Abstract:
A study of aquatic invasive plants of Vembanad Lake south of
Thannermukkam bund and AC (Alappuzha-Changanassery) canal in the Kuttanad
wetland ecosystem using multispectral imageries was undertaken to assess the
spatiotemporal variation in the aquatic weed area, its distribution, extent and trends in
variation. The study also aimed to analyse the contribution of climatic factors in
explaining the changes in the spatiotemporal distribution of aquatic invasive plants.
The study employed medium resolution LANDSAT imageries for mapping and
monitoring the aquatic weed distribution. The digital image processing software used
for classification of the multispectral satellite imageries to estimate the area of aquatic
weeds was ILWIS 3.31 Academic version (Integrated Land and Water Information
System) by ITC. The supervised classifications of the multispectral imageries were
carried out with the help of the ground truth data collected from several Ground
control points (GCPs). The study revealed the usefulness of multispectral satellite
imageries obtained from satellites like LANDSAT in studying the spatiotemporal
changes in the aquatic invasive plant distribution. However, the effects of cloud cover
in obscuring the spectral reflectance data limits the availability of usable imageries.
The study utilised cloud free imageries for the determination of the areal
distribution of the aquatic invasive plants and cloud free imageries were not available
for the rainy period. The spatiotemporal variations in aquatic invasive plants showed
a cyclic trend in its distribution. The mean area of the aquatic invasive plants
distribution during the period under consideration was 3.25 km2. Among the months
taken for the study, monthly mean aquatic weed distribution was maximum in the
month of September with a value about 5.4 km2. In the seasonal distribution, the
southwest monsoon season (5.4 km2) had the maximum aquatic weed distribution,
followed by the winter season (4.0 km2), Northeast monsoon season (3.4 km2) and
summer season (2.3 km2).
Factor analysis and stepwise regression analysis was performed to understand
the effect of climatic parameters on the spatio-temporal distribution of aquatic
invasive plants. The dependent variable considered in the analysis was the aquatic
weed area and the independent variables were maximum temperature, minimum
temperature, average temperature, relative humidity, total rain, average rain,
evaporation and sunshine hours. Among the climatic factors, temperature has got a
negative relationship with the aquatic weed area i.e., as temperature increases the
aquatic weed area decreases. From the correlation analysis, it was seen that the
maximum temperature has got the maximum negative correlation (-0.63) with the
aquatic weed area among the variables considered.
The regression analysis revealed that there is a statistically significant
relationship between the maximum temperature and aquatic weed area as the p-value
is less than 0.001. It was found that 39.67 per cent of the variation in aquatic weed
distribution can be explained by the regression model. The regression equation
obtained is
AWA = 27.93 - 0.757 Tmax
where AWA = Aquatic weed area (km2)
Tmax. = Temperature maximum (°C)
The forward and backward step wise regression analysis also showed that the
maximum temperature has got more effect on the aquatic weed distribution than the
other climatic variables considered. The aquatic weed area was more during the rainy
season as nutrient rich sediment from the agricultural lands is swept into the lake by
the rains and floods, which lead to the proliferation of the aquatic invasive plants. The
areal spread of aquatic invasive plants is not only influenced by the temperature, but
also by other factors such as dissolved oxygen, nutrient load, light intensity and pH.
The intrusion of saline water from the sea into the fresh water lake also plays an
important role as salinity has a detrimental effect on the growth of these aquatic
invasive plants.
The regions with the menace of aquatic invasive plants in Kuttanad wetland
ecosystem need sustainable management. The study of the spatio-temporal
distribution of aquatic invasive plant area using remote sensing techniques provided
useful information about the aquatic weed growth in the study area and this method
can be utilized for mapping and monitoring the areal spread of aquatic invasive
plants.