Is zooplankton an indicator of the water trophic level in dam reservoirs?

Zooplankton communities are an important component of the pelagic food web, but they are absent on the list of biotic elements to be considered in the assessment of ecological status in Annex V of the Water Framework Directive (2000/60/EC) (Caroni & Irvine 2010). The Directive 2000/60/EC requires the quality of waters to be determined on the basis of biological aspects, with other parameters complementing and supporting such an assessment. The communities of organisms that may be used for this purpose include phytoplankton, macrophytes, phytobenthos, benthos and fish, and these should be supported by a set of chemical and hydromorphological quality data (Annex V, 2000/60/EC) (Jeppesen et al. 2011). The Water Framework Directive does not specify zooplankton as an indicator applicable to water quality assessment, an omission that has attracted trenchant criticism (Moss 2008; Nõges et al. 2009; Jeppesen et al. 2011).

Zooplankton, as an intermediate trophic level in the pelagic food chain of lakes, is important in the assessment of their trophic status. These microscopic organisms are characterized by short life cycles and a relatively high metabolic rate, and such organisms, particularly rotifers and crustaceans, react quickly to changes in environmental conditions (Shurin et al. 2010). Hence, the species composition of rotifers and crustaceans as well as their abundance may be used as biological indicators that reflect changes in water quality. Parameters of the Rotifera community not only indicate the level of water pollution, but also serve to determine general tendencies in the changes of environmental conditions over time (Duggan et al. 2001; Ejsmont-Karabin 2012).

The value of zooplankton as an indicator of ecological conditions results from their position in the food web, between the top-down regulators (fish) and bottom-up factors (phytoplankton). They can thus provide information about the relative importance of the top-down and bottom-up control and their impact on water clarity (Jeppesen et al. 2011).

Danish authors have suggested that zooplankton can be used as an indicator of changes in trophic dynamics and the ecological state of lakes related to changes in nutrient loading and climate (Jeppesen et al. 2000; 2005; 2009; Søndergaard et al. 2005). According to Xu et al. (2001), a set of ecological indicators including structural, functional, and system-level aspects was proposed for the lake ecosystem health assessment, in accordance with the structural, functional, and system-level responses of lake ecosystems to chemical stresses. These indicators include acidification, eutrophication, as well as copper, oil and pesticide contamination. In many countries, zooplankton has been studied as part of lake monitoring (Mäemets 1983; Berzins & Pejler 1989; Matveeva 1991; Karabin 1985; Andronikova 1996; Ejsmont-Karabin 2012; Ejsmont-Karabin, Karabin 2013; Haberman & Haldna 2014), and as part of long-term monitoring of dam reservoirs (Fleituch & Pociecha 2000; Pociecha 2016). In dam reservoirs, zooplankton is rarely examined to determine the trophic indices of these water bodies. However, these artificial reservoirs may be considered lake areas, as there are characterized by environmental conditions similar to those found in natural lakes. For this reason, the zooplankton species composition of dam reservoirs could be used to assess the trophic status.

The objective of this paper was to analyze the usefulness of zooplankton indices based on two groups of zooplankton taxa, Rotifera and Crustacea, and to determine the trophic status of 10 different types of dam reservoir ecosystems, using formulas provided by Ejsmont-Karabin (2013).

In this paper, we demonstrate that zooplankton could be a useful indicator of the structure and function of dam reservoir ecosystems and their ecological status.

The zooplankton in the studied dam reservoirs showed significant differences, both in the qualitative and quantitative composition as well as in the density of particular groups of zooplankton. The largest number of zooplankton taxa was found in Lake Koronowskie and the lowest number in Lubachowski Dam Reservoir (Fig. 2). A small number of zooplankton taxa was also observed in Zygmunt August Lake, whose characteristics resemble a breeding pond rather than a typical dam reservoir.

Figure 2

The number of zooplankton taxa in the studied dam reservoirs in 2012. 1 – Łapińskie Nowe Lake, 2 – Mylof Dam Reservoir, 3 – Koronowskie Lake, 4 – Zygmunt August Lake, 5 – Siemiatyckie Zalewy Reservoir, 6 – Próba Dam Reservoir, 7 – Wióry Dam Reservoir, 8 – Chańcza Dam Reservoir, 9 – Leśniańskie Lake, 10 – Lubachowski Dam Reservoir

The highest densities of rotifers and crustaceans were observed in Zygmunt August Lake and Próba Dam Reservoir (Fig. 3). The lowest densities of both zooplankton communities were observed in Łapińskie Nowe Lake and Mylof Dam Reservoir (Fig. 3). A very low density of rotifers was also found in two submontane dam reservoirs: Lake Leśniański and Lubachowski Dam Reservoir.

Figure 3

Rotifera (A) and Crustacea (B) (a – Copepoda; b – Cladocera) density (ind. l⁻¹) in the studied dam reservoirs in 2012. 1 – Łapińskie Nowe Lake, 2 – Mylof Dam Reservoir, 3 – Koronowskie Lake, 4 – Zygmunt August Lake, 5 – Siemiatyckie Zalewy Reservoir, 6 – Próba Dam Reservoir, 7 – Wióry Dam Reservoir, 8 – Chańcza Dam Reservoir, 9 – Leśniańskie Lake, 10 – Lubachowski Dam Reservoir

In the most eutrophic dam reservoirs, the dominant species in the zooplankton community were Keratella cochlearis f. tecta, Pompholyx sulcata, Trichocerca pusilla, Bosmina longirostris, Chydorus sphaericus, species considered to be indicators of a high trophic state. In these reservoirs, the density of rotifers ranged from 6641 to 33 811 ind. l⁻¹ and the density of crustaceans ranged from 950 to 4195 ind. l⁻¹ (Fig. 3).

