Degree of nutrient consumption of upwelled water in the Taiwan Strait based on dissolved organic phosphorus or nitrogen

Intermittent upwelling occurs in the Taiwan Strait down to the southern tip of Taiwan in Nanwan Bay. These upwelled waters are usually supersaturated with respect to CO2, and hence may release it into the atmosphere. On the other hand, the upwelled waters also transport nutrients to the euphotic zone, thus possibly—yet somewhat paradoxically—reducing CO2 as it is consumed through enhanced biological productivity. Since it takes a period of a few days to a few weeks for phytoplankton to fully grow, naturally, only aged upwelled water has the potential to become a CO2 sink. To determine the relative aging status of upwelled water, this study employed a Degree of Nutrient Consumption (DNC) based on the ratio of dissolved organic phosphorus or nitrogen (DOP or DON), particulate organic phosphorus or nitrogen (POP or PON) and dissolved inorganic phosphorus or nitrogen (DIP or DIN): DNC=(DOX+POX)/(DIX+DOX+POX) where X is either P or N. Bottom waters in the aphotic zone in the Penghu Channel in the southeastern Taiwan Strait and newly upwelled surface waters there were found to be low in terms of their DNC. In general, a low DNC was noted alongside other traditionally used upwelling indicators, such as low temperature, less dissolved oxygen and low pH, but high salinity, apparent oxygen utilization, alkalinity, total CO2, fCO2, elevated nutrient contents and high chlorophyll a. Surface waters with a relatively lower DNC were, in fact, supersaturated with CO2; conversely, those with a higher DNC were undersaturated.