Seasonal, spatial, and high-frequency monitoring of dissolved oxygen and net ecosystem metabolism in a shallow coastal lagoon, Ria Formosa - Portugal.

Correia, Cátia; Jacob, José; Cravo, Alexandra

http://hdl.handle.net/10400.1/28026

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Authors

Correia, Cátia

Jacob, José

Cravo, Alexandra

Abstract(s)

This study provides a detailed assessment of dissolved oxygen (DO) dynamics and net ecosystem metabolism (NEM) in the Ria Formosa coastal lagoon, highlighting how spatial and temporal variability shape mesotidal ecosystems. Complementary approaches were applied: short-term (24 h) diel oxygen open-water method at two stations (eastern and western boundaries) in comparison with the bottle incubation method, and a 2.5-year high-frequency dataset at an inner station lagoon. DO showed clear seasonal patterns, largely driven by photosynthesis and respiration, but strongly modulated by water circulation and hydrodynamics. Sporadic hypoxic events (DO ≤ 2 mg L) were observed but do not represent a risk. The boundary stations are shallower and colonized by submerged vegetation, displayed stronger diel fluctuations than at the inner station, underscoring the role of biological processes in lagoon metabolism. NEM estimates revealed strong spatial contrasts: the western station was autotrophic, while the eastern station predominantly attributed to restricted water exchange. Long-term records from the inner station indicated a slightly heterotrophic status, reinforcing the value of sustained high-frequency monitoring for capturing ecosystem trends overlooked by short-term approaches. The results also highlight lagoon-coastal ocean connectivity, suggesting dual exchanges: local production may be exported offshore to sustain adjacent coastal areas, while coastal processes, such as upwelling, can also influence lagoon metabolism. Methodological comparisons confirmed that bottle incubation method underestimated NEM relative to diel oxygen open-water method, emphasizing the role of hydrodynamics in mesotidal system metabolism. Although sampling focused on western and eastern boundaries and inner lagoon, the strong tidal renewal in the main and secondary channels suggests Ria Formosa overall is close to metabolic balance, with spatial heterogeneity shaped by hydrodynamics, vegetation, and coastal forcing. By integrating short-term, high-frequency, and long-term observations, this study advances understanding of metabolism in coastal lagoons, offering key insights for predicting ecosystem responses to climate change and for guiding management of vulnerable coastal environments.