Oxygen and nutrient dynamics within mats of the filamentous macroalga Chaetomorpha linum |
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| Authors: | Dorte Krause-Jensen Peter Bondo Christensen Søren Rysgaard |
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| Institution: | (1) US Geological Survey, 431 National Center, 12201 Sunrise Valley Drive, Reston, VA 20192, USA;(2) US Geological Survey, 3215 Marine St., Boulder, CO 80303, USA;(3) US Geological Survey, 430 National Center, Reston, VA 20192, USA;(4) Department of Chemistry and Biology, Ryerson University, 350 Victoria Street, Toronto, ON, M5B 2K3, Canada;(5) Cooperative Institute for Research in Environmental Sciences, University of Colorado, UCB 216, Boulder, CO 80309-0216, USA |
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| Abstract: | Concentration profiles of O2, NH4 +, NO3 −, and PO4 3− were measured at high spatial resolution in a 12-cm thick benthic mat of the filamentous macroalga Chaetomorpha linum. Oxygen and nutrient concentration profiles varied depending on algal activity and water turbulence. High surface irradiance stimulated O2 production in the surface layers and introduced O2 to deeper parts of the mat while the bottom layers of the mat and the underlying sediment were anoxic. Nutrient concentrations were highest in the bottom layers of the mat directly above the sediment nutrient source and decreased towards the surface layers due to algal assimilation and enhanced mixing with the overlying water column. Increased turbulence during windy periods resulted in more homogeneous oxygen and nutrient concentration profiles and shifted the oxic-anoxic interface downward. Denitrification within the mat, as measured by the isotope pairing technique on addition of 15NO3 −, was found to take place directly below the oxic-anoxic interface. Denitrification activity was always due to coupled nitrification-denitrification, whereby nitrifiers in the mat utilize NH4 + diffusing from below and O2 diffusing from above. The denitrification rate in the mat ranged from 22 μmol m−2 h−1 to 28 μmol m−2 h−1, approximately equivalent to that measured in the surrounding nonvegetated sediment. Although sediment denitrification is suppressed when the sediment surface is covered by a dense macroalgal mat, the denitrification zone may migrate up into the mat. In eutrophic estuaries with a large area of macroalgal cover, the physical structure and growth stage of algal mats may thus play an important role in the regulation of nitrogen removal by denitrification. |
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