Increasing nutrient stress reduces the efficiency of energy transfer through planktonic size spectra

Angus Atkinson,Tim J Smyth,Andrew G Hirst,Martin K.S Lilley,Claire Widdicombe,Elaine S Fileman,Paul J Somerfield,Katrin Schmidt,Glen A Tarran,E Malcolm S Woodward,Andrea J Mcevoy

doi:10.1002/lno.11613

Abstract

AbstractSize‐spectral approaches quantify the efficiency of energy transfer through food webs, but theory and field studies disagree over how changes in temperature, nutrients, and extreme weather impact on this efficiency. We address this at two scales: via 6 years of weekly sampling of the plankton size spectrum at the Plymouth L4 shelf sea site, and via a new, global‐scale, meta‐analysis of aquatic size spectra. The time series showed that with summertime nutrient starvation, the energy transfer efficiency from picoplankton to macroplankton decreased (i.e., steepening slopes of the size spectra). This reflected increasing dominance by small cells and their microbial consumers. The extreme storms in winter 2013/2014 caused high metazoan mortality, steep size‐spectral slopes, and reduced plankton biomass. However, recovery was within months, demonstrating an inbuilt resilience of the system. Both L4 and our meta‐analysis showed steep slopes of normalized size spectra (median −1.11). This reflects much lower values, either of trophic transfer efficiency (3.5%) or predator–prey mass ratio (569), compared to commonly quoted values. Results from the meta‐analysis further showed that to represent energy transfer faithfully, size spectra are best constructed in units of carbon mass and not biovolume, and span a mass range of > 107. When this range is covered, both the meta‐analysis and time series show a dome‐shaped relationship between spectral slopes and plankton biomass, with steepening slopes under increasingly oligotrophic and eutrophic conditions. This suggests that ocean warming could decrease the efficiency of energy transfer through pelagic food webs via indirect effects of increasing stratification and nutrient starvation.

Highlights

IntroductionIncreases in ocean temperature, stratification, nutrient shortage, and the frequency of extreme
In a warming climate, increases in ocean temperature, stratification, nutrient shortage, and the frequency of extreme
Size-spectral approaches quantify the efficiency of energy transfer through food webs, but theory and field studies disagree over how changes in temperature, nutrients, and extreme weather impact on this efficiency

Summary

Introduction

Increases in ocean temperature, stratification, nutrient shortage, and the frequency of extreme. Temperature can affect zooplankton both directly through their physiology, and indirectly, for example, through changing stratification and nutrient supply, which impact on the abundance, size, and quality of their food (Schmidt et al 2020) Both direct and indirect effects of climatic warming impact on the efficiency of energy transfer through food webs (Morán et al 2010), making this a key emergent property to assess. Energy flow through planktonic size spectra transfer, and the number of these transfers within any given size spectrum To address such processes, size-spectrum approaches have had a long history of development (Elton 1927; Sheldon et al 1972; Platt and Denman 1978; Sprules and Barth 2016; see following section “Theoretical overview”). One of their key advantages is that the slope of the size spectrum (i.e., the rate of decrease in biomass with increasing organism size) provides a measurable index of the combined effects of the various inefficiencies, each of which is very hard to measure across a whole assemblage (Jennings et al 2002)

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Conclusion