Dissolved nutrients (PO sub(4)-P, NO sub(3)-N, NO sub(2)-N and SiO sub(4)-Si) estimated in the surface and bottom waters of five selected stations of the Paravur Lake, Kerala, India, during February 1987 to January 1988 revealed distinct seasonal variations. Rainfall and land drainage play significant roles in the nutrient economy, particularly NO sub(3)-N and SiO sub(4)-Si, of this water body. Abnormally high values of PO sub(4)-P indicated extremely polluted condition at the wetting zone of the lake during the premonsoon season. SiO sub(4)-Si showed significant negative relationship with salinity

Balakrishnan Nair, N.

Ritakumari, S.D.

Shibu, S.

Aquatic Commons

Journal of the Indian Fisheries Association 
20, 1990, 59~68 
ENVIRONMENTAL INVENTORY AND THE DISTRIBUTION OF 
INORGANIC NUTRIENTS IN A TROPICAL ESTUARY OF TilE 
SOUTH-WEST COAST OF INDIA 
S. SHIBU, S.D. RITAKUMARI AND N.BALAKRISHNAN NAIR 
Department of Aquatic Biology & Fisheries University of Kerala, Thiruvananthapuram - 695 007 
ABSTRACT 
Dissolved nutrients (P04-P, N03-N, N02-N and Si04-Si estimated in the surface and bottom 
waters of five selected stations of the Paravur lake during February 1987 to January 1988 
revealed distinct seasonal variations. Rainfall and land drainage play significant roles in the 
nutrient economy, paa·ticularly N03-N and Si04-Si, of this water body. Abnormally high values 
of PO .s"P indicated extremely polluted condition at the retting zone of the lake during the pt·e-
monsoon season. Sio4-Si showed signicifant negative relationship with salinity. 
INTRODUCTION 
Investigation on the distribution and 
seasonal variations of nutrients is a 
prerequisite for assessing the biological 
economy of an estuarine milieu. Studies on this 
aspect in tropical environments have been 
made by several workers (Sankaranarayanan 
and Qasim, 1%9; De souza et aL, 1981; Saraladevi 
et al., 1983; Nair et al., 1984; Upadhyay, 1988). 
The present paper discusses the spatial and 
ten1 poral distribution of inorganic nutrients at 
the five selected stations of the Paravur lake, a 
temporary estuary along the South-west coast 
of India (Fig. 1 ). 
MATERIAL AND METHODS 
Water samples for the analysis of P04-P, 
N03-N, N02-N and Si04-Si were collected at 
monthly intervals for a period of one year 
(February'87 - January'88) from two depths, 
surface and near bottom. The procedures 
suggested by Strickland and Parsons (1972) 
were followed for the sample collection and 
analysis. 
RESULTS AND DISCUSSION 
Results (Fig. 2) show wide fluctuations in 
phosphate-phosphorus concentrations with 
high values during pre-monsoon (February-
May). This is in contrast to earlier reports 
(Sankaranarayanan and Qasim, 1969; De Souza 
et al., 1981; Nair et al., 1984). As the bar 
59 
remained closed during this period normal 
flushingofnutrients from the lakeintothesea 
did not take place. Coupled with this, the high 
rate of evaporation prevailing at that time might 
also contribute to the replenishment of 
phosphate in this system during the pre-
monsoon. Even though all the stations, except 
station 5 showed some increase in phosphate-
phosphorus content during early monsoon, 
higher levels were not maintained for ~ny 
considerable length of time at any of the 
stations. So also the distribution ofthis nutrient 
in the water column was not uniform in the 
monsoon season even though vertically 
homogenous conditions prevailed. Hence it 
seems that even though some enrichment in 
this nutrient takes place with the first pulse of 
the monsoon, no significant contributions are 
made from the river influx. This is further 
evidenced by the fact that very low 
concentrations and even nil values were 
noted at all the stations during the post-
monsoon when there was considerable rainfall 
and river runoff. This fully agrees with- the 
