Empirical evidence reveals seasonally dependent reduction in nitrification in coastal sediments subjected to near future ocean acidification.

Ulrike Braeckman,Katja Guilini,Carl Van Colen,Magda Vincx,Karline Soetaert,Jan Vanaverbeke,Dirk Van Gansbeke,Kay C Vopel

doi:10.1371/journal.pone.0108153

Ulrike Braeckman, Katja Guilini + Show 6 more

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https://doi.org/10.1371/journal.pone.0108153

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Abstract

Research so far has provided little evidence that benthic biogeochemical cycling is affected by ocean acidification under realistic climate change scenarios. We measured nutrient exchange and sediment community oxygen consumption (SCOC) rates to estimate nitrification in natural coastal permeable and fine sandy sediments under pre-phytoplankton bloom and bloom conditions. Ocean acidification, as mimicked in the laboratory by a realistic pH decrease of 0.3, significantly reduced SCOC on average by 60% and benthic nitrification rates on average by 94% in both sediment types in February (pre-bloom period), but not in April (bloom period). No changes in macrofauna functional community (density, structural and functional diversity) were observed between ambient and acidified conditions, suggesting that changes in benthic biogeochemical cycling were predominantly mediated by changes in the activity of the microbial community during the short-term incubations (14 days), rather than by changes in engineering effects of bioturbating and bio-irrigating macrofauna. As benthic nitrification makes up the gross of ocean nitrification, a slowdown of this nitrogen cycling pathway in both permeable and fine sediments in winter, could therefore have global impacts on coupled nitrification-denitrification and hence eventually on pelagic nutrient availability.

Highlights

Over the past 250 years, the atmospheric CO2 concentrations have increased by nearly 40% as a consequence of human activities [1]
Climate change models predict a further decrease of 0.35 units by the end of the century for open ocean waters [3,4] and recent measurements for coastal zones even reveal acidification rates that are an order of magnitude higher [5,6]
Our results indicate that ocean acidification in coastal sediments reduces sediment community oxygen consumption and nitrification, but does not significantly affect total N mineralization

Summary

Introduction

Over the past 250 years, the atmospheric CO2 concentrations have increased by nearly 40% as a consequence of human activities [1] This increase is partly counteracted by the capacity of the oceans to absorb CO2, which occurs currently at a rate of about 106 metric tons of CO2 per hour [2]. Climate change models predict a further decrease of 0.35 units by the end of the century for open ocean waters [3,4] and recent measurements for coastal zones even reveal acidification rates that are an order of magnitude higher [5,6] This decrease in pH is known to have a direct or indirect negative effect on many nektonic, pelagic and benthic organisms [7,8,9,10] and marine food web structures in general [11]. A decrease in seawater pH has been shown to affect both macrobenthic bioturbation and bio-irrigation activities [18,22,24], which alter redox gradients and microbial communities that regulate the cycling of energy and matter

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