Periodic Reporting for period 1 - DBL-OA (Living in the diffusive boundary layer of seaweeds a potential refuge habitat from oceanacidification)
Reporting period: 2017-06-15 to 2019-06-14
The main objective of this project DBL-OA was to investigate the capacity of brown seaweeds in providing favorable micro-environments to help calcareous taxa in coping with OA. At the surface of all seaweeds, there is a thin (mm) layer of seawater called the “diffusive boundary layer” (DBL) whose chemistry, including pH, is controlled by the seaweed’s metabolism. Depending on algal morphology and hydrodynamics characteristics, the DBL thickness varies, forming a sometimes thick (6 cm) DBL associated with the seaweed canopy, thus providing more or less complex microhabitats for associated species. The projects aimed at understanding the link between the metabolic responses of seaweeds, the shaping of chemical micro-environment at different scales (blade and canopy) and the consecutive mitigation of environmental stress on the organisms living in theses specific habitats (e.g bryozoans on blades and coralline algae in understory), in different scenario of global warming and OA.
In Tasmania, a spot presenting a high density of Phyllospora comosa, one of the most common brown seaweed East Australian species, was monitored during summer 2018, measuring temperature, light and pH fluctuations under canopy. The data acquired informed the design of the experiments developed in the next step of the project. An experiment run in a cutting-edge fully replicated setup investigated the physiology of P. comosa under current and future scenarios of ocean acidification and in the presence or absence of an heatwave. The first results analysed tend to show that P. comosa is not affected by future pH levels. In addition, this alga is also able to resist current and future levels of heatwave without drastic physiological impediment or growth reduction. A second experiment looked at the heterogeneity of micro-environments (i.e. DBL thickness and O2 and pH gradients) at the surface of coralline algae living in understory of P. comosa canopies in different flow conditions. First results show that the presence of epiphytes’ tufts on coralline algae increases the thickness of the DBL. In these micro-habitats, pH and oxygen concentration increase at higher levels than in the surrounding seawater, providing potential shelters for small organisms as abalone larvae which usually recruit on these type of substratum.