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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
(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
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
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The old view of humic substances as refractory compounds is obsolete; most allochthonous DOM entering lakes should rather be viewed as recalcitrant.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):
- 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).
- 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
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