Slow-Release Fertilizer for Rehabilitating Oligotrophic Streams: a Physical Characterization

A combination of habitat degradation and overharvesting of anadromous salmonids in many of B.C.’s oligotrophic watersheds has prompted the addition of inorganic nutrients to streams, which increases autotrophic production and aids in the restoration of salmonid production. A new slow-release fertilizer (7-40-0, N-P₂O₅-K₂O, percent by weight) was examined to determine its phosphate (PO₄3-) release rates using laboratory and field trough experiments. A series of indoor trough experiments indicated that the fertilizer pellet dissolution rate (0.393 g ? days^-0.401) was independent of the experimental range of water velocity (0.03-0.30 m . s^-1), fertilizer pellet size (2–9 g) and water temperature (8–14.5°C). Resulting phosphate additions (0.5-5 µg P . L^-1) in outdoor trough experiments increased periphyton biomass and altered the dominance pattern of periphytic diatoms. An optimal phosphate concentration for periphyton biomass was achieved with calculated 3.0 µg P . L^-1 phosphate additions from May to June. In June to July, periphyton biomass increased proportionately to fertilizer additions. Saturation of the relative specific growth rate of the diatom community occurred with 1.0 µg P . L^-1 phosphate additions. Nitrogen analysis was not conducted since inorganic nitrogen is typically available in non-limiting concentrations (i.e., >50 µg . L^-1 dissolved inorganic nitrogen) in the majority of B.C.’s oligotrophic salmonid streams. These studies indicated that slow-release fertilizer may be effective in stimulating autotrophic production and restoring salmonid production in nutrient deficient streams.