Abstract
To understand the effects of ocean acidification (OA) on marine calcifiers, the trade-offs among different sublethal responses within individual species and the emergent effects of these trade-offs must be determined in an ecosystem setting. Crustose coralline algae (CCA) provide a model to test the ecological consequences of such sublethal effects as they are important in ecosystem functioning, service provision, carbon cycling and use dissolved inorganic carbon to calcify and photosynthesize. Settlement tiles were placed in ambient pH, low pH and extremely low pH conditions for 14 months at a natural CO2 vent. The size, magnesium (Mg) content and molecular-scale skeletal disorder of CCA patches were assessed at 3.5, 6.5 and 14 months from tile deployment. Despite reductions in their abundance in low pH, the largest CCA from ambient and low pH zones were of similar sizes and had similar Mg content and skeletal disorder. This suggests that the most resilient CCA in low pH did not trade-off skeletal structure to maintain growth. CCA that settled in the extremely low pH, however, were significantly smaller and exhibited altered skeletal mineralogy (high Mg calcite to gypsum (hydrated calcium sulfate)), although at present it is unclear if these mineralogical changes offered any fitness benefits in extreme low pH. This field assessment of biological effects of OA provides endpoint information needed to generate an ecosystem relevant understanding of calcifying system persistence.
Highlights
A primary challenge for global change biology is to better understand the role of the sublethal effects of environmental change on individuals and populations in real world ecosystems
Individual Crustose coralline algae (CCA) patches growing in low pH/high pCO2 conditions similar to those projected for 2100 can maintain growth and calcification without detectable trade-offs in skeletal structure
While results 5 presented here suggest that some CCA patches could maintain their growth and skeletal structure in low pH, a previous study [29] found the total per cent cover of CCA was significantly less in low pH than ambient pH
Summary
A primary challenge for global change biology is to better understand the role of the sublethal effects of environmental change on individuals and populations in real world ecosystems. High pCO2 induced a change in skeletal mineralogy to less soluble calcium sulfates [27] Such alteration in skeletal accretion could increase susceptibility to structural damage from grazing [26] or physical impacts [19]; for example, reduced thallus thickness in CCA may alter their competitive hierarchy and increase their susceptibility to grazing under more acidified conditions [26]. At 14 months, coralline algae were not observed growing on the tiles in the extreme low pH zone at the north site which prevented a balanced analytical design. Analyses of variance (general linear model) with Tukey’s pairwise comparisons were used to determine differences in Mg composition, carbonate ion disorder and areas between time and pH zone (both fixed) with site as a random variable with interactions being examined. Time was not treated as a repeated measure as at each time point separate tiles were collected and removed, individuals were not re-sampled
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