P-tot means ‘total Phosphorus’. While determination of dissolved phosphate gives information on the bioavailable pool of phosphorus, an assessment of the total amount of phosphorus is also essential. Total phosphorus includes all organic and inorganic forms of phosphorus present in seawater, particulate as well as dissolved. The dissolved organic phosphorus is also partly bioavailable, mainly when phosphate is exhausted. Also the particulate fraction can be used in part.