Repository logo

Invasive non-native species and the management and exploitation of freshwater ecosystems



Change log



The effects of climate warming, hydrological intensification, and eutrophication are driving unfavourable shifts in phytoplankton communities. In particular, harmful algal blooms (HABs), dominated by toxic cyanobacteria, are becoming increasingly common and often lead to ecosystem collapse and unsafe drinking water. Freshwater ecosystems are also vulnerable to the establishment of invasive non-native species (INNS), whose prevalence has rapidly accelerated through the expansion of global travel, trade, and transportation. INNS can drive significant changes in habitat suitability and native community composition. I investigated how INNS affect the tipping points of freshwater ecosystems, between clear water and phytoplankton-dominated states, and influence phytoplankton community structure and cyanobacteria abundance.

Through a meta-analysis of the global impacts of INNS on freshwater lake ecosystems, I identified that molluscs were potential drivers of a clear water state in lake ecosystems, encouraging system recovery and delaying ecosystem collapse. Invasive fish and crustaceans, however, were identified as potentially pushing systems towards a turbid state and resisting recovery.

I further assessed the effects of invasive molluscs by quantifying the impacts of quagga mussels (Dreissena rostriformis bugensis) in UK reservoirs. I identified that impacted sites demonstrated: an increase in overall reactive silica and soluble reactive phosphorus concentrations; reductions in overall diatom, cyanobacteria and Aphanizomenon sp. abundance; and an increase in chlorophyll a concentration. However, no significant changes were seen in overall green algae, Microcystis sp., Anabaena sp., or Euglena sp. abundance, ammonium concentration or turbidity.

The selective grazing behaviour of quagga mussels has been shown to vary between studies, an effect hypothesised to be driven by phenotypic plasticity or genotype selection between mussel populations. I explored putative population level differences in the discriminatory grazing behaviour of quagga mussels by observing mussels from both lentic and lotic environments, as hydrology is a known driver of phenotypic plasticity in mussels. I show that different populations of quagga mussels have the potential to drive different outcomes in phytoplankton communities. While both populations expressed comparable clearance rates and cleared green algae (Chlorella vulgaris) at a higher rate than cyanobacteria (Synechococcus elongatus), mussels from lotic environments ejected significantly greater quantities of cyanobacteria in pseudofaeces. Rejection of cyanobacteria by invasive mussels may drive HABs.

Beyond invasive mussels, I also assessed the allelopathic effect of invasive plants Hydrocotyle ranunculoides and Crassula helmsii on phytoplankton communities. The allelopathic effect of each plant was tested in both a whole, and crushed state, to test for natural excretion of allelochemicals. In its whole state, H. ranunculoides supressed the growth of green algae (C. vulgaris), whereas crushed C. helmsii supressed the growth rate of both green algae and cyanobacteria (Synechocystis sp.). The species-specific nature of these allelopathic interactions suggests that the use of allelopathy for controlling phytoplankton abundance may have to be assessed on a case-by-case basis. Although, a sophisticated understanding of this differential sensitivity may also allow for some targeted management of phytoplankton populations towards specific desirable end points.

Finally, I explored the potential role of invasive zebra mussels (Dreissena polymorpha) in the novel concept of Synthetic Ecology. I investigated how zebra mussel grazing behaviour could be harnessed to maintain favourable conditions for the cultivation of commercially valuable algae species in outdoor raceways by supressing the growth of competing cyanobacteria. Their potential was confirmed through the consistent preference for cyanobacteria (Synechocystis sp.) over green algae (C. vulgaris) observed across all treatment conditions.

The results of this thesis suggest, in certain contexts and where risks are appropriately managed, there may be opportunities to harness INNS in order to drive favourable changes in the alternative equilibria of freshwaters, reducing both overall phytoplankton and cyanobacteria abundance in biologically depleted ecosystems.





Aldridge, David


Invasive species, Freshwater ecosystem, Cyanobacteria, Zebra mussel, Quagga mussel, Allelopathy, Alternative equilibria


Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge
Biotechnology and Biological Sciences Research Council (1804793)