Coral skeletons reveal the history of nitrogen cycling in the coastal Great Barrier Reef

Dirk V Erler,Natasha L Carlson-Perret,Hanieh Tohidi Farid,Janice M Lough,Thomas D Glaze

doi:10.1038/s41467-020-15278-w

Dirk V Erler, Natasha L Carlson-Perret + Show 3 more

Open Access

https://doi.org/10.1038/s41467-020-15278-w

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Abstract

Anthropogenic nutrient discharge to coastal marine environments is commonly associated with excessive algal growth and ecosystem degradation. However in the world’s largest coral reef ecosystem, the Great Barrier Reef (GBR), the response to enhanced terrestrial nutrient inputs since European settlement in the 1850’s remains unclear. Here we use a 333 year old composite record (1680–2012) of 15N/14N in coral skeleton-bound organic matter to understand how nitrogen cycling in the coastal GBR has responded to increased anthropogenic nutrient inputs. Our major robust finding is that the coral record shows a long-term decline in skeletal 15N/14N towards the present. We argue that this decline is evidence for increased coastal nitrogen fixation rather than a direct reflection of anthropogenic nitrogen inputs. Reducing phosphorus discharge and availability would short-circuit the nitrogen fixation feedback loop and help avoid future acute and chronic eutrophication in the coastal GBR.

Highlights

Anthropogenic nutrient discharge to coastal marine environments is commonly associated with excessive algal growth and ecosystem degradation
On the Great Barrier Reef (GBR), the world’s largest coral reef ecosystem, numerical estimates[11,12,13,14] and coral skeleton proxy data[15,16,17,18] clearly show that particulate and dissolved nutrient inputs have been increasing since European settlement in the
How has N cycling in the coastal GBR responded to increased nutrient input since the 1850s, and how can we address this question without long-term records of N cycle process rates in the coastal GBR? Here, we use the 15N/14N (i.e. δ15N, where δ15N = [(15N/14N)sample/(15N/14N)air] − 1) of N trapped in the organic skeletal matrix of massive reef-building corals to understand how N cycling in the coastal GBR has responded to anthropogenic nutrient discharge since European settlement

Summary

Introduction

Anthropogenic nutrient discharge to coastal marine environments is commonly associated with excessive algal growth and ecosystem degradation. The idea that N2 fixation has increased in the coastal GBR as a result of European activity has been contentious and difficult to prove without long-term (i.e. pre-European) records[34]. When combined with the water sample δ15N data, these trends suggest that N2 fixation has increased in the coastal GBR since European settlement.

Results

Conclusion