Methods description:
Between April 2014 and January 2015, discrete samples for total alkalinity (TA), dissolved inorganic carbon (DIC), total organic carbon (TOC), and nutrients (nitrate, nitrite, ammonium, phosphate, and silicate) were collected hourly from high tide to low tide at the mouth of SDL, both during the day and during the night. Samples for TA, DIC, and nutrients were also collected at ten different sites across the SDL during the day at high tide. For all samples collected, surface temperature, salinity, and dissolved oxygen were measured with handheld YSI multiprobe while discrete seawater samples were collected by hand at the surface following standard protocols.

Analytical Methods:
Surface seawater samples were collected by hand at ~0.25 m depth using 250 ml Pyrex glass bottles and immediately fixed with 100 µL HgCl2 as per standard protocols (Dickson et al. 2007). A handheld YSI multiprobe Pro2030 was calibrated and used to measure temperature, salinity, and dissolved oxygen saturation at the time of sampling. All seawater samples were transported to the Scripps Coastal and Open Ocean Biogeochemistry lab and analyzed for TA via an open-cell potentiometric acid titration system developed at Scripps Institution of Oceanography (SIO) by A. Dickson (Dickson et al. 2007) and DIC via an automated infra-red inorganic carbon analyzer (AIRICA, Marianda Inc). Nutrient samples were filtered through a 0.45 um filter into acid-leached HDPE-containers, and kept frozen until analysis. They were analyzed at Oceanographic Data Facility at SIO (http://odf.ucsd.edu) using a Seal Analytical continuous-flow AutoAnalyzer. TOC samples were collected in combusted glass-vial and acidified with trace-metal grade HCl. They were analyzed in the Aluwihare lab at SIO according to Pedler et al. (2014) by combusting an unfiltered sample and measuring CO2 gas concentration using a non-dispersive infrared detector.

Quality Control:
Standard protocols were followed for sampling and analysis of seawater all parameters. The YSI multiprobe was calibrated prior to each sampling with an accuracy of ±0.2°C for temperature, ±0.3 g kg-1 for salinity, and ±2% for DO. The mean accuracy (TA±1 µmol kg-1, DIC ±1 µmol kg-1) and precision (TA±2 µmol kg-1, DIC ±2 µmol kg-1) of TA and DIC measurements were evaluated using certified reference materials (CRM) provided by the laboratory of A. Dickson at SIO and analyzed every 5 samples for DIC and ~10 samples for TA. The uncertainty of the nutrient measurements were reported as nitrate ±0.05 uM, nitrite ±0.05 uM, ammonium ±0.03 uM, silicate ±2-4 uM, and phosphate ±0.004 uM by ODF. The TOC method was reported with an uncertainty of ±5%.