We measure water column total particulate matter (TPM) monthly during open water at the deepest part of the lake. The sample depths are selected based on thermal stratification: we sample top and bottom of the epilimnion, mid thermocline, and top, middle, bottom of the hypolimnion. Using a peristaltic pump and tubing, we pump lake water through preweighed 0.45u membrane filters until the filters clog or until we have pumped 500ml.
Upon return to the lab we store the filters in the freezer until the following week at which time they are dried in a 60°C drying oven for at least 24 hours. We weigh the dried filters on a Sartorius Research analytical balance and calculate particulates (mg/L) from the mass on the filter and the volume of lake water filtered.
Protocol Log: 1986 -- began collecting TPM samples. Filters were weighed in Madison.
2000 -- began weighing filters at Trout Lake Station. A Cahn Model 31 electrobalance is used.
2007 -- began using Sartorius Research analytical balance for filter weighing.
field & laboratory