Incorporation of pulp mill effluent solids in aquatic food webs: use of carbon and nitrogen stable isotopes.

D.J. Velinsky, C.A. Flinders, N.E. Saxe, and R.L. Thomas. Patrick Center for Environmental Research, The Academy of Natural Sciences, Philadelphia, PA 19103

Stable isotopes of C and N have been used to evaluate the relationships between food sources and consumers in aquatic food webs. In this study, we used the isotopic signature of pulp mill discharge to determine how the nutrient-enriched effluent influenced isotopic compositions of material transported downstream and its potential incorporation into benthic algae, macroinvertebrates, and fishes. Effluent solids (> 0.7 μm) were enriched in ¹³C and depleted in ¹⁵N (δ¹³C = -21±1‰ and δ¹⁵N = 0.7±0.4‰) compared to upstream suspended material (SPM) (δ¹³C = -29±3‰ and δ¹⁵N = 5.5±0.9‰). Periphyton isotopic signatures also reflected a shift in the source of C and N. Downstream (>14km) from the discharge both the periphyton and SPM’s isotopic compositions became similar to upstream values. The carbon isotopic composition of filter feeding macroinvertebrates (δ¹³C = -21±0.1‰) was similar to effluent solids just below the discharge. Farther downstream, macroinvertebrate C values (δ¹³C = -26±0.1‰) were comparable to those above the facility. In addition, the isotopic enrichment of nitrogen between the effluent SPM and macroinvertebrates is approximately 3.7‰, well within the expected shift in δ¹⁵N from related studies. This study shows that paper-pulp mill effluent solids can be used to trace aquatic food webs and are a source of C and N to downstream organisms.