Introduction

Importance of Deepor Beel nutrient management

Deepor Beel is the largest lake in Assam, northeast India. It is located to

the south-west of Guwahati city, in Kamrup Metropolitan district. The lake
is a permanent freshwater lake, in a former channel of the Brahmaputra
River, to the south of the main river. It is having length of 4,014 hectares
(15.50 sq mi) and varies in depth from approximately 1 m to 4 m
depending on the season.

The Assam government has designated Deepor Beel and its environs as a
wildlife sanctuary. Since August 2002, the lake has been recognised as a
protected Ramsar site. Its position places it within the biogeographic
region of the Burma monsoon forest, which is home to a wide variety of
plant and animal species. The lake is a significant part of the Deepor Beel
Sanctuary and contributes to its rich biodiversity.

The lake is particularly rich in a variety of birds, including endemic bird

species. The water is inhabited by common and unique species of fishes.
Different species of prawns, snails, crabs, turtles and tortoises are also
found here. More than 70 species of birds have been recorded from the
lake and its shore. Among unique species are Spotbilled Pelican (Pelicanus
philippensis), Lesser and Greater Adjutant Stork (Leptoptilos javanicus and
dubius), and Baer’s Pochard (Aythya baeri), which are rarely seen in other
parts of the region. In addition, aquatic birds and wild ducks in Deepor
Beel are found nowhere else in Assam.

The health of these species and the surrounding communities are directly
dependent on the health of this wetland ecosystem. Potential threats
include over-fishing and hunting pressure upon waterbirds, pollution from
pesticides and fertilizers, and infestation by water hyacinth Eichhornia
crassipes. Already, a sewage canal from the city to the beel is considered
destroying in its.

Methodology
Sample collection

Field work was conducted to collect water quality samples during the 10 th
- 11th May, 2023 in and around the Deepor Beel. Spatially, a total of 18
water quality samples were collected (duplicate samples) from the lake
(Figure 1). In-situ measurements of water quality (salinity, pH, turbidity,
dissolved oxygen, chlorophyll-a (Chl-a) and temperature) were carried out
using a pre-calibrated water quality probe. After collection, nutrients
samples were filtered through cellulose acetate membranes filters (GF/F
47 mm, 0.45-μm, Whatman) and then preserved in 500 ml high-density

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polyethylene (HDPE) plastic containers. To avoid sample deterioration,
filtrates were preserved with HgCl2. Considering the use of short-term,
other samples were refrigerated storage (4 °C), and laboratory analysis
was conducted within 7 days after sampling.

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Figure 1: Spatially, a total of 18 water quality samples were collected from the Deepor Beel

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Results

Dissolved inorganic nitrogen (DIN)

Dissolved inorganic nitrogen (DIN) levels are critical in determining the
nutritional status and ecological health of aquatic habitats such as
Deepore Beel. DIN encompasses various forms of nitrogen, including
nitrate, nitrite, and ammonium, which are essential nutrients for aquatic
plants and algae but can become pollutants at elevated concentrations.
DIN concentrations vary significantly between measurement stations,
demonstrating regional variability in nutrient delivery within Deepore
Beel. The concentration of DIN is lowest at DB-08 (2.43 µM/L) and greatest
at DB-04 (785.11 µM/L)
900
800
DIN [µM/L]

700
600
500
400
300
200
100
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Figure 2: Spatial variation of DIN (µM/L) in Deepor Beel Waters

Dissolved inorganic phosphorus (DIP)

Dissolved inorganic phosphorus (DIP) concentrations are important

markers of nutrient dynamics in aquatic environments such as Deepore
Beel. DIP levels are relatively high at many measurement locations,
including DB-03 (62.90 µM/L), DB-05 (65.65 µM/L), and DB-06 (68.82
µM/L), indicating phosphorus enrichment. Agricultural runoff, wastewater
discharge, and sediment resuspension are all potential causes of elevated
DIP concentrations. Excessive phosphorus inputs can promote algal
blooms, resulting in eutrophication, diminished water clarity, and oxygen
depletion, all of which harm aquatic ecosystems and biodiversity.
Monitoring DIP levels is critical for measuring nutrient loading, identifying
pollution sources, and guiding management strategies to reduce
phosphorus pollution while preserving Deepore Beel's ecological integrity.

