Underwater light field patterns in subtropical coastal waters: A case study from the Gulf of Eilat (Aqaba)

In this study, we describe the seasonal patterns and the depth dependence of the underwater distribution of ultraviolet radiation (UVR) and photosynthetically active radiation (PAR) in the Gulf of Eilat (Aqaba). Our data show a remarkable seasonal variation in the daily irradiance dose that reaches the benthos, including seaweeds, turf algae, seagrasses, and the endosymbiotic algae of corals. Corals experience 7 times less light in the winter than in summer, which is equivalent to an almost 30 m deeper location. This seasonal variation in irradiance to which high-latitude benthic phototrophs are exposed, is due to a synergetic effect of changes in solar elevation and in water transparency. Unlike the major seasonal difference in insolation in subtropical regions, low-latitude coral reefs and benthic macrophytes are exposed to only much subtler differences in irradiance. One of the northernmost coral reefs in the world, yet a very highly diverse one, is found at the northern tip of the Gulf of Eilat (Aqaba) at 29 N. Together with its geographical marginality, this reef is also characterized by low cloud cover, making it an appropriate model for demonstrating the dynamics of a "clean" seasonal light field, in contrast to coral reefs at high latitudes with much higher cloud cover. Based on these data, we present a model allowing calculation of the seasonal variation of a light field as a function of depth and latitude under clear-sky conditions, and variable water transparency.