Fish, Lettuce and Bacteria: Modeling a Greenhouse Aquaponics Loop

Fish Under the Lettuce

1The Greenhouse Crew at Alder Ridge Middle School runs an aquaponics system: a way of raising fish and plants together in one loop of water. A 400 L tank holds six young tilapia. A small pump lifts tank water into two shallow beds of gravel where lettuce grows, and the water trickles through the gravel and back down into the tank. Once a month, the crew harvests the lettuce for the cafeteria salad bar.

2Club advisor Mr. Ignacio Ferraro asked seventh grader Tomasz Kowal to model the loop. The lettuce uses light energy to make sugars from carbon dioxide and water, and its roots take up nitrate and other minerals from the water flowing past them. The tilapia eat food pellets made from soybeans and fish meal. The fish give off a waste called ammonia through their gills and in their droppings, and too much ammonia can harm fish. Red worms, fungi and many kinds of bacteria in the gravel break down droppings, uneaten pellets and dead roots, giving off carbon dioxide and more ammonia. Other bacteria, most of them living on the gravel, change ammonia into nitrate.

3When the crew built a second, brand-new system, Tomasz tested its water once a week (Table 1). The gravel was freshly washed, the six fish were fed the same amount each day, and no plants were added until week 6.

4The next term, the crew ran the two systems side by side for 4 weeks (Table 2). Each tank had six tilapia, water at 26 °C and 25 g of pellets a day. System A had 24 lettuce seedlings in its gravel. System B had gravel only. Both sat under the same greenhouse glass, with grow lights on cloudy days.

5Mr. Ferraro pointed out two things that leave the loop. Each harvest carries lettuce out of the greenhouse, and heat from the fish, the plants and the busy decomposers escapes through the glass into the winter air.