Increasing Carbon-To-Phosphorus Ratio (C:P) From Seston As A Prime Indicator For The Initiation Of Lake Reoligotrophication

Decline in total phosphorus (TP) during lake reoligotrophication does not apparently immediately influence carbon assimilation or deep-water oxygen levels. Traditional monitoring and interpretation do not typically consider the amount of organic carbon exported from the productive zone into the hypolimnion as a measure of net ecosystem production. This research investigated the carbon-to-phosphorus ratios of suspended particles in the epilimnion, (C:P)(epi), as indicators of changing productivity. We report sestonic C:P ratios, phytoplankton biomass, and hypolimnetic oxygen depletion rates in Lake Hallwil, a lake whose recovery from eutrophic conditions has been documented in 35 years of historic water-monitoring data. This study also interpreted long-term (C:P)(epi) ratios from reoligotrophication occurring in four other lakes. Lake Hallwil exhibited three distinct phases. (i) The (C:P)(epi) ratio remained low when TP concentrations did not limit production. (ii) (C:P)(epi) increased steadily when phytoplankton began optimizing the declining P and biomass remained stable. (iii) Below a critical TP threshold of similar to 15 to similar to 20 mg P m(-3), (C:P) epi remained high and the biomass eventually declined. This analysis showed that the (C:P)(epi) ratio indicates the reduction of productivity prior to classic indicators such as deep-water oxygen depletion.