Assessment of hydroecological changes at the Slave River Delta, NWT, using diatoms in seasonal, inter-annual and paleolimnological experiments.

摘要

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 nonflooded 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. (Abstract shortened by UMI.)