Phosphorus cycling in tundra ponds

Abstract

Phosphorus concentrations in several low-centered polygon ponds on the Arctic coastal plan at Barrow, Alaska were monitored at least weekly in summers 1970 and 1971, and less frequently in 1972 and 1973. Phosphate was always low, averaging less than µg P/liter. Dissolved unreactive P was the most common form of phosphorus, with concentrations about five- to tenfold that of phosphate. About 30% of this was in the form of labile primary or secondary plankton secretion products. Particulate P was the second most concentrated form, at 4 to 11 µg/liter. The major phosphorus source for the pondwater system was the emergent vegetation inhabiting pond shallows. Phosphate originating in the sediment root zone was incorporated into above-sediment phytomass and in part secreted while the plant was alive and in part leached after senescence or death. Secreted or leached phosphorus resided only briefly in the water before being resorbed by surface sediment. The equilibrium between water and sediment in tundra ponds appeared to be unfavorable for phytoplankton growth even though sediment inorganic and water column phosphorus concentration were not especially low. The limited competition to this cycle that plankton organisms were able to supply was derived from an ability to take up phosphate in excess of growth requirements and to excrete the excess in form of labile, recyclible, dissolved organic phosphorus. This in turn was the dominant phosphorus cycle within the water column. Phosphate availability to pondwater biota was found ultimately limited by retention of most sediment P in organic or reductant-soluble inorganic form and to strong sediment sorption of the remainder.