Understanding the role of animal defecation in aquatic nutrient cycles

Halvor M. Halvorson, Delaney J. Hall, and Michelle A. Evans-White

A central question in ecology is how organisms affect the ecosystems they inhabit. Ecologists have historically addressed this question from many angles; one of the most compelling over the last two decades has been a focus on animal wastes. This is because wastes depend on diets and vary widely across animal species, leading to substantial diversity, and when wastes are released, they enter the environment and affect ecosystem processes including the transformation and transport of nutrients. In aquatic settings like streams, animal wastes predominately occur as faeces and excreta, released in particle and dissolved forms respectively. Previous research has focused on animal excreta because these dissolved wastes – especially ammonium and phosphate – are nutrient-rich and highly bio-available for uptake. Through excretion, animals can be substantial sources of dissolved nutrients in many ecosystems.

Animal faeces are a different form of waste that, compared to excreta, are less bio-available and more nutrient-depleted, but may still be ecologically important because many animals defecate substantial material (many times their own body weight) during their lifespan. However, we poorly understand whether defecation serves as a “source” (net production, as in the case of excretion) or a “sink” (net storage or loss) of nutrients in ecosystems. In our study, we collected 3 different co-occurring stream macroinvertebrate species – the stonefly Allocapnia, the cranefly Tipula, and the isopod Lirceus – and fed them low- or high-nutrient leaf litter. We then collected their faeces and compared faecal nutrient contents as well as release and uptake of dissolved nutrients as faeces decomposed over several months. We found that both diet and source animal affected faecal nitrogen and phosphorus contents, ammonium release, and uptake of nitrate over time. Importantly, we found that animal faeces can remove dissolved nutrients from the water column due to microbial uptake, serving as sinks, in direct contrast to net production of dissolved nutrients by animals via excretion. These findings are helpful to understand how animals may affect their ecosystems by changing nutrient cycling.

Image caption: London Creek, the source stream for aquatic macroinvertebrates. Photo credit: Hal Halvorson.
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