The future of the northeast Atlantic benthic flora in a high CO2 world

Abstract

Seaweed and seagrass communities in the northeast Atlantic have been profoundly impacted by humans, and the rate of change is accelerating rapidly due to runaway CO2 emissions and mounting pressures on coastlines associated with human population growth and increased consumption of finite resources. Here, we predict how rapid warming and acidification are likely to affect benthic flora and coastal ecosystems of the northeast Atlantic in this century, based on global evidence from the literature as interpreted by the collective knowledge of the authorship. We predict that warming will kill off kelp forests in the south and that ocean acidification will remove maerl habitat in the north. Seagrasses will proliferate, and associated epiphytes switch from calcified algae to diatoms and filamentous species. Invasive species will thrive in niches liberated by loss of native species and spread via exponential development of artificial marine structures. Combined impacts of seawater warming, ocean acidification, and increased storminess may replace structurally diverse seaweed canopies, with associated calcified and noncalcified flora, with simple habitats dominated by noncalcified, turf-forming seaweeds.

Keywords: Calcified algae; climate change; invasive species; macroalgae; microphytobenthos; seagrasses; volatile gases.

Figure 1

Present distribution of habitat-forming species in the northeast Atlantic, and an estimate of environmental change by 2100. SST anomaly (change from the present) is based on annual mean from an A1B scenario ensemble as Jueterbock et al. (2013). Many species' ranges such as the kelp L. hyperborea are thought to be limited by summer and winter thermoclines (van den Hoek ; Dieck 1993). Temperature changes are expected to impact distributions as species' ranges track these limits (Harley et al. 2012). Maerl are calcifying species utilizing high magnesium calcite, which has a similar saturation state to aragonite in the northeast Atlantic (Andersson et al. 2008). Most maerl are currently found in locations supersaturated for aragonite (Ω > 2). Predictions of the saturation state for 2100 (Steinacher et al. 2009) suggest that most of the northeast Atlantic will be outside this range.

Figure 2

Predicted change in northeast Atlantic benthic marine flora if CO₂ emissions continue unabated. (A) Arctic region: warming will be detrimental to cold-adapted species, and acidification will corrode maerl (M.). Pacific species, for example, Alaria marginata (Am), will invade as polar ice melts, competing with native species such as Laminaria hyperborea (Lh) and Alaria esculenta (Ae). Fleshy invasives, for example, Sargassum muticum (Sm), will move north competing with fucoids, for example, Fucus distichus (Fd), in the intertidal. Acidification will corrode epiphytic calcified algae, for example, Titanoderma pustulatum (Tp), and increased CO₂ levels will stimulate growth of diatoms (D.) (magnified circles) and seagrasses such as Zostera marina (Zm). (B) Boreal region: Laminaria hyperborea (Lh) forests will be increasingly dominated by Laminaria ochroleuca (Lo), with the loss of Alaria esculenta (Ae) and fucoids, for example, Fucus vesiculosus (Fv) and the continued spread of invasive Undaria pinnatifida (Up), Sargassum muticum (Sm), and Grateloupia turuturu (Gt). As in the Arctic, maerl beds will be corroded, seagrasses will thrive, but epiphytic calcified algae will be reduced or replaced with diatoms and filamentous seaweeds (magnified circles). (C) Lusitanian region: kelps will be replaced by smaller, fleshy algae and invasive species, for example, Caulerpa taxifolia (Ct) will proliferate. Fucoids will be replaced by invasives such as Asparagopsis armata (Aa). Seagrasses will thrive, and it is expected that maerl and epiphytic calcified algae will be retained (magnified circles).