Oligotrophication after a nutrient reduction in a shallow sand

Artnls
Limnol.
28 (3) 1992 : 253-262
Oligotrophication after a nutrient reduction in a shallow sand-pit lake
(Créteil Lake, Paris suburbs, France) : a case of rapid restoration
J.
A.
1
Gamier *
Chestérikoff2
P.
Testardl
B.
Garban2
K e y w o r d s : shallow lake, sand-pit, nutrient reduction, o l i g o t r o p h i c a t i o n .
A l i m n o l o g i c a l study was performed o u t , o v e r 8 years from 1979 t o 1986, in a s h a l l o w recently created sand-pit lake,
L a k e Créteil (Paris suburb, France). The l a k e is mainly supplied by phreatic waters. Typical o f shallow waters, t h e r m a l
stratification w h i c h occurred f r o m M a y t o O c t o b e r was intermittently broken by w i n d , leading t o high seasonal fluctuations a n d allowing a reoxygenation o f the total water c o l u m n . The interannual d e v e l o p m e n t o f the lake was characterized by a regular increase in transparency ( f r o m 1 m in 1979 t o 2.7 m in 1985) which w a s partly d u e t o n o t only stabilizat i o n o f the b o t t o m sediment a n d the b a n k s , b u t also t o a reduction o f the biological c o m p o n e n t . A decrease in c h l o rophyll a c o n c e n t r a t i o n s resulted from nutrient diversion o f domestic inputs collected t h r o u g h the rainwater pipe c o m i n g
i n t o the lake. T h e rapid restoration o f water quality s h o w s that this small artificial lake is very sensitive t o h u m a n
interventions.
O l i g o t r o p h i s a t i o n d'une sablière en e a u ( L a c de Créteil, région parisienne, France) après une réduction des a p p o r t s e n
é l é m e n t s nutritifs : un c a s d e rapide restauration
M o t s clés : lac p e u p r o f o n d , sablière en e a u , réduction e n éléments nutritifs, o l i g o t r o p h i s a t i o n
La tendance é v o l u t i v e d'un lac artificiel d e création récente a été étudiée pendant 8 années consécutives, d e 1979 à
1986 (Lac d e Créteil, banlieu parisienne, F r a n c e ) . C e lac d e sablière, p e u p r o f o n d , est essentiellement alimenté p a r les
e a u x d'une nappe superficielle. E n raison d e s stratifications et destratifications intermittentes liées au vent, invariablement observées d'avril à o c t o b r e , l'évolution saisonnière d u lac se caractérise par des fluctuations importantes ; les phases d e déstratification permettent une r é o x y g é n a t i o n de la c o l o n n e d'eau. L'évolution interannuelle d u lac s'est caractérisée par une a u g m e n t a t i o n régulière d e la transparence des e a u x ( - 1 m e n 1979 à - 2 . 7 m en 1985) due à la stabilisation
des berges et des s é d i m e n t s , mais aussi à la d i m i n u t i o n de la c o m p o s a n t e b i o l o g i q u e . L a diminution d u stock p h y t o p l a n c t o n i q u e (concentration en c h l o r o p h y l l e a) est clairement liée à la réduction des a p p o r t s d o m e s t i q u e s rejetés d a n s le lac
j u s q u ' e n mai 1981 par un collecteur d'eau d e pluie. L'amélioration particulièrement rapide d e la qualité de l'eau m o n t r e
q u e c e milieu est très sensible a u x a m é n a g e m e n t s .
1. Introduction
t o r i a , B a l a t o n or W i n d e r m e r e l a k e s . . . ) whereas t h e
knowledge of o t h e r s h a v e even been recently i n t e -
Limnological
studies h a v e mainly
focused
on
grated i n t o b o o k s ( M e n d o t a L a k e : Billings et a l .
n a t u r a l lakes ; s o m e of t h e m a r e particularly well
(1985), M i r r o r Lake : Likens (1985), Stechlin L a k e .
d o c u m e n t e d in literature ( C o n s t a n c e , G e n e v a , Vic-
Caspers (1985)). M o r e recently, a b o o k was p r o p o sed analysing reservoir functioning ( T h o r n t o n et al.
1990).
1. Laboratoire de Géologie Appliquée, U.A. 1367 C . N . R . S . ,
4, Place Jussieu, Tour 26, 5' étage, 75005 Paris, France.
* Present adress : Groupe de Microbiologie des Milieux Aquatiques, Université Libre de Bruxelles, Campus de la Plaine, CP
221, 1050 Bruxelles, Belgique.
