Evidence of trace metal limited photosynthesis in eutrophic estuarine and coastal waters

Based on field observations and incubation experiments, this study provides evidence that trace metal limited (e.g., Fe) photosynthesis may function in eutrophic coastal waters. In the Zhujiang (Pearl River) Estuary and the adjacent coastal area, analyses of water samples indicate that phytoplankton photosynthesis is apparently phosphorus limited in the freshwater and upper estuary and nitrogen and/or silica limited in the coastal waters farther offshore. The incubation experiments using in situ water samples show, however, that in both cases photosynthesis can be enhanced by addition of trace elements (e.g., Fe and Cu). This is presumably because exogenous influx of major plant nutrients has skewed the primary production toward trace-element limitation, which in turn limits the increase in chlorophyll biomass and full consumption of major nutrients in eutrophic coastal waters as compared to the nutrient-poor oligotrophic open ocean.