Analysing structural ecosystem change in high latitude lakes

Abstract

High latitude freshwater lakes are highly sensitive to climate change with numerous lake ecosystems already indicating signs of being impacted by the stress of global warming. With the current trajectory of climate change, it is anticipated that these impacts will continue and increase over the coming decades. Climate change is thought to be a likely driver of nonlinear response and abrupt transitions in ecosystems. Therefore, understanding lake ecosystem response to increasing climate stress is a complex issue yet is critical to forecasting ecosystem trajectories and predicting the vulnerability of lake ecosystems to abrupt ecological transitions. The majority of research surrounding nonlinear responses, tipping points, and critical transitions has focused on an approach centred on regime shifts from one community state to another over time. One element of this approach is the idea of early warnings signals within the frequency domain that may anticipate ecosystem instability prior to transitions between states. However, identifying early warning signals has been limited because of difficulties in detecting clear ecosystem shifts and valid statistical signals in time series data. As an alternative approach, research has recently expanded to consider testing ecosystem stability in the spatial domain, through analysing taxonomic organisation and connectivity within biotic networks. The thesis tests the effectiveness of structural metrics as parameters of lake ecosystem stability using climatically and environmentally sensitive chironomids (Diptera: Chironomidae), a key macroinvertebrate component of freshwater ecosystems. Chironomid ecosystem structure is analysed over spatial temperature gradients, periods of past known abrupt high-magnitude and gradual low-magnitude climate change, and in response to modern (c. 200 years) climate warming. Model-simulated datasets are used to test metric expectations of community composition and structural change across temperature gradients and through network development. The work demonstrates the effectiveness of four ecological metrics, taxon richness, beta diversity, compositional disorder and network skewness, as indicators of chironomid community composition and structure in response to climate change. Analyses indicate that chironomid assemblages in high latitude locations were likely to be experiencing some level of structural stress due to climate extremes. Climatic warming may have a two-fold effect on high latitude lakes. First, initial warming may allow ecological release and promote increased biological productivity, thus expanding the diversity of lake habitats and chironomid assemblages. This was recognised in North American and Norwegian datasets through increased beta diversity, compositional disorder and network skewness values. Second, further temperature stress, often with the additional effects of secondary drivers, may cause ecological degradation through habitat homogenisation and increased breakdown of existing network structures. Chironomid communities at the warmest end of the temperature gradients in North American and Norwegian datasets showed decreased beta diversity, compositional disorder and network skewness values. Evaluation of palaeoecological chironomid records spanning the Late Glacial and Holocene suggested that climate, as an exogenous stress, requires abrupt, high-magnitude change to alter aquatic community structures. Chironomid records spanning the abrupt, high-magnitude climate change at the Bølling-Allerød - Younger Dryas transition exhibited high magnitude changes in taxon richness and beta diversity. However, changes in the ecosystem structure, represented by °disorder and skewness, were smaller. This suggested some functional resilience within the chironomid communities, perhaps through the replacement of the same functional-type taxa in the network structure. Chironomid records spanning the Holocene indicated low taxonomic turnover in association with low levels of climate change, with little indication of climate-driven structural change. Changes within the chironomid assemblages suggested that other environmental factors were having a greater influence on the chironomid assemblages during this period of lower climatic stress. Recent changes within the chironomid community composition in three subarctic Alaskan lakes indicated signs of climate stress, with increased beta diversity associated with increased lake biological productivity. Small changes in chironomid community structure suggested some resilience within the structure of the chironomid communities to the current levels of climate and environmental change. Overall, changes in structural metrics for macroinvertebrate assemblages can provide insight into ecosystem stability and resilience. Ecosystems with impacted structures, as indicated by lower compositional disorder and network skewness values, are likely to be less resilient to increasing stress, and thus more vulnerable to system-wide transitions. Wider use and application of structural metrics may help identify reduced ecosystem resilience and vulnerability to potential tipping points in other major assemblages and ecosystems.

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Identifiers

ORCID for Peter Langdon: orcid.org/0000-0003-2724-2643

ORCID for Charles Doncaster: orcid.org/0000-0001-9406-0693

ORCID for John Dearing: orcid.org/0000-0002-1466-9640

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