The invasive gold clam (Corbicula fluminea) can reach very high population densities, which can harm freshwater ecosystems. Harm that could impacts plants includes the clam's excretion of ammonium and the dense coverage of dead shells.
Impact of ammonium nitrogen excretion
The gold clam excretes a lot of ammonium nitrogen (NH4-N). Some of this ammonium ion (NH4) turns into ammonia (NH3), which can be toxic to invertebrates, fish, and submerged plants. The balance between NH4 and NH3 (together called ammoniacal-N) in water depends on the pH level, with more NH3 produced as pH increases.
To study the impact of clams on native submerged plants, researchers exposed two native species (Potamogeton ochreatus and Myriophyllum triphyllum) to six different concentrations of ammoniacal-N. The plants were kept in a room with a constant temperature of 20°C, 16 hours of light, and 8 hours of dark. The plants were exposed to ammoniacal-N for three days in dosed water and for 21 days in jars. The concentrations of NH4-N were 1, 2, 5, 10, and 20 mg/l, with a control group that had no added NH4-N.
The water was exchanged every three days to keep the concentrations constant. To assess the efficiency of photosynthesis and understand how these plants respond to the environmental conditions researchers (Dr Ben Woodward and Dr Yu Cao) measured photosynthetic yield and efficiency using Pulse-Amplitude-Modulation (PAM) fluorometry.
Preliminary updates - May 2025
Preliminary results suggest that Potamogeton ochreatus is more sensitive to ammoniacal-N than Myriophyllum triphyllum, showing greater declines in photosynthetic efficiency and yield. The final results are expected to be submitted for publication by the end of 2025.
Impact of dead shell coverage of sediments
The dense coverage of dead clam shells may inhibit the germination of native charophyte oospores.
To test this, researchers from NIWA, Dr Yu Cao (Wuhan Botanic Gardens) and mana whenua collected sediments from Lake Tarawera. This lake sediment was known to be rich in viable oospores. They added different densities of clam shells (whole or smashed) to the containers. The clam densities tested were 4000 and 2000 ind./m2 as whole shells and 20000, 10000, 4000, 2000 ind./m2 as smashed shells. Two types of controls were included: one where all germinating charophytes were allowed to grow, and one where all germinating charophytes were removed and counted. This allowed researchers to quantify the total charophyte biomass and the number of germlings (new plants emerging from oospores).
Preliminary updates - May 2025
Photos show that crushed shells may allow more oospore germination, while high densities of smashed shells prevent germlings from emerging.
This experiment is being repeated at NIWA’s Ruakura Aquatic Research Facility and by our international collaborator Dr Yu Cao (Wuhan Botanic Gardens). Results, including datasets, from our initial and repeat experiments will be published by the end of 2025.
