Cascade Project at North Temperate Lakes LTER: Nutrients 1991 - 2007

Physical and chemical variables are measured at one central station near the deepest
point of each lake. In most cases these measurements are made in the morning (0800 to 0900).
Vertical profiles are taken at varied depth intervals. Chemical measurements are sometimes
made in a pooled mixed layer sample (PML); sometimes in the epilimnion, metalimnion, and
hypolimnion; and sometimes in vertical profiles. In the latter case, depths for sampling
usually correspond to the surface plus depths of 50percent, 25percent, 10percent, 5percent
and 1percent of surface irradiance. The 1991-1999 chemistry data obtained from the Lachat
auto-analyzer. Like the process data, there are up to seven samples per sampling date due to
Van Dorn collections across a depth interval according to percent irradiance. Voichick and
LeBouton (1994) describe the autoanalyzer procedures in detail. Nutrient samples were sent
to the Cary Institute of Ecosystem Studies for analysis beginning in 2000. The Kjeldahl
method for measuring nitrogen is not used at IES, and so measurements reported from 2000
onwards are Total Nitrogen.