Season-specific impacts of climate change on canopy-forming seaweed
communities.
Abstract
Understory assemblages associated with canopy-forming species such as
trees, kelps, and rockweeds should respond strongly to climate stressors
due to strong interaction strengths. Climate change can directly and
indirectly modify these assemblages, particularly during more stressful
seasons and climate scenarios. However, fully understanding the seasonal
impacts of different climate conditions on canopy-reliant assemblages is
difficult due to a continued emphasis on studying single species
responses to a single future climate scenario during a single season. To
examine these more complex interactions, we used mesocosm experiments to
expose intertidal assemblages associated with the canopy-forming golden
rockweed, Silvetia compressa, to elevated temperature and pCO2
conditions reflecting two projected greenhouse emission scenarios [RCP
2.6 (low) & RCP 4.5 (moderate)]. Assemblages were grown in the
presence and absence of Silvetia, and in two seasons. Relative to
ambient conditions, predicted climate scenarios generally suppressed
Silvetia biomass and photosynthetic efficiency. However, these effects
varied seasonally - both future scenarios reduced Silvetia biomass in
summer, but only the moderate scenario did so in winter. These
reductions shifted the assemblage, with more extreme shifts occurring in
summer. Contrarily, future scenarios did not shift assemblages within
Silvetia Absent treatments, suggesting that climate primarily affected
assemblages indirectly through changes in Silvetia. Mesocosm experiments
were coupled with a field Silvetia-removal experiment to simulate the
effects of climate-mediated Silvetia loss on natural assemblages.
Consistent with the mesocosm experiment, Silvetia loss resulted in
season-specific assemblage shifts, with weaker effects observed in
winter. Together, our study supports the hypotheses that
climate-mediated changes to canopy-forming species can indirectly affect
the associated assemblage, and that these effects vary seasonally. Such
seasonality is important to consider as it may provide periods of
recovery when conditions are less stressful, especially if we can reduce
the severity of future climate scenarios.