Slime algae make lakes clearer

Posted on July 1, 2013

Shallow lakes often show alternating regime shifts between clear-water and turbid-water states.  Because these changes in water clarity have wide-ranging impacts on the structure and function of the aquatic ecosystem, a knowledge of the underlying processes can be useful in lake management.   For example, removing fish from lakes can release predation pressure on zooplankton, which allows zooplankton populations to increase.  This strengthens the grazing pressure on phytoplankton, which increases light penetration and encourages the growth of larger plants (macrophytes).  Macrophytes help to perpetuate the clear-water state by providing refuge for zooplankton and reducing the resuspension of bottom sediment.  Although research has confirmed the macrophyte – clear water connection,  it’s been observed that lakes without macrophytes can also switch to the clear water state.  This finding has prompted researchers to investigate whether clear-water can be maintained solely by processes involving periphyton (films of algae and bacteria on underwater surfaces).  To do this they developed a new state transition model and tested it with data from a Danish lake (LakeEngelsholm) and elsewhere.  The model describes dynamic changes in the density of phytoplankton and the concentration of phosphorus.  Model simulations showed that periphyton can indeed maintain clear-water conditions in lakes, regardless of macrophytes.  The crucial mechanism turned out to be the ability of periphyton to regulate lake phosphorus levels by taking up phosphorus from the sediment and by oxidising the sediment surface through the action of photosynthesis.  The growth of periphyton depended in turn on the amount of light reaching the lake bottom.  In the case of LakeEngelsholm, when light levels at the bottom were greater than 13% of those at the surface,  phosphorus was bound to periphyton and its release to the water was effectively zero.  Clear-lake control by periphyton is likely to be most strongest in shallow lakes where levels of nutrients and conductivity are low. 

Reference:   Genkai-Kato, M., Vadeboncoeur, Y., Liboriussen, L. & Jeppesen, E.  2012.  Benthic–planktonic coupling, regime shifts, and whole-lake primary production in shallow lakes.  Ecology 93(3), 619–631.