Marine intrusions drive short-term and long lasting changes on limnological dynamics along the water column of a tropical coastal lagoon

Non-tidal coastal lagoons are widespread aquatic ecosystems distributed along coastlines. They exhibit complex limnological dynamics influenced by geomorphology, land use, atmospheric processes, and oceanic exchange. Although such shallow ecosystems are described as vertically homogeneous, sandbar breaching events can quickly modify the water column by promoting seawater intrusion, which is the primary driver of changes in water quality and ecological functioning in the short term. However, the duration of sandbar breaching effects on the vertical distribution of limnological variables was rarely evaluated. In this study, we investigated the impact of an episodic marine intrusion on the water column characteristics of Jurubatiba lagoon, a tropical coastal ecosystem, for 12 months following a sandbar breaching event to elucidate the duration of its effects. Marine intrusion resulted in salt wedges formation in the short-term, leading to stratification in dissolved oxygen, pH, salinity, and chlorophyll-a concentration along the water column for at least four months. However, rainfall and temperature also emerged as crucial factors driving vertical heterogeneity in limnological variables, influencing water density, pH, and turbidity throughout the study period. Wind speed had marginal effects on thermal stratification, probably due to the lagoon's orientation in the landscape. Our data unveiled that, despite their shallowness and wind exposure, coastal lagoons can exhibit significant vertical heterogeneity in limnological variables due to various climatic and hydrodynamic drivers. Notably, the effects of marine intrusions on salinity and ectogenic column stratifications can persist for several months, affecting water quality and, probably, biotic communities and ecosystem functioning. Once human activities and climate change may intensify marine intrusions on coastal lagoons, understanding these dynamics and their drivers is fundamental to anticipating their effects and conserving such vulnerable ecosystems.