SEDIMENT AND WATER AS SOURCES OF NITROGEN AND PHOSPHORUS FOR SUBMERGED ROOTED AQUATIC MACROPHYTES

Two rooted submerged freshwater macrophytes (Lagarosiphon major (Ridley) Moss; Myriophyllum triphyllum Orchard) grown in large in situ experiments in two lakes of differing trophic status (Lake Taupo, oligotrophic; Lake Rotorua, eutrophic), exhibited two different nutrient limitations. Plants were grown on both oligotrophic and eutrophic sediments in each of the lakes. In the oligotrophic lake, plants grown on the eutrophic sediments were approximately twice the size of those grown on the oligotrophic sediments. Tissue nutrient analysis demonstrated that phosphorus levels were approximately doubled in plants grown on the eutrophic sediments, suggesting that phosphorus was limiting the growth of plants grown on the oligotrophic sediments. In the eutrophic lake, this sediment-related response was negligible with no consistent differences between the plants grown on the different sediment types. As the sediments were comparable in all the experiments, this suggested that the water phase was playing a role in the nutrition of these species.