Mussels, algae and climate change

The effects of climate change on coastal biodiversity are a major concern
because altered community compositions may change associated rates of
ecosystem functioning and services. While responses of single species or
taxa have been studied extensively, it remains challenging to estimate
responses to climate change across different levels of biological
organisation. Studies that consider the effects of moderate realistic
near-future levels of ocean warming and acidification are needed to
identify scope for adaptation and evolution. Also, studies including
different levels of biological complexity may reveal opportunities for
amelioration or facilitation under changing environmental conditions. To
test experimentally for independent and combined effects of predicted
near-future warming and acidification on key benthic species, we
manipulated three levels of temperature (ambient, +0.8 °C, +2 °C) and two
levels of pCO2 (ambient at 450 ppm, elevated at 645 ppm) and quantified
their effects on mussels and algae growing separately or together (to also
test for inter-specific interactions). Warming increased mussel clearance
and mortality rates simultaneously, which meant that total biomass peaked
at + 0.8 °C. Surprisingly, however, no effects of elevated pCO2 were
identified on mussels or algae. Moreover, when kept together, mussels and
algae had mutually positive effects on each other’s performance (i.e.
mussel survival and condition index, mussel and algal biomass, and proxies
for algal productivity including relative maximum electron transport rate
[rETRmax], saturating light intensity [Ik], and maximum quantum yield
[Fv/Fm]), independent of warming and acidification. Our results show that
even moderate warming affected the functioning of key benthic species, and
we identified a level of resistance to predicted ocean acidification.
Importantly, we show that the presence of a second functional group
enhanced the functioning of both groups (mussels and algae), independent
of changing environmental conditions, which highlights the ecological and
potential economic benefits of conserving biodiversity in marine
ecosystems.