Restoring a Saltwater Ecosystem – Studying Midge Larvae Populations

Students at Provincetown High School have been monitoring the salinity and midge larvae density at a habitat known as Pilgrim Lake on the Truro/Provincetown border. This site, also known as East Harbor, was formerly a saltwater estuary and served as part of the town's harbor. It was diked one hundred years ago during the construction of a railroad. Slowly it evolved into a fresh/brackish ecosystem that periodically experienced fish kills due to low dissolved oxygen levels from introduced species such as Carp.The Cape Cod National Seashore, which manages the site, began allowing seawater in last year. Many of the Carp and invasive plants began to die off as the saltwater ecosystem was restored. Coinciding with this, however, was an unusual outbreak in Chironimids (midges) which many attributed to the restoration project. We are trying to demonstrate that the midge population should decline as salinity continues to increase. We measure the salinity and count the density of midge larvae monthly in two sites in the lake with hopes of demonstrating a correlation. We use a refractometer and a simple device consisting of a plunger and PVC pipe to collect sediment samples. The larvae are bright red and easily counted. Back in the lab, midge survival is sharply curtailed above a salinity of 15ppt, which is consistent with our hypothesis. We will continue to monitor this through the next school year.