Photosynthetic active radiation (PAR) was measured during a nutrient enrichment and exclusion field experiment on seagrass. 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). Data was collected between July 19th and September 20th of 2017 in four sampling times (Day 0 (19.07.2017), Day 20 (09.08.2017), Day 38 (31.08.2017) and Day 63 (19.09.2017)). Data collection took place in Changuu Island (Zanzibar Archipelago, Tanzania) in the area that the experiment took place, and not per experimental plot. Changuu Island is located 3 km from Stone Town, Zanzibar's busiest town. The study area is characterised by a fringing reef around a multi-specific seagrass ecosystem. The substrate is primarily carbonate sediment. PAR and the calculation of the attenuation coefficient were collected at each sampling time to account for potential confounding variables during the experiment. Photosynthetically active radiation (PAR) was measured every 2 hours at each sampling time using a LiCor-1500 with a PAR sensor. The PAR sensor (μmol photons m−2 s−1) was placed at the water surface and approximately half a meter underwater. The light attenuation coefficient was calculated using the Bouguer-Lambert's law, which follows the equation: I(z)=I0 e -µz, where z is the depth in meters, I(z) is the PAR intensity at a depth z, I0 is the PAR intensity at the water surface and µ is the light attenuation coefficient.