Contrasting effects of ocean acidification on tropical fleshy and calcareous algae

Abstract

Despite the heightened awareness of ocean acidification (OA) effects on marine organisms, few studies empirically juxtapose biological responses to CO2 manipulations across functionally distinct primary producers, particularly benthic algae. Algal responses to OA may vary because increasing CO2 has the potential to fertilize photosynthesis but impair biomineralization. Using a series of repeated experiments on Palmyra Atoll, simulated OA effects were tested across a suite of ecologically important coral reef algae, including five fleshy and six calcareous species. Growth, calcification and photophysiology were measured for each species independently and metrics were combined from each experiment using a meta-analysis to examine overall trends across functional groups categorized as fleshy, upright calcareous, and crustose coralline algae (CCA). The magnitude of the effect of OA on algal growth response varied by species, but the direction was consistent within functional groups. Exposure to OA conditions generally enhanced growth in fleshy macroalgae, reduced net calcification in upright calcareous algae, and caused net dissolution in CCA. Additionally, three of the five fleshy seaweeds tested became reproductive upon exposure to OA conditions. There was no consistent effect of OA on algal photophysiology. Our study provides experimental evidence to support the hypothesis that OA will reduce the ability of calcareous algae to biomineralize. Further, we show that CO2 enrichment either will stimulate population or somatic growth in some species of fleshy macroalgae. Thus, our results suggest that projected OA conditions may favor non-calcifying algae and influence the relative dominance of fleshy macroalgae on reefs, perpetuating or exacerbating existing shifts in reef community structure.

Characterizing the vulnerability of intertidal organisms in Olympic National Park to ocean acidification

“…These taxa are positively sensitive to OA and have the highest percent cover at ONP rocky intertidal monitoring sites (Figure 4). Experimental laboratory treatments (Johnson et al, 2014), modeling exercises (Wootton et al, 2008), and volcanic reef field surveys (Enochs et al, 2015) support this taxonomic transition from calcareous taxa to fleshy macroalgae when exposed to decreasing pH, but this trend has not yet been detected in the ONP community survey data.…”

“…These taxa are positively sensitive to OA and have the highest percent cover at ONP rocky intertidal monitoring sites (Figure 4). Experimental laboratory treatments (Johnson et al, 2014), modeling exercises (Wootton et al, 2008), and volcanic reef field surveys (Enochs et al, 2015) support this taxonomic transition from calcareous taxa to fleshy macroalgae when exposed to decreasing pH, but this trend has not yet been detected in the ONP community survey data.In contrast, mobile organisms almost exclusively rely upon calcareous structures and demonstrate some degree of OA sensitivity (Figure 4). The most sensitive belong to the phylum Mollusca, a taxonomic group with known OA sensitivity (Gazeau et al, 2013;Parker et al, 2013).…”

Twenty-first century climate change and submerged aquatic vegetation in a temperate estuary: the case of Chesapeake Bay

“…In the Chesapeake, we must consider the impacts of coastal acidification on the competitive balance between submerged vegetation and competing macroalgae and epiphytes. On one hand, acidification may inhibit the growth of calcifying epiphytes and coralline macroalgae, benefiting seagrasses (Newcomb et al 2015;Johnson, Price, and Smith 2014;but see Johnson et al 2012). On the other hand, these same conditions may fuel the overgrowth of other fouling organisms.…”

“…Acidification is also likely to benefit macroalgae, especially those without effective carbon-concentrating mechanisms. Acidification often increases rates of photosynthesis, nutrient assimilation, growth, and reproduction of fleshy seaweed species (Koch et al 2013;Baggini et al 2014;Burnell et al 2014;Johnson, Price, and Smith 2014;Duarte et al 2016;Kübler and Dudgeon 2013). Kroeker et al (2013b) noted that in acidified conditions, fleshy seaweeds can rapidly overgrow other species, dominate ecosystems, and cause phase changes in plant communities.…”

Vulnerability of macronutrients to the concurrent effects of enhanced temperature and atmospheric pCO2 in representative shelf sea sediment habitats

“…Global climate change clearly has the potential to modify the composition of species assemblages, by promoting tolerant species and reducing sensitive species, as has been demonstrated along natural CO 2 gradients, where community structure shifts from one containing calcifying organisms to a community dominated by non-calcifying species, with a concomitant reduction in local biodiversity and species abundance (Hall-Spencer et al 2008;Hale et al 2011;Johnson et al 2014;Meadows et al 2015;Crook et al 2016;Gambi et al 2016). Similarly, elevated temperatures can increase the metabolic rate of organisms within their thermal tolerance window (Pörtner and Farrell 2008), accelerating infaunal burrowing and ventilation activity (Ouellette et al 2004).…”