A decline in primary production in the North Sea over 25 years, associated with reductions in zooplankton abundance and fish stock recruitment

Abstract

Phytoplankton primary production is at the base of the marine food web; changes in primary production have direct or indirect effects on higher trophic levels, from zooplankton organisms to marine mammals and seabirds. Here, we present a new time-series on gross primary production in the North Sea, from 1988 to 2013, estimated using in situ measurements of chlorophyll and underwater light. This shows that recent decades have seen a significant decline in primary production in the North Sea. Moreover, primary production differs in magnitude between six hydrodynamic regions within the North Sea. Sea surface warming and reduced riverine nutrient inputs are found to be likely contributors to the declining levels of primary production. In turn, significant correlations are found between observed changes in primary production and the dynamics of higher trophic levels including (small) copepods and a standardized index of fish recruitment, averaged over seven stocks of high commercial significance in the North Sea. Given positive (bottom-up) associations between primary production, zooplankton abundance and fish stock recruitment, this study provides strong evidence that if the decline in primary production continues, knock-on effects upon the productivity of fisheries are to be expected unless these fisheries are managed effectively and cautiously.

Reconstruction of Atlantic herring (Clupea harengus) recruitment in the North Sea for the past 455 years based on the δ13C from annual shell increments of the ocean quahog (Arctica islandica)

“…The correlation ( r = 0.45) between primary production and fish recruitment index observed by Capuzzo et al. () is similar to that which we found between MD and HR ( r = 0.50, Equation ). The slightly stronger covariance is not surprising as small copepods show a preference for consuming diatoms (Frederiksen, Edwards, Richardson, Halliday, & Wanless, ; Irigoien et al., , ) while constituting a staple of North Sea herring diet (Last, ; Segers, Dickey‐Collas, & Rijnsdorp, ).…”

“…In their study, Capuzzo et al. () found that the 25‐year decline in primary production caused a decline in small copepod abundances which, in turn, caused a decline in fish recruitment in seven commercially important fish stocks in the North Sea. The correlation ( r = 0.45) between primary production and fish recruitment index observed by Capuzzo et al.…”

“…Capuzzo et al. () showed that a decline in primary production in the North Sea for the years 1988–2013 produced a knock‐on effect on higher trophic levels, indicating a bottom‐up control of the local ecosystem. In their study, Capuzzo et al.…”

Land use alters trophic redundancy and resource flow through stream food webs

“…The changes in diets of primary consumers and predators in response to composition of resources across different land use indicate strong bottom‐up control of macroinvertebrate communities. Such bottom‐up control has been reported in various freshwater (Kiffney, Buhle, Naman, Pess, & Klett, ; Shurin, Clasen, Greig, Kratina, & Thompson, ), marine (Capuzzo et al., ) and terrestrial (Halvorson, Fuller, Entrekin, Scott, & Evans‐White, ; Lister & Garcia, ) ecosystems. Our results show that the bottom‐up effects of land use may extend beyond individual consumer–resource interactions and cascade to the structure of the entire food web.…”

What is multispecies MSY? A worked example from the North Sea

“…Whilst this may be a reasonable simplification for the North Sea, it would be instructive to compare it with alternative model frameworks that do not make this assumption (Blanchard et al, ; Spence et al, ). The study also assumes a constant environment, whereas in practice overall productivity and hence MSY changes with time (Capuzzo et al, ; Larkin, ).…”