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| Study of Nutrient Release Potentials of Some Macrophytes of Ajmer and Their Relevance in Trophic Assessment of a Shallow Waterbody | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Paper Id :
15905 Submission Date :
11/04/2022 Acceptance Date :
18/04/2022 Publication Date :
25/04/2022
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| Abstract | Nutrient release potentials of dominant macrophytes in trophic evaluation of Lake Anasagar, an urban wetland of Ajmer, was assessed. It was observed that besides anthropogenic activities, macrophyte species i.e. Azolla pinnata, Trapa bispinosa as floating and Potamageton crispes, Vallisnaria spiralis as submerged, release nutrients (N, P, Ca and Mg) in Lake waters and form a natural culture medium for phytoplanktons. It was estimated that comparatively more amount of nitrogen and phosphorus added to the nutrient pool is released by Azolla and Trapa respectively. Floating species contribute much to the nutrient - enrichment then submerged macrophytes. It has been emphasised that along with algal diversity and characteristic vegetation dynamics, values of decomposition constants (k), nutrient losses and biomass may be utilised for trophic evaluations of shallow urban wetlands in semi - arid regions. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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| Keywords | Macrophytes, Trophic evaluation, Nutrients, Decomposition, Biomass. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Introduction |
The fresh waterbodies of tropical and sub-tropical regions like India are characterised by large seasonal fluctuations in water level which has been recognised as a component of the normal environment for the vegetation adapted to such habitats (Gopal, 1986). Numerous small and large, natural, and man-made water bodies experience both hydrosere and xerosere operating in the same area at different times of the year (Misra, 1946). However, in waterbodies of arid and semi-arid regions of Rajasthan, such a dynamic series in communal units is clearly demarcated due to very low precipitation largely confined to rainy season and extremely high temperatures. These waterbodies are characterised by low water level during peak summers and excessive phytoplankton and macrophyte production throughout the year. |
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| Aim of study | The objectives of this study were to determine the decay rates and nutrient release potential of dominant macrophytes of the Lake Anasagar, a shallow urban fresh waterbody of Ajmer, Rajasthan. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Review of Literature | Water level drawdown followed by rewetting in
reservoirs/lakes can shift macrophytes from a nutrient sink to a source, thus
changing nutrient dynamics and causing water quality problems (Lu et al. 2018).
Therefore, rates of weight loss and nutrient release potentials of dominant
macrophytes provide valuable information pertaining to the trophic state and
cycles which involve complex interactions between phytoplankton, macrophytes
and nutrients (Melzer, 1981; Wiegleb, 1984). Nutrient release by decomposition
of macrophytes constitute a specific natural medium which also dictates a
‘type' of algal diversity. Therefore, a comparative study of dominant
macrophyte species has significance in determining the vegetation and nutrient
dynamics of an aquatic ecosystem. Various aspects of macrophytes have been
worked out for inland waterbodies of Indian subcontinent (Gopal, 1990),
however, no report is available on aquatic plant species of Aravalli region of
Ajmer. |
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| Methodology | Total 15 bags for each macrophyte were suspended in a concrete-cement tank (75x45x60 cm) filled 3/4th with lake water. Three bags were retrieved at each interval of 5, 10, 20, 40 and 60 days and remaining material was dried at 80oC in oven for the estimation of loss in weight. Decomposition constant (k) and time required for 50% and 95% decay were calculated with the help of exponential equation given by Olson (1963).
X/Xo = e-kt
(X = weight of remaining at time t, Xo = initial weight, e = base of natural logarithms and k = decomposition constant).