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 70.0
DIP [µM/L]

60.0
50.0
40.0
30.0
20.0
10.0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Figure3: Spatial variation of DIP (µM/L) in Deepor Beel Waters

16
14
N/P ratio

12
10
8
6
4
2
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Figure4: Spatial variation of N/P ratio in Deepor Beel Waters

Dissolved Silicate (DSi)

DSi concentrations vary from 244.40 µM/L to 254.58 µM/L at different
sample sites.
The very constant DSi values indicate a homogenous distribution of
silicate inside Deebor Beel. High DSi levels can promote diatom
development, resulting in eutrophication and algae blooms in the lake.
Identify and reduce DSi inputs such as agricultural runoff, industrial
discharge, and sewage effluent. Develop and execute nutrient
management techniques to prevent eutrophication, taking into account
DSi as well as other important nutrients such as nitrogen and phosphorus.
Encourage sustainable agricultural methods, such as precision farming

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and nutrient management planning, to reduce nutrient runoff into the
lake.

256
254
DISi [µM/L]

252
250
248
246
244
242
240
238
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Figure5: Spatial variation of DSi (µM/L) in Deepor Beel Waters

Primary production
Chlorophyll values vary between 18.28 µg/L and 105.6 µg/L among
sample locations. High chlorophyll levels detected at many measurement
locations suggest the existence of phytoplankton biomass and strong
photosynthetic activity. Dissolved oxygen concentrations vary from 4.05
to 24.70 mg/L. High chlorophyll concentrations indicate that Deebor Beel
has vigorous phytoplankton growth and primary production, which
contributes to the lake's productivity and food web dynamics. Elevated DO
levels suggest vigorous photosynthesis, with phytoplankton emitting
oxygen as a byproduct during daylight hours. Nutrient availability, light
penetration, and water temperature are all important elements impacting
primary production in Deebor Beel. Nutrient inputs from nearby land
regions, like as agricultural runoff and sedimentation, can boost
phytoplankton growth and primary production.Based on chlorophyll
concentration lake ecosystem can be divided in to four diffrent trophic
states1:

1. Oligotrophic (nutrient-poor): Less than 2 μg/L

2. Mesotrophic (moderately productive): 2 μg/L to 6.0 μg/L
3. Eutrophic (highly productive): 6.0 μg/L to 40.0 μg/L
4. Hypereutrophic (excessively productive): Greater than 40.0 μg/L
Results from the present study indicated prevalence of eutrophic to
Hyper-eutrophic conditions in surface waters of Deepor Beel wetland. In
particular station 2-6, towards the east and southeastern part of Deepor
Beel indicated extreme eutrohic conditionpossibly due to high input from
land based activites.

1
https://files.knowyourh2o.com/pdfs/FS-WQ-Parameters.pdf

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 120.0
Chlorophyll [µg/l]

100.0
80.0
60.0
40.0
20.0
0.0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Figure 6: Spatial variation of Chl-a (µg/L) in Deebor Beel Waters

24.0
DO [mg/l]

20.0
16.0
12.0
8.0
4.0
0.0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Figure7: Spatial variation of DO (mg/L) in Deebor Beel Waters

The main target of Deepor Beel restoration is controlling the sources of

nutrients that result in eutrophication. The excessive enrichment of waters
with anthropogenic sources of nutrients leads to the transition of
oligotrophic water bodies to mesotrophic, eutrophic and finally
hypertrophic conditions. Phosphorus is the most important nutrient in
regulating primary productivity in this type of land-locked lakes and is a
major contributor to eutrophication in aquatic systems. Eutrophication
restricts water use for fisheries, recreation, industry and drinking due to
increased growth of undesirable algae and oxygen shortage cause by
algal decomposition. Eutrophication is still one of the most important
problems to solve through lake management. Trophic state of the Lake
(TSI) is often used to estimate the trophic condition of the lake ecosystem,

