The influence of exotic fish on an emerging zooplankton community: Impacts to zooplankton abundance and composition

The introduction of invasive species into aquatic ecosystems can contribute to changes in predator-prey dynamics, water chemistry, and shift the structure and function of aquatic food webs. This thesis examines the impacts of invasive fish species on emerging zooplankton communities. Using sediment collected from wetlands along the Murrumbidgee River in south-eastern Australia, several mesocosm experiments were set up, which focused on understanding top-down and bottom-up drivers of zooplankton community composition and abundance. The experiments studied the influence of predation and sediment disturbance on the emerging zooplankton community abundance and composition, using four test fish species: Murray cod (Maccullochella peelii), gambusia (Gambusia holbrooki), carp (Cyprinus carpio) and weather loach (Misgurnus anguillicaudatus).
Predation can have an important role in structuring zooplankton communities, and the presence of invasive predators may change food availability patterns within the food web. Using mesocosm experiments, the impact of predation by similarly sized individuals of native (juvenile Murray cod) and introduced (gambusia) fish species on a zooplankton community emerging from re-wetted wetland sediment was tested. It was expected the presence of gambusia and Murray cod to decrease zooplankton abundance, however, being a less selective feeder gambusia would decrease all zooplankton taxa whereas Murray cod would be more selective and therefore decrease certain zooplankton taxonomic groups. The impacts of Murray cod were compared to gambusia and to a Control community where fish were not present. As expected, there were decreases in overall zooplankton abundance in both the Murray Cod and Gambusia treatments when compared to the Control, with gambusia suppressing zooplankton abundance to a greater extent than Murray cod. Gambusia also decreased the abundances of all zooplankton taxa compared with Murray cod which selectively decreased the abundance of cladocerans.
Using mesocosm experiments, the impact of fish density on an emerging zooplankton community was tested. It was expected for there to be a lower zooplankton abundance in the High Fish Density treatments than the Low Fish Density treatments and that prey selectivity would be displayed more in the Low Fish Density treatments due to higher food availability. Zooplankton abundance decreased at all gambusia densities but not for Murray cod when compared to the Control. Gambusia also had density-dependent effects on zooplankton community composition, with there being fewer rotifers with higher fish density.
Sediment disturbance (or bioturbation) by benthic feeding fish can impact zooplankton emergence and alter water chemistry through sediment and nutrient release. To test the influence of juvenile carp and weather loach bioturbation on water chemistry, an experiment was designed comparing mesocosms where fish had free access to the sediment and the zooplankton community (Fish treatment), fish mesocosms where fish were present but their access to sediment and zooplankton was restricted (Water Cue treatment), no fish mesocosms where sediment was disturbed manually (Manual Disturbance treatment) and Control mesocosms where no fish were added and there was no manual disturbance (Control). It was expected that in the Fish and Manual Disturbance treatments, fish addition would increase turbidity due to sediment disturbance and in the Fish and Water Cue treatments fish addition would increase labile nutrients due to ingestions and egestion and follow-on increases in Chlorophyll-a production due to increased nutrient availability. Both carp and weather loach significantly increased turbidity, ammonia concentrations, and also Chlorophyll-a production, in particular in the Fish treatment, then the Water Cue treatment, when compared to the Control treatment where no fish were added. The presence of both species also significantly decreased dissolved oxygen in the Fish treatment and to a lesser extent the Water Cue treatment.
The impacts of carp and weather loach on zooplankton communities in Australian wetlands are less studied than that of their effects on water chemistry; however, given the high densities of carp occurring in many wetland systems, they could play an important role in shaping emerging zooplankton communities. Using the same mesocosm treatments as in the water chemistry experiment, carp and weather loach predation impacts on the emerging zooplankton community abundance and composition were assessed. The indirect effects of the water chemistry on the emerging zooplankton community may have explained some of the changes seen in the community composition. It was expected that the addition of juvenile carp and weather loach would decrease overall zooplankton abundance due to sediment disturbance burying the egg bank, although they would increase abundances in the absence of predation due to changes in nutrient availability. Carp and weather loach both increased zooplankton abundance in the Fish treatment, in particular rotifers. The presence of carp contributed to an increase in cladoceran abundance when predation was restricted (water cue), and this may be due to the increased nutrient availability, whereas carp decreased cladoceran abundance in the free-roaming (fish) mesocosms due to predation. Weather loach did not decrease cladoceran abundance in the Fish treatment; however, the zooplankton community did become more rotifer-dominant.
The four research chapters of this thesis demonstrated that the three invasive fish species, gambusia, carp and weather loach, influenced zooplankton abundance and community composition and altered patterns of nutrient availability, which could have flow-on effects through the food web. In some cases, while overall zooplankton abundances increased, key food groups (cladocerans) occurred in lower abundance. The absence of certain food groups, such as cladocerans, could negatively impact native fish species like Murray cod, who would normally selectively feed on these key zooplankton. This thesis found strong evidence that the invasive species gambusia and carp are able to limit food availability which could put native fish species at risk. This thesis also found that the activity of sediment disturbance and nutrient egestion by carp and weather loach increased nutrient availability and contributed to bottom-up effects on the food web.