Multi-year satellite observations of Lake Ladoga's biogeochemical dynamics in relation to the lake's trophic status

Abstract

Multi-decadal extensive ship borne biochemical investigations conducted by the Limnological Institute in St. Petersburg (Russian Academy of Sciences) have shown that since the mid-1980s the ecosystem of Lake Ladoga (at that time a mesotrophic water body) has undergone significant and multifaceted alterations primarily as a result of a dramatic cutting of external phosphorus input enforced by the administrative measures taken at the Russian federal and regional levels. Using the SeaWiFS data for the time period 1998–2004, the seasonal dynamics of spatial distributions of phytoplankton chlorophyll and dissolved organic matter in Lake Ladoga have been documented and analyzed. It is shown that the hydro-optical characteristics of the lake's water have undergone appreciable modifications as compared to the mid-1980s. Contrary to expectations that these changes would bring about some amelioration of the lake's ecological state, ship-based and spaceborne observations indicate that the attained decrease in phosphorous input has not resulted in “healing” of the lake's ecology. It is proposed that during the lake's mesotrophic period, a specific mechanism of phosphorus release from dissolved organic matter has gradually developed. As a result, the lake's phosphorous concentrations have continued to be higher than expected. In addition changes in local atmospheric forcing are manifested not only in remotely sensed color data but also in the inter-annual behavior of the spring thermal bar derived from the AVHRR multi-year observations.