Investigation of pelagic food web resilience and potential of biomanipulation techniques to help restore ecological integrity in two eutrophic lakes in New Zealand

Lakes are exposed to a wide range of human pressures including invasive species and nutrient enrichment. Restoration of eutrophic lakes is commonly attempted, but many lakes have failed to recover when external nutrient loading is reduced. In such lakes, the manipulation of pelagic food webs to reduce phytoplankton biomass has been useful, resulting in improved water clarity. To devise effective top-down biomanipulation techniques, a combination of long-term and short-term assessments are required. Long-term studies are useful in examining the effects of non-indigenous fish and nutrient enrichment, as these dynamics can take place over several decades. However, intra-annual assessments of fish, zooplankton and phytoplankton communities help in identification of key species, their significant life stages, and the timings of trophic cascades. I reconstructed the historical pelagic food web structure and dynamics in Lake Hayes and Lake Johnson, both seasonally stratified eutrophic lakes, using palaeolimnological techniques. Then, I correlated these reconstructions with limnological and fisheries records. Results revealed that the study lakes were in a relatively stable, oligotrophic state before the 1940s, and that, contrary to prediction, game fish introduction showed little discernible effect on phytoplankton assemblages and community composition for more than seven decades. The effects of agricultural nutrient enrichment became apparent in the 1940s substantially increasing inferred phytoplankton biomasses. My results also confirmed that the introduced Daphnia pulicaria first established in Lake Hayes in 1972 and in Lake Johnson in 1992, coexisting with the native Daphnia species and leading to an average 2-fold increase in Daphnia spp. abundance. Contrary to prediction, I did not find evidence of a significant increase in grazing pressure on phytoplankton biomass. I studied phytoplankton and zooplankton communities along with associated physico-chemical variables in Lake Hayes, in the months before and after perch recruitment. My results reveal that in the months following perch recruitment, the zooplankton biomass decreased marginally, and no significant changes in zooplankton community structure were observed, compared with before. However, after perch recruitment, the biomass of the phytoplankton significantly increased, and their compositional variability halved, compared with before. Furthermore, my results suggest that phytoplankton community structure is strongly influenced by the dynamics of Ceratium hirundinella, while zooplankton community structure is strongly influenced by dynamics in Daphnia pulicaria. An inverse association was observed between the biomass of D. pulicaria and that of C. hirundinella both before and after perch spawning, indicating that D. pulicaria had a strong grazing impact, even on the larger phytoplankton taxa which tend to dominate the lake. I also studied seasonal and ontogenetic dietary and habitat selection shifts in European perch with the aim of gaining understanding of perch interactions with potential pelagic grazers (Daphnia) and water clarity. I used the complementary techniques of gut content analysis and stable isotope (δ13C, δ15N) analysis to quantify the absolute and relative abundance of prey items ingested and assimilated by perch. I predicted that during the summer and autumn after perch recruitment, young-of-the-year (YOY) perch predation would decrease Daphnia abundance, and that this would result in increased chlorophyll a concentration and decreased water clarity. My results suggest that the perch exhibited an extended spawning season, resulting in high abundances of perch larvae in the littoral zone in spring. In summer the abundances of both YOY and adult perch increased substantially in the pelagic zone, while declining in the littoral zone. YOY perch fed primarily on Daphnia and zoobenthos in spring and the early summer. In late summer and autumn both YOY and adult perch mainly fed on Daphnia. Gut content analysis showed that in summer adult perch were cannibalistic, however stable isotope analysis contradicted this, suggesting that zooplanktivory was predominant at that time. In summer and autumn, YOY and adult perch suppressed Daphnia abundance, and this may have decreased herbivory and therefore increased phytoplankton abundance and decreased water clarity. It appears that the recruitment of perch into zooplanktivorous size classes seems to have induced a trophic cascade. The presence of perch may therefore be an impediment to the restoration of water clarity in Lake Hayes. The use of classical pelagic food web biomanipulation approaches may hence facilitate the restoration of water clarity in this lake and in other similar lakes