Science Serving Maryland's Coasts

Barbara Beckingham, Hobert and William Smith College

Class Year: 

Project Title: 

Redox Chemistry Transitions Across the Pycnocline in Chesapeake Bay


Vertical profiles of nitrate, nitrite, ammonium, hydrogen sulfide, iron(II), N2 gas, phosphate and physical parameters (salinity, temperature, dissolved oxygen) in the water column were determined with especially high resolution across the pycnocline (1 m) at the LMER mid-Chesapeake Bay site over four sampling dates during Summer 2004. Shifts in the significance of redox pathways over vertical and temporal scales are recognizable. The transition of redox chemistry across the pycnocline may mark changes in bacterioplankton communities since the chemistry and biology involved are critically linked (Peranteau and Krump, REU submission). Oxic respiration must account for the majority of organic matter oxidation above the pycnocline (0-10 m). Denitrification in the water column is important (5-10 m) depending upon the availability of nitrate and labile Fe(II). At the ecosystem level, hypoxia/anoxia reduces denitrification via the reduction of nitrification (Kemp et al. 1990). The appearance of higher concentrations of Fe(II) below the pycnocline (+10 m) may indicate iron(III) hydroxide reduction except under reducing conditions when most Fe(II) is in the form of a Fe(HS)+ precipitate (O'Sullivan 1997). The reduction of sulfide or thiosulfate is likely to be important at the oxic/anoxic interface (Ho et al. 2004). The development of higher hydrogen sulfide concentrations below the pycnocline suggests that the predominant organic matter oxidation pathway is sulfate/ide reduction as anoxia developed in late July with the bacterial source originating in both the water column and underlying sediments.