2. Institut d'Hydrobiologie et de Climatologie, 4, Place Jussieu, Tour 26, 5 étage, 75005 Paris, France.
e
A l t h o u g h sand-pit lakes h a v e become m o r e a n d
m o r e n u m e r o u s in t h e last few years, d u e t o t h e
increasing need in building m a t e r i a l s , less a t t e n t i o n
has been paid to these kind of ecosystems. S o m e ecological characteristics of several sand-pit lakes have
however been given in G a m i e r et a l . (1987).
Article available at http://www.limnology-journal.org
or http://dx.doi.org/10.1051/limn/1992022
254
(2)
J. GARNIER, A. CHESTERIKOFF, P. TESTARD, B. GARBAN
T h e sand-pit lake of Créteil (Paris suburb, France)
has been intensively studied for 8 years. T h e study
started in 1979 soon after the quarrying of sand has
finished in 1976. T h e study period can therefore be
considered as a field experiment a r o u n d the follo­
wing question : how long would be necessary for this
y o u n g system t o achieve a relative stability ? Such
a recently created system was a priori a p p r o p r i a t e
t o interpret the possible process of naturalization of
a n artificial aquatic system. However, situated in a
t o w n , the role of L a k e Créteil as a recreational and
o r n a m e n t a l area has required several management
m e a s u r e s which interfered with natural processes.
T h e p a p e r analyses the seasonal and interannual
changes of that y o u n g artificial aquatic system.
in 1986, show that the ionic composition of the lake
water has been rather stable for the period studied,
although it slightly decreased in 1986 (Table 2).
2 . Site study
1981 : The rainwater run off collected o n the
urbanized watershed and brought to the lake by a
pipe was accompanied by polluted domestic water
at the beginning of the study. In May 1981, the pol­
luted inputs were lowered.
Créteil lake is situated in an urbanized area near
the confluence of the M a r n e a n d Seine rivers 15 km
SE of P a r i s , F r a n c e (Fig. 1). Parisian Basin has
a t e m p e r a t e climate a n d h a s been rather stable for
t h e period studied, although some year-to-year
variability has been observed. A s u m m a r y of
a n n u a l m e a n a n d extreme values is given for global
incident r a d i a t i o n , t e m p e r a t u r e , rainfall a n d wind
(Table 1).
T h e Créteil sand-pit lake originated from the exca­
v a t i o n , between 1947 t o 1976, of the alluvial sedi­
m e n t s . T h e alluvial plain is delimited by the Seine
(West) a n d M a r n e rivers ( N o r t h ) a n d by Tertiary
o u t c r o p s (East, S o u t h ) . Créteil Lake is n o t connec­
ted with the rivers. T h e lake is mainly supplied by
anoxic phreatic waters of t h e alluvial aquifer, water
circulating mainly t h r o u g h diverse filling materials
which h a v e replaced the n a t u r a l alluvium dug out
in the past. Besides direct rainfall, the lake also recei­
ves r a i n w a t e r runoff from the 0.5 k m of the M o n t
Mesly urbanized z o n e . T h e water residence time in
the lake was estimated at 6-12 m o n t h s (Chestérikoff
1981). T h e high total salinity (1.4 g l - i ) of Lake
Créteil water originates from the composition of the
supplying u n d e r g r o u n d water (Table 2). T h e domi­
nance of calcium a n d sulfate is mainly caused by the
g y p s u m s t r a t a of M o n t Mesly outcrops whereas the
high c o n c e n t r a t i o n of sodium a n d chloride is due
t o t h e filling materials. Total salinity in the water
c o m i n g f r o m the rainwater pipe is lower (0.87 g
I - ) . D a t a collected from 1978 t o 1980 a n d again
3
T h e surface area of the lake is 0.42 k m ; maxi­
mal length and width are 1.5 a n d 0.4 km respecti­
vely following a NS axis. The banks are steep so that
macrophytes occupy only 15 % of their length. The
lake is s u r r o u n d e d by buildings at East and N o r t h East, an artificial hill at West ; it is exposed to wind
from the South and South-East. The depth of the
lake is h o m o g e n o u s within most of its surface area
but maximal depth has varied between 4 and 5 m.
During the study period, different management
measures were successively t a k e n , which all pertur­
bed the trend of the lake functioning.
1983 : The water level of the lake regularly increa­
sed (1.27 c m / m o n t h ) from 1979 to 1982 due t o the
consolidation of ground materials following the
construction of an artificial hill (Chestérikoff & Che­
vreuil 1987), although this increase might also be
caused by the increase in rainfall observed during
that period (Table 1). At the beginning of 1983, a
drainage pipe of 5001/sec in capacity was set to regu­
late the level of the lake, decreasing the residence
time.
1985 : From December 1985 to August 1986,
intensive pumpings of phreatic waters led to a regu­
lar decrease of the water level (3 cm/week, i.e. 1 m
in total) ; initial values were then found when p u m ­
ping ceased.
