Samples were collected from the Sage Lot Pond salt marsh tidal creek in Waquoit Bay, MA at approx. 41.5546N, 70.5071W.

In situ, high-frequency sensors for DIC and salinity were deployed at the mouth of the tidal creek in Sage Lot Pond at the latitude and longitude listed in the location above. DIC was measured with an autonomous sensor called CHANnelized Optical Sensor (CHANOS). An EXO2 Multiparameter Sonde (YSI Inc., Yellow Springs, OH) was submerged in the tidal creek to measure temperature and salinity. The YSI EXO2 recorded at intervals ranging from 2 min to 8 min. Reported YSI EXO2 sensor accuracy specifications are: 1% of the reading for salinity and 0.05 °C for temperature. CHANOS was placed on a platform atop the marsh adjacent to the creek with the inlet pumping from the creek at the same depth and within 30 cm of the EXO2 Sonde. This setup avoided any interference by CHANOS on water flow in the creek. There is no significant concentration difference with depth in the creek (data not shown). In order to prevent fouling, sample water was filtered by a 100μm plastic disc filter (Keller Products, Acton, MA) followed by a copper mesh filter. CHANOS was powered by two 12 V batteries that were charged with two 250W solar panels (Renogy, Ontario, CA).

CHANOS uses spectrophotometric principles to measure DIC and pH using two independent channels (Wang et al., 2015). Briefly, CHANOS consisted of syringe pumps for delivery of reagents, junction boxes containing valves, thermistors, and optical and fluidic components for DIC and pH analysis, and an electronics housing, as well as reagent bags for storage of CRM, hydrochloric acid, reference solution, and pH- sensitive indicator solution. For this study, only [DIC] measurements were used. The DIC channel uses an improved spectrophotometric method described in detail in Wang et al. (2013) whereby a countercurrent flow configuration between acidified seawater and a pH-sensitive indicator solution in a tube-in-tube design achieves fast, continuous CO2 equilibration across highly CO2-permeable Teflon AF 2400 tubing. After CO2 exchange in the countercurrent flow cell, the indicator solution is directed into an optical cell for detection. Each measurement cycle is ~15min. The system achieved a precision of ~ ± 2.5 μmol kg-1 and an accuracy of ~ ± 5.0 μmol kg-1 during coastal deployments (Wang et al., 2015).