Iris Stiers

Impact of three aquatic invasive species on native plants and macroinvertebrates in temperate ponds

Biological plant invasions pose a serious threat to native biodiversity and have received much attention, especially in terrestrial habitats. In freshwater ecosystems impacts of invasive plant species are less studied. We hypothesized an impact on organisms from the water column and from the sediment. We then assessed the impact of three aquatic invasive species on the plants and macroinvertebrates: Hydrocotyle ranunculoides, Ludwigia grandiflora and Myriophyllum aquaticum. Our research on 32 ponds in Belgium indicated that the reduction in the native plant species richness was a common pattern to invasion. However, the magnitude of impacts were species specific. A strong negative relationship to invasive species cover was found, with submerged vegetation the most vulnerable to the invasion. Invertebrate richness, diversity and abundance were measured in sediments of invaded and uninvaded ponds along a gradient of H. ranunculoides, L. grandiflora, and M. aquaticum species cover. We found a strong negative relationship between invasive species cover and invertebrate abundance, probably due to unsuitable conditions of the detritus for invertebrate colonization. Taxonomic compositions of aquatic invertebrate assemblages in invaded ponds differed from uninvaded ponds. Sensitive benthos, such as mayflies were completely absent in invaded ponds. The introduction of H. ranunculoides, L. grandiflora, and M. aquaticum in Belgian ponds has caused significant ecological alterations in the aquatic vegetation and the detritus community of ponds.

Harmonia + and Pandora + : risk screening tools for potentially invasive plants, animals and their pathogens

AbstractGiven the large number of alien species that may potentially develop into invasives, there is a clear need for robust schemes that allow to screen species for such risks. The Harmonia+ framework presented here brings together 30 questions that refer to distinct components of invasion. Together, they cover the stages of introduction, establishment, spread, and multiple kinds of impacts, viz. referring to the health of the environment (including wild species), cultivated plants, domesticated animals and man. In a complete assessment, input is provided by choosing among predefined ordinal answers and by supplementing these with textual clarification. Uncertainty is covered by indicating levels of confidence. By converting answers into scores, which are then condensed into summary statistics, Harmonia+ allows for quantitative output on stage-specific and general risks. Test assessments on five species emerging in Belgium showed the perceived environmental risks of Procambarus clarkii to be highest (0.72), and that of Threskiornis aethiopicus to be lowest (0.13). Given the considerable parallels that exist between invasive alien species and emerging infectious diseases, we additionally created Pandora, which is a risk analysis scheme for pathogens and parasites. It consists of 13 key questions and has the same structure as Harmonia+. Since diseases play a paramount role in biological invasions, results of Pandora assessments may feed into Harmonia+ through a slightly adapted, host-specific version named Pandora+. Harmonia+, Pandora and Pandora+ may be used both for prioritization purposes and for underpinning detailed risk analyses, and can be consulted online through http://ias.biodiversity.be.

Biomanipulation as a nature-based solution to reduce cyanobacterial blooms

We considered the limnological literature for an overview of biomanipulation methods that were implemented to avoid or reduce cyanobacterial bloom development in ponds and lakes. For this purpose, we reviewed 48 publications representing 34 whole-lake and large-scale case studies of different biomanipulation approaches clearly mentioning the extent of a cyanobacteria bloom problem and the cyanobacteria taxa involved. This delivered complementary information to the suite of review papers already providing elaborated syntheses on biomanipulation and associated ecotechnological measures as a restoration tool for overall eutrophication reduction and control. We considered nature-based solutions such as fish removal and associated water drawdown, addition of piscivorous fish, filter-feeding planktivorous fish, Daphnia or bivalves, re-introduction of macrophytes and a combination of accompanying restoration methods. Reasons for success or failure to control cyanobacterial blooms of especially Anabaena,Pseudanabaena, Aphanizomenon, Aphanocapsa, Limnothrix, Microcystis, Oscillatoria or Spirulina spp. could be explained through bottlenecks encountered with fish removal, stocking densities, cascading effects, associated zooplankton grazing, diet shifts away from cyanobacteria, macrophyte recovery, nutrient or pH status. Threshold values to avoid failures are synthesized from experiments or monitoring studies and presented in a conceptual scheme about cyanobacteria reduction through (1) direct abatement of existing blooms and forcing/maximization of biotic key interactions (2) reducing risk of blooms and improving lake or pond multi-functionality and (3) avoiding blooms, balancing biotic communities and enhancing existing ecosystem services. More information will be required on temporal dynamics and abundances of cyanobacteria taxa in whole-lake pre- and post-biomanipulation conditions to better evaluate the applicability and effectiveness of such nature-based solutions.