1
1
3. Material and methods
T h e study was carried out in the central area of
the lake from 1979 to 1986 at intervals ranging from
a week in summer to a m o n t h in winter.
Measurements of temperature and oxygen were
performed in depth profiles with intervalls of 50 cm,
with a YSI (model 57) p r o b e . Wind speeds were
recorded at a meteorological station in the nearby
N U T R I E N T R E D U C T I O N IN A S A N D - P I T
flow p a t h of p h r e a t i c
f
LAKE
aquifer
aquifer
Fig. 1. Situation of Créteil Lake. Cross-section of the plain of Créteil (according to Chestérikoff & Chevreuil 1987).
Fig. 1. Situation du lac de Créteil. Coupe géologique de la plaine de Créteil {D'après Chestérikoff & Chevreuil 1987).
256
I. GARNIER, A. CHESTERIKOFF, P. TESTARD, B. GARBAN
(4)
Table 1. Interannual variations, from 1979 to 1986, of annual means (x) and extreme values (min, max) for global incident radiation
(Io : cal c m - d a y ) , air temperature (T : °C), wind speed (wind : m s e c ) and rainfall {mm year" ).
2
- 1
_ !
1
Tableau 1. Variations interannuelles, de 1979 à 1986, des moyennes annuelles (x) ei des valeurs extrêmes (min, max) de la radiation globale incidente
(lo : cal c m j ) , de la température de l'air (T : °C), de la vitesse du vent {m sec~ ') et de la hauteur annuelle des précipitations (mm).
- 2
- 1
197 9
Io
T
1981
1 9 82
1983
1984
1985
19B6
24 9
X
244
239
241
2 62
263
255
256
min
9.1
8.8
17.2
9.8
14.3
9.3
16.7
max
649
667
715
705
656
706
698
x
Wind
1980
6
812
10.6
10.6
11.7
11.9
11.6
10.9
10.8
10.8
min
-10.8
-7.3
-5.3
-8.1
-5.2
-5.3
-16.4
-10.6
max
33.9
33.4
33.8
34.1
36.1
36.3
33.2
33.7
1.96
2.38
2.63
2.77
2.46
2.45
2.6
0.3
0.1
0.1
0
0.1
8.1
8.8
8.1
7.8
7.9
784.0
590.7
733.2
482.5
640.6
x
1.92
min
0
0
max
6.5
7.3
571.3
641.8
Rainfall
0
10.4
743.6
-
Table 2. Comparative conductivity (^S c m ') and major elements (mg 1" ') of the waters. Phreactic waters : range of values for the
7 to 9 different piezometers and exceptional high or low values in parentheses (Chestérikoff et al., 1981). Pipe waters : extremes
values (ext) and mean (x) (Chestérikoff et al. 1981). Lake waters : mean values in 1978-1979 (Chestérikoff et al. 1981), in 1980
(Chestérikoff 1981) and in 1986.
- 2
Tableau 2. Valeurs comparatives de la conductivité (^S c m ) et des éléments majeurs dans l'eau et dans la nappe. Eau de la nappe :
amplitude des valeurs pour 7 à 9 piézomètres et valeurs exceptionnellement basses ou élevées entre parenthèses (Chestérikoff et
al., 1981). Eaux du collecteur : valeurs extrêmes (ext) et moyenne (x) (Chestérikoff et al., 1981). Eaux du lac : valeurs moyennes
en 1978-1979 {Chestérikoff et al. 1981), en 1980 (Chestérikoff 1981) et en 1986.
phreatic
waters
pipe
waters
lake
water
1978-1980
1978-1980
1978-79
1980
ext
conductivité
u\S c m "
1000-3000
(5000)
1
HCO3" mg l "
Cl"
50 - 2 0 0
(1600)
SO4"
+
Ca
Na
K
+
+ +
Mg
1
+
+
total
alization
830-1260
296
-
397
347
120
115
101
64
-
220
127
230
231
214 . 5
270
-1700
90
-
200
141
650
560
558.2
144
-1000
100
-
154
127
250
220
219.5
16.5 - 29
(7)
(211.6)
23 100
(506)
25.6
111.3
(5)
257
NUTRIENT REDUCTION IN A SAND-PIT LAKE
e n v i r o n m e n t . G l o b a l radiation was also continuously recorded with a pyranograph (Eppley) equiped with a n i n t e g r a t o r (Mecilec) a t the m e t e o r o l o gical station. Light a t t e n u a t i o n profiles were established every 20 c m , with a q u a n t a m e t e r coupled
with an integrator (LI 193 S, LI 188). Vertical atten u a t i o n coefficient (K) was calculated from a linear
regression given b y the equation : L n Q = Kz +
L n Q where Q a n d Q are the irradiance u n d e r the
surface a n d at the depth z. T r a n s p a r e n c y was estimated w i t h a Secchi disk (white, 30 cm diameter).
filter, extraction in 90 % acetone and centrifugation (Lorenzen 1967). The volume of filtered water
varied from 0.5 t o 3 L, and samples were duplicated. Values represent a n average of the six d e p t h s .
