Seagrass abundance and morphological, nutritional and biochemical traits in relation to sea urchin Tripneustes gratilla overgrazing

We studied the grazing of tropical seagrass species by the sea urchin Tripnesutes gratilla in relation to seagrass traits and abundance. The inception of this project happened after witnessing the overgrazing of the seagrass species Thalassodendron ciliatum by the sea urchin Tripneustes gratilla. To study if seagrass abundance and/or traits were driving the urchin grazing, we mapped the seagrass meadows to quantify their abundance and measured their morphological (leaf area and canopy height), nutritional (carbon, nitrogen and phosphorus content) and biochemical (free phenolics content) traits. To assess sea urchin grazing preference, we studied the gut content of the urchins and measured the amount of each seagrass species in the gut. With the seagrass abundance and gut content, we measured an electivity index to assess sea urchin grazing preference. In addition, we measured sea urchin density in meadows of different seagrass species. Finally, in order to assess if the overgrazing is due to the lack of top-down control on the urchin population, we quantified the abundance of predatory fishes in the area doing underwater video surveys. The data was collected in Unguja Island (Zanzibar Archipealgo, Tanzania) in two sites (Changuu and Snake Island) and 8 sampling days (seven sampling edays in Changuu Island and one in Snake Island), making a total of 8 sampling events (Changuu1, Changuu2, Changuu3, Changuu4, Changuu5, Changuu6, Changuu7 ans Snake). All the samplings were made between the years 2015 and 2017. Both Changuu and Snake Island are surrounded by seagrass meadows, mostly composed of the seagrass species Thalassodendron ciliatum, Thalassia hemprichii, Syringodium isoetifolium, Halodule uninervis and Cymodocea serrulata. The sampling sites consisted of subtidal meadows (1-4 meters depth) of around 2500 square meters, delimited by the coastline and a fringing reef (Changuu Island) or patchy reef (Snake Island).