Seagrass leaf carbon, nitrogen and phosphorus content and their ratios in Changuu Island (Unguja Island, Zanzibar Archipelago, Tanzania)

We measured the % carbon (C), nitrogen (N) and phosphorus (P) content of seagrass leaves. We took three seagrass leaf samples in five meadows of each seagrass species (T. ciliatum, T. hemprichii, H. uninervis, S. isoetifolium, C. serrulata) in Changuu Island and transported them in a cooled container to the Institute of Marine Sciences (Stone Town, Zanzibar). The leaves were cleaned of epiphytes and rinsed with distilled water. We separated the second leaf of each shoot for the measurement, dried them in the oven at 60 degrees for 48 hours, and ground them to a fine powder with mortar and pestle. The samples were then transported to the Leibniz Centre of Tropical Marine Research in Bremen (Germany). Carbon and nitrogen % content was measured in a Euro EA 3000 (EuroVector) analyzer. For the determination of the % phosphorous content of the leaves, we used an alkaline persulphate oxidation method (Koroleff, 1983). We transferred 1 mg of ground sample into a vial and added 4.5 ml of distilled water and 0.5 ml of Oxisolv reagent (Merck). The vials were closed and placed in the oven for one hour at 120 °C. After letting the samples cool down at room temperature, they were centrifuged at 4700 rpm for ten minutes. Three mL of supernatant were placed into another vial and 0.0626 mL of ascorbic acid reagent were added. After mixing, 0.0626 mL of molybdate mix-reagent was added. After a reaction time of 10 minutes, the samples were transferred to a 1 cm macrocuvette. We then measured absorbance at 880 nm in a Shimadzu UV-1700 UV-VIS dual-beam photometer for the % P determination. Finally, we calculated the ratios between C, N and P: C:N.