What's that floating on the water surface?

Parallel lines on the water surface

A. Thurnherr photo
See more of Andreas Thurnherr's Langmuir photos

On a windy day, lines of foam or debris may form along the length of a lake, reservoir, or river. These lines are called windrows or Langmuir streaks, after the first person to study the phenomenon. Irving Langmuir noticed patterns of floating seaweed when crossing the Atlantic Ocean in 1938. Intrigued, he conducted experiments in a lake and discovered that wind can cause water to circulate in a pattern that makes material collect in lines on the surface. The lines are roughly parallel to the wind direction, and the windier it is, the further apart the lines.

Pollen

C. Smith photo

C. Smith photo

After becoming water-logged, the pollen sinks to the bottom or may collect in coves along the shore.

Lines of pollen may be left on rocks as water levels drop in early summer.

Oily sheens

An oily sheen that reminds you of rainbow puddles in an asphalt parking lot might be from spilled petroleum. A spill of just one gallon of oil is enough to form a film across the surface of a four-acre lake.

But what about when you are in the woods far from any roads, or walking along the undeveloped shore of a lake? Oily sheens can also come from natural sources. Some bacteria (Leptothrix discophora) that live in waterlogged places get their energy from iron and manganese, and as these harmless bacteria grow and decompose, the iron may appear oily or form red or orange films, fluffs, and coatings. Leptothrix can also excrete manganese, which looks like black slime.

How to tell the difference between petroleum spills and natural oil sheens? Poke the sheen with a stick. If the sheen swirls back together immediately, it's petroleum. If the sheen breaks apart and does not flow back together, it is from bacteria or other natural source.

More information on bacteria and oily sheens from the U.S. Geological Survey

The breakdown of organic matter (plant and animal material) also can leave an oily sheen on the water surface. In the spring and summer, a dark cloud in the water accompanied by an oily sheen could be the outer skins of insect cases left behind from a hatch of aquatic insects. The larvae of mayflies and some other aquatic insects molt and shed their skins as they leave the water and become flying adults. The skins are called exuvia. Exuvia can be seen floating on the water or can accumulate on wave-swept shores, where they are sometimes mistaken for fish kills. You can find dragonfly skins attached to docks, plants, and objects near shore. As exuvia decompose, an oily film sometimes forms on the water surface. A diatom bloom can also leave oil behind as the algal cells die.

Floating green stuff

Fuzzy, green floating dots on lakes and in the top few feet of water, or tiny tapioca-like balls might be an alga (microscopic plant) called Gleotrichia echinulata . Gleotrichia (glee-oh-trick-ee-ah) usually appear mid-summer for brief periods, but can persist longer in some lakes. The presence of Gleotrichia does not necessarily indicate poor water quality since it is commonly present in Maine lakes that have good water clarity. Wind and currents can concentrate them in one part of the lake and high densities can collect in coves.

Gleotrichia
R. Hill Photo

Don't mistake floating plants like duckweed and water meal for algae. Duckweed ( Lemna spp.) look like miniature lily pads, with a flat, round floating leaf and a tiny root. Water meal ( Wolffia spp.) also floats but does not have a root, it is a round grain-like plant, about the size of a poppy seed.

More about Gleotrichia from Maine COLA

Gleotrichia in Belgrade Lakes

Green or bluish-green scum or film on the surface of a lake, pond, or stream might be a bloom of blue-green algae (not really algae but a group of organisms called cyanobacteria). Lots of algae can also color the water green.

Should you be concerned about algae?

The presence of algae in a lake or stream does not mean the water is polluted. A diverse community of algae is healthy. Algae are an important source of food and oxygen for other plants and animals in the water.

Photo of an algae bloom by W. Garland

Sometimes, certain conditions might favor a species that is normally rare in a lake or stream. With the right temperature, light, and nutrients in the water, the rare organism might multiply rapidly, forming a bloom. When an algae bloom is persistent or occurs routinely, too many nutrients may be entering the water. Nutrients (especially phosphorus) fertilize a lake just as they fertilize your lawn or garden, causing microscopic plants in the lake to grow.

If you suspect an algae bloom, contact the DEP Bureau of Land and Water Quality 287-3901. For more information, visit DEP's algae bloom page.

Algae links:

Unidentified Floating Objects

Foam is often seen along lake shores and on streams and rivers. Most foam is natural and does not indicate pollution. Foam forms when water is mixed with air, such as by a waterfall or waves breaking against shore. Plants and animals release organic compounds as they decompose, and these compounds lessen the surface tension of water and create bubbles.

More about foam from ME DEP

Foam on the Great Lakes

Foam in Canada

Foam on New Hampshire lakes