report of U padhyay (1988). Ithikkara river 
carries very high sediment load in the flood 
waters (Azis, 1978) which might be the reason 
for phosphate removal by adsorption causing 
low values during the post-monsoon. The 
surface and bottom water showed almost the 
same trend of variation in the phosphate-
60 SHIBU, RITAKUMARI AND NAIR 
80 J'~ ~ ..., ~k t.. a' Gt u~ 
76° 39
1 ~( 
Fig. 1. Location of sampling stations (Paravur lake). 
phosphorus concentration. On most of the 
occasions, bottom water values were slightly 
higher than those of the surface except at 
station 5, where a reverse condition was 
discernible. 
In general, the phosphat.e-phosphorus 
concentration of the medium ranged from 
nil to 1.46 J.tg. at. I-1, when stations 1, 2, 3 and 
4 are taken into account barring a stray 
.incident at station 4 in February when an 
unusually high value of 5.07 J.tg~ at. 1"1 was. 
recorded. The cause of this variation can by 
all probability be attributable to the incursion 
.of polluted water from the nearby retting 
zone (station 5). Unlike the other ·stations, 
station 5, a retting enclosure highly poUuted 
N 
t 
• 1 ITHIKKARA 
• 2 MAMOOTY KADAVU 
• 3 MUKKAM 
• 4 POZHIKARA 
• 5 MANIYANKULAM 
~ 
:E 80 
t..B' 
... 
.,. 
76° t..2' 
with the ret liquor and H2S coupled with anoxic 
condition, recorded abnormally high 
phosphate values throughout the pre-monsoon 
period. Similar results have been reported by 
earlier workers (Azis, 1978; Nair et al., 1983). 
Inorganic phosphate concentration is a 
useful index of the state of eutrophication of 
water bodies. Ketchum .(1967) held the view 
that 2.55 J.tg. at. 1"1 is the upper limit of 
phosphate ~ncentration in eutrophication of 
estuaries. In the present study stations 1,2,3 and 
4 had consistantly lower values throughout 
the year whereas station 5 surpassed this limit 
· during the entire pre-monsoon period. 
As . reported by several workers, the prime 
12·0 
10·0 
8·0 
6·0 
l.· 
z.o 
0 
1·4 
1·2 
,___. 
1·0 
....:. 
0 
0'1 0-8 
;L 
~06 
a:: 
0 
:r:: 
CL 0·4 
\I) 
0 
I0·2 CL 
UJ 0 
f-
<( 1· 
:r:: 
CL 
\I) 
0 1·2 
:r:: 
CL 
1· 
0'8 
0·6 
O.L, 
0·2 
0 
\ 
\ 
INORGANIC NUTRIENTS IN A TROPICAL ESTUARY 
STATION 5 
" I \ 
I I 
I I 
I I 
I 
STATION 3 
STATION 1 
-SURFACE 
0 J 
1988 
---BOTTOM 
s.nJ 
4·0{ 
F M A M J 
\987 
Fig. 2. Seasonal variations in Phosphate-phosphorus 
61 
STATION 4-
STATION 2 
J A S 0 
MONTHS N 0 ldsa 
62 SHIBU, RITAKUMARI AND NAIR 
source of nitrogencous nutrients in an estuary 
arc from the river run off during the monsoon 
rains and also from in situ fixation. In the 
present study also the nitrate-nitrogen 
concentration registered a sharp increase 
during the onset of the monsoon and a 
decrease as the season progresses (Fig. 3). 
After the peak in May-June, the nitrate-
nitrogen values dropped during the following 
months prior to another increase in October 
with the advent of the north-east monsoon. 
Influence of nitrate rich fresh rainwater could 
explain for the higher values recorded. This 
agrees fairly well with the earlier reports 
(Azis, 1978; Saraladevi eta l., 1983; Gopinathan, 
1985). Nitrate concentrations were fairly g~ 
in the medium during February to April (pre-
monsoon). The high rate of evaporation and 
the decomposition of dead organic matter 
account for this (Zafar, 1964). 
A comparative study between station 5, a 
highly polluted zone, and the other stations 
revealed that in the former nitrate values were 
much lower than those in the latter throughout 
the period of study. The low values for the 
polluted region could be due to low rate of 
replenishment during the degradation of 
organic matter and also possibly due to 
reduction as suggested by Nair et al. (1984). 
the concentration of nitrite-nitrogen was 
generally found to be much lower than that 
of nitrate. Nitrite-nitrogen exhibited a' bimodal 
