Based on long-term data of measurements of nutrient and chlorophyll a concentrations as well as estimates of primary production, the response of phytoplankton to nutrient enrichment was evaluated in the highly urbanized Jiaozhou Bay. Results showed that phytoplankton biomass, as indicated by chlorophyll a concentration, did not show a direct linear response to increased nutrient concentrations. Instead, chlorophyll a concentration was maintained at a constant level in the last two decades in Jiaozhou Bay, so did primary productivity. However, a reduction of zooplankton biomass was observed during the same period. Analysis showed that although the scale of phytoplankton blooms might be limited by availability of silicate due to low Si/N ratio and low concentration, top-down control on phytoplankton biomass by bivalve grazing may be an important factor in Jiaozhou Bay ecosystem. 1. Introduction In shallow coastal marine waters, total primary production and biomass of phytoplankton is generally assumed to be hyperbolically related to nutrient loadings from land [1]. Many coastal marine waters have been nutrient enriched, and their nutrient ratios altered, as a result of changes in riverine inputs related to changes in land use and anthropogenic nutrient emissions over the last century [2, 3]. Coastal ecosystems often respond to such changes in nutrient budgets by altering phytoplankton structure and abundance, thereby impacting the rest of the food web [4]. Long-term observations are important for documenting such variations and understanding the impact of anthropogenic perturbations in coastal ecosystems fwithin the context of natural variability [5]. Jiaozhou Bay is a semienclosed coastal ecosystem with a surface area of 367？km2 and an average depth of 7？m and is connected to the Yellow Sea through a narrow mouth. Previous studies reveal that concentrations of dissolved inorganic nitrogen (DIN) and phosphate (DIP) have increased from the 1960s to the 1990s, while concentration of dissolved silicate (DSi) has decreased from the 1980s to the 1990s [6]. However, phytoplankton biomass, as indicated by chlorophyll a concentration, has remained roughly unchanged from the mid-1980s to the 1990s, while abundance of net phytoplankton has decreased since the 1960s [6, 7]. Therefore, it was hypothesized that phytoplankton growth was limited by silicate, and phytoplankton composition might have shifted from large-size species to small-size species [6, 7]. In this paper, we present data on long-term trends in primary productivity and phytoplankton biomass (measured
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