Short-term dynamics of nutrients influenced by upwelling in a small oligotrophic coastal ecosystem, Gan Bay, in the northwest Philippines

Abstract

We present a time-series analysis of nutrient and pCO 2 (partial pressure of CO 2) levels in an oligotrophic coastal ecosystem (Gan Bay), which was likely to be influenced by upwelled subsurface water. Gan Bay is off Currimao Harbor, in the northwest Philippines and is located at the boundary of the South China Sea (SCS). This 42-h time-series observation was conducted in December 2006. In addition to continuous observations of dissolved oxygen (DO) and pCO 2, discrete samples were collected at a depth of 5 m every 3 h for measurements of nutrients, including soluble reactive phosphorus (SRP) and inorganic nitrogen (NO 3 − + NO 2 −) in order to examine their dynamics and possible physical and biological controls. We observed remarkably large short-term variations in the surface water, spanning a 10-fold change for SRP (32–330 nM) and from <0.3 μM to 4.3 μM for NO 3 − + NO 2 −. DO also varied substantially from a lower end of 171 to 205 μM O 2. Surface water pCO 2 changed from an equilibrium stage with the atmosphere (∼386 μatm) to a stage where it was a significant source for the atmospheric CO 2 (seawater pCO 2 ∼469 μatm). We found that the variation of nutrients was driven neither by tidal mixing nor by biological activities, as was suggested by the variations in the total bacterial abundance and chlorophyll a. Instead, our inverse T– S relationship suggested a two end-member mixing process during the observation period. The N:P ratio throughout the observation period was ∼13.2, which is characteristic of SCS subsurface and deep waters. Moreover, pCO 2 was correlated inversely with the sea surface temperature. It is likely, therefore, that an upwelled subsurface cold water with high nutrients, low-temperature and high- pCO 2 existed. It should be noted that this upwelled cold water did not appear to impact the entire observation period (approximately 35 h of 42 h), which might suggest an extremely dynamic nature for this upwelled cold water mass.