EXPERIMENTAL-EVIDENCE FOR NITROGEN LIMITATION IN A NORTHERN OZARK STREAM

Nutrient enrichment experiments were conducted during low flow periods in 1985 and 1986 in a northern Ozark stream characterized by low nitrate-N concentrations and molar ratios of TN:TP less than 20:1. Enrichment with nitrate to a concentration 3-6 times greater than ambient, either alone or in combination with phosphate addition, stimulated periphytic chlorophyll a by 4-6 x and AFDM by 2-5 x over unenriched controls in two 30-d experiments. An intermediate increase in chlorophyll a in response to phosphate addition was observed in 1985 when TN : TP averaged 19:1. Differences in periphyton accural among treatments were influenced by snail grazing (Goniobsis) during 1986 when periphyton biomass in N-enriched treatments continued to increase over the 30-d enrichment period, but remained at low levels in control and P-enriched treatments. Low nitrate-N concentrations and TN:TP less-than-or-equal-to 20:1 were also characteristic of 16 sites on 10 streams in the northern Ozark Plateau during low flow periods in 1985 and 1986, suggesting that nitrogen limitation may be common in the region.