Functional and Taxonomic Differentiation of Macrophyte Assemblages Across the Yangtze River Floodplain Under Human Impacts

Beyond taxonomic diversity patterns - investigating how α and β components of macrophyte functional diversity respond to environmental gradients in lotic ecosystems of Greece

In addition to quantifying the taxonomic diversity of aquatic communities, understanding the patterns of alpha functional diversity (α-diversity) and exploring changes in functional dissimilarity (β-diversity) can improve our understanding on how ecosystems respond to environmental changes. In this study, we quantified functional alpha (α) and beta (β) diversity of macrophytic assemblages from river sites in Greece and then, examined relationships with water quality parameters and hydromorphological factors. We assigned 6 traits (Ellenberg nutrients indicator, Ellenberg light indicator, growth form, leaf size, leaf type, fruit size) to a total of 36 hydrophyte species and calculated three indices of functional diversity (functional richness, functional dispersion and functional evenness). We also estimated the total β-functional diversity and its' main components, turnover and nestedness. To assess the effects of water quality (including physical and chemical variables) we used Generalized Additive Models (GAM) for alpha functional diversity indices and Generalized Dissimilarity Models (GDM) for beta functional diversity. We performed Kruskal-Wallis tests and permutational multivariate analysis of variance (PERMANOVA) to search for significant variations of α- and β-diversity among the hydromorphological factors. Our results showed that macrophyte growth form and light preference were important trait characteristics that explained a large share of the total variance of functional composition. We also found relatively low taxonomic and functional richness, whereas taxonomic and functional dissimilarity were mostly attributed to species turnover, which expresses the changes in taxonomic and functional composition. We found significant relationships between functional dispersion and functional evenness with pH and oxygen saturation, whereas functional dissimilarity was driven only by geographic distance, although the GDM explained a small portion of the total variance. Functional richness, dispersion and evenness were significantly higher at systems with fine substrates and deep waters with low or high flow compared to systems with coarser substrates and riffle habitats. We also found significant variation in functional dissimilarity among the hydromorphological factors, although much of the total variance remained unexplained. Overall, our study highlights the importance of considering the functional diversity of aquatic plant assemblages within the frame of freshwater monitoring and conservation plans.

Application of artificial spawning substrates to support lacustrine fish recruitment and fisheries enhancement in a Chinese lake

Lake littoral zones with abundant submerged vegetation are considered to harbor diverse biotic communities and additionally serve as spawning grounds for important fishery resources. In recent decades, due to various human activities submerged vegetation in lakes have declined dramatically resulting in the function loss of spawning grounds. In order to enhance the recruitment of lacustrine fish (mainly Carassius auratus and Cyprinus carpio) and the proliferation of fishery resources, we explored a method of constructing artificial spawning ground based on artificial spawning substrate in lake. The results of present study indicate that the artificial substrates with complex and dense three-dimensional structure which have enough space to enable material exchange and oxygen circulation could be efficient to enhance fish recruitment. The water temperature of spawning peaks of artificial substrates were within the range of 20 ~ 25°C, and the water temperature provides a basis for the deployment period of artificial spawning grounds. The spawning substrate in the upper layer of water column was more attractive for fish to spawn. The above results provide guidance for the deployment of artificial spawning grounds from both temporal and spatial scales. Observations on the behavior of aggregations found that spawning activities mainly occurred at night and in the early morning. The abundance of larger individuals (>20 cm), as potential spawning groups, increased during this period. The above findings are informative to the construction and management of artificial spawning ground in lakes.

