Estimation of the Phytoplankton Productivity in Three Estonian Lakes/ Futoplanktoni Produktiivsuse Hindamine Kolmes Eesti Jarves

INTRODUCTION Light provides the energy necessary for the transformation of inorganic matter into organic matter by the planktonic algae and all other photoautotrophic plants. Primary production is the direct product of photosynthesis, and primary productivity is the sum of all photosynthetic rates in an ecosystem (Fee, 1998). Information on the primary production enables to improve the understanding of food web relationships in aquatic ecosystems. Because of changing light conditions, primary production has a pronounced diel pattern. In order to acquire integrated results over longer time periods (days, months, years), many consecutive measurements of instantaneous photosynthesis rate should be carried out and integrated. In some studies (Joniak et al., 2003; Yoshida et al., 2003; Forget et al., 2007) the values of daily primary production integrated over the photic zone were estimated from in situ incubations. However, such approach gives reliable results only in clear waters, while in highly productive waters incubation cannot be performed during a long period (e.g. from morning to evening) as part of the [sup.14]C-label gets lost from the cells during long-term incubation due to respiration of photosynthetic products (Lancelot & Mathot, 1986) and release of extracellular products (holler Jensen, 1985). Bio-optical model calculations could provide an alternative to the time-consuming 14C method. Several studies estimate primary production from light intensity and abundance of phytoplankton pigments (cited in Arst et al., 2008a). In cases the processes of interest occur on a longer time-scale, it is common to ignore diurnal variations and use mean daily photosynthetically available irradiance (PAR) to force models of primary producers. Widely used methods include those where the dependence of photosynthesis on available light is expressed by an equation containing two parameters: the initial slope [[alpha].sup.[[beta] and the assimilation number [P.sup.B.sub.m] (Sathyendranath et al., 1989). In these models also data on the vertical profiles of the photosynthetically active radiation (PAR) (Einst [h.sup.-1] [m.sup.-2]) are needed. This is a rather complicated way and gives results on the basis of radiation integrated over the PAR region. For reliable description of primary production profiles, however, it is preferable to use a 'spectral approach', in which the model is based on spectral data of underwater quantum irradiance and absorption coefficients of phytoplankton (Sathyendranath et al., 1989; Smith et al., 1989; Schofield et al., 1990; Kyewalyanga et al., 1992; Kirk, 1994; Sosik, 1996; Arst et al., 2006, 2008a). Two versions (spectral and integral) of a semi-empirical model for calculation of the vertical profiles of primary production in lakes were elaborated by Arst et al. (2008a). The main difference between the models resides in the data on underwater irradiance (spectral or integral). Quantification of these models was performed using the data of in situ measurements of bio-optical parameters in three turbid Estonian lakes (Peipsi, Vortsjarv, and Harku) in 2003-2005. The objectives of the present study were (1) to demonstrate the variability of underwater quantum irradiance by calculating its spectral and diurnal variations at different depths of three lakes for all measurement days; (2) to compare in situ primary production measurement results obtained in 2007 2008 with corresponding model results; and (3) using model calculations to estimate the diurnal variability of the vertical profiles of primary production as well as the corresponding integrated (over water column) values. MATERIALS AND METHODS Study sites The measurements were carried out in three Estonian lakes: Peipsi, Vortsjarv, and Harku. Morphometric data and other parameters of the lakes are shown in Table 1. Lake Peipsi is a large shallow lake on the border of Estonia and Russia. …