Terrestrial Support of Aquatic Food Webs
Aquatic ecosystems are intimately tied to their terrestrial watersheds. Terrestrial systems provide essential carbon inputs (subsidies) to aquatic systems in multiple ways, such as particulate organic matter (e.g. leaves) that fall into or are washed into the lake as runoff. Dissolved terrestrial carbon in groundwater can also become integrated into aquatic ecosystems. While it has been recognized that benthic organisms rely partially on terrestrially derived (allochthonous) carbon, the idea that pelagic organisms (e.g. zooplankton, fishes) also rely partially on these allochthonous inputs, and not solely on carbon derived within the lake ecosystem (autochthonous), has been controversial.

By using stable isotopes of carbon, hydrogen, and nitrogen we are able to estimate the allochthony of an organism using a mixing model (algebraic equation). Terrestrial and aquatic sources (mixing model end members) of carbon have different isotopic signatures. Therefore, by determining the isotopic signature of aquatic consumers (somewhere between the two end member signatures), we are able to determine to what degree the aquatic food web utilizes terrestrial carbon.

In order to experimentally test whether this terrestrial carbon can sustain aquatic consumers, we are adding Aquashade (a blue dye) to a lake. This dye will shade the lake and reduce incident light, thereby greatly reducing autochthonous production of carbon and driving the lake to become “allochthonous”.

Importance of Metalimnetic Production
Lake food webs are supported by three different resources: 1) algae production at the surface of the lake (in the epilimnion), 2) algae production deep in the lake (in the metalimnion), and 3) terrestrial production in the watershed that washes into the lake. Understanding the importance of different resources to aquatic consumer populations (such as fish) allows lake managers to make better management and restoration decisions. However, the current food web subsidy paradigm for lakes assumes that terrestrial and metalimnetic algal production are not important resources. 

In an effort to quantify the importance of terrestrial production to lake consumers, this group did a series of isotope additions to four lakes in northern Wisconsin. They used inorganic carbon enriched in 13C to isotopically label surface algae production. They found that aquatic consumers were supported by terrestrial production in all of the lakes. Other studies have since called into question the validity of the surface 13C addition methods, claiming that metalimnetic algal production was unaccounted for in the analysis. They provide evidence that this metalimnetic production in combination with surface production provides the sole support for aquatic consumers. Our group then used deuterium (stable isotope of hydrogen) and other ambient isotopes in two lakes to confirm that terrestrial production subsidized aquatic food webs and that metalimnetic production provided negligible support. 

This summer, in an effort to definitively parse out the contributions of each of the three resources - epilimnetic algae, metalimnetic algae, and terrestrial production -  inorganic 13C will be added to the metalimnion of one of the four lakes previously studied. The metalimnetic algae isotope label in combination with the new deuterium method pioneered by this group will be able to conclusively determine the relative resource contributions to aquatic consumers in this lake. To our knowledge, this approach is novel in that it is the first deep water isotope addition of its kind.

Website maintained by: Robert Johnson
Last update: 23 April 2012
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