4. Results
4 . 1 . Physical and chemical parameters of the lake
z
0
G
z
Nutrient analyses were carried out o n t h e water
taken at 1.5 m from 1979 to 1981 a n d o n a mixed
water s a m p l e of six depths (0, 0 . 5 , 1, 1.5, 2.5 and
4 m) from 1982 to 1986. Water samples for P - P 0 ,
N-NO3 a n d N - N H 4 were filtered t h r o u g h G F / F
m e m b r a n e filters a n d frozen until analyses. P - P O 4
was measured by spectrophotometer at 630 n m after
a reaction with a m m o n i u m m o l y b d a t e a n d a reduction to m o l y b d e n i u m blue by ascorbic acid. T o t a l
P h o s p h o r u s (P-tot) was determined as a b o v e o n
unfiltered lake water after sodium persulfate digestion a n d mineralization at 1 1 0 C in acidic p h a s e .
N-NO3 was spectrophotometrically m e a s u r e d at
415 n m after the f o r m a t i o n of N a - p a r a n i t r o salicylate whereas N - N H was determined a t 635 n m
by the indophenol method. As relatively small values
of N - N O 2 were measured at the beginning of t h e
study, t o t a l inorganic nitrogen (N-tot) was considered to be the s u m of N-NO3 and N - N H , a l t h o u g h
N-NO2 might have developped transiently as a result
of d e g r a d a t i o n of organic m a t t e r .
4
9
4
4
C h l o r o p h y l l a was determined s p e c t r o p h o t o m e trically after filtration t h r o u g h a W h a t m a n G F / C
W a t e r temperatures were lowest in January (2°C
within the whole water c o l u m n ) . Then, together with
the increase of global radiation a n d day length,
water t e m p e r a t u r e regulary increased from midFebruary o n w a r d s and reached 15°C in April (Fig.
2). At that time (end of April t o begirming of M a y ) ,
a thermal gradient. (4-5 °C as a maximum) regularly
appeared ; it disappeared entirely in mid-October
when the temperature h a d decreased t o 15°C. Maximal surface temperatures varied from 22° C (1982)
t o 25°C) (1983 and 1986). Intermittent mixing occurring during that period under wind effect was of
major significance for p h y t o p l a n k t o n seasonal
periodicity ( G a m i e r 1989, 1992). Periods of stratification were generally short a n d never lasted for
m o r e t h a n 6 weeks. In 1985 a n d especially in 1986
when the level of the water was lowered, t h e a m p l i t u d e of the t h e r m a l gradient was reduced.
A c o n t i n u o u s increase in water t r a n s p a r e n c y , Z
(a decrease in light attenuation coefficient, K) occurred t h r o u g h o u t the whole p e r i o d of the study (Fig.
3). Increases of Z between 1979 and 1985 were
significant according to Kruskal-Wallis test a n d multiple c o m p a r i s o n s . A decrease of Z was o b s e r v e d
in 1986 when t h e water level was lowered. A l s o ,
during the first four years of the study (1979 t o
1982), a seasonal t r e n d in water transparency was
observed, with values lower in summer ( M a y t o
s
s
s
Fig. 2. Interannual variations of temperature (T : °C) and percentage saturation of oxygen ( 0 : %) at the surface (—) and 4 m (—)
from 1979 to 1986.
2
Fig. 2. Variations interannuelles, de 1979 à 1986, de la température (T : °C) et du pourcentage de saturation en oxygène ( 0 : %) en
surface (—) et à 4 m (—).
2
258
J. GARNIER, A. CHESTERIKOFF, P. TESTARD, B. GARBAN
Fig. 3. Interannual variations of water transparency (Zj. : m), of the light attenuation coefficient (K : m
concentrations (chl a : ^g
(6)
_ 1
) and of chlorophyll a
from 1979 to 1986.
Fig. 3 . Variations interannuelles, de 1979 à 1986, de la transparence de l'eau (Zs : m), du coefficient d'extinction (K : m " ') et de la
concentration en chlorophylle a (chl a :
l ).
_ l
O c t o b e r ) t h a n in winter ( N o v e m b e r t o April)
(Wilcoxon-Mann-WMtney bilateral test). F r o m 1983
o n w a r d s , the increase in water t r a n s p a r e n c y was
m o r e p r o n o u n c e d , so that n o seasonal tendency was
observed in 1984 a n d 1986, m e a n s u m m e r values
being even higher t h a n winter values.
