Abstract
The interface between terrestrial ecosystems and inland waters is an important link in the global carbon cycle. However, the extent to which allochthonous organic matter entering freshwater systems plays a major role in microbial and higher-trophic-level processes is under debate. Human perturbations can alter fluxes of terrestrial carbon to aquatic environments in complex ways. The biomass and production of aquatic microbes are traditionally thought to be resource limited via stoichiometric constraints such as nutrient ratios or the carbon standing stock at a given timepoint. Low concentrations of a particular constituent, however, can be strong evidence of its importance in food webs. High fluxes of a constituent are often associated with low concentrations due to high uptake rates, particularly in aquatic food webs. A focus on biomass rather than turnover can lead investigators to misconstrue dissolved organic carbon use by bacteria. By combining tracer methods with mass balance calculations, we reveal hidden patterns in aquatic ecosystems that emphasize fluxes, turnover rates, and molecular interactions. We suggest that this approach will improve forecasts of aquatic ecosystem responses to warming or altered nitrogen usage.
Highlights
Aquatic primary producers perform 50% of global carbon fixation [1], of which half is directly used by heterotrophic bacteria [2]
Since many important resources are found at low concentrations with high turnover, measurements of pool sizes or magnitudes may not reveal the degree to which organisms are limited by the rates of resource supply
We modeled bacterial processing of terrestrially derived dissolved organic carbon (t-DOC) and autochthonous primary producer derived DOC (PPr-DOC) in a hypothetical “Median Lake” (Figure 1, Table 1, and Supplementary Calculations)
Summary
Aquatic primary producers perform 50% of global carbon fixation [1], of which half is directly used by heterotrophic bacteria [2]. Based on its abundance, terrestrially derived dissolved organic carbon (t-DOC) in lakes may appear to be the primary resource available for heterotrophic bacteria, unless turnover rates and DOC bioavailability are considered. If we further assume that algal-derived DOC supports bacterial production with a bacterial growth efficiency (BGE) of 0.5 and t-DOC has a BGE of 0.1 [15], we can calculate the proportion of bacterial production supported by these two sources, which is equivalent to the (flux of DOC)*Removal*BGE (Table 1) This shows that 96% of the bacterial production in the lake was supported by algal exudates and only 4% was supported by terrestrial inputs. The constituent that is present at high concentrations in Median Lake (i.e., t-DOC) is far less important for bacterial metabolism than the constituent present at much lower concentrations (i.e., PPr-DOC), despite both having very similar mass fluxes.
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