Long-term changes in Zooplankton community composition in North Island lakes: What are the drivers of change?
Permanent link to Research Commons versionhttps://hdl.handle.net/10289/16200
Zooplankton are an important component of freshwater ecosystems, forming a trophic bridge between primary producers and higher trophic levels. They are highly responsive to changes in their environment and have therefore been recognised as a useful tool for monitoring and assessing aquatic ecosystem health. However, long-term studies of changes in zooplankton communities are rare globally. Long-term studies of zooplankton composition are even more scarce within New Zealand. Past researches on New Zealand zooplankton have primarily focused on the seasonal variation of zooplankton composition, rather than long-term temporal changes, and have been undertaken over time periods of two years or less. This study utilised zooplankton compositional data from past research papers (1950-1990) conducted within North Island, New Zealand lakes and data from two 2021 sampling events, to investigate long-term temporal zooplankton community composition change. A historical dataset was constructed by conducting a desktop investigation of past studies that included zooplankton species lists. Further, zooplankton samples were collected in 2021 using vertical net hauls from nine lakes within the Bay of Plenty and Waikato regions, using zooplankton nets with three different mesh sizes (37 μm, 75 μm and 100 μm). Species lists from the historical and 2021 samples were statistically analysed to identify changes in zooplankton diversity and community composition through time. The samples from each of the three nets were also analysed independently of the historical samples, to determine the influence of mesh size on zooplankton composition and species richness estimates from semi-quantitative samples. Three major changes in zooplankton communities were evident over time. Firstly, high frequencies of the Holarctic cladoceran Daphnia galeata were found in the samples collected in 2021 yet were absent in samples taken prior to 1990. This result is consistent with the reported invasion of this species into New Zealand lakes since the early 1990s. Secondly, the cyclopoid copepod Macrocyclops albidus was not observed within the 2021 samples yet has been frequently present in species lists from the historical literature. This species has also not been commonly identified in recent research papers. Their low frequencies within the 2021 samples also coincides with increases in the importance of the D. galeata and cyclopoid copepod Mesocyclops australiensis in the 2021 samples and recent literature relative to historical datasets. Finally, there was an increase in the occurrence of many rotifer taxa within the 2021 samples compared to the historical samples. Interestingly, these taxa were largely deemed as an uncommon constituent of the plankton within New Zealand waters in the 1950s to 1970s. The increased importance of rotifer species within recent samples appeared to be a product of underestimation due to improper mesh selection by past researchers. Analysis of the semi-quantitative data of the three zooplankton nets with different mesh sizes indicated that zooplankton composition and species richness using the 75 μm net was more representative of that collected by the 35 μm samples than the 100 μm samples. This result indicates that a 75 μm net would be adequate when comparing zooplankton results within a 35 μm net among the studied lakes. Species richness within the two smaller meshed nets were higher than the 100 μm net, indicating a net with mesh size of 100 μm and greater would result in an underestimation of zooplankton community composition and species richness.
The University of Waikato
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