Abstract
Diatoms are so important in ocean food-webs that any human induced changes in their abundance could have major effects on the ecology of our seas. The large chain-forming diatom Biddulphia biddulphiana greatly increases in abundance as pCO2 increases along natural seawater CO2 gradients in the north Pacific Ocean. In areas with reference levels of pCO2, it was hard to find, but as seawater carbon dioxide levels rose, it replaced seaweeds and became the main habitat-forming species on the seabed. This diatom algal turf supported a marine invertebrate community that was much less diverse and completely differed from the benthic communities found at present-day levels of pCO2. Seawater CO2 enrichment stimulated the growth and photosynthetic efficiency of benthic diatoms, but reduced the abundance of calcified grazers such as gastropods and sea urchins. These observations suggest that ocean acidification will shift photic zone community composition so that coastal food-web structure and ecosystem function are homogenised, simplified, and more strongly affected by seasonal algal blooms.
Highlights
Diatoms are dominant marine primary producers, accounting for ~40% of ocean primary production [1]
Research into the response of diatoms to ocean acidification has mostly focussed on their growth, productivity, and community composition by using mesocosms or flask culture experiments [5], with predominantly positive effects observed
Projections of the effects of ocean acidification on diatoms suggest that increased availability of CO2 as a substrate for photosynthesis will benefit these algae where sufficient nutrients are available [6], and that these algae may indirectly benefit through reduced grazing pressure [7]
Summary
Diatoms are dominant marine primary producers, accounting for ~40% of ocean primary production [1]. Research into the response of diatoms to ocean acidification has mostly focussed on their growth, productivity, and community composition by using mesocosms or flask culture experiments [5], with predominantly positive effects observed. Carbon dioxide-driven shifts towards larger diatoms has been observed in ocean acidification experiments using natural communities [7,11,12,14,15,16,17], suggesting that larger diatoms may become favoured in the future [5]. Experiments using natural communities of marine organisms are useful when projecting the impacts of ocean acidification as these help assess cascading effects through trophic levels that can only be assessed when interactions and competition among species are considered [18]. We investigated how ocean acidification influenced the abundance, photophysiology, and habitat-provisioning of this large diatom
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