Assessing the ecological impact of invasive zebra mussels (Dreissena polymorpha) on native freshwater fish diversity and trophic interactions in a temperate river ecosystem

Project Abstract

In temperate river systems, invasive zebra mussels (Dreissena polymorpha) are known to restructure benthic communities, alter nutrient cycling, and modify fish assemblages through changes in habitat complexity and trophic pathways, yet the magnitude and mechanisms of their effects on native freshwater fish diversity and interactions remain insufficiently resolved. This study combines field surveys, mesocosm experiments, and stable isotope analyses to quantify how zebra mussel colonization influences fish species richness, abundance, and community composition across spatial scales, and to disentangle direct and indirect pathways of impact on trophic interactions. We hypothesize that mussel proliferation reduces macroinvertebrate prey diversity and availability, shifts benthic prey communities toward smaller, slower-reproducing taxa, and increases habitat heterogeneity at the microhabitat scale, thereby modifying predator–prey dynamics and competition among native fish species. Field sampling was conducted across ten reaches with varying mussel densities in a temperate river system over two seasons to capture temporal variability. Fish communities were assessed using standardized electrofishing and seine netting, while mussel abundance, biomass, and occupancy were quantified via quadrat benthic sampling and transect surveys. Invertebrate prey communities were analyzed to determine shifts in functional groups and trophic structure. Stable isotope ratios of nitrogen and carbon in fish tissues and in representative invertebrate taxa were measured to reconstruct trophic food webs and to estimate trophic position and carbon source incorporation under different mussel densities. Mesocosm experiments (n = 24 tanks) manipulated mussel presence (absent, low, high) under controlled flow regimes to isolate effects on fish growth, foraging efficiency, and predator–prey interactions using native cyprinids and darters, with standardized prey assemblages and prey availability manipulated to reflect field conditions. Growth rates, condition indices, and behavioral proxies for foraging were recorded over an eight-week period. Initial findings indicate a negative association between zebra mussel density and overall native fish diversity, with significant declines in drift-feeding and intermediate-sized species in high-density mussel sites. Invertebrate prey diversity and functional richness decreased with mussel colonization, correlating with reduced prey capture efficiency for many fishes. Stable isotope analyses reveal a trophic niche contraction for several species and a shift toward meiofaunal and detrital carbon sources in high-mussel scenarios, suggesting altered energy pathways. Mesocosms demonstrate that mussels can reduce prey encounter rates and microhabitat accessibility for ambush and mid-water foragers, while potentially enhancing shelter for some juvenile taxa, creating nuanced, species-specific responses. The integrated results highlight cascading effects where zebra mussels modulate fish community structure through prey limitation, habitat modification, and altered trophic links, with implications for nutrient cycling and ecosystem resilience under ongoing invasion. The study advances understanding of invasion biology in freshwater systems and provides evidence-based guidance for monitoring programs and management strategies aimed at mitigating biodiversity losses and preserving functional integrity in temperate rivers.

Project Overview

What This Project Is About

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The project looks at how invasive zebra mussels affect native freshwater fish, their variety, and how they feed in a temperate river. It aims to keep things simple: what changes happen in fish communities and who eats whom when zebra mussels are present?

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The Problem It Addresses

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Zebra mussels can clog water pipes, outcompete many species, and alter water quality and habitat structure. These changes may reduce fish diversity and shift food webs, but the exact effects on local fish communities and feeding interactions are not fully understood in many temperate rivers.

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Objectives of the Project

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1. Describe the fish species present before and after mussel invasion (or compare invaded vs. non-invaded sites).
2. Assess changes in fish abundance, size, and diversity associated with mussel presence.
3. Examine trophic interactions to see if fish diets shift due to new prey or competition.
4. Identify potential ecological indicators of mussel impact for monitoring.

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What You Will Do Step by Step

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1) Review basic literature on zebra mussels and river ecosystems. 2) Select study sites with and without mussel presence. 3) Collect fish samples and count species, sizes, and numbers. 4) Sample prey availability and analyze gut contents or stable isotopes to infer diet. 5) Compare diversity and diet between sites. 6) Use simple stats to find patterns. 7) Discuss what the results suggest for management and future work.

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Expected Outcome

Expect clearer evidence of how zebra mussels influence fish diversity and feeding. The project should produce practical findings for river managers, such as which fish groups are most affected and how the food web changes, along with simple recommendations for monitoring and conservation.