TEMPORAL AND SPATIAL DISTRIBUTION OF MACROPHYTES IN THE GRUŽA RESERVOIR (SERBIA)

A survey of aquatic macrophytes in the Gruža Reservoir (Serbia) was conducted during the period 20072008. Detailed data on the actual condition of macrophytes were established and compared between four parts of the reservoir. The distributions of most aquatic macrophytes of the Gruža Reservoir are heterogeneous, with different values of the MMT and MMO indices and low values of the distribution ratio. Quantitative analysis indicates that the greatest diversity and abundance of species can be found in those parts of the reservoir with the shallowest water and greatest indentation of the shoreline.


INTRODUCTION
Macrophytes as primary producers have an important role in maintaining the stability of dynamic equilibrium, nutrient cycling, sedimentation processes, and biofiltration in aquatic ecosystems (Wetzel, 1983).They are also valuable as indicators of water quality.
Although their presence is regulated by water quality, water depth, and substrate characteristics, morphometric characteristics of reservoirs on the whole show better correlations with vegetation structure than any measured chemical parameters (Heegaard et al., 2001;Mäkelä et al., 2004).Favorable morphometrical conditions make quick colonization possible in still water (Neiff, 2000).A high level of macrophyte development and especially overgrowth of emerged plants cause eutrophication of reservoirs (Wetzel, 1983;Moss, 1990).
We investigated macrophytes in a lowland eutrophic reservoir, one whose water quality supports development of this vegetation, but whose different habitat characteristics along the shoreline cause different distribution of macrophytes.Knowledge of the diversity, composition, abundance, and habitat preference of macrophytes provides important information about the ecological status of this aquatic ecosystem and makes it possible to predict possible changes in ecosystem quality.

MATERIAL AND METHODS
The Gruža Reservoir was formed in 1985, in the central region in Serbia, at an elevation of 238 m above sea level.The flooded area had a well-developed pedological substrate and represented one of the most fertile regions of Serbia.The lake is 10 km long and 0.2-1.5 km wide, with surface area of 934 ha.Oscillations of the water level comprise 3-5 m.Maximum depth of the water is 31 m near the dam, whereas two thirds of the reservoir are shallow (3-9 m).
Field work was carried bimonthly from September to July during the period 2007-2008.Macrophytes were surveyed in four parts of variable length (A, B, C, and D in Fig. 1), depending on vegetation distribution patterns, morphometric characteristics of the reservoir, water depth, and human impact.Part A of the shoreline is 1 km and lies next to both sides of the dam.Its banks are steep, unindentend, naked, and rocky, with great depths occur- Every homogeneous shoreline stretch with macrophyte colonization was investigated by the transect method.Plants growing on the shore and in water to a depth of 5 m were collected (with a grapple) and determinated according to Javorka and Shapody (1979).
Because several phytocenological areas are covered by the transect method, this paper gives a synthetic view of the presence of species of aquatic macrophytes during the period of investigations.The relative abundance was evaluated using a five-point scale (Kohler and Schneider, 2003) as follows: 1very rare, 2 -infrequent, 3 -common, 4 -frequent, and 5 -abundant, predominant.The obtained data form the base for a mathematical model with: relative plant mass (RPM) for each species; cumulative RPM for growth forms; mean mass indices for all macrophytes in total stretches (MMT); mean mass indices for stretches with their occurrence (MMO); and the distribution ratio (d) (Pall and Janauer, 1995).These data sets are used to describe quantitative relationships of aquatic macrophyte vegetation.
The Sørenson similarity index S (Schwerdtfeger, 1975) was used to calculate the degree of similarity between populations of macrophytes today and at the time of earlier investigations, based on the results of Veljović et al. (1986).

RESULTS
The main physical, chemical (Table 1), biological (chlorophyll a, zooplankton), and microbiological parameters (Ostojić et al., 2005) indicated that the reservoir's water belongs to the eutrophic category.
Table 1.Average annual values of some physical and chemical water characteristics of Gruža Reservoir, Serbia (Ostojić et al. 2005).
Data on the actual condition of aquatic macrophytes were established and compared between four parts of the reservoir (Table 2).Thus, in part A next to the dam (Fig. 1, Table 2) -where banks are steep, unindented, and naked and where great depth occurs just off shore -submerged species are represented by few species, while floating and emerged species are not present.In this part, macrophytes were not recorded in 1986 (Veljović et al., 1986).
Part B (Fig. 1, Table 2) -whose shoreline is somewhat more indented and less steep, and where the water is less deep -has more recorded species, but still small coverage.Ceratopyllum demersum and Polygonum amphibium are dominant here.In 1986 there existed very poor macrophyte vegetation.Low values of the Sørenson similarity index indicate that there are great differences between the earlier and current state of macrophytes, which over 22 years settled this unfavorable area.
In the zone of submerged vegetation in the next part of the reservoir (part C in Table 2 and Fig. 1), Myriophyllum spicatum (which builds underwater  2) show a high degree of similarity between the two times of analysis.
Part D (Fig. 1, Table 2) -whose shoreline is shallow, muddy, indented, and surrounded by agricultural fields, with a very gently-sloping littoral -is rich in species and coverage, both at the present time and in the past, and values of the Sørenson similarity index indicate that there are small differences between the earlier and current state of macrophytes.
Myriophyllum spicatum is the most common submerged species and together with M. verticillatum builds underwater meadows.Less frequently and in the deep water other submerged species are also present, but shallow water does not permit massive development of these species.Floating vegetation is represented by the dominant species Polygonum amphibium.Heleocharis palustris, Mentha aquatica, and Lycopus exaltatus are the most common emerged species.Values of the Sørenson similarity index (Table 2) indicate that there are small differences between the earlier and current state of macrophytes.
Our survey showed that the highest percentage of RPM was recorded for Polygonum amphibium, Myriophyllum spicatum, Ceratophyllum demersum, and Heleocharis palustris (Fig. 2).The cumulative RPM is 51.77% for emerged species, 33.33% for sub- merged species, and 14.90% for floating species.
The MMT and MMO values are low for most plant species, indicating that the species occur in small populations (Fig. 3).The MMT and MMO values are levelled and low for the species Ceratophyllum demersum, C. submersum, Myriophyllum spicatum, and M. verticillatum because these species are present with low plant masses in all surveyed sections (a homogeneous distribution pattern).
In this case, the distribution ratio (d) in Fig. 4 has a value of 1.For the Gruža Reservoir as a whole, the distributions of most aquatic macrophytes at the present time are heterogeneous, with different MMT and MMO values and a low value of the distribution ratio (Figs. 3 and 4).

