Editor|Can duck literary history
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«——[Introduction·] ——»
Zebra mussels are considered one of the most worrisome freshwater invasive species. Fresh- and saltwater environments native to the Caspian and Black Seas are now widely distributed throughout the Northern Hemisphere. Zebra mussels compete with most native invertebrates for space and food resources, resulting in a decrease in biodiversity, density and biomass, and it alters entire ecosystems through efficient suspended predation behavior.
In the case of reduced phytoplankton, zooplankton and organic matter, the transparency and light permeability of the water body will increase. So zebra mussel invasions can have a huge impact on zooplankton and fish production.
Studies have shown that the key carbon isotope ratio (δ13C) of native fish increases after the invasion of zebra mussels, while the trend of nitrogen isotope ratio (δ15N) is not clear, and the nitrogen isotope ratio generally decreases in most lakes. This result supports the theory of energy path changes in the food chain of lakes after the invasion of zebra mussels.
«——[Sampling and testing methods of isotopes.] ——»
1. Lake Constance fish sampling
Lake Constance is located between Austria, Germany and Switzerland and is part of the Rhine River basin (see Figure 1). The lake has a total area of 536 square kilometers, including 472 square kilometers of the large, deep and scarce resource Lake Constance Upper Bourstance and 63 square kilometers of the smaller, resource-rich Lower Lake Constance. Between 1990 and 2010, the lake underwent several captivity processes and its fish community included at least 30 species, about 10 of which were targeted by fisheries. Among these target species, whitefish is the one with the highest economic value.
Figure 1.Location of Lake Constance in Europe (left, black square) and location of Upper and Lower Lake Constance (right).
This study mainly investigated fish from different fishing areas, including zooplankton, white trout, benthic whitefish, carp and zooplankton from Lake Constance, benthic whitefish, herbivorous red fish, and barracuda common to both lakes, and a total of 2020 white muscle tissue samples were collected.
2. Fish isotope extraction
They are first freeze-dried at -50 °C and subjected to pressure treatment. A mixing mill is then used to grind the sample into a homogeneous powder. The sample powder (0.3-0.4 mg) is weighed into a tin capsule and burned in an isotope ratio mass spectrometer. The combustion process is connected to the mass spectrometer interface via an elemental analyzer.
3. Statistical analysis
Use piecewise least squares linear regression, also known as "breakpoint analysis", to study trends in stable isotope ratios in white trout over time. The aim is to identify potential breakpoints in δ15N and δ13C and test their relationship to ULC settling time. Set the statistical significance of the breakpoint to P<0.05 and test with the appropriate confidence interval.
«——[Analysis of isotope test results.] ——»
1. Analysis of the proportion of fish predation
Of the two lakes, zebra mussels have the highest weight ratio in the digestive tract of catfish, for which the other food groups are relatively less important. For carps, snails are more important than mussels, accounting for almost 60% of all prey. Carp feed more on snails and other benthic macroinvertebrates, not zebra mussels.
For benthic whitefish in the two lakes, the most abundant food group in the digestive tract is benthic macroinvertebrates, especially lateral vertebrates, while whitefish have a diet dominated by insect larvae. Breams feed only on zooplankton, red fish on aquatic plants, and pikes feed only on fish (Table 1).
2. Whitefish breakpoint analysis
In piecewise regression analysis of planktonic whitefish sampled annually between 2014 and 2020, a significant breakpoint was found in the slope of the 2017 delta 13C value, one year after the zebra mussel established in the lake. On the other hand, δ15N remained relatively stable in the late invasion of zebra mussels.
Figure II
Figure 2 shows the segmented regression of white trout δ13C with the regression line shown in the figure above. No significant breakpoints were observed at δ15N, as shown in the figure below. The vertical dotted line indicates the period during which zebra mussels were established (between 2016 and 2017).
Each point represents a separate measurement and the δ13C value has been adjusted for the 2016 reference year using the Suess effect. Significant differences in δ15N and δ15N were observed in all other fish species before and after the establishment of herring, and in all other fish that were analyzed. In both lakes, the delta 15N value of benthic whitefish increased from 2014 to 2020 (Figure 3), while the value of benthic whitefish decreased over the same period (Figure 3).
A significant difference in δ15N was observed only in the fish of LLC (Figure 3). Both lakes had lower δ15N values compared to 2014, similar to the red fish observed only in ULC (Figure 3). For barracudas in both lakes, δ15N increased from 2014 to 2020 (Figure 3).
These observations were confirmed by GLM analysis of δ15N (adjusted r2=0.31, n=346, P<0.0001), indicating that periodicity and lake had a significant effect on the model results (P<0.0001). The overall δ15N value recorded by LLC fish was significantly higher than that of ULC (t-test, α<0.05), and the post-invasion δ2020N value of zebra mussels was significantly lower in both lakes than in the pre-invasion period (t-test, α<0.05). Feeding behavior and the interaction between feeding behavior and cycle had no effect on the model results.
Figure 3
3. Mussel stable isotope value
Table 3 shows significant differences in stable isotope values for zebra mussels and snails in the two lakes. Zebra mussels have lower δ15N values than snails, while only large pond snails exhibit lower δ13C values than fish.