At a seasonal scale, three p e r i o d s can be distin­
g u i s h e d with respect t o water oxygenation (Fig. 2).
F r o m J a n u a r y t o A p r i l , the whole water column was
s a t u r a t e d (values between 90 t o 100 %) ; from M a y
t o O c t o b e r , d e p e n d i n g o n the stratificationdestratification episodes, water was at times super­
s a t u r a t e d in t h e u p p e r c o l u m n whereas oxygen
d e p l e t i o n sometines occurred deeper ; at the end of
O c t o b e r , oxygen d e p t h profiles were h o m o g e n e o u s
b u t a slight depletion (saturation at 70 % ) was obser­
ved, saturation being progressively restored towards
December.
F r o m M a y t o O c t o b e r , oxygen depth profiles
d e p e n d e d o n t h e t h e r m a l s t r u c t u r e . Oxygen deple­
t i o n a t t h e b o t t o m w a s m o r e p r o n o u n c e d when the
thermal gradient has been important for a long
period ; oxygen depletion at the b o t t o m was accen­
tuated b y the supply of anoxic phreatic waters, cir­
culating t h r o u g h strongly reducing filling material.
In addition, when transparency was low at the begin­
ning of the study, as photosynthetic processes were
restricted within the upper water layer, differences
between surface and b o t t o m were accentuated. The­
refore, wind induced intermittent mixing events had
an important role in the re-oxygenation of the water
column in s u m m e r . T h e u n d e r s a t u r a t i o n during
a u t u m n a l mixing was caused by a considerable
demand for dissolved oxygen for the decomposition
of o r g a n i c material produced during the period of
m a x i m u m biological activity.
A l t h o u g h , as a general rule, oxygen followed the
same year t o year seasonal p a t t e r n , changes have
nethertheless been observed throughout the studied
period. Differences between surface a n d b o t t o m
have become of lesser importance at the end of the
study when the increase in water transparency deepened
(7)
259
NUTRIENT REDUCTION IN A SAND-PIT LAKE
photosynthetic activity (Fig. 2). A s an average for
the summer period, saturation percentage regularly
increased from 48.8 % in 1979 to 88.7 % in 1985.
In 1985 a n d 1986, m i n i m u m values at 4 m equalled
50 % whereas they decreased t o 5 % from 1979 to
1982 (Fig. 2).
4 . 2 . Changes in chlorophyll a level
Whereas transparency increase, concentrations in
chlorophyll a decrease significantly during the period
of the study ; values, in annual means, were the highest ( a b o u t 15 j i g l " ) from 1979 to 1981, decreased significantly in 1982 a n d 1983 (7 a n d 9.5 ^ g l - 1
respectively) a n d decreased still further in 1984 a n d
1985 (4.1 a n d 3.6 ^ g l ) whereas a slight increase
occurred in 1986 (5.4 u£\~ ),
(Fig- 3). W i d e variations in chlorophyll a concentrations were observed
at a seasonnal scale. High value in winter were often
associated to early increases in a b u n d a n c e (not in
biomass) of small algal species (Garnier 1989) ; the
highest algal b i o m a s s found in summer a n d determined from cell volume (Garnier 1989) were not
accompanied by the the highest values in chlorophyll
so t h a t a seasonal t r e n d did not appear clearly.
1
_ 1
l
4.3. Nutrients in the lake and sources of nutrients
A nutrient budget is difficult to establish for the
Lake Créteil because the hydrodynamic a n d chemistry of the supplying phreatic water are very complex. T h e rainwater pipe furthermore complicates
the system because, until M a y 1981, it also b r o u g h t
polluted domestic waters besides rainwaters.
T h e year-to-year trends in total p h o s p h o r u s were
clearly related t o h u m a n intervention on t h e rainwater pipe which decreased the polluted effluents.
Total p h o s p h o r u s concentrations steadily increased
in lake water from J a n u a r y 1979 t o May 1981, values
averaging 32.8 „ g I~ in 1979, 48.5 ^ g l " in 1980
and 71 ^ g l - from J a n u a r y to May 1981 (Fig. 4) ;
the nutrient diversion caused a decrease t o an average of 37.6 fig l - in t h e second p a r t of 1981.
F r o m 1982 to 1986, a n n u a l average of total phosp h o r u s (between 42 a n d 50 ^ g l ') stabilized in the
range of the values found before the increase,
a l t h o u g h high values were occasionally still observed. T h e concentrations of orthophosphates decreased at the beginning of the study period a n d remained at a low level since then (Fig. 4). There was no
seasonal trends for b o t h total phosphorus and orthophosphates even when averaging summer (from May
to October) a n d winter d a t a (November t o April).
