Assessment of hydroecological changes at the Slave River Delta, NWT, using diatoms in seasonal, inter-annual and paleolimnological experiments
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Relationships between hydrology, limnology and ecology are analyzed in a comprehensive study of water bodies in the Slave River Delta, Northwest Territories, Canada, at a variety of temporal and spatial scales, including seasonal, inter-annual and multi-decadal timescales at individual sites to delta-wide perspectives. Water chemistry and surface sediments were analyzed from 41 shallow lakes representing three previously-defined hydrological categories in the Slave River Delta, in order to identify relationships between hydrological and limnological conditions and their associations with recently deposited diatom assemblages. Evaporation-dominated lakes are physically removed from the influence of the Slave River, and are characterized by high alkalinity and high concentrations of nutrients and ions. In contrast, flood-dominated lakes tend to receive a pulse of floodwater from the Slave River during the spring thaw and have low alkalinity and low concentrations of most nutrients and ions. Exchange-dominated lakes are variably influenced by floodwaters from the Slave River and seiche events from Great Slave Lake throughout the spring thaw and open-water season, and are characterized by a broad array of limnological conditions that are largely dependent on the strength of the connection to these sources of floodwater. Specific diatom ‘indicator’ taxa have been identified that can discriminate these three hydrological lake categories. Evaporation-dominated lakes are associated with high relative abundance of common epiphytic diatom taxa, while diatoms indicative of flood- and exchange-dominated lakes span a wide range of habitat types (epiphytic, benthic) but also include unique planktonic diatoms (Stephanodiscus and Cyclostephanos taxa) that were not found in surface sediments of evaporation-dominated lakes. Water chemistry, diatom phytoplankton communities and macrophyte biomass were monitored seasonally over three years (2003-05) from six hydrologically-diverse lakes of varying flood susceptibility to determine the effects of river flooding on the seasonal and inter-annual hydroecological conditions of lakes in the Slave River Delta. Results indicate that river flooding is the dominant hydrological process controlling the temporal dynamics of limnological and ecological conditions in lakes of the Slave River Delta. In the absence of river flooding, lakes have relatively high concentrations of nutrients and low concentrations of most ions, but when flooded, concentrations of nutrients decrease and ions increase. The limnological and ecological conditions in frequently-flooded and non-flooded lakes are relatively stable from year to year, whereas lakes that are intermittently flooded fluctuate widely and are subject to variable conditions depending on whether or not they flood. Lakes that do not flood lack planktonic diatom communities, while spring flooding from the Slave River introduces an abundance of planktonic, centric diatoms that persist only for a few weeks in the water column before settling out. Flooding reduces lake water transparency, which decreases macrophyte biomass, while non-flooded lakes exhibit higher macrophyte biomass. To improve understanding of the role of river flooding and other hydrological factors on epiphytic diatom communities, a natural experiment was conducted to compare diatom communities in two hydrologically distinct lakes in the Slave River Delta (Northwest Territories, Canada) over two years (2004 and 2005) of varying spring flood magnitude of the Slave River. Magnitude and spatial extent of flooding was low in 2004 and high in 2005. Replicate samples were collected from three dominant macrophyte species (Lake SD28: Potamogeton friesii, Myriophyllum exalbescens and a species of Equisetum; and, Lake SD29: Potamogeton friesii, Ceratophyllum demersum, and a species of floating filamentous green algae) in three separate basins from each lake. Multivariate analyses (PCA, analysis of similarities) of the common macrophyte in both lakes (Potamogeton friesii) revealed that yearly differences due to frequency and intensity of flooding account for the greatest differences in epiphytic diatom community composition (PCA axis 1 = 0.302 and ANOSIM SD29 – 2004 versus 2005 Global R value = 0.982, p < 0.001 and d.f. = 23), followed by hydrolimnological differences between the two study lakes (PCA axis 2 = 0.262 and ANOSIM SD28 versus SD29 – 2004 and 2005 Global R value = 0.814, p < 0.001 and d.f. = 47), and by spatial variability within lakes (variation along both PCA axes and ANOSIM from both lakes in both years Global R value = 0.940-1.000, p < 0.001 and d.f. = 47). Epiphytic diatom community composition differed significantly (p < 0.001) at all levels of comparison. Observed patterns of epiphytic diatom composition between years and lakes indicated that hydrological effects on the limnological conditions, coupled with changes in the light environment, exert the strongest control on epiphytic diatom community composition, whereas spatial variability of limnological conditions within lakes and host-macrophyte specificity appear to play statistically significant but less important roles. Epiphytic diatom community composition is a sensitive indicator of hydrological change and knowledge gained concerning the roles of hydrological factors, limnological conditions and macrophyte host species on epiphytic diatom communities is important to inform interpretations of hydroecological changes from analyses of sedimentary diatom assemblages, which are often dominated by epiphytic taxa, in ongoing monitoring and paleolimnological studies in the Slave River Delta and other floodplain environments. Sediment cores were collected from a lake in each of the three hydrological lake categories in the Slave River Delta, to evaluate the ability of diatom assemblages to detect differences in the hydrological state and temporal variability of hydroecological conditions within individual lakes of this complex deltaic environment. Results indicate that diatoms provide sensitive records of environmental change and important information on past changes in hydrological conditions such as river flooding. The composition and temporal patterns of variation in diatom assemblages differ among lakes from different hydrological categories in consistent and predictable ways. The flood-dominated lake (SD2) provides a high resolution record of hydroecological variability and changes in flood-frequency over time. Sedimentary diatom assemblages repeatedly shift in dominance from taxa indicative of low river influence to taxa indicative of high river influence. Sedimentary diatom assemblages from the exchange-dominated lake (SD28) are dominated by taxa indicative of high river influence and were relatively static during the past ~100 years, but the total sum of planktonic diatoms (% abundance) can provide robust records of large, spatially extensive flood events in the Slave River Delta. Both flood- and exchange-dominated lakes show marked correspondence with gauged Slave River discharge levels over the past 46 years indicating an ability of diatoms to track periods of higher and lower flood frequency and high- magnitude flood events. Sedimentary diatom assemblages from the evaporation-dominated lake (SD20) in this study were poorly preserved below 6 cm depth, but assemblages from above 6 cm had distinct community composition similar to assemblages in surface sediments of evaporation-dominated lakes of the Slave River Delta. These diatoms correctly identify this lake as having evaporation-dominated hydrology. Overall, the sediment records provide no evidence for reduced flood frequency or a decrease in high magnitude flood events to indicate that upstream river regulation or another driver of change may be causing perceived changes of lower flood frequency and low water levels throughout the delta.
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Michael Andrew Sokal (2007). Assessment of hydroecological changes at the Slave River Delta, NWT, using diatoms in seasonal, inter-annual and paleolimnological experiments. UWSpace. http://hdl.handle.net/10012/3364