Spatial variation in photosynthetic recovery of intertidal turf algae from acute UVB and temperature stress associated with low tides along the central coast of Chile

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Ecological consequences of punctuated, extreme climate events depend largely upon species' physiological capacity to tolerate and recover from such stressful events. However, physiological responses of intertidal organisms to repetitive exposure to severe conditions using natural patterns are rarely explored, and we know little about how physiological sensitivity within a species varies over local scales or among natural populations, making it difficult to extrapolate results to the naturally variable environmental conditions encountered in the field. Here, we simulate realistic scenarios of the exposure of the turf-forming alga (Gelidium chilense) to atmospheric stressors (UVB and temperature) associated with natural mild or harsh weather conditions that occur when summertime midday low tides coincide with unusually calm seas and particularly warm, sunny days along the central Chilean coast. We show that sudden, short-term exposure to artificial UVB radiation during daytime low tides represents a strong stress factor for Gelidium turf algae as measured by the changes in maximal photosynthetic quantum yield (Fv/Fm), and repetitive exposure over a period of 5d, as it occurs naturally during spring–summer days, can critically compromise the ability of the fronds to recover once the stressor is removed. In contrast, while increased aerial temperature had important effects on the photosynthetic system, this source of stress had no lasting effects on the fronds' capacity to recover, nor did it modify the UVB effects on photosynthesis (i.e. no synergistic effects between these common stressors). The ability of Gelidium turf to recover from UVB-induced damage to the photosynthetic apparatus differed between tidal heights and among populations from different sites along the central coast. These results suggest that for many shore communities, abrupt, unpredictable short-term stress events will have greater consequences lower on the shore. We found no evidence that greater upwelling intensity and associated nutrient enrichment of coastal waters could consistently ameliorate recovery from the negative effects of UVB. The complex spatial variation in the turf's ability to recover rapidly from a stress event may alter predictions regarding the effects of extreme climate events and/or climate change on species interactions and species' geographic distribution.