Diversity of carbon use strategies in a kelp forest community: implications for a high CO2 ocean

Abstract

Mechanisms for inorganic carbon acquisition in macroalgal assemblages today could indicate how coastal ecosystems will respond to predicted changes in ocean chemistry due to elevated carbon dioxide (CO 2 ). We identified the proportion of noncalcifying macroalgae with particular carbon use strategies using the natural abundance of carbon isotopes and pH drift experiments in a kelp forest. We also identified all calcifying macroalgae in this system; these were the dominant component of the benthos (by % cover) at all depths and seasons while cover of noncalcareous macroalgae increased at shallower depths and during summer. All large canopy-forming macroalgae had attributes suggestive of active uptake of inorganic carbon and the presence of a CO 2 concentration mechanism (CCM). CCM species covered, on average, 15-45% of the benthos and were most common at shallow depths and during summer. There was a high level of variability in carbon isotope discrimination within CCM species, probably a result of energetic constraints on active carbon uptake in a low light environment. Over 50% of red noncalcifying species exhibited values below À30% suggesting a reliance on diffusive CO 2 uptake and no functional CCM. Non-CCM macroalgae covered on average 0-8.9% of rock surfaces and were most common in deep, low light habitats. Elevated CO 2 has the potential to influence competition between dominant coralline species (that will be negatively affected by increased CO 2 ) and noncalcareous CCM macroalgae (neutral or positive effects) and relatively rare (on a % cover basis) non-CCM species (positive effects). Responses of macroalgae to elevated CO 2 will be strongly modified by light and any responses are likely to be different at times or locations where energy constrains photosynthesis. Increased growth and competitive ability of noncalcareous macroalgae alongside negative impacts of acidification on calcifying species could have major implications for the functioning of coastal reef systems at elevated CO 2 concentrations.

The other ocean acidification problem: CO 2 as a resource among competitors for ecosystem dominance

“…Such variation among species may be due to species-specific variation in CCMs themselves [27,28] or the ability of species to acquire other limiting resources (e.g. light or nutrients) [29].…”

How will Ocean Acidification Affect Baltic Sea Ecosystems? An Assessment of Plausible Impacts on Key Functional Groups

“…Equivalent patterns can be found in literature on the effects of ocean warming (e.g. Kordas et al 2011), and therefore it seems clear that changing abiotic factors will alter competitive, and trophic, interactions of key functional groups (Hepburn et al 2011). Understanding these changes will be fundamental to understanding how ecosystems will respond in a high CO 2 ocean.…”

“…Therefore, these taxa have been assumed to be largely insensitive to acidification (Giordano et al 2005). There are no published results for the effects of ocean acidification on Baltic Sea macroalgae, but recent work elsewhere confirms that species with CCMs are likely to be unaffected by, or may benefit marginally from, ocean acidification (Hepburn et al 2011). These authors also note that the responses to ocean acidification of macroalgae, such as the Fucus spp.…”

Divergent responses in growth and nutritional quality of coastal macroalgae to the combination of increased pCO2 and nutrients

“…Due to highly abundant bicarbonate ions, most macroalgae rely on CCMs to convert HCO 3 to CO 2 for use in photosynthesis (Hepburn et al, 2011). In addition, many species also have the ability to passively diffuse CO 2 and may gain an advantage under future conditions due to reduced reliance and downregulation of CCMs.…”