MEPS 219:99-107 (2001)  -  doi:10.3354/meps219099

The role of benthic vegetation as a sink for elevated inputs of ammonium and nitrate in a mesotrophic estuary

Bernard J. Dudley*, Anna M. E. Gahnström, Diana I. Walker

Botany Department, The University of Western Australia, Perth Western Australia 6907, Australia

ABSTRACT: Benthic vegetation plays an important role in determining the fate of nitrogen inputs to estuaries, thus influencing their degree of eutrophication. This study investigated the role of benthic vegetation as a sink for anthropogenic inputs of nitrate and ammonium into Wilson Inlet, a mesotrophic estuary in southwestern Australia. The dominant aquatic angiosperm in Wilson Inlet is Ruppia megacarpa Mason. We examined: (1) whether R. megacarpa leaves remove inorganic N (as nitrate and/or ammonium) from the water column, despite the presence of a layer of epiphytes; (2) whether the macrophyte and its epiphytes are equally important in the removal of inorganic N from the water column; and (3) whether inorganic N taken up by leaves is translocated to other plant parts. We added inorganic 15N nitrogen, as ammonium, nitrate, or both, to aquaria containing intact cores of sediment, R. megacarpa and attached epiphytes, and unfiltered estuary water. We measured depletion of nitrogen species from the water column and incorporation of 15N into components of the core. Epiphytes removed more nitrate and ammonium from the water column than R. megacarpa, despite having 25% of the biomass of the macrophyte. Maximum rates of nitrate uptake were 4.6 (for epiphytes) and 2.0 µmol h-1 g-1 DW (for R. megacarpa), and maximium rates of ammonium uptake were 35 (for epiphytes) and 23 µmol h-1 g-1 DW (for R. megacarpa). The presence of ammonium reduced rates of nitrate uptake, indicating that benthic vegetation prefers ammonium as a nitrogen source. Using mass spectrometry, we recovered between 37 and 45% of the added 15N nitrogen. The remainder was transformed to either organic nitrogen in the water column by algal epiphytes or nitrogen gas via coupled nitrification-denitrification in the sediment. This experiment indicates ecosystem-scale responses to dissolved inorganic nitrogen which would not have been observable from experiments conducted with isolated plants. Benthic vegetation in Wilson Inlet removes nitrate and ammonium quickly from the water column. Depending on water mixing, it may reduce transient increases in the concentration of these nutrients to background levels within 30 h. This process may be responsible for maintaining low water-column concentrations and reducing the likelihood of algal blooms.

KEY WORDS: Benthic vegetation · Eutrophication · Estuary · 15N · Nutrient sink

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