Habitat complexity in the freshwater ecosystem plays an important role in the growth and distribution of aquatic organisms (Meerhoff et al., 2007; Debastiani-Júnior et al., 2016). Generally, animal abundance and species richness are considerably higher where submerged macrophytes are present (Hemminga and Duarte, 2000). Macrophytes are an important habitat component in shallow lakes, as complex macrophyte architecture of dissected leaves and stems often supports more macroinvertebrate populations (Xie et al., 2006; Hinojosa-Garro et al., 2010). It is well established that macrophytes with finely dissected leaves provide more advantages (shelter against predation, food resources and cover or substrate) for the growth and reproduction of invertebrates than do macrophytes with broad leaves (Warfe and Barmuta, 2006; Vermonden et al., 2012). The increased surface area of macrophytes with dissected leaves is more suited to colonization by large numbers of invertebrates, augmenting the species richness in freshwater ecosystems (Heino, 2005; Renniie and Jackson, 2005). Indeed, the increase in habitats, spawning places, and numbers of some invertebrate species living on dissected leaved macrophytes is enough to decrease the risk of extinction. In addition, the complex architecture of macrophytes provides more cover from predators, resulting in increased population density of aquatic invertebrates (Vermonden et al., 2009, 2012). The theoretical support for the aforementioned benefits mostly hinges upon on the reasoning that macrophyte surface area and spatial heterogeneity increase with leaf dissection.
J. Limnol., 2017; 76(3): 581-590
This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License (CC BY-NC 4.0).
However, the relationship that complex macrophyte habitats support higher animal abundance and biomass is not always found (Soszka, 1975; Cry and Downing, 1988; Taniguchi et al., 2003; Cremona et al., 2008). Soszka (1975) reported that Myriophyllum verticillatum L. with dissected leaves supported fewer invertebrates than the other 12 macrophytes with dissected and broad leaves, and Ceratophyllum demersum L. and Utricularia sp. with dissected leaves did not sustain significantly more invertebrates than broad-leaved macrophytes. Moreover, broad-leaved macrophytes sometimes have also been found to support more invertebrates than dissected leaved macrophytes. For example, more invertebrates were reported in Potamogeton amplifolius Tuckerm than in P. rohbinsii Oakes or Vallisneria spiralis L. Stands (Cry and Downing, 1988). Taniguchi (2003) also stated that there was no difference in the abundance of invertebrates between simple and complex forms on the natural macrophytes. Therefore, submerged macrophytes with a complex architecture did not always host significantly greater macroinvertebrate biomass than plants with a simpler architecture (Cremona et al., 2008).
Thus, the hypothesis that submerged macrophytes with complex architectures support higher invertebrate abundance than macrophytes with simple architectures requires additional empirical studies for verification. In this study, four submerged macrophytes with two contrasting architectures (finely dissected = complex architecture and broad-leaved = simple architecture) and zooplankton community were selected to evaluate the effects of macrophyte architectures on zooplanktons.
For this experiment, 15 pottery pots (diameter: 48 cm; height: 36 cm) were prepared in the laboratory with 12 pots for 4 test treatments and 3 pots for a control treatment. Each pot contained a 10-cm high mixed sediment of sand and mud in a ratio of 1 to 1, in which the sand was bought from the construction market and the mud was collected from the West Lake (30°15′N, 120°09′E) in Hangzhou, China. This lake is one of the most famous lakes in China and was listed in UNESCO’S World Culture Heritage catalogue in 2011. After tap water was injected into the 15 pots, they were allowed to precipitate for 3 days to reduce the turbidity caused by watering.
Four submerged macrophytes (broad leaved: V. spiralis and Potamogeton malaianus Miq.; finely dissected: C. demersum and M. verticillatum) were also collected by an iron clamp and carefully cleaned to remove attaching organisms in the West Lake. Moreover, 15 experimental zooplankton samples collecting from this lake were gained respectively by filtering 100 L of water through a 112-μm plankton net and concentrating it into a 100-mL volume of plastic bottle. Individuals of the four macrophytes in analysis with nearly the same mean biomass of 150 g, were planted in triplicate into 12 pots, but no macrophyte was planted in the control group. After 15 zooplankton samples were added to the 15 pots one by one, they were collected every 10 days during the later next 60 days to analyze the changes in zooplankton community.
