A paleoecological investigation of recent cyanobacterial blooms and their drivers in two dune lakes : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Ecology at Massey University, Manawatū, New Zealand

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Cyanobacteria have a major impact on many of Aotearoa New Zealand's lake ecosystems. Blooms of the prokaryotic organisms can cause hypoxia and fish kills, while their potential to produce a range of toxins pose a risk to social and cultural values. Cyanobacteria blooms are common in dune lakes along coastal Manawatū-Whanganui, with little information to guide the management of these globally-rare ecosystems. Setting appropriate targets for restoration programmes is therefore difficult, as cultural eutrophication effects cannot be disentangled from baseline conditions. Paleolimnology has been used within this thesis to reconstruct the environmental and in-lake changes of Lakes Alice and Wiritoa, two dune lakes of similar size that have different depths. Surface water samples from Lakes Alice and Wiritoa were collected monthly between November 2021 and April 2022 AD, while surface sediment samples were collected in April 2022 AD. Metabarcoding analysis indicated the cyanobacterial communities within the surface sediment were very similar to the seasonal average water column communities, with some likely effects from accumulated settling, lake depth and length of the water sampling period. Sediment cores were collected from the depocenters of both lakes and analysed for pollen, trace metals, trace elements and autochthonous chlorophyll-a. Historic cyanobacterial communities in each lake were assessed via environmental DNA, offering taxonomic and quantitative insight into the changing structure of each lake's ecosystem. Both lakes have experienced significant environmental change with human settlement. Their catchments were likely covered in podocarp forest prior to human arrival, followed by significant deforestation during Māori settlement. European arrival then signals the onset of intensifying agriculture with ongoing deforestation and conversion of scrub to pasture. Increased cadmium concentrations in both lakes identify the onset of aerial superphosphate topdressing and intensive agriculture. Cyanobacterial communities in both lakes transition from the picocyanobacterial Cyanobium to bloom-forming, potentially toxigenic taxa including Dolichospermum and Microcystis; the most significant magnitude shift in cyanobacterial communities occurs with the onset of intensive agriculture in both lakes. Nutrient reduction is likely key to reducing the impact of cyanobacterial blooms in Lakes Alice and Wiritoa, although in neither lake will nutrient reduction alone result in complete water quality restoration. Deforestation of the catchments was identified as an additional driver of potential cyanobacterial blooms. Additional unknown drivers or in-lake thresholds may be aggravating the cyanobacterial blooms seen today, and further research to identify these would be valuable. Both lakes likely have internal mechanisms that will maintain their elevated trophic status if external nutrient inputs are reduced, however these differ between the lakes due to their depth differences.
Figure 2.1 is re-used with permission.