Abiotic and biotic interactions in the diffusive boundary layer of kelp blades create a potential refuge from ocean acidification
Noisette, F and Hurd, C, Abiotic and biotic interactions in the diffusive boundary layer of kelp blades create a potential refuge from ocean acidification, Functional Ecology, 32, (5) pp. 1329-1342. ISSN 0269-8463 (2018) [Refereed Article]
Seaweeds are able to modify the chemical environment at their surface, in a micro‐zone
called the diffusive boundary layer (DBL), via their metabolic processes controlled
by light intensity. Depending on the thickness of the DBL, sessile invertebrates such
as calcifying bryozoans or tube‐forming polychaetes living on the surface of the blades
can be affected by the chemical variations occurring in this microlayer. Especially
in the context of ocean acidification (OA), these microhabitats might be considered
as a refuge from lower pH, because during the day photosynthesis temporarily raises
the pH to values higher than in the mainstream seawater.
We assessed the thickness and the characteristics of the DBL at two pH levels (today's
average surface ocean pH 8.1 and a reduced pH predicted for the end of the century,
pH 7.7) and seawater flows (slow, 0.5 and fast, >8 cm/s) on Ecklonia radiata (kelp) blades. Oxygen and pH profiles from the blade surface to the mainstream seawater
were measured with O2 and pH microsensors for both bare blades and blades colonized by the bryozoan Membranipora membranacea.
The DBL was thicker in slow flow compared with fast flow and the presence of bryozoans
increased the DBL thickness and shaped the DBL gradient in dark conditions. Net production
was increased in the low pH condition, increasing the amount of oxygen in the DBL
in both bare and epiphytized blades. This increase drove the daily pH fluctuations
at the blade surface, shifting them towards higher values compared with today's pH.
The presence of bryozoans led to lower oxygen concentrations in the DBL and more complex
pH fluctuations at the blade surface, particularly at pH 7.7.
Overall, this study, based on microprofiles, shows that, in slow flow, DBL microenvironments
at the surface of the kelps may constitute a refuge from OA with pH values higher
than those of the mainstream seawater. For calcifying organisms, it could also represent
training ground for harsh conditions, with broad daily pH and oxygen fluctuations.
These chemical microenvironments, biologically shaped by the macrophytes, are of great
interest for the resilience of coastal ecosystems in the context of global change.