Algae and hippo dung a threat to fish in water bodies

Freshwater bodies like Lake Victoria could be unable to sustain fish as humans empty their waste into the water, leading to algae growth that depletes oxygen for marine life.

Scientists attending a three-day symposium on water quality in Kisii said that Lake Victoria’s water is increasingly becoming an uncontrollable breeding ground for algae.

Algal bloom – when microscopic algae breed and grow uncontrollably – makes the lake’s water toxic to humans, fish and other organisms. Algae discolours the water, turning it green. Increased algae has been linked to discharge of untreated sewage and increased fertiliser usage on farms. The latter is washed into the lake when it rains.

“These are more than environmental issues; they encompass food, security, employment and survival,” said Kisii University Vice Chancellor John Akama while opening the symposium.

Hippo waste in the Mara River also causes low oxygen conditions that kill fish, according to new research published in Nature Communications.

“The Mara’s resident hippos add about 8,500 kg of partially digested plant material into the river each day. We were interested in how this massive influx of organic matter and nutrients influenced aquatic life,” said one of the authors Emma Rosi, a freshwater ecologist. Researchers monitored water chemistry and flow downstream from 171 hippo pools.

When rain increased river flow to twice its normal rate, dissolved oxygen concentrations declined, falling low enough to cause fish deaths.

“During dry periods, oxygen-poor water accumulates in hippo pools which contain hippo waste that decomposes and depletes oxygen. Periodic intense rains flush the pool water downstream. This sudden pulse of deoxygenated water can cause temporary hypoxia and fish kills,” noted Rosi.

When low-oxygen water from hippo pools mixes with river water, it reduces the river’s overall dissolved oxygen concentration. At the same time, hippo pool sediments washed out during flushing flows continue to use up oxygen as they decompose downstream. Researchers say that altering a river’s flow through dams or water withdrawals could have ecological consequences in rivers with hippos.

“In the Mara River system, flushing flows are important for cleaning hippo waste out of pools, but the accumulated toxic chemicals and deoxygenated water have severe impacts on aquatic life downstream,” said co-author David Post. The researchers, however, observed that the dead fish are a source of food for birds and crocodiles.