Abstract. To investigate variations in nitrite, nitrate, phosphate and silicate in a spring-neap tide in a coral reef system influenced by groundwater discharge, we carried out a time-series observation of these nutrients and and 2.68 M, respectively. Nitrate and phosphate were negatively correlated with water depth during the spring tide, but showed no correlation during the neap tide. Nitrite was positively correlated with water depth in the spring and neap tide.They were also significantly correlated with salinity at the ebb flow of the spring tide. We quantified variations in oxidized nitrogen (NO x ) and phosphate contributed by biological processes based on mixing lines of these nutrients. During both the spring and neap tide biologically contributed NO x and phosphate were significantly correlated with regression slopes of 4.60 20 in the spring tide and 13.37 in the neap tide, similar to the composition of these nutrients in the water column, 5.43 and 14.18, respectively. This similarity indicates that the composition of nutrients in the water column of the reef system was closely related with biological processes during both tidal periods, but the biological influence appeared to be less as inferred from the less significant correlations during the spring tide when groundwater discharge was more prominent.