1
At the beginning of t h e study, t h e rainwater pipe
constituted a considerable input of phosphorus, b u t
fortunately 80 °/o of the p h o s p h o r u s h a v e been
shown to precipitate in the s e d i m e n t (Chestérikoff
et al. 1981). The precipitation would explain the relatively low increase in total p h o s p h o r u s c o n c e n t r a tion in the lake c o m p a r e d to that in the r a i n w a t e r
pipe : from 1979 to 1980, concentrations in total
p h o s p h o r u s in the rainwater pipe increased indeed
from 1500 to 9000 ^ g l " , (Table 3 , Chestérikoff
et al. 1981). In 1985, total p h o s p h o r u s was analysed in lake water, just below the p i p e : high values
of p h o s p h o r u s (P-tot : 119 ^ g l
in a n n u a l average ranging from 36 to 250 ^ g l " ; P - P 0 4 :
60.6 ^ g 1 ~ ranging from 4 to 193 ^ g 1~ ' ) showed
that u r b a n waters b r o u g h t by the rainwater pipe at
the end of the study period still constituted a source
of nutrients, despite the sewage diversion in M a y
1981. N o a p p a r e n t increase in phosphorus c o n t e n t
has been observed in the lake sediment from 1980
(0.33 mg c m - 3 ) t o 1987 (0.34 mg c m - 3 ) .
1
- 1
1
1
Phreatic waters also appeared t o be a source of
phosphorus for the lake ; measurement p e r f o r m e d
at the beginning of the study from piezometers were
indeed higher than those of the lake (Table 3 , Chestérikoff et al. 1981).
Total inorganic nitrogen (N-tot) decreased at the
beginning of the study, from 1.08 t o 0.75 m g 1between 1979 a n d 1980 as a n a n n u a l average (from
3.9 to 1.15 mg l - i for maximal values) b u t also
drastically decreased in M a y 1981 ; values stabilized a r o u n d 0.3 m g 1- i from 1982 to 1986 (Fig. 5).
T h e decrease in total inorganic nitrogen resulted
from a simultaneous decrease of b o t h a m m o n i u m
and nitrate representing respectively 2 / 3 and 1/3 of
total inorganic nitrogen.
1
1
1
1
-
Despite some irregularity in the seasonal p a t t e r n ,
nitrate tended to decrease in s u m m e r , this t e n d e n c y
being apparent at the beginning of the study when
higher values were found (Fig. 5). N o seasonal trend
was found for a m m o n i u m , excepted in 1979.
Similarly to p h o s p h o r u s , the pipe and p h r e a t i c
waters constituted a source of nitrogen ; concentration in total inorganic nitrogen (N-tot) in t h e pipe
waters (18.7 mg 1 ) as well as in the p h r e a t i c
waters (from zero to 150 mg l
depending on the
situation of piezometers, median value of 12 mg 1(Table 3) were m u c h higher t h a n in t h e lake water
(1 m g l - ' ) - In 1985, nitrate concentrations (N-NO3)
_ 1
-
1
1
260
(8)
J. GARNIER, A. CHESTERIKOFF, P. TESTARD, B. GARBAN
_! P . I O K , I - ' I
H
! "
1
J p_po if g i- )
4
I
YFWAM J JÏI(SY&
I960
1979
>
19B1
I
19S2
1983
19B4
'
|
198 ï
19»6
Fig. 4. Interannual variations of total phosphorus (P-tot) and orthophosphates ( P - P 0 ) from 1979 to 1986. Concentrations in ^gP 1
1
Fig. 4. Variations interannuelles, de 1979 à 1986, du phosphore total (P-tot) et des orthophosphates (P-PO,,). Concentrations en ^ gP 1
_ 1
4
Table 3. Concentrations in nutrients of phreatic and pipe waters
between 1978 and 1980 (Chestérikoff et al. 1981). Phreatics
waters : mean (x) and extreme values (ext) for 8 piezometers, *12
mg 1 ~ N-tot is the median value for 8 piezometers. Pipe waters :
**1500 and **9000 ^.g 1~ P-tot are values obtained in January
1979 and December 1980, values increasing in an exponential way
during that time interval (cf. p. 27, fig. 16 : Chestérikoff et al.
1981) ; mean (x) and extreme values (ext) for N-tot and N - N H ,
***2,7 mg 1~' N - N O 3 represents the difference between 18,7 (Ntot) and 16 ( N - N 0 ) (Chestérikoff et al. 1981). Concentration
in nutrients of lake water below the pipe outfall : Mean (x) and
extreme values (ext) for 13 sampling dates.
