Effects of future climate on coral-coral competition

As carbon dioxide (CO 2 ) levels increase, coral reefs and other marine
systems will be affected by the joint stressors of ocean acidification
(OA) and warming. The effects of these two stressors on coral physiology
are relatively well studied, but their impact on biotic interactions
between corals are poorly understood. While coral-coral interactions are
less common on modern reefs, it is important to document the nature of
these interactions to better inform restoration strategies in the face of
climate change. Using a mesocosm study, we evaluated whether the combined
effects of ocean acidification and warming alter the competitive
interactions between the common coral Porites astreoides and two other
mounding corals ( Montastraea cavernosa or Orbicella faveolata ) common in
the Caribbean. After 7 days of direct contact, P. astreoides suppressed
the photosynthetic potential of M. cavernosa by 100% in areas of contact
under both present (~28.5°C and ~400 μatm p CO 2 ) and predicted future
(~30.0°C and ~1000 μatm p CO 2 ) conditions. In contrast, under present
conditions M. cavernosa reduced the photosynthetic potential of P.
astreoides by only 38% in areas of contact, while under future conditions
reduction was 100%.  A similar pattern occurred between P.
astreoides and O. faveolata at day 7 post contact, but by day 14, each
coral had reduced the photosynthetic potential of the other by 100% at the
point of contact, and O. faveolata was generating larger lesions on P.
astreoides than the reverse.  In the absence of competition, OA
and warming did not affect the photosynthetic potential of any coral.
These results suggest that OA and warming can alter the severity of
initial coral-coral interactions, with potential cascading effects due to
corals serving as foundation species on coral reefs.