Effects of spatial subsidies and canopy cover on ponds ecosystems

Background/Question/Methods

Spatial subsidies are resources that move from one ecosystem to another.  In aquatic systems, canopy cover often determines both light availability (a major determinant of system productivity) and subsidy input in the form of senescing leaves and terrestrial invertebrates. This creates a system where subsidies are more prevalent in areas with low primary productivity and less prevalent in areas with high primary productivity.  We manipulated subsidy input in in situ pond mesocosms placed along a canopy cover (and thus light) gradient to examine these effects and their interaction on ecosystem, community, and environmental variables in central Missouri.  For subsidy input treatments, we had subsidy input (leaves), a within-system input (an aquatic grass), and no input.  The aquatic grass had higher concentrations of both nitrogen and phosphorus than the leaves.  In 2008 and 2009, we sampled mesocosms in spring, summer, and fall for primary productivity, community respiration, macroinvertebrates, zooplankton, and water chemistry.

Results/Conclusions

We found that canopy cover and subsidy input were important for community and environmental variables, though these often varied with season.  Many (but not all) variables showed the same pattern in both 2008 and 2009.  We also found that mesocosms with no input differed from tanks with either grass or leaf input in water nutrients, primary productivity, and invertebrate communities.  Dissolved nutrients increased with canopy cover in mesocosms with no input but decreased with canopy cover in grass and leaf mesocosms.  Gross primary productivity declined with canopy cover and was lower in mesocosms with no input than leaf or grass input.  Community metabolism did not change with canopy cover in spring and fall, but declined with canopy cover in summer.  Chironomid larvae accounted for a high percentage of macro-invertebrates.  Total abundances of macro-invertebrates were higher in mesocosms with high shade than low shade and were much lower in mesocosms with no input.  Overall, we found that detritus input affected many community and environmental variables differently when compared to no input.  These differences sometimes even changed the relationship between environmental variables and canopy cover.  Additionally, we found that subsidy input was not different from within-system inputs, despite differences in litter quality and decomposition rate.