Tableau 3. Concentrations en elements nutritifs dans la nappe
et dans le collecteur entre 1978 et 1980 (Chestérikoff et al. 1981)
et à la sortie du collecteur en 1985. Dans la nappe : valeurs moyennes (x) et extrêmes (ext) pour 8 piezomètres, *12 mg 1~ N-tot
est la valeur médiane pour 8 piezomètres. Eaux du collecteur :
* * 1500 and * *9000 ^g I ~ sont des valeurs obtenues en janvier
1979 et en décembre 1980, les valeurs augmentant exponentiellement pendant cet intervalle de temps (cf. p. 27, Fig. 16 : Chestérikoff et al. 1981) ; moyenne (x) et valeurs extrêmes (ext) pour
N-tot et N - N H , ***2,7 mg l " , N - N O 3 représentant la différence entre 18,7 (N-tot) et 16 ( N - N 0 ) (Chestérikoff et al. 1981).
Concentrations en éléments nutritifs dans le lac à la sortie du collecteur : moyenne (x) et valeurs extrêmes (ext) pour 13 dates
échantillonnées.
1
1
4
just below the pipe (0.37 mg 1 - 1 , from 0.09 t o 0.56
mg 1~ 'in extreme values) higher than those in the
lake show that the rainwater pipe was still a source
of nitrogen.
There was n o coincidence between m a x i m u m
values of total phosphorus a n d inorganic nitrogen
in t h e lake.
3
5. Discussion
5 . 1 . Typical seasonal features of the physical and
chemical environment
1
1
1
4
3
iter pipe
OUTFALL
A s shown by the vertical distribution of oxygen
concentrations, a slight thermal gradient is sufficient
to offer a resistance to exchanges between surface
and b o t t o m but, mixing events invariably reduced
the duration of bottom deoxygenation. The increase
in water transparency, favouring a relative increase
in photosynthetic activity deeper, has attenuated the
strong heterogeneity of oxygen profiles observed at
the beginning of the study.
T h e decrease in transparency during s u m m e r for
1979 t o 1982 coincided to the maximum of p h y t o plankton biomass. However, after 1982, the summer p h y t o p l a n k t o n biomass was t o o low to
influence t h e seasonal trend in transparency. Seasonal fluctuations of chlorophyll a concentrations
were clearly associated to the main phases of phytop l a n k t o n growth and decline, although high chlorophyll values, compared t o p h y t o p l a n k t o n biomass, were found in winter (Gamier 1989, Lacroix
et al. 1989). These wide variations appear to be typical of shallow waters, where mixing events create
new conditions (nutrient release) which enhance
phytoplankton growth.
261
NUTRIENT REDUCTION IN A SAND-PIT LAKE
(9)
•JN-NH^msl-')
|
j
1
|
1
|
1
1
1
1
j
IN NO, (mg
1
I- )
]
]
]
1
J'F W A V J "j V S ' S V C I j*f WA'M" J ' J 'A's-o-N^V "M'A
1979
1
1980
|
-
w'J'J Ws o^^f\^}y
1911
I
«'s^V&
1982
"""v ~r ~--'
—
1
1981
1914
1
1985
1
1986
Fig. 5. Interannual variations of ammonium ( N - N H ) and nitrate ( N - N 0 ) from 1979 to 1986. Concentrations in mgN 1 ~
4
3
1
Fig. 5. Variations interannuelles, de 1979 à 1986, en ammoniaque (N-NH ) et nitrates ( N - N 0 ) . Concentrations en mgN l ' .
4
T h e summer decrease in nitrates at t h e beginning
of t h e study must be related to p h y t o p l a n k t o n
u p t a k e b u t also, as a n a e r o b i c conditions dévelop­
pée!, to denitrification processes i m p o r t a n t in sand­
pit lake ( L a b r o u e et al. 1988). The o t h e r forms of
nutrients have not shown any clear seasonal pattern.
This could indicate a rapid biological u p t a k e toge­
ther with rapid exchanges between water a n d sedi­
ment during mixing events.
5.2. Characteristics of the physical and chemical
changes over the 8 years studied
F r o m 1979 o n w a r d s , t h e increase in water t r a n s ­
parency has been the most significant a n d directly
observable characteristic of the development. Sus­
pended m a t t e r (dry weight) have n o t been continously analysed but between 1978 (Chestérikoff et
al. 1981) and 1985 (Mourelatos 1988), a decrease
from 13 t o 6 mg 1
as an a n n u a l average has
occurred. T h e reduction in suspended m a t t e r partly
corresponded t o a decrease of the organic fraction
(a decrease in chlorophyll a concentration from 17
to 3.5 jig I
in a n n u a l means) t h r o u g h o u t t h e
period studied. H o w e v e r , as the decrease in water
transparency has been observed as early as 1980
whereas chlorophyll a concentrations began t o
decrease only in 1982, the biological c o m p o n e n t
alone cannot explain the increase in t r a n s p a r e n c y .
