|
||||||||||||
  Article by J.G. Stockner, E. Rydin and P. Hyenstrand For several decades water managers have been working hard to reduce nutrient inputs at source, to make our waterways “cleaner.” But in our haste to remove phosphorous from over-enriched lakes and streams, we have failed to consider the longer-term consequences of reduced ecosystem production on reliant fish populations. The consequences of this short-term vision has resulted in some profound declines of fresh water fisheries in both European and North American large lakes, sometimes with severe economic outcome.
Public perception of what is clean and unpolluted has been the dictum of the eutrophication crusade since the mid-1960s. Eutrophication, the process by which a lake or pond becomes rich in plant nutrient, was seen as a bad thing. Instead, society wanted waters that were oligotrophic - that is, waters that were clear because they were poor in plant nutrient minerals and organisms. The large improvement in the aesthetic appeal of lakes and streams, as nutrient supplies diminished, was one of the easily perceived positive attributes of oligotrophication, i.e the process of becoming less productive. But what is visually pleasing is not necessarily good for efficient ecosystem function and for productive recreational or commercial fisheries. For example thousands of lakes of North East North America have lost nutrient and production by acidification and are very clear but sterile in terms of fisheries. We believe that soon it may be necessary in some cases to modify our goals. Instead of making all lakes and streams look “clean,” we should make some of them look good for fisheries and protein production by restoring lost nutrients (nitrogen and phosphorus) in a balanced way. Declining fish production is an inevitable consequence of changes in food web structure that accompany the process of oligotrophication (nutrient loss). As phosphorous loads diminish and lakes become nutrient poor, their annual primary production declines as well as their carrying capacity for healthy fisheries.
In this new millennium it is apparent that very soon choices will have to be made between lakes that are less clear - but which support productive fisheries - and lakes that are esthetically clear, but which are unproductive! Marked declines of fisheries over the last 20 years in large lakes of Europe and North America have occurred because of oligotrophication. These are signs that we have not managed our nutrient resources well, especially phosphorus which is already in short supply. It is time to reverse the trend. Nutrient enrichment (adding nutrients back to low production lakes and streams) has been shown to be a very effective habitat enhancement tool, increasing fish production in oligotrophic lakes and streams in British Columbia and Alaska. We believe that the properly administered addition of nitrogen and phosphorus in a balanced proportion to lakes and streams can be very beneficial for the restoration of fisheries and in some cases essential to sustain productive fisheries. (John Stockner is a research scientist with the University of British Columbia, Fisheries Centre and Institute of Resources and Environment. He runs a consulting business: Eco-Logic Ltd./2614 Mathers Avenue/ West Vancouver, B.C./CANADA V7V 2J4 [Phone: 1 604 926 5383 or Fax: 1 604 926 3663] E. Rydin and P. Hyenstrand, are with the Department of Limnology, Uppsala University, Uppsala, Sweden.) |
||||||||||||
As nutrient levels decline food webs shift from high nutrient requiring diatom-based, short food chains to longer, less energy efficient, picoplankton-based microbial food webs that are predominant in oligotrophic systems. Short food chains occur in regions of high production, e.g. ocean upwelling, coastal regions and meso-eutrophic lakes, and support the major fisheries of the world. Moderately high nutrient concentrations required for healthy fisheries in freshwaters are usually incompatible with the low nutrient concentrations necessary to maintain visual, clear water standards in developed countries.
