Temporal and spatial dynamics of urea uptake and regeneration rates and concentrations in Chesapeake Bay

Abstract

We examined the temporal and spatial variability of urea concentrations and urea uptake and regeneration rates collected on cruises along the longitudinal axis of the Chesapeake Bay between 1972 and 1998. Interannually, mean Bay-wide surface urea concentrations ranged between 0.49 and 0.91 μg-at N l−1 with a nearly 50% decrease in surface concentrations observed between 1988 and 1998. Concentrations of urea from samples collected within ∼1 m of the bottom were generally higher and much more varable than surface samples. Seasonally, two different patterns were observed in mean Bay-wide surface urea concentrations. Urea concentrations from near surface waters exhibited a clear summer peak for 1988 through 1994, while for 1973 and 1996 to 1998 a distinct winter-spring peak in concentration was observed. Urea concentrations from deeper waters showed a similar seasonal trend each year with peak concentrations measured in spring. Spatially, urea concentrations in the surface waters decreased in a conservative-type pattern from 0.91 μg-at N I−1 at the freshwater end member to 0.46 μg-at N I−1 at the ocean end member. Mean Bay-wide surface urea uptake rates displayed a seasonal pattern throughout the data set with maximum uptake rates (up to 0.33 μg-at N I−1 h−1) consistently observed during summer. Mean Bay-wide surface regeneration rates were highest but most variable during fall (1.63±0.82 μg-at N I−1 h−1). Mean urea uptake and regeneration rates displayed opposing spatial trends along the axis of the Bay with uptake rates being lowest in the North Bay where regeneration rates were highest. The average temporal and spatial patterns of urea concentration in Chesapeake Bay appear to reflect a balance between external inputs and internal biological recycling.