It must also be related to a progressive stabilization
of b o t t o m sediment a n d b a n k s , after the q u a r r y i n g
of sand has stopped, thus reducing resuspension by
wind and littoral turbulence. In 1986, when the lake
water level was lowered, the reversed t r e n d c a n
_ 1
- 1
3
conversely be explained by erosion of the b a n k s as
well as by increased resuspension of the bottom sedi­
ments. A n indirect b u t i m p o r t a n t effect of the
increase in water t r a n s p a r e n c y was t o improve the
oxygenation of the water c o l u m n .
W h e r e a s direct superficial i n p u t s in nutrients are
obviously an i m p o r t a n t source for eutrophication,
the rapid decrease in chlorophyll after n u t r i e n t
reduction has shown the lake t o be very sensitive t o
restoration.
The diversion of domestic water brought by the
rainwater pipe until M a y 1981 is the main cause for
the decrease in p h o s p h o r u s in the lake. T h e early
decrease in nitrogen, but also in o r t h o p h o s p h a t e s ,
tends to support the idea of a reduction of nutrients
brought by the phreatic waters, after progressive
wash out of nutrients from polluted material. Later
on in 1983, the regulation of the water level of t h e
lake by a drainage pipe, decreasing the residence
time, coincided with an accentuation of oligotrophication, but no effect of residence time can b e shown
for the nutrient conditions found in the lake in 1983
(Garnier & Billen, in press). O t h e r processes inclu­
ding colonisation of m a c r o p h y t e s could h a v e con­
tributed t o a progress in the oligotrophication of the
lake.
Internal processes m u s t have also contributed t o
the elimination of nutrients.
T h e hard waters of Lake Créteil represented
favourable conditions for a transfer of large frac­
tion of the phosphorus input t o calcium, explaining
the precipitation as calcium phosphates a n d t h u s
262
J. GARNIER, A. CHESTERIKOFF, P. TESTARD, B. GARBAN
limiting t h e increase in p h o s p h o r u s concentration in
t h e l a k e ( T i r e n & P e t e r s s o n 1985 ; H e r o d e k & Istv a n o v i c s 1986). W h e r e a s a large part of p h o s p h o r u s was t h e r e f o r e
(Bates & Neafus
immobilized
in the
sediments
1980, B o s t r o m et al. 1982, F o y
1986), a n o t h e r p a r t m u s t b e h o w e v e r p e r m a n e n t l y
recycled
either
u n d e r intermittent
stratification-
destratifications, by mineralization of o r g a n i c p h o s p h o r u s u n d e r oxic c o n d i t i o n s (Kamp-Nielsen 1975,
Lee
et
al.
1977)
or
by
biological
uptake
and
a d s o r p t i o n - d e s o r p t i o n processes o n s u s p e n d e d matter within t h e lake water ( C a p b l a n c q et a l . 1986).
C o n s i d e r i n g n i t r o g e n , denitrification c o u l d be a
m a j o r p r o c e s s for its e l i m i n a t i o n . In L a k e Créteil
chiefly at t h e beginning of t h e study, the supply with
anoxic water a n d intermittent oxygen depletion toget h e r w i t h a high algal b i o m a s s in s u m m e r represent e d s u i t a b l e c o n d i t i o n s for denitrification ( B a r r o i n
1985,
L a b r o u e et a l . 1988). Denitrification
might
p a r t l y explain the difference between n i t r a t e concent r a t i o n s in the lake (1 mg 1~ ') and in phreatic waters
a n d in w a t e r s b r o u g h t by t h e pipe (12 a n d 18 m g
l
-
1
respectively) ; it is i m p r o b a b l e t h a t t h e diffe-
r e n c e c o u l d h a v e b e e n mobilized by p h y t o p l a n k t o n .
I n 1985, p a r t i c u l a t e nitrogen for the fraction less
t h a n 35 ^ m were 0.1 mg 1~
1
; taking into account
t h e r e d u c t i o n in p h y t o p l a n k t o n b i o m a s s (from 17
t o 3.5 fig 1 -
1
in c h l o r o p h y l l a) p a r t i c u l a t e n i t r o g e n
1
s h o u l d be less t h a n 1 mg 1^ at the beginning of the
study.
References
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franc. Sci. Eau 4 : 79-92.
Bates N.H. & Neafus N. Jo E. 1980. — Phosphorus release from
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1477-1481.
Bostrom B., Janson M. & Forberg C. 1982. — Phosphorus
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:
Capblancq J., Labroue L. & Fardeau J.C. 1986. — Echanges
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r
Likens G.E. (ed). 1985. — An ecosystem approach to aquatic
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e
Tiren T. & Pettersson K. 1985. — The influence of nitrate on
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