|Long-term monitoring is essential for distinguishing between natural and anthropogenically induced changes within ecosystems, particularly those with high degrees of natural fluctuation. In order to gain further understanding of the links between physical, chemical and biotic factors, zooplankton have been highlighted as useful bioindicators within aquatic ecosystems. Long-term zooplankton monitoring studies have been utilized to highlight changes in lake trophic state, changes within catchments, fisheries practices and invasions.
Lake Taupō, New Zealand’s largest lake, has been subject to a long-term monitoring programme by NIWA since 1994, commissioned by the Waikato Regional Council, with the primary purpose of detecting changes in the lake’s trophic state through time. Since January 2000, the monitoring programme has included biweekly zooplankton sampling; no results of this have yet been published. The purpose of my research was to assess if there have been any significant changes in the zooplankton community composition over the monitoring period (2000-2020), with a particular focus on the last 12 years of data (2009-2020), and determine the importance of measured environmental variables in influencing changes in the zooplankton community composition.
Lake Taupō was assessed as microtrophic to oligotrophic throughout the monitoring period (Trophic Level Index (TLI) between 1.5 and 2.9), and there were no significant linear changes in any of the measured environmental variables (R-squared values all <0.07, p values all >0.05). The zooplankton data was separated into two separate datasets; a shorter February 2009 – December 2020 dataset with samples analysed entirely by myself, and a longer January 2000-December 2020 dataset, with earlier samples analysed by NIWA, utilising a lower taxonomic resolution.
Multivariate analyses (nMDS and ANOSIM) indicated significant differences among three yearly groups of zooplankton data within both the shorter (Global R statistic = 0.179, p = 0.014) and longer dataset (Global R statistic = 0.353, p = 0.001). Time and temperature were inferred by Redundancy Analysis (RDA) as the most important environmental variables associated with change in zooplankton community composition in both the shorter 2009-2020 (Lambda-A = 0.08 for both, p = 0.002 for both) and longer 2000-2020 datasets (Lambda-A = 0.15, p = 0.002 for time, and Lambda-A = 0.06, p = 0.002 for temperature). Both ordinations indicated that changes associated with time (long-term) and changes in temperature were primarily unrelated. Changes in zooplankton community composition influenced by temperature were attributed to seasonal succession.
Zooplankton community composition differed between the beginning and end of the monitoring periods in both datasets. However, the direction of the community change seemingly reversed around 2009-2011, with assemblages late in the study reverting to communities similar to those observed earlier in the study. Decreases in the native large Daphnia thomsoni and increased importance of the non-native Daphnia galeata indicate that colonisation by the invading species has influenced the observed changes in zooplankton community composition. The influences of top-down control in the form of predation on the zooplankton community by fish (an unmeasured variable) may have also influenced communities, with shifts from large- to small-sized species observed. Rotifers indicative of both low and high TLI are present throughout the study (e.g., Polyarthra dolichoptera and Keratella cochlearis respectively), but the community is dominated throughout by species indicative of low trophic state (e.g., Conochiloides dossuarius), which is consistent with previous zooplankton studies within Lake Taupō. Continued monitoring of zooplankton, physical and chemical lake properties, algae, and bacteria is advised, with the inclusion of fish monitoring recommended.