The timing and composition of organic matter (OM) inputs to rivers are important as
carbon plays a major role in river functioning. Management of Australian rivers since European
settlement has altered inputs of organic matter to these systems. Heterotrophic microbes play a
critical role in the transformation of OM in rivers, allowing transfer of carbon to other biota.
Alteration to the proportions of OM from different sources affects microbial functioning due to
differences in OM composition. Macrophytes can represent important sources of carbon to
rivers, however their inputs and in-stream processing are poorly understood. The aim of my
study was to examine inputs and microbial processing of macrophyte OM in Australian lowland
rivers under different flows.
Distributions of dominant macrophytes (Typha orientalis, Phragmites australis,
Vallisneria gigantea and Persicaria prostrata) were mapped in three lowland river reaches in
south eastern Australia. Integration with flow data in a GIS allowed the determination of
macrophyte inundation patterns under different flows. Resource allocation (biomass and
nutrients), live and dead shoot densities and litter production were monitored in the field over 18
months. DOM release from different macrophyte tissues was examined in the laboratory and
leachate composition was assessed using nutrient and spectral analyses. Responses of riverine
microbial communities to different OM sources were assessed from substrate-induced respiration
and enzyme activity experiments and field measurements of respiration and enzymatic responses
to varied OM inputs. Finally, all data were integrated into a model of microbial responses to
macrophyte OM inputs induced by different flows.
Large populations of macrophytes occurred at all three sites, at bed level, on in-channel
benches and on banks. Bank slope, channel heterogeneity and the vertical distribution of
macrophyte beds all affected macrophyte inundation patterns. Substantial differences in biomass
allocation, nutrient dynamics and litter composition were observed among different plant growth
forms and over time. While leaves represented the major shoot component in litter for all
species, stems and reproductive structures were also important in some species. Aside from the
litter pool, translocation to rhizomes represented a major sink for annual production in emergent
plants.
Patterns of shoot density and litter production over time varied among species, providing
a source of variation for particulate, and hence dissolved OM inputs upon inundation. The
majority of DOM release from POM occurred within 24 hours of inundation. Growth form,
tissue type (blade, stem, etc.) and status (live or dead) affected rates, quantities and composition
of DOM release, with implications for microbial utilisation. Both overall activity and patterns of
carbon utilisation in riverine microbes changed in response to altered OM inputs. Patterns of
microbial carbon use were shown to be specific to the carbon source which induced them.
Modelling showed that flow regulation had a major impact on OM inputs and microbial
metabolism, through the effects of flow variability on macrophyte vertical distributions,
macrophyte bed inundation and dilution. Positive relationships between discharge, DOM inputs
and microbial metabolism were observed at the most highly regulated site (drought < current < historic < flood). While a similar pattern occurred at the less regulated site in terms of total
loading, dilution effects resulted in a reversal of this trend on a reach volume basis. Microbial
metabolism and DOM inputs were restricted to summer/autumn under regulated flows compared
to a greater emphasis on winter/spring inputs and microbial activity under unregulated flows.
Continual OM inputs during winter with pulsed inputs in spring under natural flows probably
benefit larger, slow-growing macro-invertebrates. River regulation promotes pulsed macrophyte
OM inputs during spring/summer, potentially favouring riverine microbial and zooplankton
production, although at lower levels due to the overall reduction in OM inputs.
The predictive model of macrophyte OM inputs and microbial responses developed
throughout this thesis represents a major step forward in our understanding of macrophytemicrobe
interactions and our ability to manage our river systems. This work has shown that flow
manipulation can be used to influence macrophyte organic matter inputs to rivers and microbial
responses, affecting whole stream metabolism and food web interactions.
| Identifer | oai:union.ndltd.org:ADTP/219574 |
| Date | January 2006 |
| Creators | Bowen, Patricia Margaret, N/A |
| Publisher | University of Canberra. Resource, Environmental & Heritage Sciences |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | ), Copyright Patricia Margaret Bowen |
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