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Trophic state of the Lake (TSI)
Excess nutrients can trigger Lake Eutrophication, which causes algal blooms and
poor water quality and associated ecosystems. Based on the a few key
physicochemical parameters, the lake waters of Deepor Beel was classified into one
of the four general trophic states, 1. Oligotrophic, meaning low nutrient levels and
high-water transparency, 2. Mesotrophic, a moderate elevation in nutrient
concentrations resulting in lower water transparency and a reduced desirable
biological habitat, 3. Eutrophic, the enriched state or 4. Hyper-eutrophic, the most
enriched environment which can result in substantial loss of environmental health.
Based on field data of Secchi depth, Chl-a, Total phosphorous and total nitrogen, all
the Deepor Beel lake was classified into various trophic states and the results are
given in below table 1.
Calculation of Trophic state Index
Water quality indices are used to trace the trophic state in the Deepor Beel lake. The
Sechhi Disk Depth (SDD), Chlorophyll-a (Chl-a) and Total Nitrogen (TN) and
Phosphorous (TP) concentrations were used to assess of the trophic state based on
the TSI (improved Carlson TSI
(https://lake.wateratlas.usf.edu/library/learn-more/learnmore.aspx?
toolsection=lm_tsi). The method calculates a separate component TSI for nitrogen,
phosphorus and chlorophyll a. These components are then combined, as indicated
in equations 1 to 5 and A-C below, to determine the overall TSI. This procedure is
the basis for all Water Atlas TSI calculations.

Where TSI(Chla), TSI(TN), TSI(TP), and TSI(SDD) refer to the corresponding trophic
state index of Chla (μg L−1), TN (mg L−1), TP (mg L−1) and SDD (m), respectively. The
waters with TSI <40 are grouped into “oligotrophic state”, and the waters with TSI
ranging from 40 to 50 are distinguished into “mesotrophic state”. If the TSI values
range from 50 to 70, the waters belong to “eutrophic state”. The value of TSI higher
than 70, suggests “hypertrophic state”2 (Kratzer and Brezonik 1981).

TSISD = 60 − 14.42 ln (SD)

TSITN = 54.45 + 14.43 ln (TN)
TSITP = 14.42 ln (TP) + 4.15
TSICHL = 9.81 ln (Chl) + 30.6

2
https://lake.wateratlas.usf.edu/upload/documents/1996%20Water-Quality%20Assessment%20for%20the
%20State%20of%20Florida%20Section%20305(b)%20Main%20Report.pdf

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Figure8: Spatial variation in trophic state indices calculated from Secchi disk
transparency (TSI-SDD), Chl-a (TSI-Chl), total phosphorus (TSITP) and total
nitrogen (TSITN). Dotted blue line represents transition from oligotrophic to
mesotrophic, red line represents transition from mesotrophic to eutrophic condition
and dotted orange line represents transition from eutrophic to hypereutrophic
condition.
TSI values calculated from SD, TN, TP and Chl-a at different sectors of Deepor Beel
lake were given in figure 8. The high TSISD scores indicated the clear oligotrophic
nature in terms of Secchi disk transparency. TSICHL showed lower values compared
to TSISD in the Deepor Beel Lake. These lower values of TSI CHL than TSISD, clearly
indicated that phytoplankton biomass attributed to total suspended load
It is assumed that if the TSICHL is equal to or greater than TSI TN in an aquatic system,
nitrogen is limiting nutrient to algal growth. Similarly, when TSI CHL is lower than TSITP,
it indicates that there is less algal material present than expected, based on nitrogen
and that some other factors may be limiting. The trophic state index determined with
Chl-a showed higher values than those determined with total nitrogen, indicates a
probable N deficiency during the study period.

