Historical development and past ecological state of two danish shallow lakes

Abstract

Background and Scope

Lake sediment contains information on the historical development and former conditions of lakes, which is very useful when information about the undisturbed (reference) conditions of a lake is needed. Phytoplankton contains a range of different pigments, some of which are specific for individual algal groups and can be used as diagnostic markers. Phytoplankton pigments in lake sediments are indicators of prototrophic changes in lakes, since the pigments document changes in algae biomass and the composition of phytoplankton communities. Lake sediments can be dated along with pigment analysis by use of the naturally occurring radioactive isotope²¹⁰Pb.

Objectives

The objective was to describe the historical development and the former ecological state of two shallow Danish lakes — Lake Ladegaard and Lake Nr. Soeby.

Method

Sediment was sampled either in Plexiglas cores using a Kajak sediment sampler or by use of a peat sampler. The cores were cut into 2-cm slices. Sub-samples were taken from each slice for²¹⁰Pb dating, and for determining the pigment content through HPLC analysis.

Results and Discussion

The results of the datings were excellent and there was a very strong linear relation between sediment age and the depth of both lakes (r² = 0,99). The results of the pigment analyses in Lake Ladegaard revealed a distinct eutrophication initiated in the beginning of the 1930’s. Prior to this period, the phytoplankton diversity was high, with a predominance of chromophyte algae. In Lake Nr. Soeby, the phytoplankton biomass was relatively high not only in the upper part of the sediment but also in the lowest part of the sediment. Heavy eutrophication took place between 1950 and 1980, when the biomass of chlorophytes and cyanobacteria increased. Prior to this eutrophication event, the phytoplankton population had been more varied with a relatively low amount of chlorophytes and cyanobacteria and a higher abundance of chromophyte algae.

Conclusions

²¹⁰Pb dating combined with the analysis of phytoplankton pigments in sediment cores from two Danish lakes — Lake Ladegaard and Lake Nr. Soeby — turned out to be a simple but effective method of describing the historical development and the past ecological state of the lakes. The pigment content of the sediment showed that Lake Ladegaard and Lake Nr. Soeby were exposed to intense eutrophication during the 20th century. It also showed that the ecological state of the lakes was better in former times, with a much higher algal diversity and lower algal biomass.

Recommendations and Outlook

Before beginning the restoration of the lakes it is useful to know the ecological conditions to which it is possible to bring the lakes. The investigations showed that Lake Ladegaard and Lake Nr. Soeby before the 1930’s and 1950’s respectively were in another ecological state, and it might therefore be possible to return the lakes to these past ecological levels through lake restoration activities. If the investigations had shown that the original eutrophication level of the lake was the same as the present eutrophication level, then restoration activities would be pointless.