Nitrate highs and lows: differences between lakes and streams

Posted on December 15, 2009

Nitrate levels are of great concern in aquatic management because high nitrate concentrations are a leading cause of groundwater pollution, oxygen depletion and acidification, while low levels (relative to phosphorus) encourage blue-green algal blooms.  Information on the degree of nitrate variation in a given system can therefore be used to develop intervention strategies.  Swedish water quality data collected monthly through the growing season (May-October) over ten years revealed that patterns in nitrate variation were different for lakes and streams.  The coefficient of variation (i.e., the standard deviation divided by the mean nitrate concentration) was used to express nitrate variation in relative rather than absolute terms.  Nitrate variation was low in oligotrophic (unproductive) lakes but high in eutrophic (productive) lakes.  Streams showed the opposite trend, having high nitrate variation when oligotrophic and low nitrate variation when eutrophic.  The authors explain these results in terms of the degree of influence of biological processes such as  nutrient uptake by plants and denitrification, which tend to be more active in eutrophic lakes or oligotrophic streams than in oligotrophic lakes or eutrophic streams.  When these  processes are prominent, the nitrate stock is depleted and biological activities have a greater relative impact on overall levels, leading to higher nitrate variability.

Reference:  Khalili, M.I. & Weyhenmeyer, G.A.  2009.  Growing season variability of nitrate along a trophic gradient – contrasting patterns between lakes and streams.  Aquatic Sciences 71, 25–33.