The shores of lakes and banks of streams are great places to observe aquatic phenomena. Many times, unless we are out swimming or in boats, these are the areas we frequent. We walk on trails that skirt the shore, sit on docks and stare across the water, look out the window, drive by on roads. What do you see along the shoreline?

Lines on rocks along the shore

Lines on rocks along the shore are a result of fluctuating water levels, and can be created by several different phenomena.

Algae that live on the surface of the water can adhere to rocks and dry in a line. A wet black zone of algae will also form where the water meets the air.

Bands of bare rock just above the black algae layer are areas where winter ice has scoured the rock. Above the bare scour zone is often a band of moss or lichens, where there is enough water and condensation for the lichen to live but above the zone of ice damage. Lichens grow so slowly that they are a good indicator of how high the ice is pushed in winter.

Pollen that settles on a lake or quiet stream in spring may get left behind on rocks when the water drops in summer. The pollen adheres to rocks as it dries, leaving a semipermanent line.

A white crust on rocks may be leftover diatom shells. Diatoms are a kind of algae with silica in their cells. The hard, white silica may be left behind when the algae die.

Watching changes along the shoreline may lead one to wonder, "What is the 'normal' water level for a lake or river?" Find out more from the DEP.

Fish kills

Fish kills are rarely the result of toxic pollution. Causes include decreased oxygen levels, especially during hot, windless days where nutrient runoff and algae growth have used up oxygen, viral or bacterial infections, and lack of food. Fish can be stranded when water levels drop. Some fish die after migration or spawning (like suckers). Smelts die from moderate stress, such as high temperatures or low oxygen. Winter fishkills can occur when oxygen is used up beneath the ice.

Insect cases

Sometimes, piles of insect cases that wash to shore might appear to be masses of small dead fish. 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, and can be seen floating on the water or piled up on wave-swept shores, where they are sometimes mistaken for fish kills. As exuvia decompose, an oily film sometimes forms on the water surface. Only if there is a real reason to suspect poisoning or contamination should you notify authorities of a fish kill.

You can find dragonfly skins attached to docks, plants, and objects near shore.

Photo courtesy British Dragonfly Society

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