Macroinvertebrate diversity and ecosystem functioning across the eutrophication gradients of the middle and lower reaches of Yangtze River lakes (China)

Biodiversity, which strengthens ecosystem stability, ecosystem function, and ecosystem services, is threatened by anthropogenic perturbation and climate change worldwide. However, despite the study of the role of biodiversity in multiple facets of freshwater ecosystems, the linkages between macroinvertebrates diversity and ecosystem functioning have not yet been well-assessed in eutrophication gradients of lowland river-floodplain systems. In this study, we have examined the relationship between macroinvertebrates diversity (species diversity, functional diversity, phylogenetic diversity) and macroinvertebrates biomass across the three typically featured eutrophication gradients: "MACROPHYTE," "TRANSITION," and "PHYTOPLANKTON," of floodplain lakes in the middle and lower reaches of the Yangtze River (China). Our results suggest that macroinvertebrates diversity in three different lacustrine conditions, biomass, and the relationship between diversity and biomass varied along eutrophication gradients. Functional richness and variance (divergence in taxon community) were the two important macroinvertebrate diversity indices, which accounted for the largest amount of variation in the biomass (63% in PHYTOPLANKTON lakes and 57% in MACROPHYTE lakes, respectively). We also found that the macrophyte coverage is more important than the relative abundance in maintaining the macroinvertebrates diversity and biomass in lowland Yangtze floodplain lake systems, while the relative abundance of macrophyte would change the BEF relationship. Our results demonstrate the functional performance of Yangtze River lakes, which would change with increased nutrient loading and decreased macrophyte coverage and would highlight the significance of the restoration of macrophytes to reduce nutrient loads.

Heat waves rather than continuous warming exacerbate impacts of nutrient loading and herbicides on aquatic ecosystems

Submerged macrophytes are vital components in shallow aquatic ecosystems, but their abundances have declined globally. Shading by periphyton and phytoplankton/turbidity plays a major role in this decline, and the competing aquatic primary producers are subject to the complex influence of multiple stressors such as increasing temperatures, nutrient loading and herbicides. Their joint impact has rarely been tested and is difficult to predict due to potentially opposing effects on the different primary producers, their interactions and their grazers. Here, we used 48 mesocosms (2500 L) to simulate shallow lakes dominated by two typical submerged macrophytes, bottom-dwelling Vallisneria denseserrulata and canopy-forming Hydrilla verticillata, and associated food web components. We applied a combination of nutrient loading, continuous warming, heat waves and glyphosate-based herbicides to test how these stressors interactively impact the growth of submerged macrophytes, phytoplankton and periphyton as competing primary producers. Warming or heat waves alone did not affect phytoplankton and periphyton abundance, but negatively influenced the biomass of V. denseserrulata. Nutrient loading alone increased phytoplankton biomass and water turbidity and thus negatively affected submerged macrophyte biomass, particularly for V. denseserrulata, by shading. Glyphosate alone did not affect biomass of each primary producer under ambient temperatures. However, heat waves facilitated phytoplankton growth under combined nutrient loading and glyphosate treatments more than continuous warming. As a consequence, H. verticillata biomass was lowest under these conditions indicating the potential of multiple stressors for macrophyte decline. Our study demonstrated that multiple stressors interactively alter the biomass of primary producers and their interactions and can eventually lead to a loss of macrophyte communities and shift to phytoplankton dominance. These results show the risks in shallow lakes and ponds in agricultural landscapes and underline the need for multiple stressor studies as a base for their future management.

Seasonal patterns of taxonomic and functional beta diversity in submerged macrophytes at a fine scale

Spatiotemporal variation in community composition is of considerable interest in ecology. However, few studies have focused on seasonal variation patterns in taxonomic and functional community composition at the fine scale. As such, we conducted seasonal high-density sampling of the submerged macrophyte community in Hongshan Bay of Erhai Lake in China and used the generalized dissimilarity model (GDM) to evaluate the effects of environmental factors and geographic distance on taxonomic and functional beta diversity as well as corresponding turnover and nestedness components. At the fine scale, taxonomic turnover and nestedness as well as functional turnover and nestedness showed comparable contributions to corresponding taxonomic and functional beta diversity, with different importance across seasons. All taxonomic and functional dissimilarity metrics showed seasonal variation. Of note, taxonomic beta diversity was highest in summer and lowest in winter, while functional beta diversity showed the opposite pattern. Taxonomic and functional turnover showed similar change patterns as taxonomic and functional beta diversity. Taxonomic nestedness was low in summer and high in winter. Functional nestedness was also lower in summer. These results suggest that under extreme environmental conditions, both turnover and nestedness can exist at the fine scale and seasonal community composition patterns in submerged macrophytes should be considered. Future investigations on community assembly mechanisms should pay greater attention to long-term dynamic characteristics and functional information.

