The metabolic balance of an ecosystem, defined as gross photosynthesis minus community respiration, governs its potential for biomass accumulation and net carbon storage. Over much of the world’s oceans, low plankton standing stocks and high turnover rates, combined with a tight coupling between autotrophic and heterotrophic processes, pose significant challenges to quantifying in situ metabolic balances. In contrast, high productivity polar and sub-polar coastal waters exhibit a strong seasonal decoupling between photosynthesis and respiration, leading to the accumulation and subsequent loss of organic carbon from the surface mixed layer. We used high frequency measurements of pCO2 and biological oxygen saturation (ΔO2 /Ar) to examine in situ metabolic balances at the Palmer Station LTER site on the West Antarctic Peninsula. This region is among the most productive of the world’s ocean biomes, and is particularly sensitive to climate-driven ecosystem shifts. Our observations between Oct. 2012 and Mar. 2013 revealed a strong seasonal shift from net heterotrophic conditions (i.e. excess respiration over photosynthesis) to a positive metabolic balance during the large spring phytoplankton bloom. The phytoplankton bloom exerted a profound influence on surface water gas concentrations, resulting in pCO2 values below 100 ppm (among the lowest ever reported for any oceanic waters) and O2 super-saturation in excess of 180%. Superimposed on this massive seasonal signal, we observed large diel (i.e. day-night) changes in ΔO2 /Ar and pCO2, and we used these cycles to assess daily patterns in net community production (NCP). Our results demonstrate an intense, yet short-lived, period of excess carbon production during the spring, followed by a period of negative metabolic balance (i.e. excess community respiration), and a subsequent tight coupling of photosynthesis and respiration. Incident photon flux, likely in concert with grazing, appears to influence the magnitude of water column net primary production over much of the growing season. Our results yield new insight into the environmental factors controlling NCP in coastal Antarctic waters on various time-scales, and our approach provides a powerful tool for real-time observations of high latitude biogeochemical processes in the face of rapid climate change.