Pond in Wright County #151232 - Ask Extension


Pond in Wright County #151232

Asked September 05, 2013, 1:26 PM EDT

We have an approximately 2 acre pond in north central Iowa. The pond has been getting progressively greener throughout the summer, and now, suddenly it seems, it has developed a blue green almost moldy looking scum across the top of the pond. If you disturb the scum, the water underneath is clear. I will attempt to add a picture of the algae or scum. Can we do anything to rid the pond of this nasty looking substance? Is it healthy for the fish we stocked this summer? The turtles and frogs don't seem to be too concerned, but it does look nasty. Thank you for your time, Cindy Anderson

Wright County Iowa

Expert Response

Greetings anonymous,

I am not based in Iowa, so be certain to check applicable regulations in your state before taking management action.  This latest state of your algae bloom is concerning, looking potentially like Microcystin or some other blue-green algae/cyanobacteria.  Unfortunately, many species of blue-green algae have the potential to produce toxic substances.  Please note this general fact sheet I coauthored in service to Ohioans:


Do not panic, but do treat this substance with a healthy respect.  I would recommend avoiding human contact--certainly do not use the pond as a domestic water supply in its present state--and restricting access by pets and livestock unless you can determine that cyanotoxins are not present.  One potential issue with an extreme bloom regarding fish is the oxygen demand such a bloom imposes at night/under darkness.  Oxygen crashes have real potential to cause fish kills.

Blue-green algae react similarly to true algae to chemical treatments, most commonly formulae of copper (again, check your state regulations before any chemical applications).  However, the presence of such blooms is almost always indicative of nutrient issues, usually high phosphorus and/or a low nitrogen:phosphorus ratio.  A nutrient management program to prevent/minimize future blooms would likely take some time to have positive effects, but could pay off in the long term.  Can you identify any potential sources of phosphorus to or within the pond?  Are you taking any action to manage nutrients at present?


Eugene Braig Replied September 08, 2013, 11:26 PM EDT
Thanks for responding. I haven't noticed any fish kill so I'm thinking we are okay there. The wind came up and the algae is gone. The water is still green, but the blue nasty looking stuff is gone. Does it just go away?
This pond has no drainage into it. It is spring fed and I guess there is some run off into the pond on the east side from out lawn and from gutters being drained into the pond. My husband dredged the pond over the past two  years and this is the first summer, fall we have used it since the job was finished. He built dikes up around the perimeter to prevent the surrounding creeks to overflow into the pond.
We did put some crystals in the pond earlier this summer to kill some weeds growing up through the water. I'm thinking these crystals had something to do with the algae explosion. We got the crystals from the fellow we bought our fish with which we stocked the pond. He has a degree from Iowa State in this sort of thing. The heat and drought haven't helped.I would like to swim around a bit, but am hesitant. Is it dangerous just where the blue-green scum is or wherever the green tiny stuff is floating? This is a picture I took this morning, same place in pond.
Thanks for your help.
Cindy Anderson
The Question Asker Replied September 09, 2013, 10:25 AM EDT
Greetings Cindy,

At least superficially, this image very strongly resembles a Microcystis bloom, one of the genera of harmful algal bloom (HAB)-forming cyanobacteria.  Your description of its dispersal following a wind would seem to agree: Microcystis has gas vacuoles within its cells and can vary its floatation to optimize the light conditions to which it is exposed, periodically dispersed through the water column and periodically rising to form streaky scums on the surface.  I think those surface scums look like somebody spilled bright green, opaque poster paint on the water surface.  When the wind blows a bit strongly, the small, green, globular colonies often disperse into the water column.  If you look closely, you can probably still see small green globs scattered beneath the surface.

HABs don't always produce toxins.  If they do, toxins are likely their most concentrated in concentrated visible scums.  However, the toxins can be dissolved and scattered throughout the pond.  The World Health Organization (WHO) guidelines state that microcystin (a liver toxin) concentrations of only 4 - 20 ppb (that's "parts per billion") pose a moderate risk to adult human health from recreational contact; higher concentrations represent even higher risk.  Unless you get your water tested and discover otherwise, I would recommend you approach contact with the pond water as a potential health risk.

The presence of cyanobacteria in densities sufficient to be considered a HAB is almost always a symptom of a problem with excessive nutrients available in the water.
  Depending on the timing of chemical applications, suppressing the growth of some green things can remove competition for nutrients and allow an opportunistic bloom of truly nuisant things like HABs or filamentous algae.  Do you know what those "crystals" were that you used as treatment?
Eugene Braig Replied September 11, 2013, 4:50 PM EDT

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