Development and application of a nutrient‐diffusing bioassay for large rivers

Abstract

1. Laboratory and field experiments were performed to develop and then apply a nutrient‐diffusing substratum (NDS) design suitable for use in large, fast‐flowing rivers.2. Initial laboratory experiments quantified diffusion of PO4 and NO3 from new and previously used clay pots, which were soaked in deionized distilled water. Mean release rates initially exceeded 2.4 and 725 μmol l–1 day–1 P and 0.22 and 18 μmol l–1 day–1 N from new and used pots, respectively, but declined rapidly with increasing time spent in deionized distilled water and were below detectable levels after about 18 and 29 days, respectively.3. A phosphorus (P) dose–response experiment in a P‐limited reach of the Athabasca River, Alberta, Canada showed that epilithic biomass and macroinvertebrate density on NDS increased with increasing concentrations of KH2PO4 up to about 0.5 m. Beyond this threshold, biomasses and densities were unaffected by initial KH2PO4 concentration. Coefficients of variation of epilithic biomass estimates declined with increasing KH2PO4 whereas invertebrate density appeared to be unaffected by KH2PO4 levels.4. Release rates of both P and N from NDS filled with 0.5 m KH2PO4 or 0.5 m NaNO3 declined at a log‐negative rate from about 5000 μmol N‐NO3 l–1 day–1 and 3500 μmolP‐PO4 l–1 day–1 on day 2, to 200 μmol l–1 day–1 for both N and P on day 32.5. After development, we used the diffusing substrata to identify spatial patterns in nutrient limitation at seven sites along a 120 km reach in the Athabasca River, that receives two known point‐source nutrient inputs. NDS consisting of N, P, N + P and unenriched controls were attached to the river bottom for 22–23 days and then retrieved and sampled for epilithic chlorophyll a. Physicochemical parameters and epilithic biomasses on upper stone surfaces were also quantified when NDS were deployed and retrieved from each site.6. Sites located immediately downstream of the two point source inputs had higher water column concentrations of PO4 and epilithic biomasses than the site immediately upstream; epilithic biomass was positively related to PO4 in the late autumn (r2= 0.58) but not in early autumn. Sites located immediately below nutrient inputs were not nutrient‐limited, whereas upstream reference sites were P‐limited.