From the present study, it can be concluded that the trophic state of the lake was
primarily controlled by the combined effect of phosphorous availability during non-
monsoon season. Higher algal densities supported by enriched nutrient conditions
might also facilitate decay of terrestrial forest litter in the lake waters. The TSI can be
a valuable tool for monitoring lake water nutrient status and also as a simple but
effective scientific technique for investigations where an objective standard of trophic
state is necessary, but a more complete picture of the lake trophic state can be
achieved only by applying several indicators (including physical, chemical, and
biological) simultaneously.

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 Relationship Between TSI Variables Conditions

TSI(Chl) = TSI(TP) = TSI(SD) Algae dominate light attenuation; TN/TP ~ 33:1

TSI(Chl) > TSI(SD) Large particulates, such

as Aphanizomenon flakes, dominate

TSI(TP) = TSI(SD) > TSI(CHL) Non-algal particulates or color dominate light

attenuation

TSI(SD) = TSI(CHL) > TSI(TP) Phosphorus limits algal biomass (TN/TP > 33:1)

TSI(TP) >TSI(CHL) = TSI(SD) Algae dominate light attenuation but some

factor such as nitrogen limitation, zooplankton
grazing or toxics limit algal biomass.

Deepor Beel lake nutrient management

Nutrient inputs from both point and non-point sources are managed to ensure that
ambient nutrient concentrations are within the bounds of sustainable watershed
management. A substantial reduction in the current levels of algal biomass to levels
below a nuisance condition in Lake,.

Pala Lake Nutrient management Recommendation

Nutrient management for oligotrophic lakes like Deepor Beel is important because
these lakes are naturally low in nutrients and are sensitive to changes in nutrient
levels. Even small increases in nutrient levels can lead to changes in the lake's
ecosystem, such as increased algae growth and blooms. The following steps may be
followed for the future management of the land-locked Deepor Beel Lake.

Stop any degradation – Ensure no human induced eutrophication in the Lake.

Nutrient concentrations in waters are, at a minimum, to be maintained at current
levels so that oligotrophic condition can persist.
Conserve and protect waters that meet nutrient targets Maintain current nutrient
concentrations and loadings in those waters that already meet nutrient targets and
protect those waters from increases due to future land use practices.
Reduce use of detergent: Use of detergent in the watershed are a major source of
inorganic phosphorous pollution in lakes. By reducing detergent use in the
watershed, homeowners and farmers can help to protect the lake's water quality.
Restore waters that do not meet nutrient targets – Plan strategy to reduce
phosphorus concentrations and loadings in those waters by using a focused nutrient
management approach (identification of point and non-point sources) that considers
both current and future land uses.
Regularly monitor and report on the status of nutrients in the Deepor Beel Lake.
Monitor the lake's nutrient levels and water quality regularly. This will help you to
identify any trends and to assess the effectiveness of your management efforts.

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Involve the community in your nutrient management efforts. This can help to build
support for your initiatives and to ensure that everyone is working together to protect
the lake.

Conduct research to better understand nutrient cycling and the effect of human
activities, to improve management decisions.

Nutrient Management Strategy

Nutrient Management for a land-locked lake is beneficial for economic and social
growth. Implementation of the Nutrient Management Strategy and achievement of
the Vision should be based on the following key principles:
Adaptive Management – Decisions must be flexible and be adjusted in the face of
uncertainties, natural variability and new information.
Precautionary Principle and No Regrets Actions – The lack of full scientific
certainty shall not be used as a reason for postponing cost effective best
management practices.
Prevent Pollution – It is better for the environment and more cost effective to
prevent nutrient enrichment than to clean it up after the fact.
Shared Responsibility – The responsibility for policy and program development and
implementation should be shared within the mandate of all jurisdictional levels.
Promote Awareness – Connecting people with the watershed and nutrient issues
facing the Great Lakes provides a key motivating force for actions.
Accountable and Clear Actions – Actions must be coordinated and transparent
and everyone must be accountable for their actions.
Work Together – Integration and cooperation must occur across traditional
environmental, social and economic boundaries to align our actions.

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