Carbon export is facilitated by sea urchins transforming kelp detritus

Abstract

With the increasing imperative for societies to act to curb climate change by increasing carbon stores and sinks, it has become critical to understand how organic carbon is produced, released, transformed, transported, and sequestered within and across ecosystems. In freshwater and open-ocean systems, shredders play a significant and well-known role in transforming and mobilizing carbon, but their role in the carbon cycle of coastal ecosystems is largely unknown. Marine plants such as kelps produce vast amounts of detritus, which can be captured and consumed by shedders as it traverses the seafloor. We measured capture and consumption rates of kelp detritus by sea urchins across four sampling periods and over a range of kelp detritus production rates and sea urchin densities, in northern Norway. When sea urchin densities exceeded 4 m−2, the sea urchins captured and consumed a high percentage (ca. 80%) of kelp detritus on shallow reefs. We calculated that between 1.3 and 10.8 kg of kelp m−2 are shredded annually from these reefs. We used a hydrodynamic dispersal model to show that transformation of kelp blades to sea urchin feces increased its export distance fourfold. Our findings show that sea urchins can accelerate and extend the export of carbon to neighboring areas. This collector–shredder pathway could represent a significant flow of small particulate carbon from kelp forests to deep-sea areas, where it can subsidize benthic communities or contribute to the global carbon sink.

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Photographs taken by T Wernberg and K Filbee-Dexter

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Acknowledgements

This work was funded by the Norwegian Research Council through the KELPEX project (NRC Grant no. 255085). The modeling component was funded by the Norwegian Research Council through the KELPFATE project (NRC Grant no. 160016/F40) and by the Norwegian Blue Forest Network through the Norwegian Institute for Water Research’s KELPFLOAT project (NIVA project no. 180144.211). TW received funding from The Australian Research Council (DP170100023). Sabine Popp, Camilla with Fagerli, Eva Ramirez-Llodra, and Nicolai Lond Frisk assisted with field work. Eva Ramirez-Llodra and Torstein Pedersen provided insightful comments and edits to the manuscript.

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KFD, MP, TW, KMN, and SF conceived and designed the study and conducted the fieldwork. ER, TK, KFD and JA developed and analyzed the modelling component. KFD analyzed the field data and led the writing of the manuscript. All authors discussed the results and contributed to the writing.

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Correspondence to Karen Filbee-Dexter.

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Communicated by James Fourqurean.

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Filbee-Dexter, K., Pedersen, M.F., Fredriksen, S. et al. Carbon export is facilitated by sea urchins transforming kelp detritus. Oecologia 192, 213–225 (2020). https://doi.org/10.1007/s00442-019-04571-1

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Keywords

  • Shredders
  • Laminaria hyperborea
  • Marine
  • Subsidy
  • Blue carbon