The direct effects of increasing CO ₂ and temperature on non-calcifying organisms: increasing the potential for phase shifts in kelp forests

Abstract

Predictions about the ecological consequences of oceanic uptake of CO 2 have been preoccupied with the effects of ocean acidification on calcifying organisms, particularly those critical to the formation of habitats (e.g. coral reefs) or their maintenance (e.g. grazing echinoderms). This focus overlooks the direct effects of CO 2 on non-calcareous taxa, particularly those that play critical roles in ecosystem shifts. We used two experiments to investigate whether increased CO 2 could exacerbate kelp loss by facilitating non-calcareous algae that, we hypothesized, (i) inhibit the recovery of kelp forests on an urbanized coast, and (ii) form more extensive covers and greater biomass under moderate future CO 2 and associated temperature increases. Our experimental removal of turfs from a phase-shifted system (i.e. kelp-to turf-dominated) revealed that the number of kelp recruits increased, thereby indicating that turfs can inhibit kelp recruitment. Future CO 2 and temperature interacted synergistically to have a positive effect on the abundance of algal turfs, whereby they had twice the biomass and occupied over four times more available space than under current conditions. We suggest that the current preoccupation with the negative effects of ocean acidification on marine calcifiers overlooks potentially profound effects of increasing CO 2 and temperature on non-calcifying organisms.

Global patterns of kelp forest change over the past half-century

“…This result is strikingly different from that reported for many other marine and terrestrial systems that are experiencing consistent declines in species abundance across the globe (1-3, 6) and, in many ways, highlights the atypical resilience of kelps. However, where kelp resilience is eroding and leading to declines in abundance (7,18,54,64,66,67), impacts to ecosystem health and services can be far-reaching (19,25,26,68,69). Maintaining the resilience of kelp forest ecosystems into the future will rely on the continued monitoring of kelps and management of stressors on local and regional scales.…”

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

“…When combined with elevated nutrients, elevated CO 2 can release mats from co-limitation with nitrogen [31] to multiply the rate of expansion [77]. In addition, projected warming is likely to enhance mat productivity, because mats are not only better suited to the increasing availability of CO 2 [31], but these effects are also enhanced under warmer temperatures [18]. Whether additive or synergistic, such combinations of CO 2 with other stressors increase the probability of phase-shifts.…”

“…Laboratory experiments used similar techniques [18], but mesocosm experiments used floating docks in temperate conditions [26] and a Free Ocean Carbon Experiment in tropical conditions [70]. Observations at volcanic vents quantified the percentage cover of mats within space that would otherwise be suitable for recruitment (e.g.…”

“…While there is recognition that ocean acidification may alter net primary production, via the increased solubility of biogenic calcareous structures and reduced survival of calcifying species that consume algae [11,17], it may also alter production directly, especially under elevated temperature [18]. Several reviews now recognize that ocean acidification can increase carbon fixation rates in some photosynthetic organisms (review [19,20]); however, the ecological implications of this increase are largely untested.…”

Can strong consumer and producer effects be reconciled to better forecast ‘catastrophic’ phase-shifts in marine ecosystems?

“…elevated temperature and CO 2 ) represents abiotic conditions that also enable turfs to persist for longer periods of time appears to be a gap in the understanding of their future role in phase-shifts (Connell and Russell, 2010). While there is still debate surrounding whether elevated CO 2 will have direct positive effects on the productivity of macroalgae, recent experimental evidence that both temperate and tropical turfs will increase in abundance under future conditions (Table 1; Connell and Russell, 2010) illustrates the merit of increasing the breadth of consideration of elevated ocean-CO 2 beyond the effects of ocean-acidification on calcifying organisms.…”