The dominant zooplankton species were represented by 13 rotifers, 11 cladocerans and 3 copepods. Most species were characteristic of meso- to eutrophic waters. Species characteristic of eutrophic waters, such as K. cochlearis f. tecta, P. sulcata, T. pusilla (rotifers) and B. longirostris, Ch. sphaericus and Diaphanosoma brachyurum, were found in the reservoirs in densities ranging from 40 to 78 ind. l⁻¹ (Table 5).

The indices calculated for the dam reservoirs based on Rotifera and Crustacea communities were very similar. In the case of the Rotifera community, the percentage of the tecta form in the K. cochlearis population and the proportion of high-trophic species in the total species density were the best indices to describe the water trophic status. In the case of the Crustacea community, the best indices to describe the trophic state of water were the Crustacea density and the Cyclopoida biomass. The highest value of all indices calculated on the basis of density and species structure of Rotifera – above 59 – was determined for Zygmunt August Lake, but the highest value of all indices calculated on the basis of density and species structure of Crustacea – above 61 – was determined for Zygmunt August Lake and Próba Dam Reservoir (Tables 6 and 7).

In the case of one reservoir, Zygmunt August Lake, the index of the trophic state calculated for the Rotifera and Crusacea communities was the same and ranged from highly eutrophic to polytrophic. In the case of other reservoirs, the value of indices indicated mesotrophic to polytrophic state (Table 8).

The zooplankton community in freshwater ecosystems contains species identified as aquatic bioindicators. These organisms are very good indicators, because they quickly respond to environmental stress, such as pollution/nutrient enrichment, habitat loss or overexploitation (Adams 2002; Birk et al. 2012). The ecological status of water bodies is defined as the expression of the quality of the structure and functioning of aquatic ecosystems based on biological quality elements (BQEs) (CIS 2003; Jeppesen et al. 2011). When implementing the EU Water Framework Directive (WFD), the Member States must classify the ecological status of surface waters following the standardized procedures (Jeppesen et al. 2011), but zooplankton is not considered useful in this assessment.

Zooplankton is mentioned in the WFD CIS Monitoring Guidance (CIS 2003; Jeppesen et al. 2011) as a “supportive/interpretative parameter” of fish, “often/typically measured or sampled at the same time”. Nevertheless, in many countries, e.g. in Denmark, zooplankton is considered to be useful in studying the ecological state of lakes (Jeppesen et al. 2000; 2005; 2009; Søndergaard et al. 2005). Using mainly examples from Denmark, Estonia and Great Britain, Jeppesen et al. (2011) demonstrated that zooplankton is an important indicator of the ecological state of water, and they discuss straightforward indicators which, with further studies, could be useful indicators of the structure and function of lake ecosystems as well as of their ecological state.

The study of general indicators that would work well in the whole EU is very difficult due to the number of factors that influence the results, including climate, seasons and geographical location of lakes (Moss et al. 2003). Mäemets (1983) demonstrated a clear relationship between zooplankton, the type of lakes and trophic state.

The long-term research of submontane dam reservoirs has shown that the assessment of the trophic status based on the community composition, density and dry weight of zooplankton results in a change of the trophic state of water from eutrophic-mesoeutrophic to mesoeutrophic Pociecha 2016). Moreover, the geographical location of dam reservoirs, as well as reolimnic or limnic characteristics do not affect the determination of the trophic state based on the structure and density of the zooplankton community. In a dam reservoir with a relatively short retention time (below 20 days; Table 1 – Mylof Dam Reservoir and Łapińskie Nowe Lake; Rosnowski and Hajka reservoirs – Pociecha & Heese 2007), the density of zooplankton was low compared to other dam reservoirs studied.

Low density values in reolimnic reservoirs (Łapińskie Nowe Lake and Mylof Dam Reservoir) seem to have no effect on the calculations and results of Rotifera and Crustacea indices of the trophic state (Table 8).

As indicated above, zooplankton indices for Polish lakes studied by Ejsmont-Karabin (2012, 2013) and Ejsmont-Karabin & Karabin (2012) perform very well, not only in natural lakes but also in artificial reservoirs. It is relevant that the selected indices based on Rotifera and Crustacea had similar values in artificial reservoirs. The results of our study show that the indices based on the structure of Rotifera groupings are slightly more sensitive to an increase in trophic conditions compared to the indices based on the structure of Crustacea.

In dam reservoirs, the structure of the zooplankton community with species indicators is similar to that in the lakes (Karabin 1985; Matveeva 1991; Haberman & Haldna 2014). The results of the zooplankton indices used to assess the trophic state of the dam reservoirs confirm the usefulness of these indices for the assessment of the water trophic status of both submontane and lowland dam reservoirs. Our study illustrates that zooplankton is an important indicator of the structure and function of freshwater dam reservoir ecosystems and may reflect the ecological status of water bodies.