Silicate as the limiting nutrient for phytoplankton in a subtropical eutrophic estuary of Taiwan

The estuary of Tamsui River, located in the northern subtropical area of Taiwan, is eutrophic as a result of municipal sewage discharges from the Taipei metropolitan area. The primary productivity is rather low in this estuary, compared with other estuaries. The objective here was to determine whether nutrient limitation might explain the decreasing productivity that has occurred in the last two decades. Comparison of phytoplankton biomass between four sampling sites revealed a trend of decreasing growth, associated with increasing dominance of diatoms, along a gradient moving towards the mouth of the estuary. This trend corresponded with a spatial gradient in nutrient concentrations among the study sites, which resulted from a mixture of heavily polluted river water with less polluted marine water. An in vitro nutrient enrichment bioassay was done with additions of nitrogen, phosphorus, and silicate to estuarine water containing the natural phytoplankton communities. The results indicated that enrichment with silicate gave rise to a greater enhancement in phytoplankton biomass, mainly of diatoms, than did enrichment with nitrogen or phosphorus. Chemical analysis of estuarine water indicated that non-biogenic fraction of silicate might be depleted at flow tide, giving rise to imbalanced nutrient supply and thus a lowering of primary productivity. In this estuary, the imbalance of nutrient supply resulted from increasing water pollution is possibly the cause of the decrease in productivity over last several decades.