22
F. A. Khan et al.
mat. The area and volume of water body reduced further and
eventually, the duckweeds and water hyacinth formed a thick
cover on almost entire water surface.
April 2004 The duckweeds and water hyacinth were relatively denser than in April 2003, but in drying stage. The
size of the water body was far smaller and limited to central and western side of the pond. Owing to fast reduction in
water level and almost no recharging with the rain water, a
vast area of the pond had several smaller patches of Typha
angustata even in the centre of the pond. The pond dried by
the end of May 2004. Sedge grass showed slight browning
as it started drying after the maturity. Considerable drying
of duckweeds was recorded along its coast and in the central part of very small patch of water body. The water level
of 30–60 cm hindered the movement of small boats. Larger
area sediment was exposed at many places (Fig. 2.1)
The aquatic body turned into terrestrial body and by April
2005, some trees of Acacia nilotica and Prosopis species appeared on the eastern, and northern and in the centre of the
dried pond. In the following monsoon (July 2005–August
2005), the district local authorities managed the refilling of
the pond with fresh water and reduced the weirs around it.
This unusual restoration management resulted into a better
non-blooming and lesser eutrophic water body.
2.5.2 Quantitative Studies
The monthly data on seasonal variation in population dynamics of Lemna minor and Spirodela polyrrhiza in the rainfed Lal Diggi pond is summarized in Table 1. Both the duckweeds had numerical dominance in the months of February
and March. Lemna bloomed in the months of November
and August. Lemna and Spirodela did not grow in summer
(April to June) and December. The density of Lemna minor
dropped down significantly to 1,182 individuals/m
2
and of
Spirodela polyrrhiza to 2,358 individuals/ m
2
area of the
pond. The highest fresh weight of the selected macrophytes
along with blue-green algae and other floating macrophytes
(excluding water hyacinth) was recorded in the month of November 2003, March 2003, August 2003 and February 2004.
The fresh biomass was minimum in the month of January
(Table 2). Spirodela accumulated maximum dry matter in the
month of February 2004 followed by March 2003, November 2003 and August 2003. Lemna accumulated maximum
dry matter in the month of March 2004 followed by November, February and August of previous year (2003). The dry
weight of Spirodela and Lemna was least in the month of
January 2004 and September 2003, respectively. The net primary productivity (NPP) of both the duckweeds was high in
the months of February 2004, August 2003 and November
2003 (Table 2.1).
Spirodela polyrrhiza 225.82 g m
−2
yr
−1
The data in Table 2
show monthly variations in the population of Lemna minor,
Spirodela polyrrhiza, Wolffia arrhiza and chemical characteristics of water (pH, NPK, TDS, turbidity) of the selected
mesocosm, average atmospheric temperature and precipitation (rainfall in mm). Lemna minor and Spirodela polyrrhiza
bloomed between July 2003 and March 2004 (except in December 2003). The occurrence of duckweeds was recorded
in April 2003. The pH, water turbidity, total dissolved salts
(TDS) and NPK contents were high but dissolve oxygen was
very low in summer (April 2003 to July 2003). The nutrient contents (NPK) increased as the volume of water reduced in summer. The pH and turbidity reduced as the water
body swelled after 11.4 mm downpour in the month of July
(Table 2.2).
Lemna minor and Spirodela polyrrhiza bloomed in July
2003. The population ratio of Lemna:Spirodela was approximately 18:6 (105:36 individuals of Lemna and Spirodela respectively). The density of both these duckweeds increased
in the month of August 2003 (Table 2). In this month the
pH, turbidity and nutrients of the water decreased owing
to recharging with rain water (causing natural dilution)
and binding of nutrient into blooming aquatic organism. In
September 2003, the population of duckweeds decreased
substantially and few small patches (1–2 ft
2
) of Wolffia appeared. The population of Lemna, Spirodela and Wolffia
continued to bloom untill November. In the entire study
period, the Lemna:Spirodela ratio varied between 18:6 and
24:6. The water pH reduced in November. The duckweeds
dried and disappeared in December 2003. Lemna and Spirodela reappeared and bloomed in the month of January 2004.
The turbidity, NPK and pH increased from December 2003
onwards. The highest population ratio between Lemna and
Spirodela was recorded in November 2003, February 2004
and July 2003. In the month of April 2004, the duckweeds
and other floating macrophytes such as Eicchornia crassipes
(not covered in the present study) died. The water level also
decreased sharply. The major part of the pond on eastern,
southern and northern sides dried up first and completely
by the end of May 2004. The pond remained dry for several months and could be recharged after the hurdles (weirs)
around the pond were partially removed to let the mesocosm
receive the rain water from its catchments.
2.6 Discussion
In the beginning of summer (April 2003), the size and volume of the main water body of “Lal Diggi” reduced with
dirty appearance of water and moist or marshy margin
around the pond with dense population of Typha angustata
and Paspalum paspaloides. The water body size further reduced in May 2003 (Table 1). Rainfall in these 2 months
F. A. Khan et al.
mat. The area and volume of water body reduced further and
eventually, the duckweeds and water hyacinth formed a thick
cover on almost entire water surface.
