Macroalgal photosynthetic responses to light in relation to thallus morphology and depth zonation

• We show how photosynthesis and UV sensitivity of algae are related to thallus morphology and depth distributions. This was studied for typical depth zonations of red and brown macroalgae in the Skagerrak (ca. 25 psu) and the Baltic Sea (6.5 psu). The algae were collected from the water surface down to 20.5 m of depth, whereby each species was sampled at its maximum abundance depth. Altogether, we measured photosynthetic and respiratory rates of 19 red and 13 brown algal species as O2 evolution at different light intensities. Photosynthesis versus irradiance curves (PI curves) showed that light-saturated net photosynthetic rates (Pmax), respiratory rates in darkness (Rd) and the initial slope (α) were strongly related to algal morphology with higher values for thinner species. The compensation irradiance (Ic) and saturating irradiance (Ik) were strongly related to water depth with lower values at greater depth. A novel approach to analyse PI data with principal component analysis (PCA) is presented. The method makes it possible to assign a quantitative morphological gradient to algal species based on photosynthetic properties. Such a gradient can be used in ecological studies as an alternative to more subjective discrete subdivisions into functional-form groups. Another type of PCA analysis, with the relative shapes of the PI curves as input data, summarises α and convexity but discards all interference of morphology. This results in a gradient of genuine physiological responses, which in our study was strongly correlated to maximum abundance depth. The UV sensitivity of the same 32 algal species was determined as the change in net O2 evolution after exposure to UV light and the recovery after this treatment. Deeper-growing algae were more sensitive to UV and species with thinner thalli recovered better after UV treatment in the Skagerrak. No such trends were observed for the algae in the northern Baltic Sea, which suggests that no real deep-water species occur here. This is further supported by the lack of a clear pattern in Ic and Ik values with depth for the algae in the Baltic Sea. Our results advocate that the reduced species diversity of the Baltic Sea is also coupled to a loss of functional groups in the sense of general photosynthetic performance and not only in the sense of pure morphology (loss of canopy-forming species).

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