Academic journal article Estonian Journal of Ecology

Phosphorus Flux in Lake Peipsi Sensu Stricto, Eastern Europe/fosforivoog Peipsi Jarves

Academic journal article Estonian Journal of Ecology

Phosphorus Flux in Lake Peipsi Sensu Stricto, Eastern Europe/fosforivoog Peipsi Jarves

Article excerpt

INTRODUCTION

The biogeochemistry of phosphorus in lakes is rather complicated and its distribution depends on many environmental factors (Bostrom & Pettersson, 1982; Bostrom et al., 1988; Eckert et al., 1997; Sondergaard et al., 2003) including sediment properties and functioning. Lake sediments are composed of complex aggregations of minerals and organic matter and, depending on the composition and physical-chemical properties of water, they act as either sinks or sources of P in the form of readily diluted phosphates (Reddy et al., 1995; Ramm & Scheps, 1997; Zhou et al., 2001).

Because of the complex mechanisms in the exchange of P between sediments and water, part of P may be released from sediments. Here two mechanisms can be distinguished: (1) chemical and biochemical factors occurring in dysoxic conditions liberate P to the pore water and support its diffusion to the sediment surface, and (2) the mixing event, i.e. transport from bottom layers to the photic zone (Holdren & Armstrong, 1980; Moore et al., 1991; Haggard et al., 2005).

Although there are numerous detailed studies on the P dynamics in relatively shallow lakes (Anderson, 1997; Pettersson, 1998; Jordan & Rippey, 2003; Malmaeus et al., 2006), the processes in large lakes are described inadequately. Still, quite thorough descriptions of the distribution of P in sediments are available for large and shallow Lake Ladoga in Russia (Ignatieva, 1996), Lake Okeechobee in the United States (Brezonik & Engstrom, 1998), and Lake Taihu in China (You et al., 2007).

In order to properly quantify the overall loadings of P in the lake ecosystem there is an utmost need to describe the spatio-temporal patterns of sediment biogeochemistry and assess how these relate to the internal loading. In this paper, we describe the distribution of P fractions in the sediments of the largest part of Lake Peipsi and their contribution to the total P balance in the lake. The previous nutrient balance calculations showed a decrease of the external loading of P since the beginning of the 1990s due to the reorganization of agriculture and industry on the catchment after the collapse of the Soviet Union (Loigu & Leisk, 1996; Iital et al., 2005; Mourad et al., 2006).

STUDY AREA

The submeridionally elongated Lake Peipsi has a surface of 3555 [km.sup.2], maximum length of approximately 150 km, and width of 42 km. Lake Peipsi is a shallow water body with the mean water depth of about 8 m (max 15 m). It consists of three parts named from north to south: Lake Peipsi sense stricto (s.s.), Lake Lammijarv, and Lake Pskov (Pihkva) (Fig. la). The current study focuses on processes in Lake Peipsi s.s. Its central part is 9-11 m deep; the coastal zone is shallow and the rather monotonous floor descends for some kilometres in the eastern and western coasts. The formation and distribution of the structural types of bottom deposits of Lake Peipsi s.s. are determined by hydro-meteorological processes (waves, currents, seasonal water-level fluctuations, lake ice impacts) as well as by lithological and geomorphological factors. Waves in the lake are steep and short and with the wind force of 8 m [s.sup.-1] their height is 60-70 cm. The water of Lake Peipsi s.s. belongs to the hydrocarbonate class, its pH varies around 8.2, the concentration of Si is ca 1 mg [L.sup.-1], and that of Fe 0.12 mg [L.sup.-1]. The lake is classified as a eutrophic waterbody (Kangur & Mols, 2008) with a mean concentration of total P 30 mg [m.sup.-3] and P-P04 12 mg P [m.sup.-3].

[FIGURE 1 OMITTED]

METHODS

Sampling

During the spring/summer seasons of 2006 and 2007 sampling of surface deposits of Lake Peipsi s.s. from a ship was undertaken. Altogether 58 grab samples of a thickness of 5 cm were taken for analysing the lithology (Punning et al., 2008b), polycyclic aromatic hydrocarbons (PAHs) (Punning et al., 2008c), diatoms, and P fractions. …

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