Climate change: Heavy rain after drought may cause fish kills

Date:
July 9, 2020
Source:
University of Southern Denmark
Summary:
Due to climate changes, many regions are experiencing increasingly
warmer and dryer summers, followed by heavy rain. New study shows
this is a fatal combination that can cause massive fish kills in
lakes within a few hours.



FULL STORY ==========================================================================
Fish kills are a recurring phenomenon in lakes suffering from oxygen
depletion.

Often the kills are triggered by factors like an algae bloom, but now
a new study reports on a new, climate-related cause of fish kills.


==========================================================================
A grave example is reported from a lake in Denmark. In 2018, Denmark was
hit by extreme summer drought and temperatures in May, June and July were
3 degrees higher than the average for the previous 30 years. At the same
time, an average of only 27 mm of rain fell compared to normally 56 mm.

The lake, called Lake Filso/, received no rainfall until a thunderstorm
with heavy rain blew across the lake on July 28, delivering 22 mm in
only a few hours.

Resets an entire ecosystem 'Huge amounts of water flowed from the
catchment into the lake and brought with it huge amounts of organic
material. This cocktail led to massive fish kills in the lake. Events
like this are dramatic and can completely reset an entire ecosystem,'
explains biologist and Associate Professor Theis Kragh from the University
of Southern Denmark.

A few days after the heavy rainfall, 1,203 dead pike were found in the
lake. A month later, only a single pike was caught in the gill nets. The
stocks have never recovered, and this year the researchers released
8,000 pike fry and 350 kg of spawning stock biomass of perch to help
the stocks return.



==========================================================================
'And we will follow up with more pike breeding in the future. The tragedy
is that the oxygen depletion only lasted four days. On day five, oxygen
levels were normal again, but then the fish had died,' he says.

How could it happen? The reason why heavy rainfall can lead to massive
fish kills is this: When rain falls, it runs through ditches and drainage
pipes and ends up in the lake. On its way, it picks up a lot of labile
organic matter from the soil; leaves, mud, and partially degraded
organic matter.

When all this organic matter is flushed out into the lake, it becomes a
target of hungry bacteria. When the bacteria consume the organic matter,
they use oxygen and they take that oxygen from the water. This causes
oxygen depletion in the lake, and the fish die.



========================================================================== 'Lake Filso/ is not the only lake to have experienced this, and it will
happen to other lakes. Until now, we have considered extreme drought
followed by heavy rain as a 20- or 100-year event, but climate change is
a reality now and we should rather consider it a 5- or 10-year event,'
says Theis Kragh.

Similar events have probably also affected rivers and streams.

Slower drainage can help Theis Kragh and his colleagues are now working
on tracing the organic material in Lake Filso/ back to its point of
origin in the catchment surrounding the lake.

'If we know where it comes from -- which field or heath -- we can work
with the drainage of the particular area. Currently, the drainage is so effective that organic matter is flushed very quickly into the lake. If
the drainage gets slowed down, more rainwater will be absorbed by the
soil rather than being washed out into the lake,' he says.

Denmark also experienced hot weather and drought in 2019 but received
a little more rain than in 2018, and Lake Filso/ was not exposed to the
same phenomenon as in 2018.


========================================================================== Story Source: Materials provided by
University_of_Southern_Denmark. Original written by Birgitte
Svennevig. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Theis Kragh, Kenneth Thoro/ Martinsen, Emil Kristensen, Kaj
Sand-Jensen.

From drought to flood: Sudden carbon inflow causes whole-lake anoxia
and massive fish kill in a large shallow lake. Science of The Total
Environment, 2020; 739: 140072 DOI: 10.1016/j.scitotenv.2020.140072 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200709092421.htm

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