Dissolved organic matter as a direct energetic subsidy to aquatic consumers

Background/Question/Methods

Increases in the supply of dissolved organic matter (DOM) to receiving waters are typically assumed to influence food webs via effects on microbes that use DOM as a resource.  However, recent studies suggest strongly that DOM can provide a subsidy directly to aquatic consumers without the intervention of microbes.  In these studies, zebra mussels have been shown to absorb dissolved compounds directly from solution, and to use these compounds to support metabolism. Experiments suggest that zebra mussels obtain >60% of their basal metabolic needs from direct assimilation of DOM, explaining why they maintain high levels of reproduction despite having reduced their prey to levels that should induce starvation.  Direct use of extrinsic DOM may not be isolated to zebra mussels.  Other invasive bivalves also impose persistent effects on biomass of their algal prey, suggesting that they too are supported by subsidies of some sort.  Whole lake C-13 experiments suggest that trophic interactions with microbes are insufficient to explain the amount of allochthonous carbon in crustacean zooplankton.  The consequences of such direct subsidies for prey populations and ecosystem processes are largely unexplored and likely to be quite distinct from allochthonous carbon subsidies that cascade up the food chain via microbes.

Results/Conclusions

Based on field data and experiments on the use of DOM by zebra mussels, I parameterized a simple generalizable model that explicitly links zebra mussel metabolic rates and population dynamics to DOM concentrations, bacterial abundance, algal biomass, and processes influencing DOM recycling and delivery to the consumer organisms.  The model indicates that at current concentrations of DOM, extrinsic subsidies in the Hudson could explain the >80% persistent reductions in phytoplankton biomass that have been observed.  In the absence of the DOM subsidy, the zebra mussels reduced phytoplankton biomass by only ~40% and interannual variability in population filtration rate increased substantially.  The effect of changes in DOM concentrations in these dynamics depend on how readily DOM can be depleted by zebra mussels.  I adapted the model to simulate an idealized crustacean zooplankter.  The non microbial carbon subsidy was taken from whole lake C-13 results.  That model predicts lower minimum phytoplankton biomass in the presence of subsidies, but greater intra-annual variability because of the short life span of the herbivore.  The results suggest that a direct energetic subsidy via DOM can have significant impacts on ecosystem dynamics and function that are distinct from trophic interactions with the microbial loop.