Does the forest feed the pond? Evaluating the role of terrestrial leaf litter in the ecosystem metabolism and food web structure of temporary ponds
Freshwater ecosystems can receive substantial inputs of energy and nutrients, such as carbon and nitrogen, from the surrounding terrestrial environment. These inputs can be buried, exported, or incorporated into the food webs of recipient freshwater ecosystems. Terrestrial inputs often drive the metabolism of freshwater systems to be net heterotrophic, but the extent to which terrestrial inputs affect food web dynamics is often debated.
In this study, I examined how terrestrial inputs affect the metabolism and food web structure of six small, temporary ponds in northeastern Connecticut, USA. Temporary ponds are abundant throughout forests of the northern and eastern United States; yet, we know very little about their ecosystem dynamics. The physical habitat of these ponds suggests that the effect of terrestrial inputs could be quite high. The ponds are small (<1,000 m2), shallow (< 1 m deep), have a high perimeter to surface area ratio, and receive an annually renewed source of terrestrial nutrients through leaf litter. I measured oxygen, carbon dioxide (CO2), and methane (CH4) dynamics to understand ecosystem metabolism. To determine food web structure, I added leaves enriched with 15N to one pond and traced its entry into the food web in comparison to five other control ponds.
The six temporary ponds received between 188 and 253 g m-2 of dry leaf litter each year, which led to the ponds being strongly heterotrophic. Dissolved oxygen concentrations were undersaturated and declined seasonally to the point of anoxia. The CO2 and CH4 concentrations were supersaturated and were among the highest ever reported for lakes and ponds globally. The CO2 and CH4 concentrations were driven by high respiration rates and anoxic conditions, both of which can be attributed to terrestrial carbon inputs.
The food web was also influenced by terrestrial inputs. The 15N label was strongly incorporated into the food web. The label was most strongly picked up leaf biofilm, Limnephilus caddisfly larvae, and Asellus isopods. The label was also picked up by Acilius beetle larvae, spotted salamander (Ambystoma maculatum) larvae, and wood frog (Rana sylvatica) larvae. To a lesser extent, the label could be seen in periphyton, seston, and zooplankton. Overall, the results indicate that terrestrial leaf litter drives the metabolism of small temporary ponds and also provides an important food source to pond consumers.