Functional differentiation accompanies taxonomic homogenization in freshwater fish communities

The addition of nonnative species and loss of native species has modified the composition of communities globally. Although changes in β-diversity have been well documented, there is a need for studies incorporating multiple time periods, more than one dimension of biodiversity, and inclusion of nestedness and turnover components to understand the underlying mechanisms structuring community composition and assembly. Here, we examined temporal changes in functional dissimilarity of fish communities of the Laurentian Great Lakes and compared these changes to those of taxonomic dissimilarity by decade from 1870 to 2010. Jaccard-derived functional dissimilarity index was used to quantify changes in functional β-diversity within communities, between all possible pairs of communities, and using a multiple-site index among all communities. β-diversity was partitioned into components of nestedness and turnover, and changes were examined over time. Similar to patterns in taxonomic dissimilarity, each community functionally differentiated from the historical community of 1870, with Lake Superior changing the most (~24%) and Lake Ontario the least (~14%). Although communities have become taxonomically homogenized, functional β-diversity among all communities has increased over time, indicating functional differentiation. This is likely due to functional similarity between the communities being historically high (i.e., ~88% similar in 1870). The higher taxonomic relative to functional turnover indicates that the species being replaced between communities are functionally redundant, which could occur given the harsh environmental conditions of the region and/or as a result of the recent glacial history of the region. High functional nestedness across communities reflects dispersal limitations, with smaller communities being functional subsets of large communities closer to source populations. The functional differentiation observed is likely due to nonnative species with functional traits unique to the region establishing or the loss of functionally redundant native species; however, it is important to note that patterns of homogenization were periodically observed through time. Our study demonstrates the possible factors regulating diversity in the Laurentian Great Lakes fish communities, that patterns of taxonomic and functional β-diversity are dynamic over time and vary in the magnitude and direction of change, and that taxonomic diversity should not be used to predict changes in functional diversity.

Vegetation changes in temperate ombrotrophic peatlands over a 35 year period

Global changes in climate and land use are occurring at an unprecedented rate, often triggering drastic shifts in plant communities. This study aims to reconstruct the changes that occurred over 35 years in the plant communities of temperate bogs subjected to indirect human-induced disturbances. In 2015-17, we resurveyed the vascular flora of 76 plots located in 16 bogs of southern Québec (Canada) first sampled in 1982. We evaluated changes in species richness, frequency of occurrence and abundance, while considering species shade-tolerance and preferential habitat. We calculated beta diversity as between-site similarities in composition, and evaluated differences between the two surveys using tests for homogeneity in multivariate dispersion. We found a significant increase in species richness and beta diversity over the last 35 years associated with major species turnovers, indicating a biotic differentiation of the Sphagnum-bog plant communities. These changes were mostly associated with an increase in the abundance and frequency of shade-tolerant and facultative species, suggesting a global phenomenon of woody encroachment. Because the observed changes occurred in a few decades on sites free of in situ human disturbances, we suggest that they were likely induced by the synergic effect of the agricultural drainage occurring in the surrounding mineral soils, climate warming, and nitrogen atmospheric depositions. We also believe that further changes are to be expected, as the triggering factors persist. Finally, our results highlight the need for increased bog conservation or restauration efforts. Indeed, a rise in beta diversity due to the introduction of nearby terrestrial species could induce biotic homogenization of the bog flora with that of surrounding habitats and ultimately impoverish the regional species pool.

