Aquatic Pest Control
Hydrology and the permit layer
Aquatic work is regulated beyond the label: many applications to waters of the U.S. require coverage under a National Pollutant Discharge Elimination System pesticide general permit, and Maryland also enforces water-quality and wetland protections. Know the site's uses - drinking water intake, irrigation, livestock, swimming, fishing - because labels impose specific water-use restrictions after treatment.
Know the plant
Identify algae (filamentous vs. planktonic), submersed weeds (hydrilla, Eurasian watermilfoil), floating plants (duckweed, watermeal, water lilies), and emergents (cattails, phragmites) - formulations, rates, and timing differ. Copper compounds and endothall for algae; fluridone, 2,4-D, diquat, and aquatic glyphosate for various weeds; triploid grass carp are a biological tool under state permit.
Oxygen and surfactant hazards
The biggest ecological risk is dissolved-oxygen depletion when a large mass of vegetation dies and decays. Treat in strips or sections - commonly one-third to one-half of the infested area - and leave untreated refuges. Surfactants matter: many terrestrial glyphosate formulations include surfactants toxic to fish and invertebrates, so only aquatic-labeled surfactants may be used.
Traps
Treating an entire pond in one pass; ignoring irrigation or drinking-water restrictions; using a terrestrial formulation over water; forgetting that decaying algae can also crash oxygen. Rule of thumb: treat no more than one-third to one-half of a water body at a time, and read the water-use restriction table before you mix.