Impact of habitat heterogeneity on the biodiversity and density of the zooplankton community in shallow wetlands (Upo wetlands, South Korea)

Habitat heterogeneity may provide more niches and diverse ways of exploiting the environmental resources (Bazzaz 1975). Habitats with high structural heterogeneity provide refuges from predators, suitable spawning and foraging substrates, thus supporting a greater diversity of animals (Declerck et al. 2005; Vieira et al. 2007). Empirical studies have demonstrated that habitat heterogeneity in woods and stands of trees (Ganzhorn et al. 1997; Southwell et al. 1999), grass (Dennis et al. 1998), coral reefs (Roberts & Ormond 1987; Jones & Syms 1998), and landscapes (Hanowski et al. 1997) create habitat space for various wildlife. The habitat heterogeneity of freshwater ecosystems is primarily determined by the physical structure and composition of macrophytes. Thus, these aquatic plants are expected to have a major impact on the distribution and interaction of various animal groups (Thomaz et al. 2008).

Freshwater macrophytes create a heterogeneous microhabitat in the water, providing a suitable food supply for animals, as well as a refuge from predators (Lauridsen & Lodge 1996; Lauridsen et al. 1996). Thus, macrophyte habitats are able to support a large abundance of animals, such as zooplankton, macroinvertebrates, and small fish. Of the various aquatic animal groups, it has been demonstrated that the zooplankton community effectively utilizes macrophyte habitats (Kuczynska-Kippen & Nagenast 2006; Burks et al. 2002). Previous studies have shown that zooplankton are strongly affected by the density (Snickars et al. 2004), life forms (Warfe & Barmuta 2004), and structural heterogeneity of macrophytes in vegetated beds (Meerhoff et al. 2006; Thomaz et al. 2008). For example, submerged macrophytes generally increase the physical complexity of aquatic environments, and hence provide a suitable habitat for the zooplankton community. However, free-floating or floating-leaved macrophytes have also been reported to fulfil important structuring functions in wetland systems (Meerhoff et al. 2003). Therefore, it may be hypothesized that mixed structures of various macrophyte species support more zooplankton.

The importance of habitat heterogeneity to zooplankton has been central to limnological research. Zooplankton is located at the intermediate level in the food web, between phytoplankton and fish, and plays a key role in regulating the food web dynamics (Wetzel 1983). Unfortunately, the distribution of the zooplankton community has not been comprehensively evaluated in relation to different types or combinations of macrophytes. This study aimed to understand the influence of habitat heterogeneity on the zooplankton community in a wetland system. We predicted that greater macrophyte species diversity would enhance the abundance and species diversity of the zooplankton community.

During the study period, similar macrophyte species composition was observed during spring and autumn, but plant dry weight differed according to season (Table 1). A total of 6 macrophyte species were found at the study site. The helophyte and pleustophyte zones contained two species of macrophyte each, while the mixed vegetation zone contained a total of five macrophyte species. The dry weight of macrophytes was higher in autumn compared to spring, because of a different growth pattern changing with the seasons. The highest macrophyte dry weight was obtained for Potamogeton crispus, followed by Ceratophyllum demersum, and Phragmites communis.

Table 2 presents various environmental factors (i.e. depth, water temperature, dissolved oxygen, pH, conductivity and chlorophyll a) in relation to different macrophyte zones (i.e. zooplankton habitat types) at the study site. Most of the monitored environmental factors were similar in all four habitat types. The largest difference was observed for DO concentration (one-way ANOVA, p < 0.05), with relatively higher concentrations being obtained in the open water zone compared to the three other zones. Dissolved oxygen tends to be low when macrophytes are present on the water surface, because of different permeation rates (van der Valk 2006). Therefore, we expected that the zooplankton community might be affected by the structural heterogeneity of the four habitat types.

The collected zooplankton was represented by a total of 43 species, including 27 species of Rotifers and 16 species of Cladocerans and Copepods in the Upo Wetlands (Table 3). The mixed macrophyte zone supported the largest number of zooplankton species (spring – 25 species; autumn – 26 species), followed by the helophyte zone (spring – 16 species; autumn – 15 species) and the pleustophyte zone (spring – 10 species; autumn – 20 species). On the other hand, the lowest number of zooplankton species was documented in the open water zone (spring – 6 species; autumn – 5 species). Zooplankton density was statistically different among the four habitat types (one-way ANOVA, p < 0.05, n=119, Fig. 2). The highest zooplankton density was found in the mixed vegetation zone, followed by the helophyte zone and the pleustophyte zone. However, the zooplankton density in the open water zone was on average two to five times lower compared to the three other habitat types (spring – on average 32 ind. l⁻¹; autumn – on average 50 ind. l⁻¹). The pattern of species diversity was similar to that of the density.

Figure 2

Zooplankton density was greater in autumn compared to spring in all habitat types (Fig. 2). In particular, zooplankton density was three times higher in the pleustophyte zone during autumn compared to spring (on average 96 ind. l⁻¹ and 313 ind. l⁻¹ in spring and autumn, respectively). A larger number of pleustophyte species occurred in autumn. It produced a more complex habitat structure and attracted more zooplankton (Fennessy et al. 1994). Therefore, the larger zooplankton assemblages in autumn were considered to be strongly correlated with the development and growth of macrophytes.

Previous studies have suggested that the zooplankton community is more abundant in complexly structured macrophyte zones (Manatunge et al. 2000; Warfe & Barmuta 2004). Helophytes tend to have a simple structure compared to other macrophyte types in the water; thus, this group is expected to support a lower density of zooplankton. In the current study, we observed that pleustophytes often occurred together with elodeids. Hence, the coexistence of these two different macrophyte types was expected to be advantageous to zooplankton diversity due to the increased complexity of the habitat structure. Elodeids and pleustophytes are known to support different types of zooplankton (Meerhoff et al. 2003; Moss et al. 1998). Thus, the highly complex habitat structure in the mixed macrophyte zone may explain the high density and diversity of zooplankton observed in this zone compared to the three other zones.

Statistically significant correlation between the zooplankton density with the habitat type supports this observation. Two-way ANOVA (Table 4) showed that there was no seasonality in the zooplankton density and diversity, but the habitat type had a strong effect. The interaction between the season and the habitat type was not significant, which implies that zooplankton density and diversity in wetland ecosystems was primarily affected by the macrophyte type.

Even though some studies have indicated a positive relationship between zooplankton and the presence of certain types of vegetation, our findings expand on the existing information about the correlation between the zooplankton community and the structural heterogeneity of macrophytes. Empirical studies show that elodeids have a complex structure in the water, but they are also more easily agitated by the wind and water currents compared to nymphaeids and pleustophytes (Vermaat et al. 2000). Therefore, elodeids are mainly used by pelagic zooplankton species such as daphnids (Schriver et al. 1995; Beklioglu & Moss 1996; Jeppesen et al. 1998). A few studies have reported that epiphytic zooplankton often inhabit stands of nymphaeids with high biomass (Moss et al. 1998). The surface macrophyte bed is relatively stable because of the fixed location (i.e. by roots) of nymphaeid species. Thus, nymphaeids are suitable for the attachment of epiphytic zooplankton species. We found that the pleustophyte habitat supports mainly epiphytic rotifer species rather than other zooplankton species. The surface macrophytes (e.g. nymphaeids and pleustophytes) and elodeids had different spatial structures and were used by different zooplankton species. The higher zooplankton diversity in mixed wetland habitats might attract different predators, consequently leading to an ecologically healthy food web.