FISH 133 Spring 2019

Climate Change and Freshwater Ecosystems
What is climate change?
‘A change in global or regional climate patterns,
in particular a change apparent from the mid to
late 20th century onwards and attributed largely
to the increased levels of atmospheric carbon
dioxide produced by the use of fossil fuels’.
‘The climate change phenomenon refers to
seasonal changes over a long period with respect
to the growing accumulation of greenhouse
gases in the atmosphere’.
Two definitions of climate change but one refers
only to carbon dioxide and in reality greenhouse
gases can be water vapour, which acts as a
feedback, carbon dioxide (minor but important),
methane (less abundant but more active),
nitrous oxide (often related to soil cultivation) and
chlorofluorocarbons (industrial but regulated)
Impacts on freshwater
All very complicated and several studies show
many different impacts, so the whole picture
can be quite muddled. Needless to say that the
impacts on freshwater can be very damaging.
A study by Imperial College, London states
that freshwater constitutes only 2.5% of the
total global water stock and only 0.3% of this
water is available in rivers and lakes. This is so
much smaller than the marine water stock but
freshwater is so important for biodiversity and
ecosystem services crucial to humans. The use
of freshwater for aquaculture is on the increase
and this puts more pressure on the ecosystem
but also raises the question of how sustainable it
will be if temperatures rise.
The World Wide Fund for Nature estimates
in their Nature’s Living Planet index that from
the 1970s there has been an 81% decline in
freshwater species populations.
The warming of rivers and lakes will have an
effect on fish species, with cyprinid and percid
fish becoming more dominant and salmonid
species declining to a point where populations
could collapse. This is especially so in shallow
lakes where there is no thermal stratification. In
Ireland, for example, Lough Neagh supports cold
water species, often referred to as glacial relics,
such as Atlantic salmon, Arctic charr and pollan
that could be displaced by non-native species.
In lakes, the higher temperatures will also bring
about increased growth of phytoplankton and
will accelerate growth causing higher levels of
eutrophication. This could see a change in the
distribution of sensitive species by migration
to cooler locations and possibly extinction for
others. Non-native species will probably extend
their range of distribution and this includes fish,
plants and invertebrates. In rivers, species will
move to upstream areas and will upset the fine
balance of species composition.
Wetlands are particularly sensitive habitats
and provide a home to a large range of aquatic
and terrestrial species. Bogs and fens tend to
be peat-based, and as such contribute to carbon
dynamics. Any changes to temperature or
nutrient levels, or in the quantity or quality
of water supply will change the character of
the habitat and this will not only affect
species survival but will alter the benefits to
human communities.
One study carried out in Germany looked at the
effect of carbon dioxide on water fleas. These
are eaten by small fish, which are prey to larger
fish and other species. Any changes to this food
source could influence an entire lake ecosystem.
The fleas can sense chemicals given off by
fish and thereby avoid being preyed on.
The study showed that high levels of carbon
dioxide interfere with the nervous system of the
fleas and blunt their ability to sense predators.
Similarly a study of minnows, by University
of Winnipeg, showed that fish swimming
in water rich in carbon dioxide did not respond
as quickly to alarm signals released by
other minnows as in water with lower levels
of carbon dioxide.
Others issues relate to human health and the
spread of disease-bearing insects, such as
mosquitoes. A spokesperson from the British
Pest Control Association, said: “In terms of the
Asian tiger mosquito, it is particularly important
that this pest does not get a toe-hold in the UK
as it is a species of concern. They can transmit
debilitating and even deadly viruses, including
Chikungunya and Dengue fever.”
There are yet other issues such as flooding and
storms that are increasing and all these can have
a deleterious effect on freshwater ecosystems.
32