Calcium carbonate alters the functional response of coastal sediments to eutrophication-induced acidification

Tarn P Drylie,Andrew M Lohrer,Adam Hartland,Hazel R Needham,Conrad A Pilditch

doi:10.1038/s41598-019-48549-8

Abstract

Coastal ocean acidification research is dominated by laboratory-based studies that cannot necessarily predict real-world ecosystem response given its complexity. We enriched coastal sediments with increasing quantities of organic matter in the field to identify the effects of eutrophication-induced acidification on benthic structure and function, and assess whether biogenic calcium carbonate (CaCO3) would alter the response. Along the eutrophication gradient we observed declines in macrofauna biodiversity and impaired benthic net primary productivity and sediment nutrient cycling. CaCO3 addition did not alter the macrofauna community response, but significantly dampened negative effects on function (e.g. net autotrophy occurred at higher levels of organic matter enrichment in +CaCO3 treatments than −CaCO3 (1400 vs 950 g dw m−2)). By identifying the links between eutrophication, sediment biogeochemistry and benthic ecosystem structure and function in situ, our study represents a crucial step forward in understanding the ecological effects of coastal acidification and the role of biogenic CaCO3 in moderating responses.

Highlights

Acidification of seawater via the increasing absorption of atmospheric CO2 is a key contemporary issue for the marine environment[1]
There was no significant interactive effect between CaCO3 and organic matter treatments on organic content (OC) or porewater pH, but there was a significant effect of organic matter treatment (Table 2)
We observed a linear increase in OC along the organic matter treatment gradient (r2 = 0.44, p < 0.001; Fig. 1b) with mean OC doubling from 2% in 0 g dw m−2 plots to 4% in 2250 g dw m−2 plots, and a linear decrease in porewater pH from 7.3 to 6.6 (r2 = 0.47, p < 0.001; Fig. 1c)

Summary

Introduction

Acidification of seawater via the increasing absorption of atmospheric CO2 (ocean acidification; OA) is a key contemporary issue for the marine environment[1]. Coastal benthic macrofauna communities exhibit predictable responses to organic matter enrichment, such as reduced abundance and diversity at high levels of loading[17] These responses are usually attributed to the onset of hypoxia[3], empirical evidence suggests some organisms respond more strongly to fluctuations in pH than O2 (e.g. juvenile clams)[18]. Occurring gradients of acidification have been utilised to investigate the effects on benthic communities in the field, for example by monitoring communities with increasing distance from shallow volcanic CO2 vents[27,28,29] Whilst these studies have provided insights into potential changes in community structure, including decreases in the abundance of calcareous organisms, and even evolutionary adaptations, there are frequently additional environmental variables that co-vary along the gradient (such as temperature and toxic trace elements) making it difficult to attribute the responses to acidification alone[30].

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Discussion

Conclusion