This preliminary survey covered 13 aquatic macrophytes present at two sites in Lake Taupo and two sites in the Auckland region (northern half of the North Island, New Zealand). Three species previously unrecorded as CAM plants, Isoetes kirkii A. Braun, Lilaeopsis lacustris Hill, and Vallisneria spiralis Graeb., exhibited diurnal fluctuations in titratable acidity and malic acid content. The changes in titratable acidity varied seasonally from 97 (Dec. 1985) to 46 (June 1986) microequivalents per gram fresh weight for I. kirkii, and from 40 to 23 and 51 to 26 microequivalents per gram fresh weight for L. lacustris and V. spiralis, respectively. The absence of CAM both in other members of the low mixed community and in other hydrocharitacean species at the same sites suggests that CAM can not be unequivocally regarded as conferring an ecological advantage to a species.
In the submerged aquatic CAM plant, Isoetes kirkii A. Braun, light was shown to be the environmental factor having the major influence on daily did malic acid/titratable acid fluctuations. In Lake Taupo (New Zealand), a large oligotrophic lake, seasonal variation in the level of did fluctuations was demonstrated and this corresponded to changes in temperature, light, and daylength while Total Inorganic Carbon (TIC) remained relatively constant. However, in situ shading experiments demonstrated that, at any one time, the diel acid fluctuations were controlled by the ambient light levels as the other variables were constant. Laboratory experiments supported this and also showed that there was a close relationship between photosynthetic rates and deacidification when related to light intensity.
Aerial photographs of several Tennessee Valley Authority (TVA) reservoirs are taken each year to determine acreages of the dominant species of submersed aquatic macrophytes. This paper describes methods used in obtaining and interpreting the photographs. For operational work, the TVA uses large-scale color photographic prints made from a color-negative film. Although more expensive than black-and-white (BW) film, the color film allows better discrimination of submersed species of aquatic plants. While false-color (color-infrared) film has been widely used for mapping and monitoring emergent and wetland plant communities, it is less desirable than true-color film for delineating and mapping submersed plants. Scales of 1:7 200 and 1:12 000 are commonly used and provide the detail and resolution needed for accurate photointerpretation of several submersed macrophyte species.
The TVA is also experimenting with an airborne thermal line scanner for mapping aquatic plants. The imagery from the system can in some cases be used to delineate the limits of colonies of Eurasian water milfoil according to differences in surface water temperatures.
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