Food Webs in Small Ponds

Small freshwater ponds are abundant on the landscape—there may be as many as 3 billion of them globally!  These small environments are an important home to many amphibians and invertebrates that can live nowhere else.  Yet, we know relatively little about how these ecosystems work compared to larger ponds and lakes.  One critical gap in our knowledge has been food web structure—what do pond organisms eat and where does their energy come from?


A small forested pond in Connecticut, USA (picture credit: M. Holgerson)

Small forested ponds are often assumed to have leaf-based food webs.  If you look at these ponds, you see trees all around the pond and lots of leaf litter on the pond bottom.  But this had never been rigorously tested… until now.  A recent study led by Meredith Holgerson, a post-doctoral fellow in the lab, took on the task of mapping small pond food webs. Meredith and her colleagues added isotopically-labeled leaves to a pond in a Connecticut forest.  These leaves were strongly enriched with the heavy nitrogen isotope, 15N, which can be then traced through the food web.  (Think of it like a dye that doctors use to see inside your body—the nitrogen in these leaves can be traced to any organism in the food web that uses it!).  The three big findings were:

  1. Leaf litter permeated the entire food web; even algae used nitrogen derived from leaves!  Leaves and algae are usually considered separate, but parallel, food web pathways.  This is not so in small ponds where leaf and algal resources are intertwined!
  2. It’s likely that leaf nutrients are mobilized to other organisms in the pond by macroinvertebrate shredders.  Shredders, like caddisfly larvae, rip apart leaves and eat them, and their poop is a nice fine organic material that other animals love to eat (yum!).
  3. Despite the fact that small ponds are shaded and have lots of leaf litter, algae is a more important food resource to the majority of animals in the food web.  This was very surprising as it countered previous expectations.

Check out the study published in Ecology!


The relative importance of leaf litter (black) versus algae (light gray) to the biomass of organisms in a small pond. Darker gray indicates a greater importance of nutrients derived from leaf litter, whereas light gray indicates a greater importance of algal nutrients.

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