Marine Microphytobenthic Assemblage Shift along a Natural Shallow-Water CO2 Gradient Subjected to Multiple Environmental Stressors

Vivienne Johnson,Marco Milazzo,Colin Brownlee,Jason Hall-Spencer

doi:10.3390/jmse3041425

Abstract

Predicting the effects of anthropogenic CO2 emissions on coastal ecosystems requires an understanding of the responses of algae, since these are a vital functional component of shallow-water habitats. We investigated microphytobenthic assemblages on rock and sandy habitats along a shallow subtidal pCO2 gradient near volcanic seeps in the Mediterranean Sea. Field studies of natural pCO2 gradients help us understand the likely effects of ocean acidification because entire communities are subjected to a realistic suite of environmental stressors such as over-fishing and coastal pollution. Temperature, total alkalinity, salinity, light levels and sediment properties were similar at our study sites. On sand and on rock, benthic diatom abundance and the photosynthetic standing crop of biofilms increased significantly with increasing pCO2. There were also marked shifts in diatom community composition as pCO2 levels increased. Cyanobacterial abundance was only elevated at extremely high levels of pCO2 (>1400 μatm). This is the first demonstration of the tolerance of natural marine benthic microalgae assemblages to elevated CO2 in an ecosystem subjected to multiple environmental stressors. Our observations indicate that Mediterranean coastal systems will alter as pCO2 levels continue to rise, with increased photosynthetic standing crop and taxonomic shifts in microalgal assemblages.

Highlights

The current rate of CO2 release into the atmosphere is driving geochemical changes in the ocean that are thought to be unparalleled in the last 300 million years [1]
Researchers have recently begun to examine the potential effects of ocean acidification on microphytobenthic assemblages since they are known to play a crucial role in coastal ecosystems [21,22]
Since ocean acidification is occurring alongside a variety of other anthropogenic changes, studies of marine photoautotrophs have started to address the interactive effects of multiple stressors [45]

Summary

Introduction

The current rate of CO2 release into the atmosphere is driving geochemical changes in the ocean that are thought to be unparalleled in the last 300 million years [1]. Down-regulation of CCM activity has been observed when microalgae have been grown in high CO2 conditions [4,5]. The responses of microalgae to ocean acidification caused by rising pCO2 levels could have wide-ranging ramifications for ocean health globally since they drive biogeochemical cycles and contribute significantly to global primary production [8]. Ocean acidification is not proceeding in isolation; interactive effects of elevated CO2 with other changing environmental conditions such as temperature [9,10], light [11], nutrients [12], metal toxicity [13] and pollution (e.g., sewage, petroleum wastes, pesticides, agricultural run-off) [14] may occur, complicating the prediction of ecosystem responses to rising CO2 levels

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