Palaeoecological analysis of phytoplankton regime shifts in response to coastal eutrophication

We used a multiple-proxy palaeoeco logical method to reconstruct a 100 yr time series showing coastal eutrophic processes and phytoplankton re sponses. Total organic carbon, total nitrogen, diatom frustules, dinoflagellate cysts, brassicasterol and dinosterol were extracted from chronologic sediment cores in Sishili Bay, a polluted area in China. The cores showed that eutrophication occurred during about 1975 to 1985, which corresponds to increased human activity associated with China’s economic development since 1978. During eutrophication, the biomass of diatoms and dinoflagellates increased, and dominant species shifted abruptly. The small, heavily silicified diatoms Cyclotella stylorum and Paralia sulcata gradually took the place of the large dominant diatom Coscinodiscus radiatus, while dinoflagellates displayed a progressive in crease since 1975. Compared to changes in tem perature and rainfall during 1950 to 2010, increased fertilizer use, marine aquaculture and sewage discharge showed a better match to the increasing trend in biomass, species shift and nutrient concentration. Altered nutrient supply ratios caused by increased nitrogen inputs play an important role in the shifts in diatom and dinoflagellate assemblages.