Spatiotemporal variation in environmental features and elemental/isotopic composition of organic matter sources and primary producers in the Yundang Lagoon (Xiamen, China)

Aquatic eutrophication is a major problem globally, leading to significant chemical-compositional changes in the first trophic levels of a food web. These will consequently affect the whole food web dynamics in eutrophic coastal ecosystems. In this study, we analyzed the spatiotemporal variation in water chlorophyll-a, total organic carbon (TOC) and TOC/Chl-a ratio, and in the elemental/isotopic (C-at/N-at, C-13, N-15) composition of particulate organic matter (POM), phytoplankton, sedimentary organic matter, benthic microalgae, U. lactuca, and decaying leaves of mangrove plants in the eutrophic, urban lagoon of Yundang (Xiamen, China). Investigations were carried out in four different sectors of the lagoon in March (dominated by Ulva lactuca) and September (dominated by phytoplankton) 2009, and the feasible contribution of potential organic matter (OM) sources to POM was assessed in each period. The results showed significant spatiotemporal variation in C-13 and N-15 of POM, owing to changes in its carbon sources. The POM in the diversion canal (POMDC) of the lagoon originated mainly from terrestrial OM both in March and September, as evidenced by a C-at/N-at ratio of 1217 and a TOC/Chl-a ratio exceeding 400, as well as depleted C-13 (-27.3-23.7 parts per thousand) and N-15 (-2.80.1 parts per thousand). The POM in the main canal (POMMC) and the inner (POMIL) and outer (POMOL) sectors of the lagoon were largely composed of lagoon-borne phytoplankton in September. This was revealed by TOC/Chl-a values below 100 and enriched C-13 values (-22.7-17.9 parts per thousand) which are close to the values typical for fresh phytoplankton. However, these were strongly regulated by exogenous OM in March. The combined contribution of POMMC and POMDC to POM in March reached 64-99% in the inner lagoon and 67-88% in the outer lagoon. Non-living POM, which originates from terrestrial organic detritus, was the main contributor to POM (60.785.7%) both in the inner and outer sections of the lagoon. Overall, the present study demonstrates that the temporal changes in the dominance of primary producers were reflected in significant variation in the environmental features and elemental/isotopic composition of OM sources and their contribution to POM in the Yundang Lagoon. To further our understanding of the effects of eutrophication due to different primary producers on the resource partitioning in the Yundang Lagoon, additional studies on the consumers and the whole food web of the lagoon are expected.