Abstract
Flows of energy and matter across habitat boundaries can be major determinants of the functioning of recipient ecosystems. It is currently debated whether terrestrial dissolved organic matter (tDOM) is a resource subsidy or a resource subtraction in recipient lakes. We present data from a long-term field experiment in which pelagic phosphorus concentration and whole-ecosystem primary production increased with increasing tDOM input, suggesting that tDOM acted primarily as a direct nutrient subsidy. Piecewise structural equation modeling supports, however, a substantial contribution of a second mechanism: colored tDOM acted also as a resource subtraction by shading benthic algae, preventing them from intercepting nutrients released across the sediment–water interface. Inhibition of benthic algae by colored tDOM thus indirectly promoted pelagic algae and whole-ecosystem primary production. We conclude that cross-ecosystem terrestrial DOM inputs can modify light and nutrient flows between aquatic habitats and alter the relative contributions of benthic and pelagic habitats to total primary production. These results are particularly relevant for shallow northern lakes, which are projected to receive increased tDOM runoff.
Highlights
Flows of energy and matter across habitat boundaries, often referred to as allochthonous subsidies, can be major determinants of ecosystem functioning (Polis and others 1997; Loreau and others 2003)
We investigate how input of Terrestrial dissolved organic matter (tDOM) from the terrestrial catchment interacts with crosshabitat flows of nutrients and light within lakes to shape resource competition between benthic and pelagic primary producers and total in-lake primary production
The vertical light attenuation coefficient kd was on average 2.5 times higher in high versus low tDOM sections (2.34 vs. 0.92 Table 2). This implied that light supply to the benthic habitat (IZmax, equation 2) was 25 versus 3.6% of incoming light and that light supply to the pelagic habitat (Imix, equation 1) was 54 versus 28% of incoming light in high versus low tDOM sections, respectively (Figure 2, Table 2)
Summary
Flows of energy and matter across habitat boundaries, often referred to as allochthonous subsidies, can be major determinants of ecosystem functioning (Polis and others 1997; Loreau and others 2003). The relative importance of tDOM as a resource subsidy versus resource subtraction for lake systems is a matter of debate (Jones and others 2012; Kelly and others 2014; Solomon and others 2015; Carpenter and others 2016). It is currently not well understood how the simultaneous addition of nutrients and subtraction of light that come with tDOM interact in shaping lake primary production (Seekell and others 2015a, b). Understanding these processes is important, because climate change scenarios predict tDOM supply to increase in northern lakes (Solomon and others 2015), with potentially far-reaching implications for the transfer of primary production to higher trophic levels (Jansson and others 2007; Karlsson and others 2009) and for the carbon balance of the boreal landscape (Cole and others 2007; Tranvik and others 2009)
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