We report an annual cycle of surface seawater aragonite mineral saturation state (Ω_arag) during 2006–2007 at the Gray’s Reef time-series site off Georgia, USA, calculated based on three-hourly observations of carbon dioxide partial pressure (pCO₂) and salinity-derived total alkalinity. Ω_arag varied between 2.30 and 4.39 with low values (< 3.00) mainly during February–April 2007 and high values (> 3.50) during July–October 2006 and July–September 2007 as well as during two biological production spikes (April–June 2007). We first present a qualitative analysis of the drivers of Ω_arag variability based on property regressions with surface temperature, salinity and apparent oxygen utilization, and then quantify the contributions of temperature, air-sea exchange, mixing, and biological processes to monthly Ω_arag net changes using a simple 1-D mass budget model. Our analyses suggest that river inputs played the most important role in the seasonal variation of surface Ω_arag, in contrast to temperature control on pCO₂. Nevertheless, the primary processes controlling monthly Ω_arag net change varied with time of year. Furthermore, river inputs lowered Ω_arag by 0.28 and 0.48 in July–August and September–October 2007 relative to the equivalent periods of 2006. This implies that interannual Ω_arag variability at this location may be greater than that due to the influence of increased atmospheric CO₂ over the past few decades, making efforts to discern decadal coastal ocean acidification trends particularly challenging. In addition, although sea surface salinity varies substantially in coastal waters, our analysis suggests that similar to the open ocean Ω_arag is essentially determined by carbonate ion concentration ([CO₃^2 −]), not calcium ion concentration ([Ca^2 +]) or the stoichiometric solubility product (K′_sp), both varying substantially with salinity. Finally, we show that the difference between total alkalinity (TA) and dissolved inorganic carbon (DIC) is a better proxy for [CO₃^2 −] and Ω_arag compared with the ratio (TA/DIC) and helps to better elucidate processes affecting Ω_arag in coastal oceans.