Detailed seasonal analyses of phytoplankton assemblages' composition were performed on long-term datasets (20 years) of two oligotrophic Mediterranean lagoons (Diana and Urbino), in order to test if phytoplankton community and bloom events patterns rely on a seasonal basis. Our results highlight a similar phytoplankton composition between the lagoons, but different patterns in terms of phytoplankton abundances, of timing, magnitude and occurrence of Taxonomic Units, and of bloom events occurrence. Dominant diatoms group showed a seasonal repartition, with highest contribution of Skeletonema sp. during winter and spring, some other groups emerging in warmer seasons (e.g. Chaetoceros spp.), or not showing marked seasonality (e.g. Pseudo-nitzschia spp.). Dinoflagellates' abundances were higher during autumn, besides exhibiting punctual proliferations over the year. Salinity was the strongest environmental parameter in controlling Diana lagoon phytoplankton community, while Urbino community was mostly controlled by turbidity. A worrying evolution of the toxic diatom Pseudo-nitzschia spp., linked to salinity and temperature changes, was detected in both lagoons. We further demonstrated changes in the phytoplankton community are ongoing in these lagoons, despite their contrasting human pressure contexts, indicating that serious impacts on phytoplankton dynamics and blooms triggering in Mediterranean lagoons shall be foreseen within the current and forecasted climate change scenarios.
Coastal lagoons are subjected to ever-increasing direct or indirect anthropic pressures and are inexorably deteriorating with serious issues regarding their resilience. In this paper, we assessed the functioning and evolution of the highly disturbed Biguglia coastal lagoon (Mediterranean Sea, Corsica) through an ecosystem-based approach (EBA), using multiple biotic and abiotic proxies (hydro-climatic context and eutrophication), considering its connectivity to sea and watershed and biological compartments (macrophytes, phytoplankton, and invasive species) and taking into account human influence (management actions and fishing activities). The aim of this work is firstly to provide a comprehensive analysis of its long-term (2000–2021) ecological evolution trajectory and then, based on these results, to anticipate management strategies for supporting its conservation and restoration, and the maintenance of ecosystem services it offers. Results revealed that while the lagoon showed these days a good capacity to recover after disturbance and absorb change, it recently exhibited considerable changes in its phytoplankton community composition, developed an increased susceptibility to biological invasion, and experienced a drastic reduction in fish stocks. The major interannual variations of the mean salinity, strongly dependent on management interventions beyond natural climatic variability, summarized this instability. In the future, the lagoon may no longer be able to cope with even small disturbances, which could then be sufficient to reach a breakpoint and tip the system permanently into undesired/degraded states. We demonstrated that local and punctual management actions are not always beneficial for the entire ecosystem or even detrimental in some instances. Such a retrospective ecosystem-based approach is fundamental for producing the holistic insights required to implement efficient integrated ecosystem management. This further helps enhance lagoon resilience and hence preserve its ecosystem services in the context of increasing global changes. Such lessons are useful anywhere for comparable ecosystems.
Mediterranean coastal lagoons are affected by multiple threats (demographic pressures, eutrophication, climate change) expected to increase in the future and impact the ecosystem services provided. Conservation norms and scientific studies usually focus on large lagoons (> 0.5 km2) due to their economic importance, while they ignore smaller lagoons. These are poorly understood and often unprotected, despite their prevalence within the Mediterranean region and their importance. Qualitative and quantitative characterisation of small lagoons, in terms of functioning and sensitivity to global and local changes, are needed to develop appropriate management strategies. For this purpose, this work provides the first inventory of all Corsican lagoons and has investigated three of them of small size (Arasu, Santa Giulia, Balistra), characterised by contrasting anthropogenic contexts (highly modified/disturbed, medium disturbance, quasi-pristine). At the regional level, 91 of the 95 lagoons identified are < 0.5 km2, making Corsica a good example for the study of small Mediterranean lagoons. The three case studies showed differences in their seasonal biogeochemical cycles and phytoplankton communities (biomass, diversity, photosynthetic efficiency). Arasu and Santa Giulia lagoons showed an increase in watershed urbanisation (+ 12% and + 6% in 30 years), high phytoplankton biomass, low diversity and blooms of potentially harmful dinoflagellates. Conversely, Balistra lagoon showed a good status overall, but some anthropogenic pollution sources within its watershed. This study demonstrates the importance of small lagoons at regional and Mediterranean scale, and provides knowledge on studied local sites but also potential applications elsewhere. The importance of an integrated approach considering lagoons within their adjacent connected systems (watershed and sea) and anthropogenic contexts is highlighted.
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