Local and regional drivers of turnover and nestedness components of species and functional beta diversity in lake macrophyte communities in China

Beta diversity describes the variation in species composition between sites and is often influenced by both local and regional processes. Partitioning beta diversity into turnover (species replacement between sites) and nestedness (richness difference between sites) components may enhance our understanding of the mechanisms behind the local and regional drivers determining species composition across spatial scales. We sampled macrophyte communities in 24 lakes in two regions (Yangtze River basin and Yunnan-Guizhou plateau) of China covering broad climate and nutrient gradients. Based on both species and functional approaches, we calculated multiple-site beta diversity using the Sørensen dissimilarity index and partitioned it into turnover and nestedness coefficients crossed with two nested spatial scales: among depths within transects (transect scale) and among transects within lakes (lake scale). The overall species beta diversity and functional beta diversity (i.e. Sørensen coefficient) were significantly lower and thus more homogeneous at lake scale. Across spatial scales, species beta diversity was mainly explained by turnover patterns (56-61%) and functional beta diversity primarily by nestedness patterns (58-65%). Both local and regional drivers contributed to structuring species and functional beta diversity patterns, largely through changes in species turnover and functional nestedness, respectively. Overall, we observed a significant increase in species beta diversity and its turnover component while a decreasing trend in functional beta diversity and its nestedness component at high altitude. Our results further emphasized that the species beta diversity and its turnover component decreased at high total phosphorus concentration (TP) across the two spatial scales, while the functional beta diversity and its nestedness component decreased at high TP at the transect scale. We conclude that understanding of the relative role of local and regional drivers in determining macrophyte diversity patterns may help managers to select the most appropriate conservation strategies for preservation of biodiversity varying with the scale in focus.

Spatiotemporal Dynamics of Submerged Aquatic Vegetation in a Deep Lake from Sentinel-2 Data

We mapped the extent of submerged aquatic vegetation (SAV) of Lake Iseo (Northern Italy, over the 2015–2017 period based on satellite data (Sentinel 2 A-B) and in-situ measurements; the objective was to investigate its spatiotemporal variability. We focused on the southern sector of the lake, the location of the shallowest littorals and the most developed macrophyte communities, mainly dominated by Vallisneria spiralis and Najas marina. The method made use of both in-situ measurements and satellite data (22 Sentinel 2 A-B images) that were atmospherically corrected with 6SV code and processed with the BOMBER (Bio-Optical Model-Based tool for Estimating water quality and bottom properties from Remote sensing images). This modeling system was used to estimate the different substrate coverage (bare sediment, dense stands of macrophytes with high albedo, and sparse stand of macrophytes with low albedo). The presented results substantiate the existence of striking inter- and intra-annual variations in the spatial-cover patterns of SAV. Intense uprooting phenomena were also detected, mainly affecting V. spiralis, a species generally considered a highly plastic pioneer taxon. In this context, remote sensing emerges as a very reliable tool for mapping SAV with satisfactory accuracy by offering new perspectives for expanding our comprehension of lacustrine macrophyte dynamics and overcoming some limitations associated with traditional field surveys.

Spatially Structured Environmental Variation Plays a Prominent Role on the Biodiversity of Freshwater Macrophytes Across China

Different non-mutually exclusive mechanisms interactively shape large-scale diversity patterns. However, our understanding of multi-faceted diversity and their determinants in aquatic ecosystems is far from complete compared to terrestrial ones. Here, we use variation partitioning based on redundancy analysis to analyze the relative contribution of environmental and spatial variables to the patterns of phylogenetic, taxonomic, and functional diversity in macrophyte assemblages across 214 Chinese watersheds. We found extremely high spatial congruence among most aspects of biodiversity, with some important exceptions. We then used variation partitioning to estimate the proportions of variation in macrophyte biodiversity explained by environmental and spatial variables. All diversity facets were optimally explained by spatially structured environmental variables, not the pure environment effect, implying that macrophyte are taxonomically, phylogenetically, and functionally clustered in space, which might be the result of the interaction of environmental and/or evolutionary drives. We demonstrate that macrophytes might face extensive dispersal limitations across watersheds such as topography and habitat fragmentation and availability.