Highlights
- The lower Patos Lagoon Estuary displays a broad range of alkalinity and pH values, with riverine inputs marked by low buffering capacity.
- A critical period of corrosive conditions occurs from winter to mid-spring, likely driven by enhanced respiration and/or external CO₂ inputs.
- The estuary operates as a moderate to weakly buffered system, exhibiting aragonite undersaturation even under medium to high salinity conditions.
- pH sensitivity to environmental drivers is highest in summer and winter, whereas autumn presents the most uniform seasonal response.
Abstract
Coastal ecosystems exhibit a wide range of pH trends, from −0.023 to 0.023 pH units yr−1, making them particularly susceptible to acidification or basification. These variations are primarily driven by ecosystem metabolism and the influence of oceanic and riverine endmembers, as observed in the subtropical system of the Patos Lagoon Estuary (PLE, southern Brazil), where biogeochemical variability is largely governed by mixing of water masses with different properties. This study provides the first quantification of the seasonal variability of pH buffering capacity in the inner and outer zones of PLE. From May 2017 to September 2023, we assessed temporal variability using multiple approaches: (i) carbonate system parameters, (ii) sensitivity factors, (iii) buffering capacity of pH to fractional change of dissolved inorganic carbon (βDIC), (iv) metabolic effects on pH, and (v) environmental drivers of pH. The results revealed a distinct seasonal pH pattern, especially between summer with winter and spring, with consistently higher values at the outer station compared to the inner station, though spatial differences were not statistically significant. In winter and particularly in early spring, calcium carbonate (CaCO3) dissolution prevailed due to riverine input characterized by low buffering capacity. Along the salinity gradient, pH exhibited a pronounced difference, particularly between low and high salinity conditions. However, the persistent negative deviation of the metabolic effect on pH throughout the year and in salinity ranges, even under seawater conditions, supports the characterization of this coastal ecosystem as a net CO2 source, with especially high variability at mid-salinity conditions. Although the salinity gradient was comparable between stations, they exhibited differences in the magnitude of pH sensitivity to seasonal biogeochemical changes. These findings indicate that PLE functions as a system with moderate to low buffering capacity, with the outer zone showing greater resilience to pH fluctuations.
Quintana P., Machado E. & Kerr R., 2025. Resilience of pH to seasonal change in a large subtropical lagoonal estuary. Marine Chemistry 274: 104577. doi: 10.1016/j.marchem.2025.104577. Article (restricted access).
