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The role of submerged macrophytes in seasonally-flowing agricultural streams

Paice, R. (2016) The role of submerged macrophytes in seasonally-flowing agricultural streams. PhD thesis, Murdoch University.

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Although submerged macrophytes are well known to support aquatic fauna and influence physical and chemical processes in many perennial freshwaters, they are rarely studied in seasonally-flowing streams. However, these streams often support seasonal plant assemblages that may support key ecological processes, particularly in agricultural landscapes with degraded riparian vegetation. This thesis investigated the distribution and contribution to ecosystem processes of submerged macrophytes in seasonally-flowing agricultural streams in a mediterranean climate, and their capacity to improve biodiversity and water quality in degraded reaches. It comprised: a survey of macrophyte distribution in relation to environmental factors; a food web study using stable δ13C and δ15N isotopes; and a transplant experiment. Sufficient duration of flow and/or pools limited macrophyte establishment in these seasonal streams. Two plant assemblages were found with distinct distributions associated with riparian shading: strap-leaved Cycnogeton occurred as remnant populations in shaded habitats in both undisturbed and degraded reaches; whereas the more structurally-complex Potamogeton/Ottelia assemblage colonised degraded reaches with little riparian vegetation. Both assemblages supported greater macroinvertebrate abundance and richness than reaches without macrophytes. Stable isotopes indicated macrophyte and epiphyte material were a valuable food web resource in reaches with limited riparian vegetation, supporting macroinvertebrates and native fish, thereby potentially compensating for lost allochthonous resources. Low-light tolerance of Vallisneria australis (morphologically similar to Cycnogeton) and protection from waterbirds enabled successful transplantation and growth (85-100% cover after six months) in a phytoplankton-dominated lentic river reach. A regime of macrophyte/phytoplankton co-dominance achieved through transplantation delivered substantial biodiversity outcomes (macroinvertebrate abundance 18.5 times; diversity 3 times bare sediment controls) and in the longer term may contribute to improved water quality. Provision of higher faunal biodiversity and other ecosystem services suggests submerged macrophytes are worthy of conservation and inclusion in river restoration in agricultural catchments.

Publication Type: Thesis (PhD)
Murdoch Affiliation: School of Veterinary and Life Sciences
Supervisor: Chambers, Jane and Robson, Belinda
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