Nitrogen fixation in the monsoon impacted Mekong River plume
Final Report Abstract
The project focused on the role of nitrogen fixation for the biogeochemistry of the South China Sea off Vietnam. The region is heavily impacted by human change, receives the outflow of the Mekong River and is further impacted by upwelling during the SW monsoon season. The region is naturally highly dynamic and experiences negative effects from Mangrove destruction, damming of the Mekong river and changes in river water constituents. The ocean off Vietnam is an important fishing ground but increasing numbers of harmful algae blooms in coastal waters are threatening the environment. Understanding the underlying nitrogen fluxes of major importance for the protection and management of the marine system. During previous DFG funded studies the role of diatom-diazotroph associations (DDAs) for new nitrogen input in the surface ocean was first described and lead to the main hypothesis in this project that these organisms play an important role during the SW monsoon season when the Mekong River flows northeast through the study area. Two cruises with RV Falkor for the start and end of the SW monsoon season were funded by Schmidt Ocean Institute. Unfortunately, only one cruise took place the second was cancelled due to new political regulations and unknown reasons. However, we could replace this cruise with a study in Nha Trang Bay that our colleagues from the Institute of Oceanography performed. Monthly samples and experiments were carried out to measure nitrogen fixation rates and collect samples for single cell dinitrogen uptake by DDAs. Major results of our project are a new method for defining biologically relevant habitats in heterogeneous marine regions based on standard environmental data that integrate over various scales of time and space. The defined habitats in essence captured the water mass distributions and were able to resolve the spatial distribution of phytoplankton communities. By demonstrating the biological relevance of the generated habitats, we gain insights into the conditions promoting the growth of distinct communities and the factors that lead to mismatches between environmental conditions and phytoplankton community structure. These habitats then laid the foundation for interpreting our comprehensive biogeochemical data sets, which included nitrogen stable isotope measurements of DIN, POM, and zooplankton, in addition to bulk and cell-specific rates of nitrogen fixation. Stable isotope data in nitrate and particulate organic matter helped us to gain insight into the importance of fixed nitrogen for particle production and transfer into the food chain. We can clearly see that diazotrophs were present and active in all water masses and habitats, and that nitrogen fixation was a particularly significant source of bioavailable nitrogen in offshore waters where DDAs and Trichodesmium spp. were most abundant. In this region, diazotrophic N comprised up to 60% of PON, supported up to 30% of primary production, and generally made up 10-20% of zooplankton N. Finally, compound specific amino acid d15N values in a subset of POM and zooplankton samples provided deeper insight into the plankton food web and the influence of nitrogen fixation. We found that the mixed layer depth played a role in regulating both the N source and trophic structure of the plankton. Nearshore where the MLD was shallow, particularly in the vicinity of the river mouth and in the youngest upwelled waters, food webs were predominantly nitrate-based and heterotrophic. In contrast, in waters farther offshore where the MLD was much deeper, diazotrophy supported more of the plankton food web and mesozooplankton occasionally occupied trophic positions indicative of omnivory. We additionally found that the nitrate availability index (NAI) that we developed for the habitat type method correlated significantly with the δ15N of the N-source amino acid phenylalanine in mesozooplankton, suggesting that this simple measure based on water column nitrate concentrations may be a useful indicator of nitrogen sources in zooplankton in this and other systems. In summary we can state that our data covered all levels from dissolved nutrients, particles, phytoplankton into the zooplankton. The impact of nitrogen fixation can be identified everywhere and points to the outstanding role of diazotrophs and in particular DDAs for the functioning of the planktonic ecosystem in the South China Sea. Schmidt Ocean (see https://schmidtocean.org/) professionally accompanies cruises with journalists and/or artists to guarantee outreach. It is highly recommended to visit the web page of our cruise https://schmidtocean.org/cruise/changing-river-measuring-nutrient-fluxes-southchina-sea/ where the background and work is wonderfully presented e.g. in the wrap-up video. https://schmidtocean.org/cruise-log-post/expedition-wrap-video/.
Publications
- Bio-optical Provinces of the BIỂN ĐÔNG (South China Sea), AGU-Fall Meeting San Fancisco, San Francisco, 12.-16.12.2016
Subramaniam, A. I. Hay, M. Aleman, M. Voss, A. Bracco, J. P. Montoya
- “Insights from nutrient amendment experiments in the South China Sea.” 2017 Bien Dong Conference, Nha Trang, Viet Nam
Weber, S. C., H. Doan-Nhu, A. Subramaniam, J. P. Montoya, M. Voss
- “New insights on nitrogen-fixation in the South China Sea.” Ocean Sciences Meeting 2017, Portland, OR, USA
Weber, S. C., R. A. Foster, H. Doan-Nhu, L. Nguyen-Ngoc, A. Subramaniam, J. P. Montoya, M. Voss
- “Differential patterns of diatom-diazotroph association (DDAs) distributions in waters of the South China Sea.” 13th European Nitrogen Fixation Conference (ENFC 2018), Stockholm, Sweden
Weber, S. C., H. Doan-Nhu, J. Dippner, A. Subramaniam, L. Nguyen-Ngoc, J. P. Montoya, M. Voss
- (2019) Habitat delineation in highly variable marine environments. Frontiers in Marine Science, 6, 112
Weber, S.C., Subramaniam, A., Montoya, J.P., Doan-Nhu, H., Nguyen-Ngoc, L., Dippner, J.W., Voss, M.
(See online at https://doi.org/10.3389/fmars.2019.00112) - Nitrogen fixation and nitrogen dynamics in tropical river plumes. 23.02.-02.03. 2019 ASLO conference Puerto Rico, USA
Montoya, J., Lee-Patterson, D., Subramaniam, A., Peterson, R., Loick-Wilde, N., Voss, M.
