Dead leaves and the dirty ground: Sediment pollution reduces detrital resource availability to consumers in agricultural stream food webs

Background/Question/Methods

The intensification of agriculture to meet the demands of a growing world population is causing excessive ecosystem simplification, reduced ecosystem function, and biodiversity loss. Inorganic fine sediment pollution is one of the main pathways in which agriculture exerts pressure on stream and river ecosystems, and is a widespread and pervasive problem globally. Although excessive sediment can have direct and indirect effects on stream biota, there are still gaps in our knowledge regarding the effects of this stressor on food webs and ecosystem function. Ecological theory suggests that resource availability is a strong determinant of food web dynamics, and perturbations that affect basal resources can propagate up the food web. Sedimentation may dampen detrital energy flow in stream ecosystems through a variety of mechanisms including burial, which may reduce the availability of detrital resources to micro and macro-consumers. The homogenization of habitat by burial may narrow the niche breadth sufficiently so that function is impaired, reflecting predictions in ecological theory that environmental context modulates the relationship between ecosystem processes and biodiversity. We investigated these ideas by surveying detrital breakdown in nine agricultural streams along a gradient of deposited sediment, and evaluating potential mechanisms through laboratory microcosm experiments manipulating consumer-resource interactions.

Results/Conclusions

Detrital processing appeared to slow along the deposited sediment gradient with mean temperature-corrected leafpack breakdown rates exhibiting a strong curvilinear response (R² = 0.78, F_1,7 = 25, P < 0.01). Analysis of the macroinvertebrate data associated with the leafpacks suggested that the reduction of shredding caddisflies may have contributed to this loss of ecosystem function. Two laboratory experiments were conducted using sediment additions to leaf-filled stream microcosms to investigate effects on consumer-resource interactions. The first experiment demonstrated that reduced resource availability to an obligate shredding caddisfly caused changes in detrital breakdown. The second experiment corroborated these findings, and demonstrated a negative linear effect of sediment on two caddisfly taxa. Both experiments showed that sediment affected shredding caddisflies by reducing their consumption of coarse detritus, production of fine particulate matter, growth rates, and increasing mortality. It is proposed that this mortality was a consequence of increased intra-specific competition leading to cannibalism. Measurements of microbial activity showed no significant changes in response to sediment additions. These results indicate that sedimentation reduces resource availability to stream consumers with implications for biotic interactions, food web dynamics, biodiversity, and ecosystem function.