DISCUSSION
Although prior to formation of the Gruža Reservoir the terrain lacked swamp vegetation, macrophytes there developed very quickly after the reservoir's creation.Already in the first year after its formation, the entire shallow and muddy littoral was overgrown with macrophyte vegetation, most extensively in the inlets (Veljović et al., 1986).(Veljović et al., 1986).
The quickness and manner of this colonization corresponded to data published for the Kis-Balaton Reservoir (Herodek, 2001) and for the Yacireta Reservoir (Neiff et al., 2000) and show that the first changes in the flora occurred very quickly, while in recent years changes in vegetation structure seem to be much slower.Because of this, the high value of the Sørenson similarity index for the Gruža Reservoir as a whole (Table 2) shows a high degree of similarity between the two times of analysis.Comparison with results from the period 1999-2002 (Topuzović and Pavlović, 2004) shows that there are no differences in species diversity.It is known that eutrophic conditions support the growth of vascular aquatic plants (Carbinier et al., 1990).The high species diversity of macrophytes in the Gruža Reservoir is also a consequence of the diversity of habitats on the banks of the reservoir, mainly morphometric characteristics of the shoreline, its slope, water depth, and pollution by nutrients (Wetzel, 1983;Heegaard et al., 2001;Irfanullah and Moss, 2004).There is a pronounced poverty of species with small coverage in the part of the lake next to the dam, which has the form of a gorge with steep rocky banks that are virtually naked and where great depth occurs just off shore and poor physical conditions rather than water quality might be responsible for restricting biodiversity.The greatest diversity and RPM of species can be found in parts of the lake with the shallowest water and greatest indentation of the shoreline, where nutrients are constantly leached into it.
Our results on the significance of morphometric characteristics of the reservoir for macrophyte development correspond to the data of Neiff et al. (2000), Heegaard et al. (2001), and Irfanullah and Moss (2004).Mäkelä et al. (2004) came to the conclusion (based on investigation of the diversity and community structure of macrophyte vegetation in 50 lakes in Finland) that morphometric characteristics of lakes on the whole show better correlations with vegetation structure than any measured chemical parameters.
Although today increased numbers of species are present in the Gruža Reservoir as a whole, no significant increase of abundance and mass is discernible because frequent fluctuations of the water level limit their development (Tarver, 1980;Hellsten and Riihimäki, 1996).This is especially important for emerged plants, which are richest in diversity and whose death followed by accumulation of difficultly soluble lignin and cellulose leads to creation of anaerobic conditions in the sludge, thereby contributing to the process of eutrophication.Because of this, the presence of macrophytes is not excessive, and they are still a desirable component of the reservoir for regulation of the development of phytoplankton and Cyanobacteria, competition with algae for nutrients, and biofiltration of the large influx of allochthonous nutrients.The macrophytes accumulate nutrients, excluding them from the biogeochemical cycle for a certain period, during which they can be removed from the system by harvesting.Macrophyte harvesting provides the best possibility for the removal of nutrients (Engel, 1990;Moss, 1990;Heegaard et al., 2001).This is important and advisable for the Gruža Reservoir, 2/3 of which are surrounded by cultivated fields.
Together with other measures, monitoring of further development of macrophytes and their regulation are needed for conservation of the Gruža Reservoir.This is very important primarily because of the reservoir's significance as a source of water supply for settlements.

Fig. 2 .
Fig. 2. Relative plant Mass (RPM) of aquatic macrophytes in the Gruža Reservoir as a fraction of all macrophytes.

Fig. 3 .
Fig. 3. Mean mass indices (MMT, MMO) calculated for all aquatic macrophytes in stretches with their occurrence (MMT, empty column) and total stretches (MMO, dark column) in the Gruža Reservoir.

Table 2 .
Floristic composition of the macrophytes in the four parts of the Gruža Reservoir.For A, B, C and D -see Material and Methods.