WW-I-6: Start of the spring algal bloom in standing waters – case study

The picture is taken half under water, half above the surface of a lake. The water appears olive green. The shore of the lake is partly covered with trees.Click to enlarge
When the water warms up in spring, phytoplankton can proliferate very suddenly.
Source: Photograph: © Dudarev Mikhail/stock.adobe.com

2019 Monitoring Report on the German Strategy for Adaptation to Climate Change

Table of Contents

 

WW-I-6: Start of the spring algal bloom in standing waters – case study

The time of onset of spring algal bloom depends on the temperatures prevailing in late winter and spring. Particularly mild winters such as 2001/2002 or particularly cold winters such as 2013/2014 influence the processes taking place in the course of the time series. It would therefore be inappropriate to over-interpret the square trend calculated statistically.

The line graph shows the calendar week of the onset of the spring algal bloom in Lake Müggel since 1980 and in Lake Stechlin since 1994. Both graphs show a quadratically increasing trend with fluctuations between years.
WW-I-6: Start of the spring algal bloom in standing waters – case study

The line graph shows the calendar week of the onset of the spring algal bloom in Lake Müggel since 1980 and in Lake Stechlin since 1994. Both graphs show a quadratically increasing trend with fluctuations between years.

Source: Leibniz Institut für Gewässerökologie und Binnenfischerei; IGB (lake monitoring)
 

Algal bloom in spring – major fluctuations from year to year

Apart from direct and indirect impacts of water temperature on the material balance of lakes, as well as their species composition and utilisation potential, changes in the water temperature also change the layering of water bodies which, in turn, entails impacts on the pivotal processes in water ecology.

Especially in sufficiently deep lakes the phases of layers mixing and phases of temperature layering alternate. While solar radiation will warm up the surface water, it will not penetrate to water layers further down. The variety of water temperatures results in more or less stable layering of the water body – the so-called summer stagnation – which prevents the exchange of oxygen and nutrients among the layers.

Summer stagnation stops when in autumn the circulation regime restarts owing to falling surface temperatures. The cooling of surface water in winter resulting from the development of stable layers as a function of air temperature in winter and spring, is rapidly impeded again by spring circulation. Thanks to this circulation process, nutrient-rich water from the depth is transported to the lake surface where it furthers the growth of algae in this phase. The milder the winter and the higher the spring temperatures, the earlier algal bloom begins in spring; this triggers a phase of algal growth which subsides once the available nutrients have been used up and strong feeding pressure from zooplankton sets in. The clear water phase begins.

In principle, the temporal process of circulation and layering is different for each lake, depending on variables such as lake size, water depth, type of circulation and the regional climate. Deep lakes such as Lake Stechlin in Brandenburg typically form a more stable layering than shallow lakes such as the Große Müggelsee in Berlin. ´The latter can occasionally be subject to complete mixing of layers owing to windy conditions in summer. In Germany there is very limited availability of long-term time series of temperature measurements using depth profiles which might indicate the annual progress. It is therefore not possible to provide valid illustrations of time series of layering conditions in typical lakes.

The shallow Große Müggelsee and the deep Lake Stechlin illustrate that from year to year algal bloom starts at different times of spring. As far as the Müggelsee is concerned, in the years 2011 and 2014 spring algal bloom started at the earliest in the beginning of March whereas in 2016 and 2017 spring algal bloom did not start till late May. For the 70m deep Lake Stechlin the range of fluctuations is even greater: in 2009 spring algal bloom started already in early March whereas in 1997 and 2014 it did not start until mid or early June. The 1990s experienced a number of mild winters which led to a comparatively early onset of spring algal bloom. Between 2008/2009 and 2012/2013 all winters were colder than calculated for the long-term mean thus leading to a later onset of spring algal bloom.

 

Interfaces

WW-I-5: Water temperature of standing waters

GE-I-5: Contamination of recreational bathing waters with cyanobacteria

FI-I-2: Occurrence of thermophilic species in inlandwaters

 

Objectives

Management objectives for surface waters: good ecological condition and/or good ecological potential as well as good chemical condition (WHG, §27).