Sampling and data analysis
Zooplankton samples in the pots were obtained by concentrating 1 L of water fixed with 9 mL Lugol’s solution to 30 mL in a 50-mL plastic bottle. Then, a 1 mL subsample was collected using a graduated pipette into a count-frame, and quantified using a 200×magnification inverted microscope (Olympus CKX31). Zooplanktons were identified to genus/species level using the standard reference keys of Wang (1961), Chiang and Du (1979), and Shen and Tai (1979).
All data were tested for normality with a Shapiro- Wilks test. Significant differences among total zooplankton, Shannon diversity index, species richness and three zooplankton groups (e.g., rotifer, cladoceran, and copepod) living among four macrophytes were determined using a one-way analysis of variance (ANOVA), and Duncan’s multiple-range test was applied to explore the significant effects of four macrophytes on these parameters. All these analyses were completed using SPSS 21.0 for Microsoft Windows.
Variance in zooplankton community composition on four macrophytes
The analyses of zooplankton samples collected in the West Lake showed that the zooplankton community were consisted of rotifers and copepods, occupying 91.5% and 8.5% of zooplankton density (43,920 ind·L–1), respectively (Tab. 1). Among rotifers, copepods and nauplius, the dominant species were Anuraeopsis fissa (Gosse) (67.5%), Synchaeta stylata (Wierzejski) (6%), S. oblonga (Ehrenberg) (2.7%), Keratella cochlearis (Gosse) (2.7%), and nauplius (8.5%), respectively. However, we have not found cladoceran and copepod adults, probably due to the high predation pressure existing in the West Lake.
During the experiment, all the zooplankton community groups in all the macrophyte treatments in analysis exhibited drastic changes. Nevertheless, the community structures tended to be almost stable from the 40th day to the 60th day (Fig. 1). The results in the stable state showed that cladocerans and copepods were found to occupy 40% and 60% of zooplankton density in V. spiralis, approximately 60% and 40% in P. malaianus, 44% and 56% in control, 40% and 60% in C. demersum, 80% and 10% in M. verticillatum treatments, respectively. However, rotifers have completely missed in control (Fig. 1A), V. spiralis (Fig. 1D), P. malaianus (Fig. 1E) and C. demersum (Fig. 1C) treatments, but some species were found in M. verticillatum treatment occupying about 10% of zooplankton density (Fig. 1B). Moreover, in the stable state the dominant species in the five treatments had changed into Alona karua (King), Chydorus sphaericus (O.F. Müeller), Pleuroxus trigonellus (O.F. Müller), Thermocyclops taihokuensis (Harada), and nauplius (Tab. 2).
Variance in zooplankton parameters in five groups
During the whole period of investigation, the variances of zooplankton parameters in all the five treatments in analysis exhibited two distinct change trends: 1) a continuous stable state; or 2) a biphasic trend characterized by significant variations from the 10th day to the 30th day (period 1), followed by a stable state from the 40th day to the 60th day (period 2) (Fig. 2 and Tab. 3). Specially, zooplankton parameters in the two broad-leaved macrophyte treatments (V. spiralis and P. malaianus) had been in a stable state during the whole period (Tab. 3). Conversely, in the control group they significantly decreased in period 1, but tended to be stable in period 2. However, the zooplankton parameters in the two finely dissected macrophyte treatments (C. demersum and M. verticillatum) exhibited different change trends (Tab. 3 and Fig. 2). In the C. demersum treatment, cladocerans, copepods, total zooplankton biomass and diversity were in a stable state during the whole period. On the contrary, rotifers, total zooplankton density and species richness changed significantly in period 1, but were in a stable state in period 2 (Tab. 3). For zooplankton in presence of M. verticillatum, rotifers and copepods were also in a stable state in the whole period (Fig. 2 E-F,I-J). Yet, total zooplankton and cladocerans significantly increased (Fig. 2 A-B,G-H), but diversity and richness drastically decreased in period 1 (Fig. 2 C-D). Moreover, all of them were in a stable state in period 2.
