Interactions among multiple stressors, legacies of past perturbations, and the lack of historical information make it difficult to determine the influence of individual anthropogenic impacts on lakes and separate them from natural ecosystem variability. In the present study, we coupled paleolimnological approaches, historical data, and ecological experiments to disentangle the impacts of multiple long-term stressors on lake ecosystem structure and function. We found that the lake structure and function remained resistant to the impacts of catchment deforestation and erosion, and the introduction of several exotic fish species. Changes in ecosystem structure and function were consistent, with nutrient enrichment being the primary driver of change. Significant and sustained changes in the lake diatom community structure (and their nutrient requirements), bacterial community function, and paleolimnological proxies of ecosystem function coincided with nitrogen and phosphorus fertilizers in the catchment. The results highlight that the effects of increased nutrient inputs are much stronger than the influence of other, potentially significant, drivers of ecosystem change, and that the degree of nutrient impact can be underestimated by environmental monitoring due to its diffuse and accumulative nature. Delineating the effects of multiple anthropogenic drivers requires long-term records of both impacts and lake ecosystem change across multiple trophic levels.

Resolving 500 Years of Anthropogenic Impacts in a Mesotrophic Lake: Nutrients Outweigh Other Drivers of Lake Change.

With the intensification of human disturbance in urban lakes, the loss of eukaryotic biodiversity (macroinvertebrates, etc.) reduces the accuracy of the index of biotic integrity (IBI) assessment. Therefore, how to accurately evaluate the ecological status of urban lakes based on IBI has become an important issue. In this study, 17 sampling sites from four lakes in Wuhan City, China were selected to analyze the composition and diversity characteristics of benthic and microbial communities and their relationship with environmental factors based on eDNA high-throughput sequencing, and compare the application effects of the benthic index of biotic integrity (B-IBI) and the microbial index of biotic integrity (M-IBI). Canonical correspondence analysis showed that the key environmental factors affecting benthic family/genus composition were temperature, conductivity, total phosphorus (TP), and total nitrogen (TN). Redundancy analysis showed that pH, TP, conductivity, and ammonia nitrogen had the greatest impact on microbial phyla/genera. After screening, four and six core metrics were determined from candidate parameters to establish B-IBI and M-IBI. The B-IBI evaluation results showed that healthy, sub-heathy, and poor accounted for 58.8%, 35.3%, and 5.9%, respectively, in the sites. The results of the M-IBI evaluation showed that 29.4% of the sites were healthy, 47.1% were sub-healthy, and 23.5% were common. M-IBI was positively correlated with water quality (r = 0.74, P < 0.001), whereas B-IBI was not. Further results showed that M-IBI was negatively correlated with the relative abundance of bloom-forming cyanobacteria Planktothrix (r = -0.54, P < 0.05). Therefore, M-IBI is more sensitive than B-IBI and can better reflect the actual water pollution status. This study can provide a new perspective for ecological assessment and management of urban lakes strongly disturbed by human activities.

Evaluation and comparison of the benthic and microbial indices of biotic integrity for urban lakes based on environmental DNA and its management implications.

Evaluation and comparison of the benthic and microbial indices of biotic integrity for urban lakes based on environmental DNA and its management implications

Freshwater fish are in a perilous state with more than 30% of species considered critically endangered. Yet significant ecological and methodological complexities constrain our ability to determine how disturbances are impacting native fish communities. We review current methods used to assess the responses of fish communities, especially native fish, to disturbances, with a focus on lakes. These methods include contemporary population surveys, manipulative experimental approaches, paleolimnological approaches and Indigenous Knowledge and social histories. We identify knowledge gaps, such as a lack of baseline data for native fish, an inability to assess the impact of historical disturbances, stressor response dynamics in contemporary multi‐stressor environments, and natural disturbance regimes. Our assessment of the current methods highlights challenges to filling these knowledge gaps using the reviewed methods. We advocate strongly for the implementation of an integrative approach that combines emerging technologies (i.e. molecular‐based techniques in contemporary surveys and paleolimnology) and underutilised knowledge streams (i.e. Indigenous Knowledge and social histories) which should be used in concert with conventional methods. This integrative approach will allow researchers to determine the key drivers of decline and the degree of change, which will enable more informed and successful management actions.

An integrative approach to assess the impact of disturbance on native fish in lakes.

Freshwater fish biodiversity and abundance are decreasing globally. The drivers of decline are primarily anthropogenic; however, the causative links between disturbances and fish community change are complex and challenging to investigate. We used a suite of sedimentary DNA methods (droplet digital PCR and metabarcoding) and traditional paleolimnological approaches, including pollen and trace metal analysis, ITRAX X-ray fluorescence and hyperspectral core scanning to explore changes in fish abundance and drivers over 1390 years in a small lake. This period captured a disturbance trajectory from pre-human settlement through subsistence living to intensive agriculture. Generalized additive mixed models explored the relationships between catchment inputs, internal drivers, and fish community structure. Fish community composition distinctly shifted around 1350 CE, with the decline of a sensitive Galaxias species concomitant with early land use changes. Total fish abundance significantly declined around 1950 CE related to increases in ruminant bacterial DNA (a proxy for ruminant abundance) and cadmium flux (a proxy for phosphate fertilizers), implicating land use intensification as a key driver. Concurrent shifts in phytoplankton and zooplankton suggested that fish communities were likely impacted by food web dynamics. This study highlights the potential of sedDNA to elucidate the long-term disturbance impacts on biological communities in lakes. 

