Darkness Is a Resource

Sep 10, 2019

It’s the second Tuesday of the month, which is when we hear from our commentators in the field.

This week, Gretchen Gerrish of UW-Madison’s Trout Lake Station tells us about darkness as a resource.

Sitting on a boat in the middle of Crystal Lake under a full moon, I can see a few stars around the sky’s perimeter, clearly see the outline of trees, differentiate the light-colored concrete of the boat landing and see the details of my student intern’s face as she rows us out to the deep spot to collect a series of water samples. Fast-forward two weeks to the night of the new-moon, and it is dark…really dark. The night sky of the Northwoods is beautiful with clear constellations readily visible above the tall tree line and the milky way visible as a smear of starlight across the center. Once my eyes adjust, I can still make out the tree line surrounding the lake and the silhouette of my student rowing to the center of the lake. As we move farther from shore the contrasting concrete of the boat landing is no longer visible. We will use the distant red lights of cell towers and a visible dip in the tree line to navigate back to our truck once we finish sampling.

Our goal is to test how twilight and moonlight impact the behavior of microscopic organisms called zooplankton. These tiny lake-dwelling invertebrates are the food-web link between algae and fish. Each day, some types of zooplankton move down in the water column to avoid being eaten by fish and then move up at night to feed on all the algae that has grown in surface waters throughout the day. The feeding behaviors of some fish can be linked to the migration of the zooplankton and these migrations help explain why fishing is good at dawn and dusk.

Light is the main cue that initiates migration behaviors. But…’light and dark’ are two extremes and are an over-simplification when it comes to explaining our natural environment.

Light sensitive pigments and sensory receptors occur through all lineages of life and can detect many different wavelengths and intensities of light. Plants like sunflowers, use light-detecting chemicals to orient their daily dance as they shift to follow the position of the sun. When fungi living underground sense light they realize they are close to the surface so they grow mushrooms and produce spores to disperse. Eyes in animals range from simple, just able to sense daytime and nighttime, to complex, like our eyes with the ability to focus on multiple colors and shades of light to form three dimensional images.

Many animals can recognize different wavelengths of light. Humans use special pigments and structures in our eyes to perceive wavelengths as different colors. Some animals ‘see’ in wavelengths we can only imagine. Bees live in a world where red and purple flowers are almost invisible but UV pigments in the central ‘nectar guides’ of bee-pollinated flowers stand out and allow the pollinator and specific plants to connect more often. Recent studies in aquatic systems show that numerous aquatic creatures can see polarized light in ways we cannot. Polarized light occurs when light bounces off water or angles through water. Polarized light is greatest at morning or evening when the sun is most severely angled with the water surface. Some zooplankton show stronger migration responses under polarized light conditions and some fish use polarized light coming from the water surface to camouflage their outline.

The light environment of a lake is highly dynamic during dawn and dusk periods. Imagine an intensely sunny, high UV, bright day transitioning into dusk with a lower intensity angled sun creating polarized light. Finish with a full moon illuminating the lake surface. It is the impact of these changing light periods on the organisms dwelling just under the water surface that we are starting to explore in local lakes.

Why does this matter? New data derived from nighttime satellite images estimate that over 80% of the world’s population lives in regions where visualization of the Milky Way is impacted by artificial lighting. The same study esimates that 40% of the world’s children have never even seen the Milky Way. How lucky are our children here in the Northwoods! With that said, as we conclude sampling on Crystal Lake and orient back to the landing using the cell tower lights, we can readily see the glow-domes of three nearby towns.

Light pollution may not be the most pressing issue for our natural environment but there are easy local solutions that can help minimize its impact. We can start by recognizing that darkness is a resource important for the health of our natural ecosystems.

For Field Notes, this is Gretchen Gerrish of UW-Madison's Trout Lake Station.

The above photo belongs to Flickr user turn off your computer and go outside and can be found online here.