Detrital nutrient enrichment alters elemental processing by a stream detritivore

Background/Question/Methods

Consumers may respond to dietary nutrient enrichment by releasing excess nutrients in dissolved and particulate forms. Through the mechanism of consumer-driven nutrient recycling, this release may carry larger ecosystem implications. However, dietary influences on excretion and particularly egestion stoichiometry are still poorly understood among benthic detritivores. We conducted a five-week laboratory feeding experiment with the stream detritivore Pycnopsyche spp. (Trichoptera: Limnephilidae) to examine how the ratios of consumer outputs of nutrients compare between egestion and excretion as well as how nutrient ratio release changes with diet quality. Field collected caddisflies were fed post oak or sugar maple leaves conditioned under four levels of phosphorus (P) to produce a diet quality gradient (with increasing P concentration, diet carbon:phosphorus = 4064, 2494, 1538, and 1106 for oak and 3336, 2879, 1467, and 1039 for maple; diet carbon:nitrogen = 39, 34, 32, and 32 for oak and 41, 38, 32, and 34 for maple). We measured caddisfly instantaneous growth, nitrogen (N) and P excretion and egestion rates, and additionally examined diet-dependence of caddisfly feces respiration, an indicator of microbial activity and decomposition.

Results/Conclusions

Caddisfly P excretion was diet-dependent. Net P uptake, likely by the caddisflies’ cases, outpaced excretion on the lowest P diets, but caddisflies became increasingly net sources of dissolved P with higher diet nutrient content (range = -0.23 to 0.59 µg P/mg/day; P<0.05) relative to insect-absent controls. N excretion rates were net positive but not diet-dependent (mean±1SE = 0.80±0.10 µg N/mg/day; P>0.05). N and P excretion rates did not differ according to leaf type (P>0.05), suggesting that diet stoichiometry overrides leaf species effects on dissolved nutrient release. Total egestion was greater among faster-growing caddisflies, whereas feces respiration rates varied three-fold across treatments and were negatively related to caddisfly instantaneous growth rate (P<0.001). Oak diets resulted in feces with lower microbial respirations rates as compared to maple diets. We suggest that diet-dependent caddisfly assimilation of elements modifies carbon availability and stoichiometry of egesta, thereby altering the fate of consumer feces in streams. Our findings illustrate that Pycnopsyche spp. may switch from being a net sink to a net source of water column dissolved P with dietary nutrient enrichment. Further, we expand the concept of consumer-driven elemental cycling to include diet quality influences on mineralization of organic matter.