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University of Western Australia (2015)

Organic matter dynamics of hydrologically variable pools in intermittent streams of northwest Australia

Siebers, Andre Robert

Titre : Organic matter dynamics of hydrologically variable pools in intermittent streams of northwest Australia

Auteur : Siebers, Andre Robert

Université de soutenance : University of Western Australia

Grade  : Doctor of Philosophy (PhD) 2015

Flow regimes of the streams and rivers of many dryland regions are highly variable and intermittent, often characterised by intense, seasonal floods interspersed by long, dry periods in which surface water contracts into a series of isolated pools along major drainage channels. These pools are important refugia for aquatic biota and other wildlife, but their biogeochemistry, and links to hydrologically driven ecological processes more generally, remain largely unknown. This thesis examines the organic matter (OM) and nutrient dynamics of ephemeral and permanent pools within intermittent streams in the Hamersley Ranges of the semi-arid Pilbara region of northwest Australia. The relative permanence of these pools through the dry season is highly dependent on connections to alluvial water (AW). However, little is known of how ecosystems within these pools function during inter-flood periods, and particularly how AW connections influence ecosystem structure and function. This research thus sought to characterise i) how the source and availability of organic matter changes as pools evaporate and contract, ii) how microbial and metazoan utilisation of organic matter changes as pools evaporate and contract, and iii) what influence AW connectivity has on these relationships. A comparison of four catchments across the Pilbara revealed that the intensity of fluorescent fractions of dissolved organic matter (DOM) increased with the level of evaporation. However, the composition of DOM fluorescence was usually characterised by high contributions of humic-like fluorescence components and low contributions of protein-like components across pools, suggesting that the production or transformation of DOM within pools does not change as they evaporate and contract. Closer investigation of a single catchment indicated that changes in pool volume due to evaporation and contraction can control concentrations of organic carbon (C), nitrogen (N) and phosphorus (P) within the water column, but that inorganic N and P concentrations were less closely linked to changing pool volumes. The stoichiometry of C, N and P within aquatic plants, terrestrial detritus and the water column also indicated that AW may provide a source of nitrate and phosphate for aquatic plant growth, while organic matter may be tightly cycled within pool ecosystems which are strongly limited by available P.
Further research suggested tight internal cycling of organic matter within microbial communities, especially after the cessation of surface flow. Pools were all net heterotrophic in the late dry season, with primary productivity exceeded by ecosystem respiration. Ecosystem metabolism varied amongst pools, but was not related to AW connectivity. However, diel patterns in the stable isotope composition of dissolved inorganic carbon (δ13C-DIC) were highly consistent across pools, suggesting that DIC dynamics were more highly affected by temperature-solubility relationships than by ecosystem metabolism. Finally, investigation of metazoan food webs showed that macroinvertebrate densities increased in direct relation to the extent of pool evaporation. Macroinvertebrate food webs were also smaller and less diverse in the late dry season when compared with pools post-flooding. However, there was no difference between food webs in AW-connected and isolated pools. Macroinvertebrate assimilation of C and N also occurred from a wide range of sources regardless of time or AW connectivity. The trophic base of metazoan food webs in these intermittent and highly oligotrophic systems may therefore be determined by general adaptations to intermittent flow regimes rather than variation in response to changing environmental conditions. Overall, the effects of AW connections are often masked by the widerecological effects of intermittent flow. However, identifying the extent of connectivity to groundwater is essential in understanding the OM dynamics of intermittent rivers and streams, particularly where sub-surface flowpaths may contribute to increased mineralization of recalcitrant OM, chemoautotrophic production, or utilization of ancient OM sources.


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