Coastal zones receive massive terrestrial inputs of nutrients and organic matter, and play an important role in biogeochemical cycles. The interactions of river inputs, ocean currents, atmospheric exchanges, anthropogenic influences, and biologically active ecosystems make CO2 system studies in coastal waters highly challenging. This work focuses on improving our understanding of the CO2 system in coastal waters through (1) development of a new methodology for measurements of CO2 system parameters in the field; (2) observations of large spatial and temporal CO2 system variations in coastal waters; and (3) characterization of the influence of organics on CO2 system behavior in coastal waters. A novel portable light-emitting-diode (LED) photometer was developed to provide low-cost seawater pH measurements in the field. With meta cresol purple (mCP) as the indicator, the photometer produces pHT measurements within ± 0.01 units of state-of-the-art spectrophotometric measurements (7.6 ≤ pH ≤ 8.2, 30 ≤ S ≤ 36.2, and 15 oC ≤ t ≤ 30 oC). With a simple “do-it-yourself” (DIY) construction design, a hundredfold reduction in cost relative to benchtop spectrophotometric systems, and routine calibration-free operation in the field, the DIY photometer is an ideal replacement for pH test strips or consumer-level potentiometric probes. Applications of special interest include education, citizen science, coastal zone monitoring, and aquaculture and aquarium management. Subannual variability of total alkalinity (TA) distributions in the northeastern Gulf of Mexico (GOM) was examined through the use of TA data from ship-based water sampling, historical records of riverine TA, and contemporaneous model output of surface currents and salinity. Variability of TA observed in the upper 150 m of the GOM water column was primarily controlled by subannual variations in the extent of mixing between seawater and river water. A transition in TA distribution patterns between the river-dominated northern margin (near the Mississippi–Atchafalaya River System) and the ocean current-dominated eastern margin (West Florida Shelf) was observed. A riverine alkalinity input index was developed to highlight riverine TA contributions. Contributions of organic alkalinity (Org-Alk) to TA were investigated in coastal waters from three different environments (estuary, urban, mangrove) and offshore sites in the Gulf of Mexico. The difference in alkalinity (∆TA) between TA measured by direct titration (TAmeas) and calculated (TAcal) from observations of DIC and pH was used as an estimate of Org-Alk. Average values of ∆TA were 0.1 ± 5.0 µmol kg-1 at coastal sample sites outside the Mississippi-Atchafalaya River Estuary (n = 17), 1.9 ± 5.2 µmol kg-1 in offshore waters (n = 14) in the northern Gulf of Mexico, 33.6 ± 18.0 µmol kg-1 in the Suwannee River Estuary (n = 17), and 16.0 ± 25.4 µmol kg-1 in sites that included Tampa Bay, the Caloosahatchee River, and the Ten Thousand Islands area (n = 55). In addition to Org-Alk assessments based on measurements of ∆TA, a novel two-step spectophotometric titration method was developed for the characterization of Org-Alk. Direct titrations showed substantial Org-Alk in coastal samples (n = 5), and the Org-Alk values obtained from the two-step titrations showed good agreement with results from ∆TA calculations. The spectrophotometric titration data were used in model fits to evaluate the dissociation constants (pKi) of the natural organic acids. The pKi of the organic acids were within the previously reported range for riverine fulvic acids.