Charlotte Price of Wairakei Primary School, asks :-
What is the most common animal in NZ fresh water lakes?
Marc Schallenberg, a Limnologist (Freshwater Scientist) at Otago University's Department of Zoology, responded.
If you were to visit any lake in New Zealand, from ice-covered lakes high in the Southern Alps to sand dune lakes in Northland, you would likely find tiny animals called rotifers living there.
These are the smallest animals in lakes, about the size of a grain of sand. Rotifers, live in the open waters of lakes and are important because they feed on bacteria, algae, and protozoans (single celled creatures) and because they are eaten by larger animals, such as water fleas and some baby (larval) fish.
Most rotifers are attached to surfaces of plants, rocks and mud, but many are also found swimming in the open waters of lakes. I have found up to 20 rotifers living in a teaspoonful of water from Lake Waihola, Otago. That means approximately 27 trillion rotifers were swimming in the waters of the Lake Waihola at the time! Not only are rotifers very common in New Zealand lakes but they are also probably the animals found in highest numbers in most of our lakes.
Not many people know about rotifers because they are so small. You can only see them by looking at drops of lake water using a microscope. There are many different species of rotifers and each tends to prefer certain lake water conditions. Some can live in extreme conditions, such as in extremely cold and salty ponds and lakes in Antarctica. If conditions become too difficult for survival, rotifers can produce and release resting eggs which settle and remain in the lake sediment until conditions improve, when they can hatch and start a new generation of rotifers.