Phytoplankton strategies to exploit nutrients in coastal lagoons with different eutrophication status during re-oligotrophication

Abstract

We studied a mesotrophic and a hypertrophic Mediterranean coastal lagoon, both of which had been simultaneously subjected to a nutrient input reduction for 9 yr. We compared these 2 lagoons to an oligotrophic coastal lagoon. Using bioassays comprising 24 h incubations with added phosphorus and/or ammonium, we investigated the response of the phytoplankton communities to 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 3 communities, but it differed in composition among the lagoons. Green algae dominated in the hypertrophic lagoon, whereas the mesotrophic lagoon presented a higher diversity of phytoplankton groups. Picocyanobacteria and small diatoms were the most abundant groups in the oligotrophic lagoon, although they accounted for 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 nutrientdepleted 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.

Trophic states regulate assembly processes and network structures of small chromophytic phytoplankton communities in subtropical coastal ecosystem

“…Increasing evidence has shown that disturbance could select species exhibiting speci c biological traits and markedly alter the composition of SCP community (Li et al, 2020;Stegen et al, 2012). A high relative abundance of Thalassiosira, Pseudo-nitzschia and Skeletonema, were observed and widely spread in the heavy eutrophication Maowei Sea, whereas Thalassiosira, Minutocellus, Navicula, and Chaetoceros were signi cantly over-dominated in the Qinzhou Bay and Beibu Gulf open sea as eutrophic states decreased, which is consistent with previous studies (Leruste et al, 2019;Tian et al, 2017). Bacillariophyta exhibited high adaptability for oligotrophic conditions as to be dominant in nutrient-de cient areas (Zhou et al, 2017).…”

Trophic States Regulate Assembly Processes and Network Structures of Picophytoplankton Communities in Subtropical Coastal Ecosystems

“…Increasing evidence has shown that disturbance could select species exhibiting speci c biological traits and markedly alter the composition of picophytoplankton community [31,32]. A high relative abundance of Rhodophyta was observed and widely spread in the heavy eutrophication Maowei Sea, whereas Bacillariophyta was signi cantly over-dominated in the Qinzhou Bay and Beibu Gulf open sea as eutrophic states decreased, which is consistent with previous studies [33,34]. Bacillariophyta exhibited high adaptability for oligotrophic conditions as to be dominant in nutrient-de cient areas [35].…”

Long-term ecological trajectories of a disturbed Mediterranean coastal lagoon (Biguglia lagoon): Ecosystem-based approach and considering its resilience for conservation?

“…This led to the accumulation of nutrients (from runoff and reduced exchanges with seawater). Parallel to the desalinization, observed until 2010, an increase in nitrates concentration was reported, linked to agricultural activities, and still ongoing, despite management measures taken in the past few years for nitrogen reduction through improved sanitation (Pasqualini et al, 2017;Leruste et al, 2019). This inertial pollution was found to be also due to the accumulation of nitrate in the groundwater (Erostate et al, 2018;Erostate et al, 2019;Jaunat et al, 2019).…”

“…Finally, monthly phytoplankton cellular abundances (cell L −1 ) and community composition data were collected to analyze relative composition at the class level and identify dominant phytoplankton species and bloom events, defined as cellular proliferation over 100,000 cell L −1 threshold (Cecchi et al, 2016;Belin and Neaud-Masson, 2017). Data were issued from literature (Cecchi et al, 2016;Garrido et al, 2016;Leruste et al, 2019) and field sampling in 2020. All analyzed data came from optical microscopy identification (inverted microscope CKX31, OLYMPUS) and quantification through the Utermöhl method, applied on samples fixed with formaldehyde solution at 2.5% final concentration (Utermöhl, 1958;AFNOR, 2006).…”