Daily variability of pelagic metabolism in a subtropical lagoonal estuary

The influence of meteoceanographic variability on pelagic ecosystem metabolism was evaluated during late austral summer (from February to March 2021), through a daily survey for one month, at two channel sites in the subtropical Patos Lagoon Estuary, Brazil — the largest choked (restricted connection to the sea) coastal lagoon of South America. Heterotrophic predominance prevails in the studied area, with an overall mean of −205 ± 143 mmol O₂ m−² d⁻¹, with 78 ± 67 mmol O₂ m⁻² d⁻¹ from gross primary production (GPP) and 285 ± 178 mmol O₂ m⁻² d⁻¹ from respiration (R). Generalized additive models were used to identify the most relevant driving forces for GPP, which were nitrate, salinity, chlorophyll-a (chl-a), wind speed and direction, water flow rate, silicate, and turbidity. The main driving mechanisms for R were photosynthetically active radiation, temperature, wind speed, chl-a, turbidity, and nitrate. GPP was potentially co-limited by dissolved inorganic nitrogen (very low nitrogen to phosphorus ratio, N:P = 3.1 ± 2.0) throughout the period and by light in some events due to material resuspension from the bottom by wind-induced salt-wedge intrusions. Evidence of intrusions of Plata Plume Water into the estuary were observed, likely contributing as a key factor to local biogeochemistry. The main phytoplankton groups, in decreasing order of abundance, were diatoms, cryptophytes, and cyanobacteria, with their variability being controlled mainly by wind-induced salinity changes. Diatoms responded as the main contributor to metabolic rates, water carbon dioxide (CO₂), carbon dioxide partial pressure (pCO₂), and water-air CO₂ fluxes (FCO₂) by its higher biomass and uptake of total inorganic carbon. Inlet (70%) and sea-exposed (95%) stations behaved mainly as net CO₂ sinks from the atmosphere during most of the study period (−7.9 ± 30.6 mmol C m⁻² d⁻¹). The inlet inner station had a median FCO₂ of −9.8 and an average of 4.4 ± 41.2 mmol C m⁻² d⁻¹, while the sea-exposed station had −17.0 and −15.4 ± 17.3 mmol C m⁻² d⁻¹, respectively. Although total inorganic carbon and chl-a were negatively correlated, which indicates carbon assimilation by primary production, no correlation was observed between metabolic rates and FCO₂. These results suggest the need for additional investigation into the driving factors and/or sources of carbon species in the Patos Lagoon Estuary to better understand the role of aquatic ecosystem metabolism in the carbon budget. Finally, the daily investigation of pelagic ecosystem metabolism and its driving factors was unprecedented for the Patos Lagoon Estuary and showed the importance of short-term monitoring for a better understanding of highly dynamic estuarine environments.