Tracing anthropogenic inputs in stream foods webs with stable carbon and nitrogen isotope systematics along an agricultural gradient

Abstract

Stable carbon (13C) and nitrogen isotopes (15N) are useful tools in determining the presence of agricultural influences in freshwater ecosystems. Here we examined δ15N and δ13C signatures in nitrate, fish, and mussel tissues, from rivers in Southern Ontario, Canada, that vary in their catchment proportion of agriculture land use, nutrients and organic matter quality. We found comparatively 15N-enriched δ15N values in animal tissues and dissolved nitrates, relative to expected values characterized by natural sources. We also observed a strong, positive correlation between riparian agriculture percentages and δ15N values in animal tissues and nitrates, indicating a significant influence of agricultural land use and the probable dominance of organic fertilizer and manure inputs in particular. The use of a 15N-based equation for the estimation of fish trophic position confirmed dietary analyses is showing all fish species to be tertiary consumers, with a relatively consistent 15N-enrichment in animal tissues between trophic levels. This indicates that variability in 15N-trophic fractionation is minor, and that fish and mussel tissue δ15N values are largely representative of source nitrogen. However, the trophic fractionation value varied from accepted literature values, suggesting strong influences from either local environmental conditions or dietary variation. The δ13C datasets did not correlate with riparian agriculture, and animal δ13C signatures in their tissues are consistent with terrestrial C3 vegetation, suggesting the dominance of allochthonous DOC sources. We found that changes in water chemistry and dissolved organic matter quality brought about by agricultural inputs were clearly expressed in the δ15N signatures of animal tissues from all trophic levels. As such, this study confirmed the source of anthropogenic nitrogen in the studied watersheds, and demonstrated that this agriculturally-derived nitrogen could be traced with δ15N signatures through successive trophic levels in local aquatic food webs. The δ13C data was less diagnostic of local agriculture, due to the more complex interplay of carbon cycling and environmental conditions.

Giant beaver palaeoecology inferred from stable isotopes

“…Anthropogenic activities (particularly land development and agricultural practices) can contribute nitrogen sources that enter the local environment and impact the nitrogen isotope composition of primary producers. This effect is particularly pronounced in modern wetland habitats 45 .…”

Riparian buffers maintain aquatic trophic structure in agricultural landscapes

“…More recently, efforts have been made to extend these results to food web structure and function (e.g. [12][13][14]). While studies have shown that increased nutrients and riparian removal can alter energy flows into ecosystems, it is not yet well understood how different forms of local agriculture mediate stream food web responses in agriculturally dominated watersheds [15].…”

The influence of pastoral and native forest land cover, flooding disturbance, and stream size on the trophic ecology of New Zealand streams

“…Additionally, riparian land-cover vegetation type has been linked to effects on food-chain length (Fraley et al 2018), but it is unclear what mechanisms influence this. For example, does per cent catchment pastoral cover affect fish trophic position indirectly through algae and invertebrate consumers, or are effects due to isotopic enrichment from nitrate runoff influencing stable isotope (SI)derived measures of trophic position (Lake et al 2001;Anderson & Cabana 2005;Clapcott et al 2012;Lee et al 2018)? Therefore, understanding the mechanisms controlling freshwater trophic structure could give managers a tool for measuring and addressing effects of perturbations.…”

“…2012; Lee et al . 2018)? Therefore, understanding the mechanisms controlling freshwater trophic structure could give managers a tool for measuring and addressing effects of perturbations.…”