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The ecological consequences of stream sedimentation for freshwater macroinvertebrates in the Te Awanui/Tauranga Harbour catchment
Abstract
Sedimentation is a significant stressor in streams and rivers globally. The natural sediment regime
paradigm posits that human activities have greatly altered the natural transport of sediment in river
networks and that land-uses leading to increased bank erosion and surface runoff have resulted in
large quantities of fine sediment being deposited in streams and rivers. In New Zealand, land
conversion from native forest to pastoral agriculture has dramatically increased riverine sediment
loads, making deposited fine sediment (DFS) a major stressor that negatively impacts stream
ecosystem health. Consequently, DFS is an important attribute managed under the National Policy
Statement for Freshwater Management 2020. However, uncertainties remain in how best to
monitor the effects of DFS, and what impacts different land-use types have on stream ecosystems
in addition to the adverse influence of sedimentation.
In my study, I surveyed 24 stream sites allocated evenly to three land-use types (native forest,
pastoral agriculture, and horticulture, e.g. kiwifruit orchards) in the Tauranga Harbour basin. I
measured habitat attributes, including DFS, and sampled stream macroinvertebrate communities
at each site. Using macroinvertebrate data, I calculated and compared taxonomic and trait-based
metrics as indicators of ecosystem health across land-use types. I used a variety of metrics,
including the Macroinvertebrate Community Index (MCI) and its quantitative equivalent, the
QMCI, the Average Score Per Metric (ASPM) index, and community-weighted mean trait
abundances, including facets of functional diversity to assess impacts on stream health.
I found that human land uses negatively influenced stream health, indicated by greatly reduced
MCI, QMCI, and ASPM scores. Although DFS explained most of the negative influences on
macroinvertebrate communities in pastoral and horticultural streams, there were differences
between native forest and horticultural streams not accounted for by DFS that contributed to
declines in ecosystem health. I also found that sediment-specific macroinvertebrate metrics
(sediment ‘decreasers’) were the best indicators of DFS impacts on stream health. Contrary to my
predictions, I did not find that functional diversity was strongly reduced by land use or DFS,
although several key traits responded to these stressors. Notably, life history trait modalities
involving the number of reproductive cycles per year and oviposition site responded strongly to
DFS and land use, indicating potential mechanisms contributing towards the impacts of these
stressors on macroinvertebrate communities.
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My study points to the need to continuously manage the deposition of fine sediment in streams to
help mitigate the adverse effects of human land use. Future research should further explore the
role that fine sediment and other stressors associated with human land uses play in the degradation
of streams ecosystems in the Tauranga Harbour basin. Quantifying land use and land cover
(LULC) attributes using geospatial tools will help further explore the relationship between
catchment properties and macroinvertebrate responses. Additionally, issues regarding trait
syndromes, trade-offs, and database inconsistencies must be addressed to better understand the
mechanistic links between specific macroinvertebrate traits and fine sediment.
Type
Thesis
Type of thesis
Series
Citation
Date
2024
Publisher
The University of Waikato
Supervisors
Rights
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