VARIATIONS IN PHOSPHORUS FRACTIONATION ALONG STREAM TRANSECTS IN TWO DIFFERENT CLIMATE REGIMES

Phosphorus is a key nutrient in freshwater aquatic systems that can undergo transformations between the various physicochemical fractions within stream sediment. Differences in P fractions through a watershed can result from the residence time, or temporal variation, of P in the stream sediment system. As the residence time of P within a stream system increases, easily mobile or transformable phases such as Al-P and OP often decrease, while stable phases such as Ca-P will be a higher percentage of the total P concentration.

Clear Creek, a mid-latitude temperate watershed located in Iowa, and part of the Intensively Managed Landscapes CZO, shows several apparent trends. Organic matter associated P (Org-P) was at very high concentrations in Zone 1, near the headwaters, ranging from 82 to 324 mg/kg of sediment. The Org-P concentrations drop rapidly as you move downstream. Zone 2 Org-P concentrations drop to range from 18 to 46 mg/kg, slowly drop as you move further downstream. Interestingly, moving from Zone 1 to Zone 2 shows a large increase in Aluminum associate P (Al-P). The Al-P concentrations jump from 37.5 to 179 mg/kg between these two zones, but drops further downstream. Calcium associated P (Ca-P) shows a similar pattern from Zone 1 to Zone 2, but subsequently increases with distance downstream.

Rio Blanco, a watershed within the Luquillo CZO, is located within the El Yunque National Forest, a tropical rainforest at the eastern end of Puerto Rico. From the headwaters to the mouth, distinct trends are seen in Al-P, Org-P and Ca-P fractions. Both the Al-P and Org-P fractions, which dominate in the headwaters, show a halving in their relative contribution to the total P in the sediment. Sediment concentrations actually increase slightly for each as the river moves from the National Forest to agricultural lands. In contrast, the Ca-P fraction shows a relative doubling as a percentage of the total, and concentrations steadily increase from 33 to 177 mg/kg.