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Living in the diffusive boundary layer of seaweeds a potential refuge habitat from ocean acidification

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

In temperate coastal communities, seaweeds are ecosystem engineers that are of ecological importance, providing food an habitat for numerous benthic species as well as important ecosystem services such as carbon fixation and coastal stabilization. In addition to these roles, they also modify their local chemical (e.g. pH) and physical (e.g. water flow) environment because of their metabolism and structure. These modifications might offset the environmental changes generated by global changes like ocean acidification (OA), a worldwide phenomenon leading to seawater pH decrease and changes in carbonate chemistry which can affect marine organisms, particularly calcifying ones.
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.
The work accomplished in the 13-month period of this project combined a literature review, field observations and rigorous laboratory experiments. First a literature review was written, addressing recent developments, in the context of global change, on how hydrodynamic processes modulate biogenically-induced fluctuations in coastal seawater chemistry shaping chemical habitats which can change the responses of subtidal benthic organisms and communities to global change. This seminal article, almost submitted for publication, aims at showing to the scientific community the importance of (1) the local heterogeneity in physico-chemical parameters and (2) the biological shaping in coastal environments to cope with global changes.
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.
Results acquired in the context of this project provide new insights in the capacity of seaweeds in buffering the negative effects ocean acidification on benthic organisms, thereby elucidating to a certain extent how seaweed-based ecosystems can provide natural refuges from OA. Moreover, the ability of seaweeds to provide resistance to face ongoing global changes reinforce their role as engineer species in coastal ecosystems. The outcome of this project could then support management solutions and help local stakeholders in establishing rules for the protection and management of coastal zones dominated by seaweeds. From a scientific point of view this project highlighted the importance of considering local physical drivers as well as chemical heterogeneity to understand the functioning of benthic coastal communities in the context of ocean acidification and more generally, global change.
Refuge habitats shaped by seaweeds in the context of ocean acidification