Salmon subsidies alleviate nutrient limitation of benthic biofilms in southeast Alaska streams

Using nutrient-diffusing substrata (NDS) in seven streams in southeast Alaska, USA, we tested whether (i) nutrient limitation of autotrophic and heterotrophic biofilms was alleviated by salmon resource subsidies, and (ii) whether the degree of alleviation could be predicted by environmental variables. Before salmon spawners arrived, autotrophic biofilms were nitrogen (N)-limited, or co-limited by N and phosphorus (P), whereas heterotrophic biofilms were either P-limited, or co-limited by N and P. Combined N and P amendments resulted in a 2.6-fold increase in biofilm chlorophyll a, and a 3.2-fold increase in community respiration. After salmon arrived, autotroph nutrient limitation was alleviated in six of the seven streams. Heterotrophs still exhibited nutrient limitation in six streams, but most streams shifted from co-limitation to P-limitation. Nutrient-diffusing substrata amended with salmon tissue indicated that salmon could also be an important source of organic carbon for biofilms. Autotrophs responded less to N and P amendments as streamwater ammonium concentration increased with the arrival of salmon. For heterotrophs, ammonium concentration and N:P ratio best predicted changes in response following the arrival of salmon. We provide the first direct evidence that biofilm nutrient limitation can be alleviated by salmon spawners in nutrient-poor streams.

Regional patterns in periphyton accrual and diatom assemblage structure in a heterogeneous nutrient landscape

The effect of nutrient regime on periphyton community development in large rivers was examined (sites ranged from oligotrophic to eutrophic). Patterns in diatom community structure were examined at a large spatial scale (ultimate), whereas at the microhabitat scale (proximate), artificial nutrient-diffusing substrata were used to examine periphyton response to amendment with nitrogen, phosphorus, and N + P. Ratios of ambient dissolved inorganic nitrogen to total phosphorus were used to make predictions of nutrient limitation (molar total inorganic nitrogen (TIN) : total phosphorus (TP)), which matched experimental results in 8 of 12 sites. Two sites with highest ambient nutrient levels (mean NO3 + NO2 and TP, 1.49 and 0.081 mg·L–1, respectively) possessed the highest diatom richness and diversity (mean richness = 42). Lowest diatom taxa richness (19) occurred in an impounded system with low TP (0.008 mg·L–1). Principal components analysis (PCA) of diatom taxa structure among sites (control treatments only) and small-scale patterns among nutrient treatments using all sites and treatments combined indicated that sites were grouped according to drainage basin (r2 = 0.79) and that there was no unified response to enrichment (r2 = 0.43). Results suggest that large spatial scale factors are more important in determining the potential benthic diatom assemblage than small-scale, proximate variables provided by the diffusers.

Effects of Water and Substratum Nutrient Supplies on Lotic Periphyton Growth: An Integrated Bioassay

Effects of substratum and water nutrient perturbations on periphyton growth were assessed in a nutrient-poor stream by combining a substratum enrichment technique with a flow-through bioassay system. Periphyton growth (chlorophyll a, total biovolume) responded to combined influences of water and substratum enrichment in an additive manner when both compartments were amended with N and P to yield an optimal ratio [Formula: see text]. When NO3-N was added to the substratum and PO4-P to the water, algal growth response was synergistic. Analysis of the vertical distribution of P fractions in cores taken from nutrient-diffusing substrata indicates that attached microorganisms mediate P release from underlying substrata, acting as a filter or temporary sink. Nutrient-diffusing substrata are useful detectors of limiting nutrients in aquatic systems; however, their function and application differ from water enrichment assays where nutrients are added at a constant rate. Differences are partially attributed to spatial and temporal variability of nutrient release and the strictly localized influence of substratum flora on ambient water chemistry.