Tracking phosphorus enrichment and ecosystem change in wetlands

Kirsi J. Kauhanen

This thesis describes how phosphorus (P) storage of ecosystem components and ecosystem structure change simultaneously as a wetland undergoes nutrient enrichment. Three experiments at Capel Wetlands Centre (Western Australia), where treated municipal effluent was discharged into P-limited wetlands, tracked progress of eutrophication within and between different functional zones (submerged bare sediment, submerged aquatic vegetation (SAV), emergent Typha stands), over different temporal scales: a mesocosm experiment (3-month), and seasonal (9-month) and annual (3-year) monitoring. Conceptual models were developed to describe the outcomes. The experiments revealed several important aspects of the eutrophication process. Seasonal patterns in P-dynamics evident in unenriched lakes were overwhelmed by high P-loads, and progression to hypereutrophy occurred despite not all P-stores being saturated. Functional zones responded differently to enrichment, contingent on the type and number of P-stores with different uptake patterns. P concentration in stores responded more rapidly than mass, with both measures needed to assess the progress of eutrophication. Phytoplankton, periphyton and SAV responded rapidly to enrichment, but uptake was minimal, while topsoil, litter, floc and Typha responded more slowly but were larger P-stores. Bare sediment eutrophied most rapidly. Enrichment caused loss of SAV; then re-released P stimulated phytoplankton and floc production. Diverse stores in the Typha zone slowed the progress of eutrophication, but Typha itself declined due to increased depth and decreased water quality caused by wastewater discharge. Maintaining vegetation is recommended to maximise direct and indirect P uptake and biodiversity benefits. Litter and floc took up substantial P in early enrichment, but potentially become P-sources when water/sediment quality deteriorates. Their presence reduced soil/water interaction potentially limiting P-uptake by soil. The holistic approach used here chronicles eutrophication as a changing flowpath of interrelated reactions between different ecosystem components over different temporal scales. It provides a functional and transferable understanding of eutrophication, necessary for successful wetland management.

Tracking phosphorus enrichment and ecosystem change in wetlands

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