Didymo, a.k.a. "rock snot" is formally known as Didymo geminata ‑ a diatom which is a type of one‑celled, microscopic algae.  The nearest occurrences to Missouri are in four nothern Arkansan lakes.  Here is a map of its current distribution: http://www.epa.gov/region8/water/didymosphenia/na_dis.map.pdf

How it gets started

It is primarily transported via fishing gear such as boats, canoes, kayaks, waders (especially felt‑soled), trailers, bait buckets, life jackets, etc.  Diving gear is also problematic.

Since it is one‑celled, this microscopic algae can be spread very easily ‑ even in a single drop of water.  Furthermore, Didymo can remain viable for several weeks if kept moist.

Didymo is generally a northern circumpolar species, where it is found in colder, low nutrient, high clarity streams (similar to the Eleven Point and Current Rivers here in Missouri).  However, it is now being found in warmer waters with less clarity.

Impacts

Didymo is not a human health risk, though people in contact with water containing D. geminata might notice itchy eyes or irritated skin from the silica in the algal cells.

Since one picture is worth a thousand words, here is a picture of a Didymo infestation in the Mararoa river in New Zealand

The main adverse effects come from the organism’s ability to smother the riverbed, thus changing the bottom appearance, structure, and food web of a the stream.  Didymo forms dense mats, which can cover up to 100% of the available bed to a depth of 8 inches or more.  These mats can kill aquatic insect life and essentially starve out fish populations in the area.   In addition, ecologically  common macroinvertebrates (such as crayfish) found on the bottoms of well‑oxygenated streams will change to more worm‑like and snail populated communities.

Such reductions of available and preferred food have seen trout populations severely reduced or disappear altogether in some overseas streams. In some parts of the western USA, fisheries have declined by 90% in 2003 and 2004.  There have also been reports of trout and salmon parr absent from traditional rearing areas due to gill irritation or clogging, or to the changes in food availability.

Didymo is also suspected to be the causal agent in weight loss of trout (http://www.hoaff.org/newsletters/NL2007‑04.htm) in Arkansas.  The relative weight of brown trout below Bull Shoals dam in November 2006 was the lowest that has ever been recorded.  Relative weight for both brown and rainbow trout have been falling in Bull Shoals tailwater since 2004, which coincides with the appearance of Didymo.  This decrease in relative weight has been documented in other trout waters around the world where didymo has occurred.  It is believed that blooms of didymo in upper Bull Shoals tailwater have had the predicted detrimental effect on trout forage and trout foraging.  Didymo carpets the bottom of the river and makes it impossible for native macro invertebrates to colonize or survive.  Further, trout cannot forage efficiently in didymo and get no nutritional value from ingesting it.

There have also been reports overseas of the dense mats blocking water intakes, irrigation intakes, and the intake screens on hydroelectric dams.

Prevention/Control

Effective prevention and control techniques should include the cleaning (check‑clean‑dry) of any item that is transported from one water body to another.  For example, in 2004 didymo was discovered in New Zealand, the first time it was found in the southern hemisphere. To restrict its spread, the whole of the South Island of New Zealand was declared a controlled area in December 2005. All items, such as boats, fishing gear, clothing, and vehicles, that have been in a stream, river or lake, must be cleaned before they enter another waterway.

Recent experiments have identified a copper‑based compound that may be an effective and safe weapon against the invasive alga didymo, although further tests are needed to determine its effectiveness in rivers (http://www.niwascience.co.nz/ncabb/abb/2006‑19/copper).

Here is a good synopsis of Didymo: http://www.des.state.nh.us/factsheets/bb/bb-61.htm

Here is the US EPA website information for Didymo: http://www.epa.gov/region8/water/didymosphenia/

Didymo is know from 4 sites in Northern AR, all below major dams.  And, here is a poster I developed for our folks to place at boat launches, etc., esp those of the White River drainage.  Didymo poster

Some more info..........

So far Rocksnot seems to prefer low temperature, oligotrophic waters such as found at the outlets of our major dams (it has not yet been found in impounded waters).  This is why the four sites in Arkansas are in short reaches of rivers below major dams, however, pages 15‑16 of this link (http://www.epa.gov/region8/water/didymosphenia/White%20Paper%20Jan%202007.pdf) suggest that Didymo is adapting to a wider range of freshwater systems, and Figure 9 illustrates the potential area of spread worldwide for Rocksnot.

Thus, Rocksnot may not remained confined to the White River system.  The Eleven Point, Current River, Jacks Fork, and Big Piney also contain low temperature, oligotrophic water.  And, the sites below the dams of Table Rock Lake and Lake of the Ozarks are very similar to the ones in Arkansas.  If Rocksnot gets into one of these systems (or the larger Gasconade, Meramec, and Osage River systems in their entirety) that could be potentially catastrophic.  Also, if Rocksnot adapts to warmer, more nutrient rich waters, then Lake Wappapello (among many) could be affected.

Hopefully, in the Ozarks, Rocksnot won't turn into the monster it has become in New Zealand, but based on current trends with some invasive species (zebra mussel, leafy spurge, spotted knapweed) it is better to assume the worst and be proactive where possible.