AME prepress abstract  -  DOI: https://doi.org/10.3354/ame01906

Phytoplankton strategies to exploit nutrients in coastal lagoons with different eutrophication status and involved in a re-oligotrophication process

A. Leruste*, F. Guilhaumon, R. De Wit, N. Malet, Y. Collos, B. Bec

*Email: a.lerustecalpena@gmail.com

ABSTRACT: Abstract: We studied a mesotrophic and a hypertrophic Mediterranean coastal lagoon, which both have been simultaneously subjected to a nutrient input reduction since 9 years. We compared these two lagoons with an oligotrophic coastal lagoon. Using bioassays comprising 24-h incubations with added phosphorus and/or ammonium, we investigated the response of their phytoplankton communities to a nutrient enrichment during summer in terms of biomass, size class structure, abundance, and growth. For nitrogen and phosphorus, we identified which nutrient limited phytoplankton growth, and what strategies of nutrient exploitation the communities adopted to cope with these limitations. Ultraphytoplankton dominated the three communities, albeit differing in composition among the lagoons. Thus, green algae dominated in the hypertrophic lagoon. The mesotrophic lagoon presented a higher diversity of phytoplankton groups. Picocyanobacteria and small diatoms were the most abundant groups in the oligotrophic lagoon, albeit representing less biomass than green algae. The communities of the mesotrophic and the hypertrophic lagoons strongly responded to the nutrient pulse, showing that the re-oligotrophication trajectories of these lagoons were still very vulnerable to occasional eutrophication events. On the other hand, the oligotrophic lagoon marginally responded to the enrichment, indicating its adaptation to nutrient-depleted conditions. We observed a shift along the eutrophication gradient, from a co-limitation by N and P in the oligotrophic and the mesotrophic lagoons to a single and strong N limitation in the hypertrophic lagoon. Each community demonstrated specific use of internal, external or recycled nutrient pools under experimentally induced limitation.