankton and fish recruitment.

The recognition that fishing is not the sole pressure influencing North Sea fish stocks could be used to set more realistic fisheries quotas, which consider changes in plankton productivity as a key influence on commercial fish stocks. This holistic vision of the marine ecosystem is exemplified in the ecosystem approach to management, which focuses on integrated management of marine ecosystems to reach sustainability. Though information from lower trophic levels is not yet routinely used to set CFP quotas, the food web links described in this paper are already being used to manage biodiversity through the EU’s Marine Strategy Framework Directive (MSFD). In fact, one of OSPAR’s MSFD pelagic habitats indicators, Change in phytoplankton and zooplankton communities, serves as an indicator of both plankton and food web change, information which is used to set management measures for OSPAR countries. As our knowledge about food web dynamics develops, we have the opportunity to manage our marine ecosystems in an increasingly sustainable way.

Abigail, Plankton and Policy

Read more:

Capuzzo, E., Lynam, C.P., Barry, J., Stephens, D., Forster, R.M., Greenwood, N., McQuatters-Gollop, A., Silva, T., Sonja M. van Leeuwen and Engelhard, G.H., (2017). A decline in primary production in the North Sea over twenty-five years, associated with reductions in zooplankton abundance and fish stock recruitment. Global Change Biology, doi: 10.1111/gcb.13916.