A combination of light, temperature, oxygen and other abiotic variables, food resources, competition and predation (both by fish and invertebrates) affects the seasonal vertical structure of freshwater zooplankton. Many reviews and investigations emphasize this (e.g. Stich & Lampert, 1981; Primicerio & Klemetsen, 1999; Primicerio, 2000; Kehayias et al., 2004; Kessler & Lampert, 2004; Pinel-Alloul et al., 2004; Ka et al., 2006; Trifonova & Makartseva, 2006; Adamczuk, 2009; Doulka & Kehayias, 2011).
Seasonal stratification is considered to be the primary factor affecting zooplankton species composition and biomass succession (Ortega-Mayagoitia et al., 2000; Ringelberg, 2010). Variation in physical and chemical conditions impacts zooplankton both at population and individual levels, providing unique, species-specific and predictably changing (see Hutchinson, 1957), but not necessarily stable niches. Besides the constantly changing environment due to seasonality some environmental conditions can become more influential than others. For example, competition for food resources is more important than predator avoidance in the vertical habitat segregation of cladocerans (Adamczuk, 2009). Time of starvation and ability to store resources may affect an individual's success in variable environments (Kirk, 2002). In fresh waters many zooplankton species are competing for limited resources (Hebert, 1982), and the outcome for coexistence among species is largely dependent on the fluctuations of biotic and abiotic factors and vertical segregation (Jacobs, 1977; Primicerio, 2000). The complexity of zooplankton species is very dynamic between different biotopes within a lake, with no clear delineations due to ecological conditions as one species complex is replaced by another (Line, 1966). Typically the zooplankton vertical distribution is shown to be more uniform before stratification and more differentiated during the summer stratification time (Primicerio, 2000; Hudcovicova & Vranovsky, 2006; Zadereev & Tolomeyev, 2007; Doulka & Kehayias, 2011).
About 1.5% of the area of Latvia is occupied by lakes (Glazaceva, 2004). Most of the Latvian lakes are shallow and eutrophic (70% of the lakes have an average depth of 1-5 m). Only seven Latvian lakes are deeper than 40 m and like shallow lakes tend to become shallower due to eutrophication and sedimentation impacts (Klavins et al., 2002; Glazaceva, 2004).
Dridzis is the deepest lake in Latvia (and the Baltic States). It is of great ecological importance like lakes Rica and Svente as they are located in protected natural parks and are also included in NATURA 2000 (network of protected areas in the European Union). Lake Rica is located in the border area of Latvia and Belarus and is protected in both countries. In accordance with regulations of the Cabinet of Ministers of the Republic of Latvia No. 118 'On surface and groundwater quality' of 12.03.2002 four of the investigated lakes (Dridzis, Svente, Rica, and Geranimova) are categorized as priority fish lakes for salmonid fish species. Therefore the current assessment of zooplankton, which is an important fish prey, is of particular interest.
Latvian freshwater zooplankton studies have so far mostly focused on upper water layers. Little is known about the seasonal vertical distribution of zooplankton in deep Latvian lakes. The deepest parts of the hypolimnion of the lakes have begun to be explored only recently. The aim of this study was therefore to describe patterns of the vertical dynamics of the zooplankton population and species interactions in the late summer to early autumn period in relation to environmental variables from five deep lakes in Latvia (Dridzis, Svente, Geranimova, Rica, and Garais). We also examined changes in zooplankton populations over the last 50 years where relevant data were available.
MATERIAL AND METHODS
The study was carried out in five lakes situated in the southeastern part of Latvia (Fig. …