DOI: 10.6073/pasta/2a89928cd59a07e13b868169761d5e63
title: North Temperate Lakes LTER: Primary Production - Trout Lake Area 1986 - 2007

description: Crystal, Sparkling and Trout lakes are sampled approximately fortnightly throughout the ice free period and water is returned to the lab for 3 hour in vivo incubations at ambient temperature. On an annual rotation, one of the lakes is sampled to the bottom of the photic zone (ie. 0.5% of surface light) while only the epilimnion of the other 2 lakes is sampled. All samples are integrated by thermal layer and 14C uptake is deternimed at 10 light levels using a metal-halide lamp. All samples are acidified and bubbled before liquid scintillation counting and uptake is dark bottle corrected. DIC and chlorophyll are also measured. Each P-I (photosynthetic uptake vs irradiance ) set is further reduced to 3 parameters (alpha, beta, and Pmax) based on the work of Platt 1980. In addition, thermal information and lake light transparency profiles (collected approximately fortnightly on these three LTER lakes) along with daily incident PAR at 30 minute intervals (measured at Noble F. Lee Municipal Airport, Woodruff, WI) provide input to a mechanistic model of lake primary productivity. Primary productivity for a lake is calculated from productivity parameters derived from the laboratory uptake experiments, coupled with the lakes' thermal and light regimes. 1. PI parameters: Productivity (P), as a function of irradiance (I), is described by a hyperbolic tangent curve with three parameters (Platt 1980). The three parameters describe the initial slope of the curve, the I at which maximum P occurs, and the decay rate of P, following maximum P. All three parameters are fit simultaneously to laboratory observations, using minimization of a least squares objective function (Matlab v. 6, Mathworks, Inc.). The parameters allow for the calculation of P, as a function of I, for a given thermal stratum of a given lake on a given day. 2. LEC: Light extinction coefficients (LEC) for each lake are calculated as the slope of the best fit line through the natural logs of observations of light at depth. The light at depth data were obtained from the LTER core data set (light profiles are sampled approximately fortnightly throughout the ice-free period on LTER lakes 3. Thermal layers: The thermal layers for the lake are obtained from the LTER core data set (temperature profiles are sampled approximately fortnightly throughout the ice-free period on LTER lakes). 4. Lake productivity: Productivity for a given lake on a given day is calculated from the derived data in steps 1-3. Light versus depth is discretized for 0.5 m depth intervals to match the lake hypsometric data. Light for each 0.5 m disc is calculated for every 30 minutes from observed irradiance and LEC, using numerical integration through the depth of the disc. Productivity for every 30 minutes is calculated for a disc, using the light data applied to the parameters and equation described in #1. Productivity for every 30 minutes in the thermal layer is the sum of the disc productivities for that layer. Productivity for every 30 minutes in the lake is the sum of the productivity of the thermal layers. Productivity for the day is the sum of all 30 minute productivities for a given calendar day. Hypsometrically weighted productivity is calculated by weighting each of the discretized productivities by the hypsometric weight of that disc. 5. Interpolation: Rarely do all observations required to calculate productivity occur on the same day. PI parameters, LECs, and thermal layers were linearly interpolated to daily values. For any given day's productivity calculation, all data, other than observed irradiance, were considered constant Sampling Frequency: water samples are taken fortnightly; PAR measured every 30 minutes Number of sites: 3