Shade and Oxygen - Sentinel Floating Islands

Pattee Creek Retaining Pond in February

     "Cold and short" is what students recalled about their visit to the floating islands in Pattee Creek retaining pond. These students have been studying wetland health in science classes at Sentinel High School through a Floating Islands watershed health curriculum taught by Sylvia Doyle.


     A healthy wetland, defined by the students, depends on vegetation. Plants have roots which "hold the bank in," "slow the current," and "shade the algae, so less grows." The plants need light, of course. There should be animals, whose feces and dead bodies provide fertilizer for the soil. Decomposers must be around to break down the dead plants and animals. The habitat type is important, too: "It's gotta have a rocky bottom for fish to lay their eggs, and for bugs."


     Another key aspect of healthy wetlands is dissolved oxygen.

     "Our fishermen and fisherwomen know dissolved oxygen is important, like for trout. Our fish are cold water species, not just because they need cold water but because it means there's more dissolved oxygen." Colder water can hold more oxygen than warmer water.

     "So it's really good to have shade on water," a student observed.

     I thought of Stream Team expeditions I'd gone on in the past, where I dunked my hands again and again into freezing water. My hands would turn pink and then white, but I knew the cold was necessary for the river's health.

     "When you see a white cap in a river or lake, that means the water is completely saturated, over 100%, so the bubbles can't mix in," Sylvia pointed out. The water already has so much oxygen dissolved that it can't take any more. She was referring to river rapids or waves formed by wind, but the image of ocean waves crashing on the shore sprang to my mind.

     Shade not only cools the water but prevents the overgrowth of algae. Too much algae limits the amount of oxygen that the water can contain, and encourages bacterial growth.

     "Bacteria can turn nitrogen gas into soluble form," Sylvia said. Some amount of nitrogen enters the water from gas exchange with the air, but bacteria facilitate this process. The students knew nitrogen is important for the ecosystem and that it can come from decomposing matter, but too much nitrogen makes it hard for organisms to breathe. This is also why fertilizer in runoff can be a problem for wetlands, as it adds more nitrogen than the system is accustomed to.

     Even though Sylvia only brings her lessons to the class once per month, the students remember many bits and pieces of what wetlands need to stay healthy and work together to put the big picture together.

-Cassie Sevigny
AmeriCorps Team Member
Media Coordinator