Sampling was conducted during day trips on Pleasant (Maine, USA) and St. John (New Brunswick, CA) estuaries in May and October 2018 and 2019.

* See "Related Datasets" section for access to the related organic alkalinity data described below.

Sample Collection:

Four surveys were conducted of both the Pleasant and St. John estuaries, in May and October 2018, and again in May and October 2019 in order to assess potential differences in estuary conditions between spring and fall seasons. Estuary samples were collected during single-day surveys on small vessels in each system, departing from Addison Maine for Pleasant estuary surveys and from St. John, New Brunswick, for St. John surveys. Estuary water was continuously pumped to an underway measurement system, which recorded location, salinity(Seabird SBE-45), water temperature (Seabird SBE-45), and the partial pressure of carbon dioxide (pCO₂) among other parameters. Surveys were started on the incoming tide and lasted through high tide and into the ebb tide. At intervals determined from the underway salinity, surface water was captured for discrete sample collection. During the October 2017 and May 2018 surveys a Niskin bottle was lowered overboard by hand; during the later surveys a 10-liter high-density polyethylene (HDPE) carboy was rinsed and filled from the outflow of the underway system, then tightly capped until samples were drawn from a spout at the bottom of the carboy. River endmember samples were collected from above the final downstream dam on each river. For the Pleasant, this dam formed a physical tidal barrier, and the transition from river to estuary was immediate. For the St. John the closest site was in Fredericton New Brunswick, a location over 120 km from the estuary mouth along the river’s course. For both endmember sites, a plastic bucket was lowered from the center of a bridge over the river, rinsed three times with river water, and samples were collected as described above. The temperature and conductivity of samples were measured directly from the bucket with a handheld meter (YSI, Yellow Springs, Ohio).

Water from the Niskin or carboy was transferred without bubbling into individual, previously-flushed borosilicate glass BOD bottles: 500 mL for alkalinity and pH_T analyses, and 300 mL for inorganic carbon (DIC) analysis. All bottles had greased stoppers and positive closure mechanisms, were filled to leave less than 1% headspace in the bottle, and were preserved with saturated mercuric chloride solution. Samples for silicate and phosphate analysis were filtered using a plastic syringe and 0.2 µm cartridge filter into acid-washed and previously-rinsed 50 mL HDPE vials and preserved with chloroform. Samples for DOC were filtered as was done for the nutrients into acid-washed and previously-rinsed 30 mL HDPE bottles. All samples were immediately placed on ice. Alkalinity, pH, and DIC samples were refrigerated until analysis; nutrient and DOC samples were frozen until analysis.