Environmental change and its effects on inter-decadal variations of diatom production, species composition and frustule dissolution in a coastal marginal sea

The implications of climate change during the second half of the 20 th century have been reported throughout the world. Although marginal seas are sensitive to climate change and anthropogenic impacts, relatively little attention has been given to the South East Asian marginal seas. Thus, to bridge this gap in knowledge, a sediment core was collected from the coastal areas of the Leizhou Peninsula in the South China Sea(SCS) to study the inter-decadal climate change and its consequences using diatom species composition as a proxy record. Diatom absolute abundance varied from 2 300 to 68 000 and averaged 16 000 valves per gram of dry weight(v/gdw). The fractional dissolution index(Fi) was usually below 0.5, which indicates low to moderate preservation of diatom valves at coastal area of the SCS. At the inter-decadal time scale, total diatom abundance was high for the period after 1972, which coincided with 1) increased percentage of planktonic diatom abundance and Fi; 2) emergence and dominance of high productivity indicative cosmopolitan species such as T halassionema nitzschioides and Paralia sulcata(their relative abundance increased from <1.5% to >7% for the period before and after 1972, respectively); 3) decreased relative abundance of the small-sized eutrophication indicative species, C yclotella striata, from 70% to 40%. This study reveals that variations in the abundance of diatoms preserved in the sediment was a function of both production and dissolution/preservation of diatom valves, which in turn was intimately connected to the prevailing environmental/climatic conditions. In conclusion, these data reveal the existence of substantial changes in the coastal SCS in response to the 1970 s climate shift that was recorded in dif ferent parts of the world.

Decadal variations in diatoms and dinoflagellates on the inner shelf of the East China Sea, China

Diatoms and dinoflagellates are two major groups of phytoplankton that flourish in the oceans, particularly in coastal zone and upwelling systems, and their contrasting production have been reported in several world seas. However, this information is not available in the coastal East China Sea(ECS). Thus, to investigate and compare the decadal trends in diatoms and dinoflagellates, a sediment core, 47-cm long, was collected from the coastal zone of the ECS. Sediment chlorophyll-a(Chl-a), phytoplankton-group specific pigment signatures of diatoms and dinoflagellates, and diatom valve concentrations were determined. The sediment core covered the period from 1961 to 2011 AD. The chlorophyll-a contents ranged from 2.32 to 73 μg/g dry sediment(dw) and averaged 9.81 μg/g dw. Diatom absolute abundance ranged from 29 152 to 177 501 valve/gram(v/g) dw and averaged 72 137 v/g dw. Diatom valve and diatom specific pigment marker concentrations were not significantly correlated. Peridinin increased after the 1980 s in line with intensified use of fertilizer and related increases in nutrient inputs into the marine environment. The increased occurrence of dinoflagellate dominance after the 1980 s can be mostly explained by the increase in nutrients. However, the contribution of dinoflagellates to total phytoplankton production(Chl-a) decreased during the final decade of this study, probably because of the overwhelming increase in diatom production that corresponded with the construction of the Three Gorges Dam(TGD) and related light availability. Similarly, the mean ratio of fucoxanthin/peridinin for the period from 1982 to 2001 was 6% less than for 1961 to 1982, while the ratio for 2001 to 2011 was 45.3% greater than for 1982 to 2001. The decadal variation in the fucoxanthin/peridinin ratio implies that dinoflagellate production had been gradually increasing until 2001. We suggest that the observed changes can be explained by anthropogenic impacts, such as nutrient loading and dam construction.