Sedimentary ancient DNA reveals the impact of anthropogenic land use disturbance and ecological shifts on fish community structure in small lowland lake

Cyanobacterial blooms are one of the most significant threats to global water security and freshwater biodiversity. Interactions among multiple stressors, including habitat degradation, species invasions, increased nutrient runoff, and climate change, are key drivers. However, assessing the role of anthropogenic activity on the onset of cyanobacterial blooms and exploring response variation amongst lakes of varying size and depth is usually limited by lack of historical records. In the present study we applied molecular, paleolimnological (trace metal, Itrax-µ-XRF and hyperspectral scanning, chronology), paleobotanical (pollen) and historical data to reconstruct cyanobacterial abundance and community composition and anthropogenic impacts in two dune lakes over a period of up to 1200 years. Metabarcoding and droplet digital PCR results showed very low levels of picocyanobacteria present in the lakes prior to about 1854 CE (1839-1870 CE) in the smaller shallow Lake Alice and 1970 CE (1963-1875 CE) in the larger deeper Lake Wiritoa. Hereafter bloom-forming cyanobacteria were detected and increased notably in abundance post 1984 CE (1982–1985 CE) in Lake Alice and 1997 CE (1990–2007 CE) in Lake Wiritoa. Currently, the magnitude of blooms is more pronounced in Lake Wiritoa, potentially attributable to hypoxia-induced release of phosphorus from sediment, introducing an additional source of nutrients. Generalized linear modelling was used to investigate the contribution of nutrients (proxy = bacterial functions), temperature, redox conditions (Mn:Fe), and erosion (Ti:Inc) in driving the abundance of cyanobacteria (ddPCR). In Lake Alice nutrients and erosion had a statistically significant effect, while in Lake Wiritoa nutrients and redox conditions were significant. 

A paleoecological investigation of recent cyanobacterial blooms and their drivers in two contrasting lakes

ABSTRACT Pressure on lakes in Aotearoa New Zealand is increasing because of elevated catchment nutrient loads, establishment of non-native species, and climate change. Current government legislation requires that pressures are managed to avoid eutrophication and degradation of lake health. This approach requires information on the state of lakes at regional and national scales, which is challenging because <5% are currently monitored. In this study, we (1) modelled lake trophic status at a national scale using a highly representative dataset and lake characteristics, land use, and environmental parameters as predictor variables; and (2) identified lakes that should be prioritized for protection to prevent further degradation. Six statistical models were evaluated, with extreme boosting producing the highest predictive power and lowest error. This model indicated that for the 3738 lakes in the dataset, 44% were eutrophic or higher trophic state, 22% mesotrophic, and 34% oligotrophic or lower trophic state. These data provide a benchmark to guide management and support the call for more resourcing to restore lakes in Aotearoa New Zealand. To identify lakes to prioritise for protection, we focused on the ∼800 mesotrophic lakes. We used (1) the portion of lake catchment not within conservation estate, and (2) road access as proxies for the likelihood of land-use intensification and the introduction of non-native species, respectively. We identified >170 lakes with limited catchment protection and easy human access. Immediate attention should be given to protecting these waterbodies to prevent the need for costly and resource-intensive remediation in the future.

A national scale trophic state analysis to prioritize lakes for restoration in Aotearoa New Zealand

Lakes are becoming degraded at an accelerating rate owing to human activity, and understanding their past ecology is necessary for lake management and rehabilitation. Palaeolimnology provides numerous methods that enable the historical state of lakes to be determined. New Zealand provides an ideal setting in which to do this as human modification of the landscape occurred later here than in most regions of the world (approx. 1300 CE). Lake Oporoa is a shallow lake that is highly significant to the local indigenous Māori community. This study used multiple proxy palaeolimnology to explore how lake ecology shifted following Māori and European settlement in the catchment, and how palaeolimnological data can be used to inform lake rehabilitation and conservation measures, alongside the desires of the indigenous community. Sedimentary pollen, diatoms, bacterial communities, and elemental and hyperspectral imaging scanning were used to infer ecological changes in the lake and catchment from pre-human times to present. Following Māori settlement (approx. 1620 CE) there was gradual vegetation change and a rapid shift in diatom and bacterial assemblages, but not in phytoplankton pigments or sediment geochemistry. An increasing abundance of diatom taxa Discostella stelligera and Staurosirella cf. ovata indicates early nutrient enrichment. European pastoralism from approximately 1840 CE resulted in further deforestation, and all proxies show evidence for enhanced primary productivity driven by a combination of nutrient enrichment and changing lake levels, particularly since the 1960s. This has caused degradation in water quality and is likely to have contributed to the decline in populations of tuna (eel, Anguilla spp.). Conversations with local Māori, together with the palaeolimnological results, indicate that a culturally acceptable and realistic rehabilitation target for Lake Oporoa aligns with ecological conditions in the 1950s. The palaeoecological data provide information to guide catchment and lake revegetation and other methods of nutrient abatement, with the eventual aim of restoring culturally important tuna and native fish populations. 

Using palaeolimnology to guide rehabilitation of a culturally significant lake in New Zealand

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A paleoecological investigation of recent cyanobacterial blooms and their drivers in two contrasting lakes

A national scale trophic state analysis to prioritize lakes for restoration in Aotearoa New Zealand

Using palaeolimnology to guide rehabilitation of a culturally significant lake in New Zealand