Description: We studied if functional traits related to resource preemption (light and inorganic nutrients) exert control on space preemption of tropical seagrass meadows. Additionally, we studied if space preemption changed under different eutrophication scenarios. We took seagrass abundance data to study space preemption, seagrass traits data to study their effect on space preemption and eutrophication indicators to evaluate the level of eutrophication at each site/sampling event. The data was collected in Unguja Island (Zanzibar Archipealgo, Tanzania) in seven sites/sampling events (Harbor, Chapwani, Changuu, Bweleo, Fumba, Mangroves and Marumbi). Each site/sampling event comprised a subtidal seagrass meadow (2-4 meters depth) of around 2500 square meters, delimited by the coastline and a fringing reef. The data was taken between the 26.09.2016 to the 05.10.2016. In each site/sampling event, five 50 meters transects were deployed perpendicular to the coast and paralel to each other, approximately separated by 50 meters. The areas enclosed beweeen the transects were names A, B, C and D. We considered nine traits reportedly correlated to light and inorganic nutrients preemption, namely, leaf maximum length (cm), leaf maximum width (cm), vertical rhizome length (cm), leaves per shoot, rhizome diameter (cm), roots per meter, root maximum length (cm), leaf mass area (grams per square centimeter), and shoots per meter. To quantify the seagrass traits at each site/sampling event, we defined four zones of ~2500 m2 per site/sampling event, within which we sampled five ramets (defined as a train of at least two shoots) per species, amounting to a total of approximately 20 ramets per species and site/sampling event. The ramets were transported frozen at -4°C to the Leibniz Centre for Tropical Marine Research for trait measuring. We measured leaf maximum length, leaf maximum width and vertical rhizome length with a ruler to the nearest millimeter. Leaves per shoot were visually counted.

Description: To determine the gut content of T. gratilla, we sampled 10 sea urchins along a transect parallel to the coastline and transported them to the Institute of Marine Sciences (Stone Town, Zanzibar). The urchins were collected upon encounter along the transect with the rule of not collecting urchins from the same seagrass patch. This rule was applied to avoid bias towards specific seagrass species. For this analysis, we weighed the sea urchins, measured their diameter to the nearest millimeter with a ruler, and stored them in a 90% ethanol solution. For the gut content analysis, the sea urchins were opened and the content from the whole digestive tract was extracted. The gut content was placed in a Petri dish, and then transferred to a 250 µm mesh to remove the sediment. After the sediment removal, we weighed the gut content fresh weight. Approximately 1 gram (fresh weight) was taken as a sub-sample from the gut content and placed in a new Petri dish to observe under the magnifying glass. Each sample was compared to an example of seagrass material that was prepared specifically for identification after the grazing and digestion process of each species. We imitated this effect by grinding the seagrass species to a similar size and storing them in 90% ethanol solution. Using this method, we could separate the gut contents into the different seagrass species. Contents were then dried in the oven at 60 degrees for 24 hours to obtain their dry weight, and percentage of each seagrass species in the gut was calculated.