Abstract
Ocean warming is altering the biogeographical distribution of marine organisms. In the tropics, rising sea surface temperatures are restructuring coral reef communities with sensitive species being lost. At the biogeographical divide between temperate and tropical communities, warming is causing macroalgal forest loss and the spread of tropical corals, fishes and other species, termed "tropicalization". A lack of field research into the combined effects of warming and ocean acidification means there is a gap in our ability to understand and plan for changes in coastal ecosystems. Here, we focus on the tropicalization trajectory of temperate marine ecosystems becoming coral-dominated systems. We conducted field surveys and in situ transplants at natural analogues for present and future conditions under (i) ocean warming and (ii) both ocean warming and acidification at a transition zone between kelp and coral-dominated ecosystems. We show that increased herbivory by warm-water fishes exacerbates kelp forest loss and that ocean acidification negates any benefits of warming for range extending tropical corals growth and physiology at temperate latitudes. Our data show that, as the combined effects of ocean acidification and warming ratchet up, marine coastal ecosystems lose kelp forests but do not gain scleractinian corals. Ocean acidification plus warming leads to overall habitat loss and a shift to simple turf-dominated ecosystems, rather than the complex coral-dominated tropicalized systems often seen with warming alone. Simplification of marine habitats by increased CO2 levels cascades through the ecosystem and could have severe consequences for the provision of goods and services.
Highlights
Rising anthropogenic carbon dioxide levels are fundamentally altering the physicochemical and biological properties of the ocean, driving changes in marine biogeography
The first with present-day conditions that is representative of a temperate ecosystem with abundant kelp forests (Biodiversity Center of Japan, 2019; Serisawa, Akino, et al, 2002; Wada et al, 2007), the second as an analogue of future conditions under warming whereby the ecosystem has recently shifted from a temperate-type ecosystem to a more tropicalized one with various corals cohabiting with macroalgae (Agostini et al, 2018; Biodiversity Center of Japan, 2008; Nakabayashi et al, 2019), and the third with ocean warming plus ocean acidification which has a simplified, homogeneous ecosystem dominated by turf algae that lacks habitat complexity and supports less biodiversity (Agostini et al, 2018; Cattano et al, 2020; Harvey et al, 2019, 2021)
We assessed the impacts of ocean warming alone, as well as the more realistic scenario of ocean warming combined with ocean acidification
Summary
Rising anthropogenic carbon dioxide levels are fundamentally altering the physicochemical and biological properties of the ocean, driving changes in marine biogeography (i.e., what lives where; Tittensor et al, 2010). The first with present-day conditions that is representative of a temperate ecosystem with abundant kelp forests (Biodiversity Center of Japan, 2019; Serisawa, Akino, et al, 2002; Wada et al, 2007), the second as an analogue of future conditions under warming whereby the ecosystem has recently shifted from a temperate-type ecosystem to a more tropicalized one with various corals cohabiting with macroalgae (Agostini et al, 2018; Biodiversity Center of Japan, 2008; Nakabayashi et al, 2019), and the third with ocean warming plus ocean acidification which has a simplified, homogeneous ecosystem dominated by turf algae that lacks habitat complexity and supports less biodiversity (Agostini et al, 2018; Cattano et al, 2020; Harvey et al, 2019, 2021) These last two locations, remote from the “Present” location, provide space for time analogues for an ocean warming alone scenario and an ocean acidification and warming combined scenario. We expected kelp to suffer under warming conditions and that any potential benefit of ocean acidification, as an increased source of carbon for growth, would be overwhelmed by an increase in fish herbivory due to warming
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