Phytoplankton dynamics and bloom events in oligotrophic Mediterranean lagoons: seasonal patterns but hazardous trends

Abstract

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.

Patterns and Predictors of Phytoplankton Assemblage Structure in a Coastal Lagoon: Species-Specific Analysis Needed to Disentangle Anthropogenic Pressures from Ocean Processes

“…During our study, the phytoplankton assemblage structure exhibited significant seasonal differences, across all seasons, but was maximized between winter and summer, periods with contrasting conditions for most of the abiotic variables evaluated. Significant seasonal changes in assemblage structure have also been reported for the western sector of the RF lagoon [53,55], and other temperate [41,131,156] and tropical lagoonal systems [19,137,169]. Although the majority of the discriminating OTUs and/or indicator season specialists were associated with the spring-summer period, probably due to a higher temperature and light intensity (see Section 4.3), some OTUs emerged as autumn-winter representatives, possibly reflecting different environmental preferences and/or a variable balance between bottom-up and top-down controls (e.g., [13]; see Section 4.5).…”

“…However, nitrogen and/or phosphorus limitation has also been observed during the autumn-summer period, especially for diatoms [63][64][65]67], although not with sufficient severity to cause a reduction in Chl-a and diatoms during summer. The classical diatom-dinoflagellate succession, reported for some lagoons (e.g., [117,155,156]), was not observed in the RF lagoon, probably due to high vertical mixing and relatively high nutrient availability (see Section 4.2).…”

Harmful algae and pathogens on plastics in three mediterranean coastal lagoons

“…A regularly maintained channel connects the lagoon to the sea in the northeast and it receives freshwater principally in the north from the rivers draining its watershed (6200 ha). Salinity is reported to be the main controller of the Diana lagoon phytoplankton community [ 76 ]. The lagoon is mainly used for aquaculture and traditional fishing.…”

Small Mediterranean coastal Lagoons Under Threat: Hydro-ecological Disturbances and Local Anthropogenic Pressures (Size Matters)

“…Recently, shifts from diatoms to potentially harmful dinoflagellates have been observed in multiple environments and linked to variations in salinity, temperature and nutrient loads (Aligizaki et al 2009;Collos et al 2009;Xiao et al 2018;Trombetta et al 2019;Fischer et al 2020). Moreover, changes in timing and increased frequency and magnitude of proliferations of harmful species, known as Harmful Algal Blooms (HABs), have been observed worldwide in recent decades, and especially in the Mediterranean region (Hallegraeff 2010;Kudela et al 2015;Trombetta et al 2019;Ligorini et al 2022a).…”