Popis: |
Strong temperature fronts are commonly formed in the coastal northern South China Sea (NSCS) by mixing of the nearshore mesotrophic cold waters with the offshore oligotrophic warm waters. To understand community response to the rapid environmental change across the frontal system, a field survey was conducted during March 2019 focusing on the spatial and vertical variations of plankton size structure, including chlorophyll-a size-fractionation, picoplankton composition and biomass via flow cytometry, as well as the normalized biovolume size spectrum (NBSS) of nano- and microplankton by Flow Image Microscopy (FlowCAM). In the surface layer, we observed a substantial loss of chain-forming diatoms from the nearshore to the frontal zone. In contrast, a rapid decline of small phytoplankton including Synechococcus, picoeukaryotes, and small nanoplankton (mainly Chaetoceros) was documented from the frontal zone to the offshore. The sharp change in surface plankton community had resulted in a large spatial variation of the mean NBSS slope from −0.37 in the nearshore, to −0.78 at the frontal zone, and −0.62 in the offshore. The cross-frontal change of plankton size composition could be explained by the varying physical and biogeochemical settings given the significant correlations of the NBSS slope and intercept with both nutrient and temperature in the surface waters. In the bottom layer of the temperature front, the resuspension of sediment detrital particles was found to substantially affect the particle size spectra with the steepest NBSS slope (−0.94) but the highest NBSS intercept (18.92) appearing at the bottom of the frontal zone. When the sediment signals were removed, the spatial change in abundance of microplankton from FlowCAM showed a general agreement with that from the micro-phytoplankton chlorophyll-a revealing the importance of phytoplankton dynamics and sediment resuspension in controlling the cross-frontal distribution of plankton size structure in the bottom layer. Finally, we found a significant negative correlation of the NBSS slope with the transparent exopolymer particles (TEP) in the bottom waters, which is different from the positive correlation observed in the surface waters. This result may be related to various roles of TEP on particle size structure in different layers of the frontal system. |