An ecophysiological investigation of the growth and nutrition of three submerged macrophytes in relation to lake eutrophication

Abstract

The effect of nutrient sources and availabilities on the growth and nutrient content of three aquatic macrophytes (Lagarosiphon major (Ridley) Moss, Potamogeton ochreatus Raoul and Myriophyllum triphyllum Orchard) were investigated in four large scale in situ experiments in oligotrophic Lake Taupo and eutrophic Lake Rotorua. The species were grown in each lake on natural sediments collected from both lakes. In lake Taupo, the results clearly indicated that sediment, rather than water, nutrients affected macrophyte growth since plants grown on eutrophic sediment were significantly (ca. 2-fold) large and heavier. Shoot nutrient analysis indicated that P contents (ca. 1.5 to 2-fold higher in plants grown on eutrophic sediment) were probably primarily responsible for the growth differences but sediment analyses for Total P and acid extractable soluble-P showed that the oligotrophic sediments had much higher concentrations of this element (between 1 to 6-fold). Further sediment analyses with a range of chemical extractants showed that extraction by solutions with pH values approaching field levels (pH 6 – 7) gave best relationship with plant growth and nutrient uptake. i.e. Rotorua sediment had significantly more (2 to 12-fold) P extracted at these pH values than Taupo sediments. The variable relationship between plant contents and P extracted from different sediments by the same extractant indicated that no one extractant may be universally suitable for the determination of bioavailable P. Under laboratory conditions, the addition of P to either the sediment or water stimulated the growth and nutrient uptake of L.major and M.triphyllum. These experiments provided strong evidence that P was limiting macrophyte growth on Taupo sediments in Lake Taupo. In the eutrophic Lake Rotorua, the results of the experiments were contrary to many of the published reports on nutrient source effect on macrophytes. There ware substantial water nutrient effects with the apparent P limitation associated with the Taupo sediments not being recorded. This is illustrated by the similar growth of each species on both sediment types. Furthermore, there were significant increases in the tissue N and P concentrations (ca. 2-fold for both elements) compared to plants grown in Lake Taupo even though the plants were grown on comparable sediments and a marked decrease in the root biomass relative to the shoot biomass. Although shoot lengths were comparable in the experiments in both lakes, there was a significant reduction (ca. 1.5 to 2-fold) in the biomass in the lake Rotorua experiments even though the increased nutrient availability was initially predicated to cause an increase in growth. Analysis of the two lake waters demonstrated that Lake Rotorua water had only 10% of the dissolved inorganic carbon available in Lake Taupo water which corresponded to a 2-fold difference in available CO2. As CO2 is considered to be the major carbon source for macrophyte photosynthesis, it was considered that this was a major contributing factor in the different biomass accumulations in the two lakes. Photosynthetic experiments demonstrated that a two-fold difference in CO2 concentration at near field levels was responsible for a two-fold difference in the photosynthetic rate. This can be translated to a similar difference in biomass observed in the field experiments in the two lakes. Seasonal growth responses were also recorded in Lake Rotorua which appeared to be temperature related. Photosynthetic evidence supporting such a temperature effect was described. Three distinct growth phases exist in macrophytes developing from either shoot apices or from transplanted plantlets. In Lake Taupo, a lag phase lasting at least 30 days was recorded and corresponded to little or no shoot growth and the development of a functional root system. In Lake Rotorua, the lag phase was often not observed, presumably due to the increased nutrient availability. Laboratory experiments conducted at water nutrient concentrations comparable to those in Lake Rotorua showed that the lag still occurred but was much shorter in duration (approximately 20 days). Thus in Lake Rotorua, the lag phase was not observed solely due to the length of the harvest period. The second phase was a period of exponential growth of the harvest period. The second phase was a period of exponential growth and was recorded in nearly all treatments. The third or plateau phase primarily occurred only in plants grown on Taupo sediment in Lake Taupo and it is suggested that the slow growth of this phase was related to the decrease in the available nutrient pool in the Taupo sediments. Based on the results presented in this thesis, the most feasible aquatic weed management scheme would be the implementation of either biannual mechanical harvesting or chemical spraying in the problem areas. It is suggested that these harvests should be at the onset of both winter and summer, as at these points, the control methods would have the most effect on macrophyte growth and development.