Spatial and Temporal Patterns of Winter–Spring Oxygen Depletion in Chesapeake Bay Bottom Water

Abstract

Although seasonal hypoxia is a well-studied phenomenon in many coastal systems, most previous studies have only focused on variability and controls on low-oxygen water masses during warm months when hypoxia is most extensive. Surprisingly, little attention has been given to investigations of what controls the development of hypoxic water in the months leading up to seasonal oxygen minima in temperate ecosystems. Thus, we investigated aspects of winter–spring oxygen depletion using a 25-year time series (1985–2009) by computing rates of water column O2 depletion and the timing of hypoxia onset for bottom waters of Chesapeake Bay. On average, hypoxia (O2 <62.5 μM) initiated in the northernmost region of the deep, central channel in early May and extended southward over ensuing months; however, the range of hypoxia onset dates spanned >50 days (April 6 to May 31 in the upper Bay). O2 depletion rates were consistently highest in the upper Bay, and elevated Susquehanna River flow resulted in more rapid O2 depletion and earlier hypoxia onset. Winter–spring chlorophyll a concentration in the bottom water was highly correlated with interannual variability in hypoxia onset dates and water column O2 depletion rates in the upper and middle Bay, while stratification strength was a more significant driver in the timing of lower Bay hypoxia onset. Hypoxia started earlier in 2012 (April 6) than previously recorded, which may be related to unique climatic and biological conditions in the winter–spring of 2012, including the potential carryover of organic matter delivered to the system during a tropical storm in September 2011. In general, mid-to-late summer hypoxic volumes were not correlated to winter–spring O2 depletion rates and onset, suggesting that the maintenance of summer hypoxia is controlled more by summer algal production and physical forcing than winter-spring processes. This study provides a novel synthesis of O2 depletion rates and hypoxia onset dates for Chesapeake Bay, revealing controls on the phenology of hypoxia development in this estuary.