The Wrack Pile at Shellstep Shore
At Shellstep Shore, waves leave piles of dead eelgrass beside rocky tide pools. These piles are called wrack. A science club studies the wrack without collecting live animals. Nearby, living eelgrass grows rooted in wet sand under shallow seawater. Unlike seaweed, eelgrass is a flowering plant.
The club's field guide explains that eelgrass takes in water and carbon dioxide dissolved in seawater. Carbon dioxide can move between air and seawater. Water and carbon dioxide provide most of the matter used to build new eelgrass. Roots also take in small amounts of dissolved minerals from wet sand. Sunlight supplies energy for growth, but sunlight is not matter.
During several visits, students watch shore hoppers nibble dead eelgrass. These small animals shelter inside the damp piles. Sandpipers search the piles and swallow shore hoppers. Some matter from an animal's food becomes part of its growing body. Some leaves its body as waste. Animals also release carbon dioxide into their surroundings.
The students find fungal threads on dead eelgrass. A scientist's close-up observations show bacteria breaking down shore-hopper waste. Fungi and bacteria are decomposers. They can break down dead plants, dead animals, and animal waste. Some matter becomes part of the decomposers' bodies. As decomposers work, they also release carbon dioxide and dissolved minerals into their surroundings. Tides move water and dissolved materials between the wrack, tide pools, and eelgrass beds.
To observe changes over time, the club puts 20 grams of air-dried dead eelgrass in a mesh pouch. The pouch stays at one marked, damp location for four weeks. Its mesh keeps shore hoppers out but allows seawater, microbes, dissolved materials, and tiny plant fragments through. Students record fungal threads growing on the eelgrass. After four weeks, they air-dry the remaining eelgrass pieces using the same drying method. The pieces have a mass of 12 grams. The students measured the dry eelgrass pieces, not everything that passed through the pouch or grew inside it.
For their final model, students prepare labels for living eelgrass, dead eelgrass, shore hoppers, sandpipers, decomposers, and the nonliving surroundings. They must decide which arrows to draw, how to label them, and whether their model leaves any matter pathways out.