The end of Darwin's nightmare at Lake Victoria?

Date:
June 18, 2021
Source:
University of Liege
Summary:
Lake Victoria, which came under the spotlight in 2004 by the
documentary 'Darwin's nightmare', is not only suffering from the
introduction and commercialization of the Nile perch: A study has
highlighted other worrying phenomena, particularly climatic ones,
which have an equally important impact on the quality of the
lake's waters.



FULL STORY ==========================================================================
Lake Victoria, which came under the spotlight in 2004 by the documentary "Darwin's nightmare," is not only suffering from the introduction and commercialisation of the Nile perch. A study led by researchers from the University of Lie`ge (Belgium) has highlighted other worrying phenomena, particularly climatic ones, which have an equally important impact on
the quality of the lake's waters.


========================================================================== Located in East Africa, just south of the Equator, Lake Victoria is the
source of the Nile and is the largest tropical lake in the world. With a surface area of 68,800 km^2 (twice the size of Belgium), it is considered
to be one of the largest water and fishery resources in East Africa,
supporting more than 47 million people in the three neighbor countries
(Uganda, Tanzania and Kenya).

Lake Victoria is best known to the general public following the
release of the 2004 documentary Darwin's Nightmare, which focuses
on the environmental and social effects of the Nile perch fishing
industry. Voracious predator that can grow up to two metres long and weigh 200kg, the Nile perch is the largest freshwater fish. Its introduction
into Lake Victoria in the 1950s and its population explosion in the
1960s gradually wiped out the native fish species living in the lake,
causing a major ecological disaster. Today, the Nile perch population
remains ubiquitous but has declined slightly due to overfishing, allowing
some species to partially recover.

What is less well known -- and perhaps interacting with the presence of
the Nile perch -- but equally damaging to the ecosystem, is the general
water quality of the lake. "This declined sharply between the 1960s and
the 1990s due to eutrophication, which is caused by increased inputs of nutrients (nitrogen and phosphorus) into the water bodies (rivers and
lakes) as a result of increased human activities in the catchment area (intensive agriculture with fertilisers or domestic wastewater) resulting
from population growth and economic development, explains Alberto Borges,
FNRS Research Director at the Laboratory of Chemical Oceanography from
the University of Liege. " This eutrophication leads to a significant development of micro-algae (phytoplankton). In particular, cyanobacteria, blue-green micro-algae, can be problematic for human health as some
forms are toxic. Moreover, the excess phytoplankton biomass (the organic
matter from these algae) cannot generally be transformed by the rest of
the food web," continues the researcher. This excess remains unused and stagnant at the bottom of the lakes, creating a phenomenon of anoxia,
the absence of oxygen in the bottom waters of the lakes. This leads to
the degradation of the ecosystem. " Since the 1990s, no large-scale study
of the water quality of Lake Victoria had been undertaken. It was within
the framework of the LAVIGAS project -- funded by the FNRS and led by
Alberto Borges -- that a research team was able to study the biomass and composition of phytoplankton as well as the nutrient status of the lake
during three scientific missions (2018 -2019). This study shows that
the phytoplankton biomass has decreased by about seven times compared
to the 1990s," says the researcher, "and that the species composition
has also changed in a subtle way." What seems to be good news for the environment of Lake Victoria may only be so on the surface...

Paradoxically, the quantity of nutrients remained comparable to that of
the 1990s. This paradox can be explained, however, because in addition to nutrients, phytoplankton (like all plants) also need light to grow. In
lakes, the amount of light for phytoplankton obviously depends on the
solar radiation at the surface of the lake, but also on the depth of
the water on which the phytoplankton cells reside. This depth, known
as the mixing layer, depends mainly on the intensity of the wind. If
the wind is intense, the depth of the mixing layer is greater, and the phytoplankton cells spend less time near the surface where the light is
more intense, and do not develop as well," explains Alberto Borges. Our
work shows that current weather conditions are windier than in the 1990s,
so the depth of the mixed layer is greater and phytoplankton growth less intense than in the 1990s." The weaker winds of the 1990s were related to
the prevailing conditions of El Nin~o, a natural oscillation in global
climate that originates from the large-scale atmospheric circulation
over the Pacific Ocean and affects climate worldwide.

This rather complex story shows that the established climate regime in
the Pacific Ocean (El Nin~o) affects the ecology of a lake in Africa,
on the other side of the planet! More specifically, it shows that the
growth of phytoplankton -- and therefore the rest of the food chain --
in large tropical lakes responds to eutrophication in a complex way and
is strongly modulated by climate," says Alberto Borges. "This means that
the current improvement in water quality in Lake Victoria may only be temporary, and that conditions could deteriorate again in the future
if vertical mixing in the lake decreases due to reduced wind intensity
(a new period of prevailing El Nin~o conditions) or due to continued
climate warming.

========================================================================== Story Source: Materials provided by University_of_Liege. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. Loris Deirmendjian, Jean‐Pierre Descy, Cedric Morana, William
Okello, Maya P. Stoyneva‐Ga"rtner, Steven Bouillon, Alberto V.

Borges. Limnological changes in Lake Victoria since the mid‐20
th century. Freshwater Biology, 2021; DOI: 10.1111/fwb.13780 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/06/210618133952.htm

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