Water Quality and Fish Community Composition in the Sava Lake: The Basic Ecological Considerations - page 01

Material and Methods

Data on water quality parameters (2009-2013 period) have been obtained from the Belgrade Institute of Public Health, the institution authorized to check and monitor environmental conditions in the city of Belgrade area. Current analysis of fish community composition is based on data reported in the Annual Fishery Management Plan. The document contains results of the detailed ichthyofaunistic survey conducted in autumn 2009, in order to evaluate the effectiveness of the 6 year stocking programme (spring 2003 – spring 2009). The available data comprises species composition and assessment of their relative abundance in terms of biomass. Sampling methods and data processing are explained in the quoted document in detail. The available data allowed for ecological classification of species and determination of diversity indices. The systematic arrangement of the species in the present paper follows that of Kottelat and Fryehof (2007). Species ecological classification is based on the literature (Aarts and Nienhius, 2003; Noble et al., 2007; Terra and Araújo, 2011) and our own experience. The total number of species was regarded as a richness index. Three diversity indices were calculated taking relative abundance in biomass as an importance value: the Shannon-Weaver index (H' = -Σ Pi ln Pi), the Simpson index (C = Σ (ni/N)2), and evenness index (E = H'/lnS), where ni was the importance value for each species, N total importance value, Pi importance probability for each species, and S number of species (Odum, 1971). Forage-carnivore ratio (F/C) was determined (Swingle, 1950, according to Moehl and Davis, 1993), piscivorous species were considered carnivores and others (excluding three) were considered as forage species.

 

Results and Discussion

The mean monthly values of selected water quality parameters recorded during the 5-year period were averaged. Narrow ranges of mean monthly values confirm that the applied methods were appropriate. Figures 1 and 2 depict the monthly averaged values together with ranges of recorded monthly mean values.

The averaged mean monthly temperature ranged between 8.4 ºC (March) and 26.9 ºC (July), while the absolute amplitude of recorded mean monthly values was 20.8 ºC (7.1 – 27.9 ºC). Annual temperature variations follow a pattern that is normal for our climatic region and are closely connected with variations of air temperatures. Temperature is of primary interest for any ecosystem since temperature regulates biotic growth rates, development of life stages and defines fishery habitat. The observed ranges were within optimal ranges for metabolic activities of lowland waters fish (Alabaster and Lloyd, 1980), therefore in respect to temperature, the reservoir could be considered as a favourable habitat for fish production. The averaged mean monthly dissolved oxygen concentrations ranged between 9.2 mg/l (September) and 12.4 mg/l (March), while the recorded mean monthly values varied in the 8.3-12.5 mg/l range. Regardless of season dissolved oxygen concentrations were above 80% saturation indicating well oxygenated conditions. During the summer and early autumn months, dissolved oxygen concentrations were supersaturated clearly reflecting intensive photosynthetic activities of phytoplankton and water macrophytes. Dissolved oxygen is an important indicator of water quality, ecological status, productivity and the health of a reservoir (Mustapha, 2008). As is temperature, dissolved oxygen is a parameter of primary importance because it is necessary to sustain aquatic life due to its use in biological and chemical reactions.

Alabaster and Lloyd (1980) reported that for resident populations of moderately tolerant freshwater fish species, a dissolved oxygen concentration greater than 5 mg/l is sufficient for promoting fish growth and survival, thus confirming suitable conditions of the Sava Lake. The fluctuations of pH values indicate alkaline conditions in the water of the Sava Lake. The averaged mean monthly pH values ranged between 8.1 (March) and 8.5 (summer), while recorded mean monthly values varied in the 7.9-8.9 range. Higher pH values during warmer periods of the year are a consequence of intensive photosynthetic activity. Suspended solids concentrations were rather low, never reaching 10 mg/l. The averaged mean monthly suspended solid concentration ranged between 1 mg/l (April) and 3.7 mg/l (August), while recorded mean monthly values varied in the 0.9-8.2 mg/l range. However, from April to August an increasing trend of suspended solids concentrations was observed, followed by a decrease in concentrations afterwards. The observed pattern most probably reflects phytoplankton abundance. The mean monthly biochemical oxygen demand (BOD) values varied between 1.1 mg/l (April) and 2.2 mg/l (August), while recorded mean monthly values varied in the 0.8-3.5 mg/l range. BOD values were relatively homogenous throughout the entire annual cycle and recorded concentrations indicate low water pollution. Wider ranges of mean monthly values observed during swimming season give the impression of slightly higher pollution which could be explained by anthropogenic influence. The averaged mean monthly chemical oxygen demand values varied between 1.7 mg/l (December) and 3 mg/l (August), while recorded mean monthly values varied in the 1.1-4.9 mg/l range. Recorded values express an increasing trend during the summer months, both in averaged mean monthly and ranges of mean monthly values, thus indicating increased anthropogenic influence. Both, total phosphorous (TP) and total nitrogen (TN) concentration slightly fluctuated over the annual cycle without an observable pattern. The averaged mean monthly total phosphorous concentrations varied between 0.01 mg/l (October) and 0.046 mg/l (June), while recorded mean monthly values varied in the 0.002-0.088 mg/l range. The averaged mean monthly total nitrogen concentrations varied between 0.145 mg/l (December) and 0.695 mg/l (April), while recorded mean monthly values varied in the 0.05-1.25 mg/l range.

In general, all selected water quality parameters indicate high water quality of the Sava Lake. This is especially true in respect to dissolved oxygen, suspended solids and COD, whose concentrations during the whole annual cycle were within limits issued for water quality class I (Official Gazette of RS 50/2012). Total phosphorous, total nitrogen and BOD concentrations only occasionally reached limits issued for water quality class II (ibidem). In respect to pH values, these were mainly within limits issued for I-IV, but during the summer months they were very close and occasionally exceeded the limit issued for water quality class V (ibidem). The reservoir is supplied with water from a sedimentation unit, which partly serves as a biological filter and an exchange rate of approximately 20 exchanges per year facilitate high water quality. Additionally, maintenance activities which include, regular removal of water macrophytes, beach maintenance and artificial water oxygenation, surely contribute in maintaining water quality conditions that are satisfactory for reservoir purposes.

The Sava Lake is inhabited by 20 fish species from 19 genera, belonging to 8 families (Tab. 1). Among the registered taxa, 2 families, 4 genera and 6 species are allochthonous.

 

Fig01
Figure 1: The 5-year monthly mean values and ranges of temperature (open circles), dissolved oxygen concentrations (solid circles), pH (open rectangles) and suspended solids (solid rectangles); WQ (broken lines) – national criteria for determined water quality classes; solid lines – swimming season.

 

Fig02
Figure 2: The 5-year monthly mean values and ranges of BOD5 (open rhombs), COD (solid rhombs), total nitrogen (solid triangles) and total phosphorous (open triangles); WQ (broken lines) – national criteria for determined water quality classes; solid lines – swimming season.

 

Tab01