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Ecosystems

, Volume 5, Issue 7, pp 0719–0725 | Cite as

Ecological and Biogeochemical Interactions Constrain Planktonic Nitrogen Fixation in Estuaries

  • Roxanne Marino
  • Francis Chan
  • Robert W. Howarth
  • Michael Pace
  • Gene E. Likens

Abstract

Many types of ecosystems have little or no N2 fixation even when nitrogen (N) is strongly limiting to primary production. Estuaries generally fit this pattern. In contrast to lakes, where blooms of N2-fixing cyanobacteria are often sufficient to alleviate N deficits relative to phosphorus (P) availability, planktonic N2 fixation is unimportant in most N-limited estuaries. Heterocystic cyanobacteria capable of N2 fixation are seldom observed in estuaries where the salinity exceeds 8–10 ppt, and blooms have never been reported in such estuaries in North America. However, we provided conditions in estuarine mesocosms (salinity over 27 ppt) that allowed heterocystic cyanobacteria to grow and fix N2 when zooplankton populations were kept low. Grazing by macrozooplankton at population densities encountered in estuaries strongly suppressed cyanobacterial populations and N2 fixation. The cyanobacteria grew more slowly than observed in fresh waters, at least in part due to the inhibitory effect of sulfate (SO42−), and this slow rate of growth increased their vulnerability to grazing. We conclude that interactions between physiological (bottom–up) factors that slow the growth rate of cyanobacteria and ecological (top–down) factors such as grazing are likely to be important regulators excluding planktonic N2 fixation from most Temperate Zone estuaries.

Key words: nitrogen fixation; estuaries; eutrophication; nitrogen limitation; grazing; heterocystic cyanobacteria. 

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Copyright information

© Springer-Verlag New York Inc. 2002

Authors and Affiliations

  • Roxanne Marino
    • 1
  • Francis Chan
    • 1
  • Robert W. Howarth
    • 1
  • Michael Pace
    • 2
  • Gene E. Likens
    • 2
  1. 1.Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, New York 14853, USAUS
  2. 2.Institute of Ecosystem Studies, Millbrook, New York 12545, USAUS

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