Periphyton carbon and nitrogen stable isotopes detect agricultural stressors in low-order streams

Shifts in the stable isotope signatures of carbon (C) and nitrogen (N) in
ecological materials have the potential to indicate environmental
disturbances. This study examined the δ13C‰ and δ15N‰ ratios of stream
water and periphyton from low-order streams in a landscape influenced by
agricultural activities. Our key purpose was to assess the influence of
best management practice (BMP) presence and age on C and N isotope values
as a potential water-quality assessment. We collected stream water and
periphyton from 19 streams within the Upper Delaware River watershed in
New York, USA, in each of 4 management categories: 1) recently applied BMP
treatments, 2) long-standing BMPs, 3) streams lacking BMPs, and 4)
minimally disturbed reference streams. We sampled and analyzed water and
periphyton for δ13C‰ and δ15N‰ in a repeated-measures design (BMP category
´ time) from April to November 2013. There were large seasonal differences
in stream water δ13C-dissolved organic C (DIC)‰ and δ15NO3-N‰, with strong
differences between reference and agricultural streams. Periphyton δ13C‰
and δ15N‰ values also differed strongly across streams draining land with
agricultural activities, with 85% higher periphyton δ15N‰ signals in all
agricultural categories vs reference streams. Periphyton diatom and
chlorophyte taxonomic proportions showed the strongest relationship with
periphyton δ13C‰ values, where diatoms were negatively associated with
increasing δ13C‰. These results suggest that aqueous and periphytic stable
isotopes were sensitive in detecting persistent effects of agriculture on
these streams despite BMP mitigation, where nutrient (orthosphosphate,
nitrite, and ammonia) levels were non-indicative. These results also
suggest that BMPs may not have fully eliminated the negative impacts of
agricultural stressors on water quality in impacted streams.