The prediction of internal phosphorus load in lakes with anoxic hypolimnia1

Abstract

Lakes with anoxic hypolimnia (anoxic lakes) have significantly lower values for phosphorus retention than do lakes with aerobic hypolimnia (oxic lakes). This difference may reflect an increased internal phosphorus load from the anoxic hypolimnia.Two models are given to predict internal phosphorus load (L;,,) in such lakes. The first predicts internal load as the difference between the observed phosphorus retention in anoxic lakes and that predicted (Rpred) by a formula that adequately describes phosphorus retention in oxic lakes. The second predicts internal load as the product of an average rate of phosphorus release from anoxic sediments, the surface area of the anoxic sediment, and the period of anoxia. Predictions of the first model compare favorably with 17 observed values of internal load; further data are required to test the second model. These models suggest that mean phosphorus concentration (TP) in anoxic lakes can be predicted in two ways. One can use whole-lake phosphorus budget models which implicilly incorporate internal phosphorus load, because they include a measurement of phosphorus retention. Alternatively, a term to account for the internal load can be added to current models based on external load (L,,,) and predicted retention (I&J, where qs is areal water load:

Eutrophication of aquatic ecosystems: Bistability and soil phosphorus

“…Rapid recycling rates are known from some enriched lakes (14,19). Various mechanisms for increased phosphorus release from enriched sediments of different lakes have been investigated (14,(19)(20)(21)(22). Some eutrophic lakes have failed to respond, or responded with long delays, to decreases in phosphorus input because of rapid recycling from sediments (12,17,(23)(24)(25), thereby demonstrating the effectiveness of recycling in maintaining a eutrophic state.…”

“…If the phosphorus cycle was near intersection 3, returning it to intersection 1 might require extreme reductions of input, long time delays, or manipulations to cross the unstable equilibrium at point 2, depending on details of rate curves (18). Rapid recycling rates are known from some enriched lakes (14,19). Various mechanisms for increased phosphorus release from enriched sediments of different lakes have been investigated (14,(19)(20)(21)(22).…”

“…Phosphorus can be recycled to lake water from sediments (14) and consumers (15). Recycling from sediments varies with annual cycles of mixing, and also over longer time horizons of decades to millennia as sediments build up in lakes.…”

Effects of nutrient loading on the trophic state of Lake Brunner

“…11) is commonly predicted by the equation (Vollenweider 1976) where L p is the loading rate of P per area of the lake surface (mg m À2 year À1 ), equal to total annual load (mg year À1 ) divided by the lake surface area (m 2 ), q s is the hydraulic load (m year À1 ) and T w is the hydraulic residence time ( mean P concentration in the lake close to that observed ( Table 2), suggests that the Vollenweider equation adequately models the retention of P in the lake (Nürnberg 1984;Ahlgren et al 1988). There are no data on atmospheric nutrient inputs (rainfall þ dry fall directly on the lake) to Lake Brunner but they are expected to be small relative to the catchment inputs.…”

“…In lakes where part or all of the bottom temporarily or permanently becomes anoxic, the method would underestimate TP concentrations in the lake. When external P loading and lake productivity exceed a certain threshold, resulting in reduced oxygenation of bottom water by sedimentation of organic matter, a smaller proportion of P will be permanently buried and additional inputs to the water column of dissolved P will appear through release of P from the sediments (Nürnberg 1984).…”