Physico-Chemical and Carbonate Chemistry Parameters in the Shallow Coastal Zone of Pointe-au-Père (St. Lawrence Estuary)

This dataset contains all physico-chemical parameters measured during the ice-free months in 2023 and 2024 in the shallow coastal waters of Pointe-au-Père, located in the Lower St. Lawrence Estuary. During these periods, high-frequency measurements were taken every 15 minutes using autonomous data loggers (HOBO) attached to a mooring to monitor pH and other physicochemical parameters. This structure was deployed in the subtidal zone, near rocks covered by crustose coralline algae and kelp, located less than 50 m from the shore, with up to 6 m of water column depending on the tide.

To mitigate biofouling, divers cleaned all sensors every 1-2 weeks by gently brushing them underwater. Note that in 2024, there is a gap from May 15th to 21st, following a partial burial after an important sediment resuspension event. In addition to continuous monitoring, discrete water samples were collected just above the mooring, every 1-2 weeks to measure total alkalinity, dissolved inorganic carbon and pH. Calibration and processing of certain data measured by the HOBO sensors were performed:

• Voltage (mV) measurements from the pH HOBO were converted to total scale (pHT) using a multi-step calibration with TRIS buffer solutions.
• Light data from the HOBO sensor (lux) were converted to Photosynthetic Active Radiation (PAR; μmol photons m⁻² s⁻¹) using a linear calibration against a miniPAR sensor.
• Water level was calculated from the absolute pressure (kPa) of the HOBO and corrected for the atmospheric pressure of the Pointe-au-Père meteorological station.

The data are divided into two databases. The first contains all high-frequency measurements, taken every 15 minutes:

• pH (mV)
• pH (total scale)
• Temperature (°C)
• Practical salinity
• Light (lux)
• Light (PAR; μmol photons m⁻² s⁻¹)
• Absolute pressure (kPa)
• Water level (m)
• Aragonite saturation state (omega aragonite; calculated)
• Omega calcite (calculated)

The second database contains the data of the discrete samples:

• pH (total scale) measured by spectrophotometer at 25°C.
• Total alkalinity (µmol/kg) measured by potentiometric titration.
• Dissolved inorganic carbon (µmol/kg) measured by a DIC analyzer.