Phosphorus limitation in the northern South China Sea during late summer: Influence of the Pearl River

The Pearl River outflow influences a large area of the northern South China Sea (SCS) under the prevalence of the southwest monsoon winds in summer. The Pearl River has high concentrations of nitrate (∼100 μM) and relatively low phosphate (∼1 μM). Little is known about how the input of the Pearl River discharge with high N:P ratios influences nutrient limitation in the coastal and offshore waters of the northern SCS. In the late summer of 2005 and 2006, two cruises were conducted to examine which nutrient, N or P, is the primary nutrient limiting the growth of phytoplankton and bacteria in the northwestern SCS by using a combination of nutrient enrichment bioassays, the induction of the enzyme alkaline phosphatase and the uptake rate of the 33P isotope tracer. Our results showed that 33P turnover times always agreed with the turnover times of alkaline phosphatase activity. Turnover times were short in P-limited areas influenced by the Pearl River, but long in oceanic-dominated N-limited areas (not influenced by the Pearl River). Nitrogen and phosphorus availability was relatively balanced in the SCS basin as nutrient enrichment bioassays indicated that phytoplankton was potentially co-limited by N and P, with N limitation occurring before P. However, in river-impacted areas of the shelf, the balance of available N and P changed due to a high riverine N:P ratio of ∼100:1 and phytoplankton biomass was elevated (up to 3.4 μg Chl a L−1) due to the relatively high nutrient input of the Pearl River discharge. As a result, phytoplankton and bacterial growth was actually P-limited due to the contribution of excess N from the Pearl River plume in late summer. The degree of P limitation was the most severe at the inner shelf stations with the lowest salinity (28.4) and declined in the offshore direction as salinity increased. The input of relatively high nutrients from the Pearl River discharge resulted in a high contribution of phytoplankton (>66%) to total particulate organic phosphorus. Therefore, whether N or P is the most limiting nutrient for the shelf waters in the northern SCS depends on whether the seawater is influenced by the Pearl River estuarine coastal plume, the size of which is influenced by river discharge, rainfall and winds.