Macronutrient and iron limitation of phytoplankton growth in Hong Kong coastal waters

The western side of Hong Kong possesses estuarine characteristics (from the Pearl River estuary) and the eastern side is mainly influenced by the oligotrophic South China Sea, while the southern side has intermediate hydrological characteristics. We investigated the concentrations of dissolved inorganic nutrients and limitations to phytoplankton growth at these 3 locations around Hong Kong. As expected, measurements of dissolved inorganic nitrogen (DIN) and dissolved silicon (DISi) concentrations decreased from the western (DIN: 42.7 to 112 μM; DISi: 24.1 to 81 μM), to the southern and finally the eastern side (<5 μM for both DIN and DISi). The dissolved inorganic phosphorus (DIP) concentration was relatively constant among all areas (1.1 to 1.7 μM). C:P and N:P ratios in the phytoplankton were much higher than the Redfield ratio at stations on the western and southern sides, and decreased with the addition of P. Therefore, phytoplankton assemblages at these locations were mainly limited by P as a result of high N and Si inputs from the Pearl River relative to P. Our summer bioassay indicated that Fe was not the primary limiting element in the estuarine area. Although P stimulation of phytoplankton growth was also observed on the eastern side, additions of N, P, Si and Fe indicated that all of these nutrients except Fe had a strong effect on growth. C:Si and N:Si ratios on the eastern side were >>6.6:1 and >>1:1, respectively, but decreased with the addition of Si. Furthermore, ‘luxury uptake’ of P and Si was observed after replenishment of these nutrients. Thus, it was hypothesized that phytoplankton on the eastern side were limited by several nutrients sequentially in the following order: P > Si > N/Fe. A shift in limitation from one nutrient to another may frequently take place, as concentrations of all these nutrients are low. Furthermore, temporal and spatial variations of nutrient concentration and phytoplankton growth limitation were also observed in the estuary.