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Typology of (northern) lakes based on metabolic balance and ecosystem stoichiometry

"All lakes are humic, i.e, dystrophic ("misfed"), but some are more humic than others"

Soil & Water Conservation Society of Metro Halifax (SWCSMH)

July 29, 2006      LIMNOLOGY

(Mostly excerpts from Jones, 2005. Verh. Internat. Verein. Limnol., International Association of Theoretical and Applied Limnology. Additions will be made in the future from other sources!)


Proposed alternative typology of (northern) lakes

The humic content of waters (allochthonous loading of organic matter) should be seen as a primary characteristic of natural waters while the degree of nutrient enrichment should be seen as a secondary modifier! All lakes receive some allochthonous loading of humic organic matter and have some inherent heterotrophic metabolic capacity.

Net heterotrophy is arguably the "natural" condition of most northern lakes, with net autotrophy arising only because of anthropogenic impact on catchments. The natural development of many lakes (atleast of boreal lakes) is to become more dilute but richer in DOC and, by inference, more strongly net heterotrophic.

Too many conclusions about net heterotrophy of lakes are based on the "ice-free" or "growing" seasons. When continued respiration during winter conditions is taken into account, even quite eutrophic lakes can be net heterotrophic on an annual basis. This is particularly apparent in lakes with winter ice cover in which considerable under-ice accumulation of CO2 can take place, giving rise to a large springtime CO2 pulse to the atmosphere when the water degasses after ice-melt.


All lakes are humic, but some are more humic than others

Humic waters are usually perceived as being brown in colour, reflecting a high content of allochthonous dissolved organic matter (DOM) originating from drainage of humified terrestrial detritus from the organic-rich soils in their catchments. Of great potential significance is the allochthonous input in drainage water of CO2 produced within soil horizons by soil respiration of terrestrial organic matter.

The old view of humic substances as refractory compounds is obsolete; most allochthonous DOM entering lakes should rather be viewed as recalcitrant. Allochthonous DOM in freshwaters provides an important substrate for bacterial metabolism. This has two broad consequences (figure below):

  1. Since bacterial growth efficiency on recalcitrant, allochthonous DOM is low, so that much of the utilised DOM is respired as CO2, lake may produce more CO2 than they consume, making them net sources of atmospheric carbon in the regional landscape (net heterotrophy).
  2. Since bacterial growth on allochthonous DOM is possible, this growth can provide an alternative source of carbon/energy, alongside autochthonous primary production, at the base of freshwater food webs. Quantifying the relative contribution of these two parallel and interlocking food webs under diverse circumstances has become a crucial challenge in limnology.


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