annual cycle with peaks during the pre- and 
post-monsoon~ (Fig. 4). In the present study 
the nitrite-nitrogen concentration was higher 
during the post-monsoon with the peak in 
October except at station 5 where seasonal 
averages showed more or less similar pattern 
during all the seasons. This increase in nitrite 
levels in the water column during the post-
monsoon period is probably related to the north-
cast monsoon showers. Nonetheless, a clear 
relationship with the seasons as in the case 
of nitrate is not apparent. Similar observations 
were made by different workers in various 
waters (Saraladevi et al., 1983; Nair eta!., 1984; 
Kahar, 1988). During the pre-monsoon period 
moderate concentrations of this nutrient 
detected in the present study might be due to 
the oxidation of ammonia, reduction process 
of nitrate, excretion of phytoplankton and by 
the recycling of nitrogen (Sankaranarayanan 
and Qasim, 1969; Rajendran, 1974). The 
seasonal variation observed in the nitrite-nitrogen 
concentration may thus be due to the 
predominance of any one of these processes 
depending upon the biological activity 
prevailing in the area. 
In estuaries the mixing of the silicon-rich 
freshwater with silicon-poor saline water plays 
an important role in governing the dissolved 
silicon distribuqon. In the present study when 
the salinity was low during the monsoon and 
post-monsoon periods the concentration of 
silicate-silicon was found to be high (Fig. 5). 
This inverse relationship between salinity 
and silicate concentration agrees well with 
earlier reports (Nair et al., 1983; Gopinathan, 
1985). Seasonal averages have indicated low 
concentration of silicate-silicon in the medium 
during pre-monsoon when the salinity was high 
confirming the views referred to above. An 
abrupt increase in this nutrient was evident 
at all stations early in the monsoon season 
(June). Higher levels were maintained for a 
longer period i.e., till December with a peak in 
October during which time rainfall in this area 
was intense. The data of the present study 
clearly indicate the riverine input of silica 
into the estuary. The influence of oceanic 
contribution of dissolved silicon seems to be 
insignificant com pared to that from the 
riverine source. 
Variations in salinity (Table 1) and Si04-Si (Fig. 5) show the inverse relationship between 
these two factors. Higher values of Si04-Si 
during the rainy season can be attributed to the 
heavy river discharge during the monsoon. 
Pre-monsoon showed the reverse condition 
coupled with higher salinity values. These 
observations indicate the riverine input of 
silica into this estuary. 
In general the nutrient budget particularly 
INORGANIC NUTRIENTS IN A TROPICAL ESTIJARY 63 
2·4 STATION 5 
1·6 NITRATE NITROGEN IN IJ9 at 1-
1 
SURFACE WATER-
1·2 
BOTTOM WATER ----
0·8 
0·4 
0 
3 2 STATION 4 
STATION 3 
2 8 
24 
20 
'i-
0 16 ' 
' l\ 
' Ol \ I 
' :::2 I I ' I ' I \ z 12 I I \ I I \ 
z I I \ 
w I l 
\ 
(!) I 
' 0 8 I ). 0::: I t:: 
z 
w 4 
~ 
0::: 
t:: 
z 0 
26 
STATION 1 STATION 2 
22 
18 
A 
I \ 
I \ 
I \ 
14 \ \ 
\ I 
' ' I 
1 0 \1 
6 
2 
() 
F M A M J J A s 0 N D J F M A M J J A s 0 N D J 
1987 MONTHS 1988 1987 MONTHS 1988 
Fig. 3. Seasonal variations in Nitrate-Nitrogen concentration 
64 SHIBU, RITAKUMARI AND NAIR 
I 
.... 
0 
en 
:::1 
~ 
z 
w 
C) 
0 
0::: 
I-
z 
w 
!:: 
0::: 
I-
z 
0·55 
0·50 
0·45 
0·40 
0·35 
0·30 
0·25 
0·20 
0·15 
0·10 
0·05 
0 
0·55 
0·50 
0·45 
0·40 
0·35 
0·30 
0 ·25 
0 ·20 
0 ·15 
0·10 
0·05 
0 
0·55 
0·50 
0 ·45 
0·40 
0·35 
0·30 
0 ·25 
0·20 
0·15 
0·10 
0·05 
STATION 5 
STATION 3 
STATION 1 
o~~--r-~-.--~~~r-4-~--r--r~ 
F M A M 
1987 
J J A S 
MONTHS 
0 N D J 
1988 
Fig. 4. Seasonal variations in Nitrite-Nitrogen 
NITRITE NITROGEN IN A.Jg at 1-1 
1 
SURFACE WATER --
BOTTOM WATER ----
STATION 4 
STATION 2 