Analyzing the effects of macrophyte architectures on zooplanktons
As all of the zooplankton parameters in the five treatments were in a stable state on the 40th day, the variance analysis of zooplankton on the 40th day may be utilized to highlight the effects of macrophyte architecture on zooplankton. The obtained results showed that the architecture seemed to have no significant effects on total zooplankton, diversity and richness (Fig. 3). Specially, finely dissectedleaved M. verticillatum could support more total zooplankton and higher species diversity and species richness than the other three macrophytes. Hence, the finely dissectedleaved C. demersum exhibited no significant effects on total zooplankton biomass, diversity and richness, and even lesser effects on total zooplankton density compared to the two broad-leaved macrophytes (V. spiralis and P. malaianus).
The architecture also seemed to have no significant effects on three main zooplankton groups (e.g., rotifer, cladoceran and copepod) (Fig. 4). For the rotifers, significant variances in the density and biomass were only found at M. verticillatum treatments with complex architecture and, no significant variances existed at the three remaining treatments, the broad-leaved macrophytes (V. spiralis and P. malaianus) and the complex C. demersum (Fig. 4A). Moreover, the effects of macrophytes on density and biomass of cladocerans were also independent on macrophyte architecture, as for rotifers (Fig. 4B). In general, all macrophytes could encourage cladocerans, but only M. verticillatum showed significant effects on rotifers. For the copepods, macrophytes could provide various effects on density and biomass (Fig. 4C). Specially, the broad-leaved P. malaianus and the finely dissected M. verticillatum did not show more effects on copepod density than control group; on the contrary, V. spiralis and M. verticillatum supported higher density than the other three groups. Yet, except for V. spiralis, all of the other three macrophytes could significantly decrease copepod biomass.
Which is more important: macrophyte species or architecture?
The effect of macrophyte architecture on zooplankton community seemed to be very limited in this study. Our results showed that only M. verticillatum could significantly support more zooplanktons and higher diversity and species richness; whereas the other three macrophytes almost showed no significant differences compared to the control conditions. Even broad-leaved V. spiralis and P. malaianus could support higher zooplankton density than finely dissected-leaved C. demersum. Moreover, we also found that only finely dissected-leaved M. verticillatum provided more advantages for cladocerans than the two broad-leaved macrophytes (V. spiralis, and P. malaianus), but C. demersum did not support cladocerans largely than does broad-leaved macrophytes. These results do not support the hypothesis that submerged macrophytes with complex architecture should have a more abundance and biomass of zooplankton than macrophytes with simple morphology, indicating that macrophyte architecture may not be a good indicator of zooplankton abundance or biomass. This conclusion was also consistent with the results of many other studies (Lalonde and Downing, 1992; Cremona et al., 2008; Hansen et al., 2010). For example, Cremona et al. (2008) reported that submerged macrophytes with a complex architecture did not host significantly greater macroinvertebrate biomass than macrophytes with a simpler architecture. Hansen (2010) also did not find significant variances in invertebrate abundance on morphologically more complex macrophytes (M. spicatum and Chara baltica Bruzelius) than on the structurally simpler plant (Potamogeton perfoliatus L.). Consequently, the relationship between structural complexity of macrophytes and zooplankton abundance or biomass is itself complex, and does not always respond to general predictions.
In addition, other factors besides macrophyte architecture (such as, different allelochemicals from diverse macrophytes) could mediate the zooplankton community in macrophyte habitats. In this study, cladoceran density and biomass in all macrophyte treatments were higher than those in the control treatment, indicating that four macrophytes were conducive to the cladoceran growth. Moreover, M. verticillatum supported more cladocerans than the other three macrophytes, but the effects of the three macrophytes on cladocerans were not significant. The architecture seemed to have no significant effects on facilitating cladocerans between four macrophytes. Some studies also found that macrophytes with a similar architecture did not always exhibit similar effects on some organisms. For example, Kairesalo (1983) reported that simple structural Vallisneria americana Michx with its long and narrow, tape-shaped leaves, allowed more light penetration and nutrient exchange, thus favoring periphyton growth, and then supported sometimes larger communities of gastropods than Potamogeton species with similar architecture. Furthermore, macrophyte species characteristic seemed to exert stronger effects on some aquatic organisms than macrophyte architecture. Additionally, greater water circulation in V. americana stands could guarantee other critical conditions for macroinvertebrates like higher dissolved oxygen levels than other macrophytes (Caraco and Cole, 2002). In addition, allelochemicals releasing from macrophytes decreased survival or changed behaviors of some invertebrate taxa (dipteran larvae, Dhillon et al., 1982; mysids, Lindén and Lehtiniemi, 2005), but the concentration and effectiveness of such substances varied with macrophyte species. Gross (2003) reported that allelopathic substances have been documented for M. spicatum but, are rare among submerged Potamogeton species. Therefore, macrophyte-specific allelopathic influences on some aquatic organisms also seemed to be independent of macrophyte architecture.
