PHOSPHORUS, IRON, AND CARBON CYCLING IN A SALT STRATIFIED COASTAL POND (MEROMICTIC, LIMITATION, BACTERIAL AUTOTROPHY)

Primary production in most lakes is limited by the supply of phosphorus to surface waters. Two important sources of phosphorus are loading from the watershed, via stream and ground water inputs, and input from nutrient rich bottom waters (hypolimnetic loading). This study compared the relative importance of these two P sources in Siders Pond, a salt-stratified pond with permanently anoxic bottom waters. Ground water measurements and budgeting demonstrated that watershed loading of P to Siders Pond was large (40 mmol m('-2) y('-1)). Concentrations of dissolved P in anoxic bottom waters of Siders Pond were extremely high (ca. 90 uM), suggesting that hypolimnetic P loading could also be large. The extent to which dissolved P in anoxic bottom waters is available to phytoplankton depends on: (1) the amount of vertical mixing between bottom and surface waters, and (2) the extent to which dissolved P is taken up into particles as water is mixed passed the oxic-anoxic border.;Vertical mixing in the water column of Siders Pond was calculated using a mixing model based on tritium-helium-3 dating of the water in Siders Pond. Bottom waters of Siders Pond exchanged relatively rapidly with surface waters and, because of this rapid exchange, there was a large export of P from bottom waters (55 mmol m('-2) y('-1)). Removal of P into particles by chemical scavenging or uptake by autotrophic bacteria, however, significantly decreased the amount of P which actually reached surface dwelling phytoplankton. A mass balance budget of dissolved P at the oxic-anoxic border of Siders Pond showed that some 25 mmol P m('-2) y('-1) was removed into particles at the oxic-anoxic border of Siders Pond. Analysis of suspended and sedimenting particles indicated that chemical scavenging by iron oxides was responsible for much of this P removal. The low biological productivity (as measured by ('14)C-DIC uptake) at the oxic-anoxic border of Siders Pond suggested that autotrophic bacteria were not important in controlling the supply of P to phytoplankton in surface waters. In Siders Pond chemical scavenging of P decreased phytoplankton P supply by some 25%. Thus, this study shows that chemical scavenging in the water column of lakes can compete with biological processes for available P and lower the potential rate of lake eutrophication.