12. Living in the boundary layer of kelp blades: refuge from ocean acidification or training for harsh conditions?

Fanny Noisette (1), Catriona Hurd (1)*

1 Institute for Marine and Antarctic Studies, University of Tasmania, 20 Castray Esplanade, Battery Point, 7004 TAS Hobart, Australia

Background
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 the thickness of the DBL, sessile invertebrates such as calcifying bryozoans or tube-forming polychaetes living at the surface of the blades can be affected by the chemical variations occurring in this layer. In the context of ocean acidification, these microhabitats might be considered as a refuge from lower pH during light as photosynthesis temporarily raises the pH to values higher than the mainstream seawater.

Methods
The thickness and the characteristics of the DBL were assessed at current day pH 8.1 and that predicted for the end of the century, pH 7.7, and seawater flows (slow: 0.5 and fast: 10 cm s-1) on blades of the kelp Ecklonia radiata. Oxygen and pH profiles from the blade surface to the mainstream seawater were measured with microsensors in both bare blades and blades colonized by the bryozoan Membranipora membranacea.

Findings
As predicted, the DBL was thicker in slow than in fast flow. We also assumed that the DBL would thinner on blades colonized by bryozoans whereas actually, their presence increased the DBL by creating their own one in addition to the kelp one. The oxygen concentrations and pH levels in the DBL were affected by the presence of bryozoans and the mainstream pH in different ways depending the flow.

Conclusions
These results show that living in the DBL of the kelps can constitute a refuge from ocean acidification or a training for harsh conditions for calcifying organisms living there, particularly in slow flow conditions.