Close but distant: Emersion promotes ecophysiological differentiation between two rhodophytes within an estuarine intertidal zone.

Abstract

Environmental factors associated with emersion determine, among others, the spatial distribution of intertidal seaweeds. This study investigates the emersion tolerance of Bostrychia scorpioides and Catenella caespitosa, in relation to their zonation pattern in a soft bottom intertidal zone. The two species are vertically segregated, B. scorpioides occupying the uppermost horizon and C. caespitosa the lowest one, but they coexist at intermediate tidal parts. Three sets of laboratory-controlled experiments were carried out at ecologically 0meaningful conditions to achieve the following specific objectives: ) to identify differences in their water loss and retention; ) to assess the interactive effect of emersion and temperature (E × T) and emersion and salinity (E × S) on their growth, % DW and elemental composition; ) to analyse the effect of short-term ( h) and mid-term (- d) desiccation periods on their photosynthetic performance and its recovery following reimmersion. Our main findings demonstrated that combination of high levels of emersion and temperature ( h · d- , 5°C) had a synergistic negative effect on the growth of both species leading to growth imbalance, especially in Catenella caespitosa, the lowermost intertidal species. Both species could grow under continuous emersion if thalli do not completely dehydrate, but only B. scorpioides maintained similar growth rate and C:N ratio as under continuous submersion, evidencing its higher emersion tolerance.

In contrast, the combination of emersion and salinity had an antagonistic outcome, since the influence of salinity decreased as daily emersion period increased. After d of continuous air exposure, both species recuperated 50% of their net photosynthetic rate at saturating irradiance, but after d B. scorpioides was able to photosynthetically recover to a greater extent than C. caespitosa (0% vs. % recovery), showing a better physiological performance of recovery following reimmersion. We demonstrated that the uppermost growing species (B. scorpioides) possessed higher emersion tolerance and recovery abilities of physiological performance following reimmersion. The different effect of emersion on species growth and mid-term abilities to recover their physiological fitness clearly supported the observed zonation pattern.