Table 3
4. Isotope niche overlap
According to Figure 4, the core isotope niche of each fishing area changed in Lake Constance before and after the zebra mussel invasion. In the pre-invasion ULC, SEAc-corrected standard elliptical areas ranged from 1.6 for zooplankton fish to 6.6 for benthic fish, while in LLC the range for fish-eaters was 2.7 and benthic-zooplankton whitefish was 3.3 (Figure 4).
In 2020 after the zebra mussel invasion, the SEAc value was marginally higher, with ULC from 2.0 for fish-eating species to 7.8 for benthic fish, and LLC from 2.0 for fish-eating fish to 5.1 for benthic fish. Prior to invasion, 19.1% ecological niche overlap was observed in ULC and 51.2% in LLC (Figure 4), but by 2020, this overlap was not evident in either ULC or LLC despite the larger standard elliptical area (Figure 4).
Figure 4
Figure 4 shows a double plot of the mean isotopic values of different fish species above and below Lake Constance (ULC) and below (LLC) before and after the invasion of zebra mussels (2014 and 2020, respectively). The horizontal and vertical bars represent the ± standard deviation of the total combined data. Standard elliptical areas (SEAcs), which represent the core isotope niche of each fishing area, account for about 40% of the data.
«——[Discussion of isotope test results.] ——»
After the zebra mussel invasion, Lake Constance's white trout were forced into coastal waters, similar to their sister species in the Great Lakes. The change in δ13C was confirmed by breakpoint analysis and richness, i.e. shortly after the mussel invasion (within one year), the δ13C value of white trout changed significantly. Primary producers in shallow waters are known to be rich in δ13C relative to deep waters, and this trend appears to apply to higher nutrient levels as well.
Therefore, δ13C values can be used as indicators of dietary location, and higher δ13C values indicate increased importance of nearshore sources. Interestingly, in Lake Constance, no changes in feeding habits of pelagic whitefish were observed. Analysis of stomach contents revealed that no macrobenthic animals were found and that the diet of white trout remained predominantly zooplankton, consistent with decades of observations.
The also observed increase in δ13C is explained by changes in energy pathways throughout the lake system, including pelagic zooplankton, now more driven by benthic primary producers. Therefore, analyzing changes in stable isotopic patterns in fish may be part of an increase in δ13C, involving primary and secondary consumers of nearshore consumers.
If this is the case, profound changes in resources at the bottom of the food chain may be responsible for the observed changes, rather than changes in dietary composition. The δ15N model showed that zebra mussel invasion significantly altered isotopic values in other analyzed fish in Lake Upper and Lower Lake Constance. Similar delta 15N changes have been observed in other lakes, such as Lake Simcoe in Canada. For herbivorous redfish fish in Lake Constance Upper Boden, the observed changes may be related to alterations in benthic communities after the invasion of Quaga.
For carp, loach, and benthic whitefish, changes in isotopic characteristics may be associated with changes in benthic macrobenthic communities, as the aggregation of zebra mussels rapidly develops into three-dimensional reef-like structures that provide shelter and food for invertebrates. This change at the bottom of the food chain can easily affect consumers with higher levels of nutrition, such as fish-eating fish, and thus may explain the difference in delta 15N values of fish-eating fish in Lake Constance before and after the Kwaga invasion.
While there are objective effects, such as the effects of sewage or agricultural inputs in watersheds or increases in nitrogen-fixing substances in algal communities, which may lead to changes in the delta 15N values of fish, there is currently no evidence that the effects are widespread and therefore cannot explain the observed changes in different directions. In fact, there is no single explanation for the change in δ15N values for all different fish.
Lake pollution
Therefore, further research is needed to determine the causes of the different δ15N values, especially for benthic fish in Lake Constance. Changes in isotopic characteristics of benthic fish endemic to Lake Constance indicate general changes in feeding behaviour. Prior to zebra mussel invasion, benthic whitefish found only a zebra mussel in a small number of individuals.
In recent years, more than 1,000 zebra mussel individuals have been found in large benthic whitefish. The δ13C model showed that consumers of zebra mussels significantly exhibited lower δ13C values, which were related to the interaction of consumers and feeding behavior of zebra mussels. Analysis of gastric contents showed that carp and loach from Lake Constance consumed large quantities of zebra mussels. Carp seem to have adapted from the previously preferred diet of zooplankton and insect larvae to this new, surplus food source.
According to the above study, the fish community of Lake Constance underwent significant changes after the invasion of zebra mussels, which was confirmed in stable isotope analysis. The delta 13C value of pelagic whitefish showed a significant change, indicating a change in energy source and energy path. Changes in isotopic characteristics in benthic and endemic benthic fish indicate changes in feeding behaviour, including feeding on zebra mussels.
«——[·Conclusion·] ——»
This study proves that the large aquatic ecosystem of Lake Constance has significantly changed its energy path after the invasion of zebra mussels. For the white trout whitefish, the observed increase in stable isotope values is most likely due to the primary species of benthic origin, not to changes in its food composition.
Other fish, including benthic whitefish, carp and tinkerel, have shown significant changes in feeding behavior and now rely at least in part on zebra mussels as a food source.
These findings are critical to our understanding of the impact of invasive species on ecosystems, as well as ecosystem management. We should pay close attention to similar changes and take appropriate management measures to maintain the health and balance of the ecosystem.
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