Beyond tides: surge-dominated submersion regimes on rocky shores of central Chile

Abstract

By definition, intertidal organisms are exposed to fluctuations between submersion and exposure to air due to the variation in the sea level caused, primarily, by tides and waves. Such submersion regimes have great consequences on the evolution, physiology, behavior, and ecology of intertidal marine organisms, but, paradoxically, our quantitative knowledge about submersion regimes is extremely limited, especially in wave-exposed shores. We here quantify submersion regimes during summer (February 2012) in a wave-exposed rocky shore of central Chile (Las Cruces, 33°30′S, 71°38′W) using a remote sensing method that permits high-frequency measurements. We provide metrics of submersion for the high (barnacle), mid (mussel) and low (upper kelp) intertidal zones and developed statistical models to hindcast submersion regimes from tide and wave data. Results revealed that the submersion consists solely of numerous but brief submersion events produced by waves. The three intertidal zones therefore experience intermittent submersion most of the time, and submersion events are so brief that, in these three zones, emersion predominates over submersion. In this region, submersion cannot be determined from tidal charts, because most of the intertidal zone lies above the highest tidal level, but still the tidal cycle strongly modulates submersion regimes, by altering wave swash into the shore. Combined statistical models including tides and waves are necessary to reproduce the most biologically relevant aspects of submersion. We discuss the consequences of these findings for our understanding of adaptation to intertidal life, and the design of studies on responses to current environmental variability and future climate change.