Seagrass biochemical traits (%N, %C, %P in leaf, rhizome and roots tissues) were measured during a nutrient enrichment and macrofauna exclusion experiment. A total of 24 plots were set up parallel to the shore, with presence of the three seagrass species (Syringodium isoetifolium, Thalassodendron ciliatum and Thalassia hemprichii) in each plot. The experiment was the factorial combination of two treatments: macrofauna exclusion using cages (three levels: open, closed and uncaged) and nutrient enrichment using garden NPK fertilizer (two levels: ambient and enriched). Each treatment combination was replicated four times. Exclusion treatment simulates the consequences for the food web of losing a top predators and macrograzers. Nutrient enrichment was simulated by issuing nitrogen-phosphorus-potassium (NPK) [15:9:20] fertilizer pellets. NPK fertilizer fast release pellets were packed into cotton tubes and then into a perforated plastic tubes to simulate slow release of nutrients. The tubes were buried half-way into the sediment to ensure enrichment of both the water column and the sediment. Data was collected at the end of the experiment, on Day 63 (19.09.2017). Data collection and experiment took place in Changuu Island (Zanzibar Archipelago, Tanzania; 06˚11'S, 39˚16'E). Changuu Island is located 3 km from Stone Town, Zanzibar's busiest town. Changuu remains relatively unaffected by nutrient runoff pollution. The study area is characterised by a fringing reef around a multi-specific seagrass ecosystem. The substrate is primarily carbonate sediment. Average water depth is approximately 30 cm at Spring Low and 5 m at Spring high tide with an average depth of 2 m. Seagrass biochemical traits were measured to understand the biochemical responses of the three seagrass species (in their leaf, rhizome and root tissues) present in Changuu Island to the nutrient enrichment and macrofauna exclusion treatments. To determine the biochemical responses of seagrass species to the treatments, Carbon (C), Nitrogen (N) and Phosphorus (P) content of the seagrass tissues were measured. The second leaf of the seagrass shoot, a section of roots, and of rhizomes from three samples of each species per treatment plot and sampling time were collected. They were scraped of epiphytes, washed with distilled water and placed in aluminium foil envelopes. Samples were oven-dried at 60°C for 48 h until constant dry weight and envelopes were then sealed and stored in the dark until analysis. Tissue samples were ground into a fine powder and stored in Eppendorf tubes. For C and N content determination, grounded dry seagrass samples were weighed and deposited into tin cups, folded and measured in a Euro EA 3000 (EuroVector) analyser for their C and N content. For the estimation of the P content in the seagrass material, the basic peroxodisulfate digestion method was used. After this procedure, the samples were put in a Shimadzu UV-1700 spectrophotometer for absorption measurements at 880 nm wavelength. With the absorption and weight measurements of the dried seagrass samples, the micrograms per gram of dry seagrass weight was calculated, and then transformed to percentage of phosphorus in the seagrass samples.