Is phosphorus a limiting factor to regulate the growth of phytoplankton in Daya Bay, northern South China Sea: a mesocosm experiment

Previous field investigations implied a potential phosphorus (P)-limitation on the growth of phytoplankton in Daya Bay, a mesotrophic bay in the northern South China Sea. Using a total of 15 mesocosms (3 × 3 × 1.5 m, with ~10.8 m 3 natural seawater containing phytoplankton assemblages for each), we found P-enrichment caused no obvious effect on phytoplankton (Chl a ) growth across 8-day’s cultivation in neither winter nor summer, while nitrogen (N)-enrichment greatly increased Chl a in both seasons. N plus P-enrichment further increased Chl a content. The N- or N plus P-enrichments increased the allocation of nano-Chl a but decreased micro-Chl a in most cases, with no obvious effect by P-alone. Coincided with nutrients effect on Chl a content, N- or N plus P-enrichments significantly enhanced the maximum photochemical quantum yield of Photosystem II (F V /F M ) and maximum relative electron transport rate (rETR max ), but declined the non-photochemical quenching (NPQ), as well as the threshold for light saturation of electron transport (E K ); again, P-enrichment had no significant effect. Moreover, the absorption cross section for PSII photochemistry (σ PSII ) and electron transport efficiency (α) increased due to N- or N plus P-enrichments, indicating the increased nutrients enhance the light utilization efficiency through promoting PSII light harvesting ability, and thus to enhance phytoplankton growth. Our findings indicate that N- or N plus P-enrichments rigorously fuel phytoplankton blooms regardless of N:P ratios, making a note of caution on the expected P-deficiency or P-limitation on the basis of Redfield N:P ratios in Daya Bay. Highlights Mesocosm P-enrichment have no effect on photosynthetic performance and growth of phytoplankton assemblages in both winter and summer periods; Nitrogen (N) enrichment enhanced photosynthetic capacity of phytoplankton and fueled their blooms; Nitrogen (N) or N plus phosphorus (P) enrichments induced an increase of allocation of nanophytoplankton, but a decrease of microphytoplankton in most cases.