The pelagic microbial food web structure in Sanggou Bay, Yellow Sea: Spatial variation over four successive seasons

Abstract. Sanggou Bay (Yellow Sea, China) is a small semi-closed bay in the eastern part of the Shandong Peninsula. In order to characterise the Sanggou Bay microbial food web (MFW) structure, we first documented, over four successive seasons, the distributions of environmental variables and abundances and biomasses of heterotrophic prokaryotes (HP), Synechococcus (SYN), picoeukaryrotes (PEUK), heterotrophic and pigmented nanoflagellates (HNF & PNF) and ciliates. The four season distributions in the Sanggou Bay of environmental variables and MFW components were submitted to cluster analysis, leading to distinguish Inner Bay and Outer Bay clusters at each season. In addition, Outer Bay MFW was found identical to the Inner Bay one but with a delay of one season, thus limiting to 4 the number of MFW characterising Sanggou Bay in that survey. We confirmed the existence of a strong relationship between HNF and HP, and extended this empirical relationship to the other MFW components: SYN, PEUK, PNF and ciliates. We also established upper and lower empirical linear boundaries for all the MFW component relationships with HP. The existence of these boundaries in the complex system made by the MFW stresses the need for systemic studies like the ones conducted for multi-enzyme systems and metabolic pathways that lead to the metabolic control theory. To better determine the MFW structure, we normalised for each sample, the biomass of the MFW components by that of HP. The normalised biomasses of SYN, PEUK, PNF and HNF had obvious seasonal variations with high values in summer or autumn, while ciliate normalised biomasses were low in summer and exhibited high values in winter. The main MFW-structure difference between Inner and Outer Bay clusters came from biomass differences for SYN, PEUK and PNF, whereas other component biomass-values were similar between Inner and Outer Bay clusters. Our study showed that the normalisation method could be used in other marine area to study the microbial food web structure. Indeed, the efficiency of this approach to determine MFW structure was demonstrated by successfully applying it to a similar data set from the literature and related to the Arabian Sea.