1 0•90 
I 
I 0•81 
I 
I 
I 
I 
I 
I 
I 
I 
I 
F M A M 
1987 
I 
I 
I 
I 
JJ SONDJ 
MONTHS 1988 
INORGANIC NU1RIENTS IN A 1ROPICAL ESTIJARY 65 
200 STATION 5 
.ug at 1-1 SILICATE SILICA IN 
I 
160 
\ SURFACE--\ 
\ BOTTOM----\ 
120 
80 
40 
0 
\ 200 STATION 3 STATION 4 
0 
0'1 160 
:::2 
\ /-----. z \ ~ / \ 
120 \ / \ A \ ,,. .... 
' <1: \ ' u \ 
::i ~ 
lJ1 80 
w 
~ 
u 40 
_j 
U) 
0 
200 STATION 1 STATION 2 
160 
1\ 
I \ 
/ \ 
120 \ h \ 
~ / I. / I. I ~ / 
80 :-- / :-. 
40 
0 
F M A M J J A s 0 N D J F M A M J J A s 0 N D J 
1987 MONTHS 1988 1987 MONTHS 1988 
Fig. 5. Seasonal variations in silicate-silicon 
66 INORGANIC NUTRIENTS IN A TROPICAL ES11JARY 
Table I. Monthly variations of salinity %oat the five stations during 1987-88. 
SALINITY 
Month Station 1 Station 2 
Feb.'87 
Mar. 
Apr. 
May 
June 
July 
Aug. 
Sept. 
Oct. 
Nov. 
Dec. 
Jan.'88 
sw 
2.05 
3.40 
4.33 
5.10 
0.20 
0.95 
1.90 
0.55 
0.20 
0.60 
1.50 
2.70 
BW 
2.15 
3.70 
4.35 
·5.15 
0.23 
0.90 
1.95 
0.70 
0.18 
0.57 
1.47 
2.85 
sw 
4.75 
7.15 
11.00 
11.90 
0.77 
1.32 
2.00 
0.50 
0.17 
0.45 
1.55 
2.95 
SW- Surface water, BW- Bottom water 
BW 
4.85 
7.25 
10.97 
11.90 
0.80 
1.37 
2.00 
0.60 
0..17 
0.47 
1.55 
3.05 
those of nitrate and silicate are considerably 
affected by the magnitude of rainfall and land 
drainage. Even though variation in the 
concentrations are seldom common to the 
water column, absolute values at the surface 
and bottom did not differ much during most of 
the time probably on account of the shallow 
nature of the water body. 
ACKNOWLEDGEMENTS 
The authors are thankful to Prof. P. 
Natarajan, Head of the Department for the 
facilities and encouragement. One of the 
authors (S.S.) is indebted to the U.G.C. for 
providing th~ Fellowship during the tenure of 
which this work was carried out. 
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Azis, P.K.A. 1978. Ecology of the retting grounds 
in the backwater system ofKerala. Ph. D.Thesis, 
University of Kerala . 
De Souza, S.N., Sen Gupta, R., Sujatha Sanzgiri 
and Rajagopal, M.D.1981. Studies on nutrients 
of Mandovi and Zuari river systems. Indian J. 
Mar. Sci.,-10 : 314-321. 
Gopinathan, K. 1985. Ecology and fisheries of certain 
inland waterbodies of Kerala. Ph.D. Thesis, 
University of Kerala . 
Kahar, A. 1988. Impact of sewage on the water 
quality of the Karamana river and the Poonthura 
Station 3 Station 4 Station 5 
sw 
7.00 
8.70 
12.60 
15.70 
2.02 
2.30 
0.90 
0.95 
0.57 
1.62 
3.00 
4.50 
BW SW BW sw BW 
8.45 5.65 5.70 4.55 4.35 
10.65 8.60 8.65 7.90 7.70 
14.75 12.17 12.10 11.90 11.90 
15.85 14.30 14.40 14.80 14.80 
4.22 0.70 1.77 6.30 7.70 
2.52 1.97 1.80 2.80 2.80 
0.95 3.55 3.60 3.40 3.40 
0.97 3.15 2.20 2.00 2.25 
0.62 0.82 0.82 1.72 1.70 
1.75 1.35 1.57 1.80 1.80 
3.75 3.05 3.20 2.90 2.90 
6.60 4.45 4.55 3.55 3.55 
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backwater systems, S.W. coast of India. Indian 
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Nutrients of the Cochin backwater in relation to 
environmental characteristics. Mar. Bioi. 
2: 236-247 
Sarala Devi, K., Venugopal, P., Remani, K.N., 
Dominic Acharias and Unnithan, R.V.1983. 
Nutrients in some estuaries of Kerala. 
Mahasagar-Bull. natn. Inst. Oceanogr. 6 (2) : 
161-174 
Strickland, J.D.H. and Parsons, T.R. 1972. A 
practical handbook of seawater analysis. J. Fish. 
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SHIBU, RITAKUMARI AND NAIR 67 
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Environmental inventory and the distribution of inorganic nutrients in a tropical estuary of the south-west coast of India

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Environmental inventory and the distribution of inorganic nutrients in a tropical estuary of the south-west coast of India

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