There are a number of agents, including algae, blue-green
algae, bacteria, fungi, helminth
worms, protozoa,
and viruses that
are common denizens of pond water.
Typically these cause dermatitis and skin irritation at the point of
infection.
If you work with water that is stagnant or laden with organic material,
you should wash your hands and arms thoroughly after contact with the
water (as when cleaning a fish tank).
If you're in a situation in which you will spend a significant amount
of time with your arms immersed in water, you should wear sturdy, impervious
gloves.
If develop a rash or other lesion on
your hands or arms, be sure to report your exposure to pond water to
your physician.
