N and P limitation of benthos abundance in the Nechako River, British Columbia

Additions of inorganic N and P to replicated stream mesocosms were used to test the hypothesis that production of the benthic food web in the flow-controlled Nechako River, B.C., was N and P limited. N, P, and N + P additions were contrasted with controls to determine the responses of periphyton and macroinvertebrates over a 33-day period in late spring. Periphyton accrual under N + P addition was greater than two times that of the controls, and N alone produced a moderate increase. Periphyton showed relatively little response to P addition compared with controls. Macroinvertebrates, especially orthocladiine midges and mayflies, increased in density under N or N + P additions, in contrast with P additions and controls. Increased emergence rates of adult aquatic insects from the mesocosms were consistent with increases observed in benthic invertebrates. The results showed nutrient limitation of benthic productivity and that algal and benthic invertebrate responses were closely coupled. N was most limiting, but with additions of inorganic N as low as 10 µg⋅L–1, P became limiting. The benthic invertebrates that increased most in abundance corresponded to the predominant prey in chinook salmon (Oncorhynchus tshawytscha) fry diets at the time of the experiment. Résumé: Nous avons ajouté du N et du P inorganiques dans une série de mésocosmes lotiques pour valider l’hypothèse selon laquelle la production de la chaîne alimentaire benthique dans la rivière Nechako, un cours d’eau de Colombie-Britannique à débit contrôlé, était limitée en N et en P. Les effets des ajouts de N, de P et de N + P ont été comparés à des valeurs témoins afin de déterminer les réponses du périphyton et des macroinvertébrés sur une période de 33 jours de la fin du printemps.