In situ measurements and model calculations of primary production in turbid waters

Data on the productivity of aquatic ecosystems helps to understand the food web relationships and functioning of these ecosystems. A semi-empirical model for calculating the phytoplankton primary production in turbid waters was elaborated. In situ measurements of the necessary bio-optical parameters were collected in 3 turbid Estonian lakes from 2003 to 2005. Secchi depth, chlorophyll a, and integrated primary production ranged from 0.1 to 3 m, 4.2 to 389 mg m(-3), and 17 to 435 mg C m(-2) h(-1), respectively. Two model versions (spectral and integral) were developed for calculating the vertical profiles of primary production, P(z). The basic equation described P(z) as a function of photosynthetically absorbed radiation and quantum yield of carbon fixation. The main difference between the models resides in the data on underwater irradiance. We also derived a new algorithm for calculating the vertical profiles of the quantum yield. According to the statistical parameters the preferable model is the spectral model, but the integral model can also be recommended. The quality of the integral model was increased by changing the coefficients in the quantum yield algorithm, by which we compensated for the errors in photosynthetically absorbed radiation in this model. We conclude that the spectral and integral models perform well in computing primary production in turbid lakes. Both models allowed estimation of the primary production profiles and integrated production of the water column, as well as the daily dynamics and daily and monthly totals.