Abstract
AbstractMassive quantities of the pelagic brown macroalgae Sargassum spp. (sargassum) have been invading the Caribbean and West African shores since 2011, causing devastating effects on the coastal ecosystem and local economy. Little is known about sargassum beaching dynamics and the capacity of the coastal system to naturally remove beached sargassum. Here, we characterize the temporal variation in arriving and beached sargassum in a reef lagoon using a 5.2‐year data set of hourly optical imagery, and identify the governing hydrometeorological conditions. Image classification reveals interannual variability in the start, duration, and intensity of the sargassum arrival season. Arrivals are associated with relatively low energy onshore directed winds and waves, and offshore abundance of sargassum. Furthermore, nearshore sargassum mat size is found to decrease with decreasing wave/wind energy. Once sargassum beaches, a berm of wrack is formed. Natural wrack removal was observed under elevated water levels and increased wave action. Three types of wrack removal were distinguished, depending on the water level with respect to the berm crest height and berm crest toe : gradual berm destruction with gaps developing in the seaward berm edge that grow larger with time (Type I; ) and abrupt berm destruction with part of the wrack depositing on the upper beach (Type II; ) or in the dunes (Type III; ). Higher energy waves activate the reef circulation, which is suspected to flush part of the wrack out of the reef lagoon. We propose a conceptual model of nearshore sargassum dynamics in a reef lagoon system.
Highlights
Large quantities of the pelagic brown macroalgae Sargassum spp., consisting of species S. fluitans and S. natans and hereafter referred to as sargassum, invaded the shores of the Caribbean and tropical West Africa in 2011 (Franks et al, 2012; Gower et al, 2013) and have been reoccurring since (Johns et al, 2020; Wang et al, 2019)
Three types of wrack removal were distinguished, depending on the water AAlevel AA with respect to the berm crest hAAeight AAcc and berm creAsAt toe AAtt : gradual berm destruction with gaps developing in the seaward berm edge that grow larger with time (TAyApe I; AAtt < ηη ηηηcc ) and abrupt berm destruction with part of the wrack depositing on the upper beach (TyApA e II; AA A AAcc ) or in the dunes (Type AA III; AA A AAcc )
We propose a conceptual model of nearshore sargassum dynamics in a reef lagoon system
Summary
Large quantities of the pelagic brown macroalgae Sargassum spp., consisting of species S. fluitans and S. natans and hereafter referred to as sargassum, invaded the shores of the Caribbean and tropical West Africa in 2011 (Franks et al, 2012; Gower et al, 2013) and have been reoccurring since (Johns et al, 2020; Wang et al, 2019). Appendini Software: Jantien Rutten, Jaime Arriaga, Leonardo D. Appendini Validation: Jantien Rutten, Jaime Arriaga, Leonardo D. Seagrass and coral mortality has been observed to increase due to decreasing light, PH, and oxygen levels in nearshore waters, associated with the decaying sargassum masses on the beach (Van Tussenbroek et al, 2017). Local economies suffered with a reduced income as a result of a lower number of beach visitors during the sargassum season and costs associated with artificial sargassum removal (Chávez et al, 2020). These examples illustrate the urgent need to better understand and, manage the recent massive sargassum beachings
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