Spatial Distribution of Macrofossil Assemblages in Surface Sediments of Two Small Lakes in Estonia/Makrojaanuste Ruumilisest Jaotusest Kahe Eesti Vaikejarve Pindmistes Setetes

Article excerpt

INTRODUCTION

Macrophytes play an important role in the lake ecosystem as they are the primary source of organic matter to fresh waters (Wetzel, 1983; Jeppesen et al., 1998) and in carbon and nutrient cycling (Rooney & Kalff, 2000). The plant macrofossil assemblage in lake sediments consists of two main components: terrestrial macrofossils originating from the catchment and aquatic remains from the lake itself. Their distribution in lakes is determined by many parameters. Smith & Wallsten (1986) suggest that the percent cover of emergent macrophytes in Swedish lakes could be predicted from the mean depth and other morphometric parameters of the lake. Macrophyte data combined with lake water chemistry were used to devise a typology of Estonian lakes (Maemets, 1974, 1991; Maemets, 2005).

Analysis of plant remains in lake sediments provides information on these aspects through time and palaeoecological and palaeolimnological studies that involve macrofossil analysis could answer the questions about the development of the lake ecosystem in a longer perspective. Macrofossils have been widely used in past environment studies (e.g. Birks, 1973, 1980, 2001, 2007; Watts, 1978; Birks & Birks, 2000; Gaillard & Birks, 2007) and for reconstruction of vegetation (e.g. Valiranta, 2006; Valiranta et al., 2006). The method is widely used to reconstruct past lake-level changes (Digerfeldt, 1986; Ammann, 1989; Hannon & Gaillard, 1997; Lotter, 1999; Birks & Wright, 2000; Dieffenbacher-Krall & Nurse, 2005; Koff et al., 2005) and anthropogenic impact (Rasmussen & Anderson, 2005). Application of the multiple core method will improve the reliability of results. As analysis of macrofossils is a time-consuming process, it is necessary to find the most representative set of cores. Before this selection it is necessary to consider various factors that may significantly affect the distribution of vegetation as well as the preservation of macrofossils in the course of sediment accumulation and diagenesis.

Information about the mechanism of macrofossil deposition in lake sediments can be obtained from surface sample studies (Birks, 1973; Davis et al., 1985; Dieffenbacher-Krall & Halteman, 2000; Zhao et al., 2006). It is important to know which plants will be represented by macrofossils in lake sediments, and whether it is informative to use these data to estimate the abundance of the plant in the past vegetation, lake trophy, or changes in the water level (e.g. Davidson et al., 2005; Dieffenbacher-Krall, 2007). The representation of macrofossils is not linearly or unimodally related to the variable (proportion of a species within the vegetation community) of interest. Many factors affect the relationship between macrofossil assemblages and plant communities, and studies of surface sediment samples may provide useful insights.

The aim of this study was to describe the present aquatic vegetation in two small lakes in Estonia and compare it with the macrofossil assemblages taken from the uppermost 10 cm of the lake sediment accumulated during the last decade. This was done in order to assess the relationship between macrophyte vegetation and the distribution and preservation of plant remains in sediments of lakes with different topography.

MATERIAL AND METHODS

Study sites

The study was performed in two lakes that are both small in size (Table 1) and characterized by quite stable macrophyte vegetation over the last decades, as shown by monitoring data (Ott & Koiv, 1999).

Lake Vaike-Juusa (hereafter L. Juusa) is situated in the southern part of Estonia (Fig. 1a) (58[degrees]03? N and 26[degrees]30? E) on the Otepaa Heights (Fig. 1b). The hillocks bordering the lake have steep slopes. The lake area is 3 ha, its maximum length 250 m, and width 160 m. The catchment area of L. Juusa is characterized by a semi-open cultural landscape. It is a meso-eutrophic dimictic lake with strong stratification during spring-summer. …