In initial and remaining material of each interval, nutrients were determined: ash content by ashing the remaining in muffle furnace at 5500C; nitrogen by micro-Kjeldahl method, phosphorus by colorimetry using ammonium molybdate, and calcium and magnesium by EDTA titration method. The amount of each nutrient in remaining (as a percentage of the initial amount) was calculated as described by Kok et al. (1990). For the computation of total nutrient release, biomass of each species was also estimated by taking twenty 1 x1 m quadrats during the period of occurrence in the Lake Anasagar and mean annual values were estimated. |
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| Result and Discussion | Data
on percentage remaining dry weight show rapid loss in dry weight within 5-10
days of decomposition (Table 1). On the 10th day, the weight
loss was less than 50% except for Azolla pinnata in which it
was about 60%. Further loss in dry weight continued at a slower rate in Azolla
pinnata and at a rapid rate in Potamoqeton crispus and Trapa
bispinosa. Most rapid loss was observed for Vallisneria spiralis in
which it remained only about 2% on 60th day as compared to
about 11% in Potamogeton and Trapa and about
48% in Azolla. Slow rate in weight loss of Azolla and
rapid rate of Potamogeton and Trapa followed
by Vallisneria is clear from decomposition constant (k) and
values of time required for 50% and 95% decay of these macrophyte species. Table
1: Changes
in dry weight during leaching (% remaining), decomposition constant (k) and
days required for 50% and 95% decay of various macrophyte
Analysis
of remaining at different stages of decay showed that significant amounts of
nitrogen and phosphorus were lost during decomposition. A rapid loss of P
occurred within the first five days where more than of initial P amount was
lost from all the species. On 10th and 20th day P was greater
in Azolla and Trapa while on 60th day
it was 15% in Azolla followed by 5-6% in Potamogeton and Trapa,
and only 1% in Vallisneria. Table
2: Calcium
and magnesium (as a percentage of the initial amount) in leached litter after
A
rapid loss of N was observed from Potamogeton and Vallisneria where
about 80% nitrogen was lost within first ten days. %N was found to be increased
from the initial content in Azolla and Trapa within
first five days and the same was lost at a slow rate in Azolla and
at a rapid rate in Trapa. Analysis of remaining at 60th day
showed that more than N loss occurred from Azolla while it was
more than 90% from other three species. About
40-50% calcium loss occurred on the 5th day with gradual
increase at subsequent intervals (Table 2). A maximum loss was noticed in Trapa (92%)
followed by Potamogeton (87%) and Azolla (60%)
at 60th day where remaining of Vallisneria was
almost negligible. Magnesium loss occurred on the 5th day
was significantly high in Azolla followed by Trapa as
compared with two submerged species i.e., Potamoaeton and Vallisneria.
Magnesium content in the remaining at subsequent intervals increased in Azolla while
it was decreased in other species. The
total biomass production and its nutrient content computed for different
macrophyte species are presented in Table 3. Based on total annual biomass
production per ha and total content of N, P, Ca, and Mg in individual
macrophyte, floating species i.e., Trapa and Azolla contribute
much to the nutrient-enrichment than Potamogeton and Vallisneria (submerged
species). Total nutrient output (if plant material is completely decomposed)
from floating species is about 6-10 times more than submerged plants. Among the
macrophytes studied, greater amount of nitrogen and phosphorus are released
from decay of Azolla and Trapa respectively. Table
3: Computed values of biomass and nutrients (kg ha-1 yr-1)
of various macrophytes
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| Findings | Nutrient release through macrophyte decay is an important parameter to evaluate trophic status of wetlands characterised by low depth. A large amount of total nutrients are added either by leakage from macrophyte shoots (Wetzel and Manny, 1972; Twilley et al., 1977) or released through decomposition of dead parts (Godshalk and Wetzel, 1978; Carpenter, 1980). In the shallow waterbodies of Aravalli region, the total biomass production during the peak grow ing period is converted into dead matter due to exposed sediment of peripheral zone during the summers. van der Velde (1979) has emphasised that in initial decomposition can be of a physiological nature controlled by external factors. However, the decomposition is more a physical in case of Lake Anasagar due to drying off of major area of wetland suddenly during peak summer period. Decay of dead matter (litter) starts immediately and continued till moisture is available. A greater amount of nutrients are released during this initial decomposition. In extreme dry period (May-June) decomposition is postponed till water is available in the preceding rainy months (July-August). Later part of decomposition is initiated in the rainy season and continued throughout the year even for longer time in species like Azolla where it requires comparatively longer time for complete decomposition (576 d for 95% decay). Through water surface-flow during rains or due to increased water level partially decomposed matter is added to total nutrient and DOM pool of the water body (Gopal, 1988). | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Conclusion | Therefore, besides biomass production, nutrient concentration, and ‘Intermittent’ decay rates of individual macrophyte species, seasonality of water level and morphometerics of the waterbody are also to be taken as important parameters for evaluation of trophic status of a waterbody. Gopal (1986) has explained the characteristic vegetation dynamics of such temporary and shallow water habitats. Among macrophytes, Azolla contributes more to the nutrient-enrichment of water as compared to other macrophytes in Lake Anasagar. Contribution by Trapa is even more than Azolla but confined to zones where it is being cultivated by farmers. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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