Just say “No” to didymo: Tracking the distribution and abundance of ‘Rock snot' in Gunpowder Falls, Maryland, USA

In January 2008, anglers observed a ‘strange-looking growth’ in the middle Gunpowder Falls, a well-oxygenated, moderately-enriched trout stream that drains Prettyboy Reservoir in north-central Maryland.  The growth was confirmed to be Didymosphenia geminata (aka ‘Didymo’ or ‘rock snot’) by Maryland Department of Natural Resources (MDNR) staff.  A monthly survey at nine stations was started in July 2008.  A tenth station upstream from Prettyboy Reservoir was added in July 2009.  Didymo presence was confirmed in the river below the reservoir, but not upstream from the reservoir.  Didymo abundance was highest at five stations closest to the reservoir, with peak growth in February through May.  Average daily water temperatures during peak abundance ranged from 3 to 16 C.  Didymo abundance was higher during winter-spring of 2008-2009 compared to 2009-2010, due in part to more scouring and higher flows during 2009-2010.  Effects of Didymo growth on benthic macroinvertebrates are being evaluated.  Didymo was also confirmed in the lower Savidge River in 2009.  MDNR installed six wader wash stations along the middle Gunpowder and 28 stations on other trout waters, in a public education/outreach campaign to prevent Didymo spread.  MDNR is considering a statewide ban on the use felt-soled waders and boots.