Based on the above analysis, compared to macrophyte architectures, macrophyte species characteristics seemed to be more important, and exhibit stronger effects on zooplanktons in this study.
Effects of macrophytes on zooplankton species
In this study, the appearances of cladocean and copepod adults are probably caused by the suitable environment condition, which is beneficial to the germination of their dormant eggs in the mud. Coupled with a very low predation pressure, they can dominate zooplankton community in this study (De los Ríos-Escalante, 2015). Moreover, the effects of macrophyte architectures on three dominant zooplankton groups were not always consistent. Specially, all macrophytes significantly increased cladocerans, but only M. verticillatum supported more rotifers. For copepods, V. spiralis and C. demersum increased density, but P. malaianus, M. verticillatum and C. demersum showed significant inhibitions on their biomass. These different effects of macrophytes on zooplanktons seemed to mostly depend on zooplankton species and not on macrophyte architecture. Many studies have also confirmed that the preferences of different organisms to submerged macrophytes are different (Lalonde and Downing, 1992; Kuczynska-Kippen and Cerbin, 2003; Cerbin et al., 2007; Cremona et al., 2008). For example, Kuczynska-Kippen and Cerbin (2003) found that the periphyton from M. verticillatumhas the higher biomass than those from Chara and Typha. Greater macroinvertebrate (e.g., gastropods) biomass sometime appeared on macrophytes with a simpler architecture than macrophytes with a complex architecture (Lalonde and Downing, 1992), and this could be related to substrate preferences of Gastropoda toward the tape grass V. americana (Cremona et al., 2008). Daphnia magna is unlikely to prefer to M. verticillatum than other macrophytes because of maturing at a smaller size and producing fewer eggs under the influence of M. verticillatum (Cerbin et al., 2007). Therefore, various organisms exhibited different responses and preferences on different macrophytes, and these responses and preferences might mostly depend on organism species rather than on macrophyte architecture.
In addition, during the stable state (day 40 to 60; period 2) rotifers were almost disappeared or at a low level in all the conditions tested, and large-sized zooplankton (e.g., cladocerans and copepods) dominated the community. These results indicated that cladocerans and copepods owned stronger competitive advantages than rotifers in this study, as confirmed by many previous studies (Gilbert, 1988; Lampert and Sommer, 1997). For example, Lampert and Sommer (1997) determined that cladocerans with a larger body size could gather food more efficiently from the medium, resulting in the starvation of rotifers. Kirk (1997) also found that larger cladocerans had more advantages in resisting starvation over smaller rotifers and thus survived for a longer time under a foodlimited condition. However, some rotifer species still existed in M. verticillatum, not in C. demersum in period 2, indicating that the species characteristic of M. verticillatum might support some rotifer species other than its finely dissected structure.
In this study, our results did not support the hypothesis that submerged macrophytes with complex architecture support more zooplanktons than simple structural macrophytes, and we found that macrophyte architecture is not a good predictor of abundance and biomass of zooplankton. The variances in promotions of C. demersum with complex architecture on cladocerans were not significant with two macrophytes with simple architecture. Moreover, broad-leaved P. malaianus and two dissected-leaved macrophytes could even significantly suppress copepod biomass. Therefore, the effects of submerged macrophytes on zooplanktons mainly depend on macrophyte speciescharacteristics and zooplankton species rather than on macrophyte architecture.