Many protozoa and even single celled plants with a cell wall, employ contractile vacuoles as their means of osmo-regulation. In general, a fresh water environment is hypotonic to the inside-cell environment (cytoplasm) and so water flows into the cell and must be removed again to prevent the cell rupturing.
Here is a good summary of what this is all about, from Wikipedia: https://en.wikipedia.org/wiki/Contractile_vacuole
From Roberts & King, Biology A Functional Approach Students’ Manual
From Clegg & Mackean, Biology Principles and Applications Study Guide
From University of British Columbia
HOW TO MAKE A HAY INFUSION and COLLECT PROTOZOA
GRAPHING QUESTION – COTHURNIA
This is a neat little question which shows how contractile vacuole activity changes in a protozoa which is subjected to different water dilutions.
Something which fascinates me is that many protozoa can inhabit fresh and salt water environments, meaning contractile vacuole activity is able to adjust to different osmotic flows of water. Yet there is also a suggestion that the organisms eventually adjust the solute concentration in their cytoplasm in order to maintain CV activity at more or less a constant speed. How long does this cytoplasmic adjustment take to kick in? Or maybe the speed of CV activity is maintained but volume of water expelled is reduced or increased?