Photosynthetic responses of an intertidal alga to emersion : the interplay of intertidal height and meteorological conditions

International audienceThe photosynthetic response of juvenile stages of the brown alga F. distichus subsp. edenatus (Fucales, Heterokontophyta) was examined at different tidal heights at an intertidal site located on the Gaspé Peninsula, Québec, Canada. During the low tide, we used pulse-amplitude modulated (PAM) fluorometry to examine the effective quantum yield (ΦPSII) and the non-photochemical quenching (NPQ) of fucoid juveniles at five intertidal levels spanning a vertical distance of 1.2 m. Observations were made under four different meteorological conditions, defined as "sunny-windy," "sunny-calm," "cloudy-windy" and "cloudy-calm." Rates of evaporation were measured at the same time. A gradient of responses was generally found: at the highest two levels, ΦPSII declined over time with concomitant increases in NPQ. At the lowest levels, there was no decline of ΦPSII over time and just a slight increase in NPQ, except under "sunny-windy" conditions when responses were similar to the higher levels. This combination of sun and wind created the most severe meteorological conditions in terms of physiological responses whereas the least intense meteorological condition was "cloudy-calm" during which only the highest level showed a decrease in quantum yields. Evaporation rates were much greater at the two highest levels and during windy conditions, suggesting that desiccation is a major stressor during emersion and is partly responsible for the smaller size and lower density of algae at the two highest level

Deterioration of sediment quality in seagrass meadows (Posidonia oceanica) invaded by macroalgae (Caulerpa sp.)

Species of the macroalgae Caulerpa sp. are increasingly being observed in meadows of the endemic Mediterranean seagrass Posidonia oceanica, and in particular Caulerpa taxifolia, has been considered as an invasive species leading to seagrass decline. Studies have so far failed to reveal the underlying mechanisms of the success of the macroalgae, and here, we examine how biogeochemical changes of the environment associated to indigenous (Caulerpa prolifera) and non-indigenous (Caulerpa racemosa and C. taxifolia) species affect the habitat of P. oceanica. Two of the species (C. prolifera and C. racemosa) affect the sediment biogeochemical conditions by increasing organic matter pools, microbial activity, and sulfide pools of the sediments, and limited effects were found for C. taxifolia. Biomass of the macroalgae contributed to the extent of impacts, and high sulfide invasion into the seagrasses and regression of the meadow were pronounced at the location with the highest Caulerpa biomass. This suggests that Caulerpa invasion contributes to seagrass decline probably because Caulerpa thrives better than the seagrasses in the modified environment. © 2009 Coastal and Estuarine Research Federation.Peer Reviewe