April 2004 The duckweeds and water hyacinth were relatively denser than in April 2003, but in drying stage. The
size of the water body was far smaller and limited to central and western side of the pond. Owing to fast reduction in
water level and almost no recharging with the rain water, a
vast area of the pond had several smaller patches of Typha
angustata even in the centre of the pond. The pond dried by
the end of May 2004. Sedge grass showed slight browning
as it started drying after the maturity. Considerable drying
of duckweeds was recorded along its coast and in the central part of very small patch of water body. The water level
of 30–60 cm hindered the movement of small boats. Larger
area sediment was exposed at many places (Fig. 2.1)
The aquatic body turned into terrestrial body and by April
2005, some trees of Acacia nilotica and Prosopis species appeared on the eastern, and northern and in the centre of the
dried pond. In the following monsoon (July 2005–August
2005), the district local authorities managed the refilling of
the pond with fresh water and reduced the weirs around it.
This unusual restoration management resulted into a better
non-blooming and lesser eutrophic water body.
2.5.2 Quantitative Studies
The monthly data on seasonal variation in population dynamics of Lemna minor and Spirodela polyrrhiza in the rainfed Lal Diggi pond is summarized in Table 1. Both the duckweeds had numerical dominance in the months of February
and March. Lemna bloomed in the months of November
and August. Lemna and Spirodela did not grow in summer
(April to June) and December. The density of Lemna minor
dropped down significantly to 1,182 individuals/m
2
and of
Spirodela polyrrhiza to 2,358 individuals/ m
2
area of the
pond. The highest fresh weight of the selected macrophytes
along with blue-green algae and other floating macrophytes
(excluding water hyacinth) was recorded in the month of November 2003, March 2003, August 2003 and February 2004.
The fresh biomass was minimum in the month of January
(Table 2). Spirodela accumulated maximum dry matter in the
month of February 2004 followed by March 2003, November 2003 and August 2003. Lemna accumulated maximum
dry matter in the month of March 2004 followed by November, February and August of previous year (2003). The dry
weight of Spirodela and Lemna was least in the month of
January 2004 and September 2003, respectively. The net primary productivity (NPP) of both the duckweeds was high in
the months of February 2004, August 2003 and November
2003 (Table 2.1).
Spirodela polyrrhiza 225.82 g m
−2
yr
−1
The data in Table 2
show monthly variations in the population of Lemna minor,
Spirodela polyrrhiza, Wolffia arrhiza and chemical characteristics of water (pH, NPK, TDS, turbidity) of the selected
mesocosm, average atmospheric temperature and precipitation (rainfall in mm). Lemna minor and Spirodela polyrrhiza
bloomed between July 2003 and March 2004 (except in December 2003). The occurrence of duckweeds was recorded
in April 2003. The pH, water turbidity, total dissolved salts
(TDS) and NPK contents were high but dissolve oxygen was
very low in summer (April 2003 to July 2003). The nutrient contents (NPK) increased as the volume of water reduced in summer. The pH and turbidity reduced as the water
body swelled after 11.4 mm downpour in the month of July
(Table 2.2).
Lemna minor and Spirodela polyrrhiza bloomed in July
2003. The population ratio of Lemna:Spirodela was approximately 18:6 (105:36 individuals of Lemna and Spirodela respectively). The density of both these duckweeds increased
in the month of August 2003 (Table 2). In this month the
pH, turbidity and nutrients of the water decreased owing
to recharging with rain water (causing natural dilution)
and binding of nutrient into blooming aquatic organism. In
September 2003, the population of duckweeds decreased
substantially and few small patches (1–2 ft
2
) of Wolffia appeared. The population of Lemna, Spirodela and Wolffia
continued to bloom untill November. In the entire study
period, the Lemna:Spirodela ratio varied between 18:6 and
24:6. The water pH reduced in November. The duckweeds
dried and disappeared in December 2003. Lemna and Spirodela reappeared and bloomed in the month of January 2004.
The turbidity, NPK and pH increased from December 2003
onwards. The highest population ratio between Lemna and
Spirodela was recorded in November 2003, February 2004
and July 2003. In the month of April 2004, the duckweeds
and other floating macrophytes such as Eicchornia crassipes
(not covered in the present study) died. The water level also
decreased sharply. The major part of the pond on eastern,
southern and northern sides dried up first and completely
by the end of May 2004. The pond remained dry for several months and could be recharged after the hurdles (weirs)
around the pond were partially removed to let the mesocosm
receive the rain water from its catchments.
2.6 Discussion
In the beginning of summer (April 2003), the size and volume of the main water body of “Lal Diggi” reduced with
dirty appearance of water and moist or marshy margin
around the pond with dense population of Typha angustata
and Paspalum paspaloides. The water body size further reduced in May 2003 (Table 1). Rainfall in these 2 months