Geniculate vs nongeniculate coralline algae: the different responses in case of stress are related also to morphology

ABSTRACT: The response of algae to stressors that affect their survival and growth appears to be strictly related to their dimension and shape. Coralline algae (Corallinophycidae, Rhodophyta) show a variety of morphological types that may play a role in their tolerance to stressful conditions. Nevertheless, only a few studies have investigated their response to multiple stressors when grouped by morphology. The aim of this work was to investigate the performance of the 2 main morpho-functional groups of coralline algae (nongeniculate and geniculate) in response to different stressors acting simultaneously (presence of fleshy macroalgae and/or benthic mucilage and sediment deposition). For this purpose, a manipulative field experiment was conducted on superficial rocky substrata (subtidal, 0.5 m depth) in the Tavolara Punta Coda Cavallo Marine Protected Area, Sardinia, in 2016-2018. The effect of each stressor, alone and in combination, was tested on the 2 groups of algae (4 treatments applied in triplicate), with the percent substratum covered by living coralline algae and the net primary production as response variables. Results showed that although some common trends were evident, different effects were observed for each morpho-functional group in relation to the considered response variable. These results highlight the importance of considering morpho-functional classification when predicting the response of coralline algae to disturbance and identifying their main threats.