Effects of lake water level fluctuations on macrophytes and littoral macroinvertebrates

Abstract

Freshwater systems are declining globally in both quality and extent and are currently losing biodiversity faster than other ecosystem types due to human impacts. Hydromorphological alterations are now the primary form of pressure on European freshwaters, occurring principally in the form of water level fluctuations (WLF) and altered flows. Modified, anthropogenic WLF remain a largely overlooked pressure on lake ecosystems, despite having a profound influence on the littoral zone, the part of a lake where biodiversity is usually concentrated. Anthropogenic alteration to lake water-level regime has been identified as a priority for investigation in order to increase understanding of ecological effects, assessment abilities and mitigation measures. There is a lack of research quantifying impacts of WLF on biota relative to other known pressures, despite the certainty that many lakes are exposed to these stressors. To address these knowledge gaps, this thesis focused on assessing the responses of littoral macrophytes and macroinvertebrates to WLFs in Scottish lakes, relative to other environmental drivers at lake and sub-lake levels. The research presented in this thesis confirms that WLFs have a dominant and overall negative association with aquatic macrophyte species richness, altering community composition, and overriding effects of other established influences (i.e. elevation, lake surface area, and phosphorus). In addition, regulation of lake water levels per se was a negative environmental factor, relative to other predictors. WLF was also the dominant and negative factor related to macroinvertebrate family richness, relative to all other significant environmental predictors, including lake elevation, perimeter, and nutrient concentrations. The range of lake WLF was also established as a key factor in explaining variation in macroinvertebrate composition. Subdivision of lakes by the morpho-edaphic index (MEI), an index formulated from lake depth and alkalinity, revealed variation in the factors associated aquatic vegetation with lake type. Macrophyte richness in lower MEI lakes, being influenced by WLFs and regulation, and with clear distinctions between plant communities in higher vs lower MEI lake types. Additional subdivision of lakes by stable or fluctuating water level regimes added further clarity. Macrophyte communities differed significantly with MEI and stability regime, with species indicative of each lake type showing contrasting life history and reproduction strategies, consistent with differences in their sensitivity to water level change. Invertebrate communities differed significantly with stability regime, with MEI having no importance. Indicator species varied by life-span, mobility and reliance on littoral vegetation. Stable regimes were characterised by Hydrachnidae, Asellidae, Haliplidae and several gastropod families, whilst Siphlonuridae were indicative of fluctuating regimes. This thesis establishes water level mediated impacts, from sub-lake factors such as littoral substrate composition and shore slope, to the robust but declining isoetid plant, Littorella uniflora. Modification of water levels resulted in uncoupling of L. uniflora from the littoral zone by way of increasing shore slope, distance, and height. Morphological traits such as root-to-shoot ratios were influenced negatively by increased wave exposure, whilst leaf length-width ratios increased with distance from the water’s edge. Biomass was negatively influenced by steeply sloping shores and sandy substrates versus coarser aggregates. This research suggests that an amplified WLF range, results in elongation of roots to access a deeper water table, may be at a cost to overall standing biomass. This thesis demonstrates that WLF is the dominant stressor on littoral zone biota, and habitat parameters, relative to other environmental factors commonly considered in studies of lakes such as elevation, perimeter, and fertility. The influence of WLF pressure should be regarded as a priority for conservationist and managers of freshwaters. In light of the expected increase in impoundments globally and high-level pressure on European freshwaters from regulation activities, as well as future projections of climate change and population growth, it is imperative to include lake water level parameters to effectively manage freshwater resources and their associated biodiversity.