Factors controlling phosphorus limitation in stream sediments1

Factors influencing the phosphorus limitation of benthic microorganisms were determined for four streams (two agricultural and two forested) in central New York State over an annual cycle. Phosphorus limitation was measured biweekly as specific alkaline phosphatase activity (APA) of sediment samples. Analysis of variance showed the forested streams (sites 2 and 4) to have significantly different sediment APA from the agricultural streams (sites 1 and 3), the higher APA for the forested sites indicating greater phosphorus limitation. Stream total reactive phosphorus concentrations showed no relationship with watershed type, with mean annual values of 10.5 and 6.0 pg liter-' for agricultural sites 1 and 3 and 4.1 and 9.6 for forested sites 2 and 4. With all sites grouped together, sediment APA had a low correlation with stream water phosphate (r = 0,184, n = 73). Phosphorus sorption isotherms showed sediments from agricultural streams to have higher phosphate sorption indices than forested sites and to sorb rapidly large amounts of phosphorus. This resulted in higher available phosphorus content for the agricultural sediments and lower