RESEARCH ARTICLE Spatiotemporal variation in environmental features and elemental/isotopic composition of organic matter sources and primary producers in the Yundang Lagoon (Xiamen, China) Xinqing Zheng 1 & Serena Como 2 & Paolo Magni 2 & Lingfeng Huang 3 Received: 7 August 2018 / Accepted: 26 February 2019 # Springer-Verlag GmbH Germany, part of Springer Nature 2019 Abstract 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 , δ 13 C, δ 15 N) 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 δ 13 C and δ 15 N of POM, owing to changes in its carbon sources. The POM in the diversion canal (POM DC ) of the lagoon originated mainly from terrestrial OM both in March and September, as evidenced by a C at /N at ratio of 12~17 and a TOC/Chl-a ratio exceeding 400, as well as depleted δ 13 C (− 27.3~− 23.7‰) and δ 15 N(− 2.8~0.1‰). The POM in the main canal (POM MC ) and the inner (POM IL ) and outer (POM OL ) 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 δ 13 C values (− 22.7~− 17.9‰) which are close to the values typical for fresh phytoplankton. However, these were strongly regulated by exogenous OM in March. The combined contribution of POM MC and POM DC 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 (60.7~85.7%) 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. Keywords Eutrophication . Stable isotope . Variation . POM . Trophic level . Coastal lagoons Introduction Aquatic eutrophication is a major problem globally and is primarily due to the discharge of large amounts of wastewater derived from agricultural, industrial, and municipal activities. Micro- and macro-algal blooms are commonly found in coast- al waters subjected to excessive anthropogenic inputs of nu- trients (Kraufvelin et al. 2006; Worm and Lotze 2006; Balducci et al. 2001). Coastal lagoons are semi-enclosed coastal waters separated by sand spits or sandbanks from inshore waters. They are one of the most productive marine ecosystems in the world, Responsible editor: Philippe Garrigues * Lingfeng Huang huanglf@xmu.edu.cn 1 Third Institute of Oceanography, Ministry of Natural Resources, Xiamen 361005, People’s Republic of China 2 National Research Council of Italy, Institute for the Study of Anthropogenic Impact and Sustainability in the Marine Environment (CNR-IAS), 09170 Oristano, Italy 3 Key Laboratory of the Ministry of Education for Coastal and Wetland Ecosystems, Xiamen University, Xiamen 361102, People’s Republic of China Environmental Science and Pollution Research https://doi.org/10.1007/s11356-019-04720-2