Domain shifts in the aquatic vegetation of shallow urban lakes: the relative roles of low light and anoxia in the catastrophic loss of the submerged angiosperm Vallisneria americana

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Abstract
The catastrophic loss of Vallisneria americana from a shallow urban lake in south-eastern Australia following nutrient enrichment has been reported previously. Two experiments are reported here to determine whether light attenuation or shifts in dissolved oxygen were more important in mediating this loss of submerged plants. The first experiment tested the response of dense beds of Vallisneria to different levels of shade in the field. The second tested the effect of (i) shading and (ii) anoxia on the performance of Vallisneria grown in the glasshouse. In the field, plants persisted after 3 months beneath 100% shade cloth, although with significantly reduced biomass (63%). In contrast, plant biomass beneath 70% shade cloth was reduced by only 9%. The field trials indicate that Vallisneria is highly tolerant to severe light attenuation. In the glasshouse, microcosms that were not artificially aerated become anoxic, and all plants died within 24 days in two of the three replicate microcosms. In shaded microcosms that were artificially aerated, plant biomass was reduced by 48% but no plants died. Severe reductions in dissolved oxygen associated with shading were primarily responsible for the rapid loss of Vallisneria, rather than light attenuation alone was concluded from the current study.

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