Terrestrial primary production is thought to subsidize the organic carbon (OC) cycle in lakes. Total ecosystem respiration of lakes usually exceeds gross primary production, and numerous studies have documented inputs of OC to lakes from the surrounding landscape.  While most of the input by mass is dissolved organic carbon (DOC), airborne inputs of particulate organic carbon (POC) could be important resources for invertebrates. However, few studies have documented airborne inputs of POC, and little is known about the factors that control inter-lake variability of these inputs. We empirically tested the assumption that airborne POC inputs vary as a function of the ratio of surface area to perimeter, and evaluated the effects of wind, exposure, and shoreline vegetation on airborne deposition of POC on lake surfaces. During the summer of 2005, we deployed collectors across a size gradient of 16 lakes (0.08 to 165 ha) to identify the magnitude of the POC flux to the lake surface. Additionally, we surveyed shoreline vegetation to determine its role as a source of deposition. Finally, we compared our results with estimates of terrestrial support of lakes derived from whole-lake stable isotope (carbon-13) additions.  

Results/Conclusions

Airborne deposition varied with lake size, but in a manner that was not consistent with our expectations. Somewhat counter-intuitively, nearshore collectors on smaller lakes received less airborne deposition than those on larger lakes. This we attribute to differences in the species composition of shoreline vegetation. Smaller lakes had sparse vegetation, more characteristic of bogs (tamarack and black spruce), while larger lakes had denser stands of mixed vegetation (maple, hemlock, and fir). Higher density and diversity of vegetation around large lakes was associated with higher deposition of POC in nearshore collectors. In contrast, the differences in terrestrial inputs were diminished when deriving a whole-lake estimate weighted by distance from shore. These findings are also contrary to our expectations based on whole-lake isotope addition experiments in which larger lakes received less terrestrial support (~10%) than smaller lakes (~50%). Thus, differences in invertebrate allochthony between lakes may not be a function of summer deposition of POC. This points to a need to re-evaluate alternate pathways such as autumnal litter fall becoming available for grazing by re-suspension and processing my macro-invertebrates, as well as dissolved pathways such as hydrologic inputs and flocculation. Overall, we found that summer inputs of terrestrial POC are non-trivial; however, these inputs alone do not appear to explain differences in allochthony between lakes.