Temporal changes of macrobenthic diversity during rapid submerged plant growth in a subtropical coastal lagoon.

Coastal lagoons are highly dynamic transitional ecosystems where mechanisms driving temporal changes in structure and functioning are highly complex. We examined changes in macrozoobenthic assemblages, climatic trends, and environmental conditions, including salinity, water depth, temperature, nutrients, and submerged macrophyte biomass, over a seven-year environmental monitoring dataset (2018-2024) in Garzón, a coastal lagoon in Uruguay. This period was characterized by regional climatic anomalies providing a broader context for the observed ecological dynamics. Two distinct phases in assemblage structure were identified: 2018-2022, characterized by high environmental variability, the absence of submersed vegetation and dominance of the estuarine gastropod Heleobia aff. australis, and 2023-2024, marked by stable low salinity, excessive growth of the submerged macrophyte Myriophyllum quitense, and the rise of freshwater-tolerant opportunistic taxa such as Heleobia parchappii and chironomids. Our results revealed substantial changes in benthic composition and abundance throughout the study period, associated with changes in hydrological conditions driven by climate variability, water quality, and habitat structure following submerged aquatic vegetation expansion. Between 2018 and 2022, species responses were influenced by salinity and temperature, whereas in 2023-2024, responses were dominated by a set of factors including macrophyte biomass. Overall, our results highlight how monitoring data can be used to detect structural reorganization in benthic assemblages during a rapid environmental change driven by climate variability.