Phytoplankton functional dynamics in a shallow polymictic tropical lake: the influence of emergent macrophytes

This study aimed at investigating the phytoplankton dynamics by employing taxonomic and functional models to test the hypothesis that emergent macrophytes could influence phytoplankton dynamics by increasing the stability of water column and harboring zooplankton. It was also hypothesized that functional models are better than taxonomic approaches to depict phytoplankton community structure in a polymictic tropical lake (Ziway, Ethiopia) with substantial macrophyte cover. Twelve sampling sites, extending from the macrophyte vegetation (Typha latifolia and Phragmites australis) covered littoral part to an open water side of the lake, were monitored monthly (January to August, 2016) for physical, chemical and biological parameters. A total of 93 taxa, distributed among six major taxonomic groups, 14 Reynolds functional groups and six morphology-based functional groups, were identified. The phytoplankton groups, which prefer stable water column and tolerate grazing dominated towards the macrophyte zones, while the phytoplankton capable of surviving strong mixing succeeded towards the completely mixing open water side of the lake. The results of the present study also suggest that the functional classification schemes are robust ways of detecting phytoplankton dynamics along the macrophyte-open water gradient in shallow tropical lakes.

Spatial and temporal distribution of submerged aquatic vegetation in a tropical coastal lagoon habitat in Viet Nam

Abstract Submerged aquatic vegetation (SAV) is considered as a keystone habitat, contributing significantly to structure and function of coastal lagoons. However, limited understanding of the factors driving SAV distribution and abundance across a wide range of salinity in tropical coastal lagoons has restricted the effectiveness of managing and preserving the ecosystem services in coastal lagoon habitats. This study examined the distribution and abundance of SAV species in the growing season in relation to water physico-chemical variables and grain sizes of sediment types in a tropical lagoon in Viet Nam. The results revealed that Najas indica and Halophila beccarii were the dominant species in the community of 7 SAV species, accounting for 70% of the total cover and 55% of the total biomass sampled. Variation partitioning showed that both water and sediment variables were important in explaining spatial distribution and abundance of SAV species across the coastal lagoon. Salinity was the most significant predictor variable that accounted for the variation of SAV species data. The study implied that changes of salinity and silt (versus sand) particles can lead to different SAV assemblages in the lagoon.

Clonal growth strategy, diversity and structure: A spatiotemporal response to sedimentation in tropical Cyperus papyrus swamps

Land degradation and soil erosion in the upper catchments of tropical lakes fringed by papyrus vegetation can result in a sediment load gradient from land to lakeward. Understanding the dynamics of clonal modules (ramets and genets) and growth strategies of plants on such a gradient in both space and time is critical for exploring a species adaptation and processes regulating population structure and differentiation. We assessed the spatial and temporal dynamics in clonal growth, diversity, and structure of an emergent macrophyte, Cyperus papyrus (papyrus), in response to two contrasting sedimentation regimes by combining morphological traits and genotype data using 20 microsatellite markers. A total of 636 ramets from six permanent plots (18 x 30 m) in three Ethiopian papyrus swamps, each with discrete sedimentation regimes (high vs. low) were sampled for two years. We found that ramets under the high sedimentation regime (HSR) were significantly clumped and denser than the sparse and spreading ramets under the low sedimentation regime (LSR). The HSR resulted in significantly different ramets with short culm height and girth diameter as compared to the LSR. These results indicated that C. papyrus ameliorates the effect of sedimentation by shifting clonal growth strategy from guerrilla (in LSR) to phalanx (in HSR). Clonal richness, size, dominance, and clonal subrange differed significantly between sediment regimes and studied time periods. Each swamp under HSR revealed a significantly high clonal richness (R = 0.80) as compared to the LSR (R = 0.48). Such discrepancy in clonal richness reflected the occurrence of initial and repeated seedling recruitment strategies as a response to different sedimentation regimes. Overall, our spatial and short-term temporal observations highlighted that HSR enhances clonal richness and decreases clonal subrange owing to repeated seedling recruitment and genets turnover.

Isotropic and anisotropic processes influence fine-scale spatial genetic structure of a keystone tropical plant

Propagule dispersal shapes fine-scale spatial genetic structure (FSGS), which is mostly a function of physical distance between individuals. However, such a relation with distance may not occur when asymmetric processes, for instance, wind direction during dispersal, are involved. We combined genetic and seed dispersal data to assess the pattern of FSGS in the keystone species Cyperus papyrus as a function of distance and wind direction. Differences in FSGS between adults and juveniles were detected as a function of distance between individuals. Fine-scale spatial genetic structure was also found to be a function of wind direction during dispersal. Our findings further suggest post-dispersal processes could also influence FSGS.