dynamics of limnetic zooplankton have been carried out in which parameters such as temperature, oxy- gen concentration, metal ions, nutrients, toxic subs-.
Annls
Limnol.
19 ( 3 } 1983 : 195-201.
High p H and the abundances of two commonly co-occurring freshwater copepods (Copepoda, Cyclopoida)
DO. J.P.
Hessen Nilssen
Mesoc\clops
1
1
letickarti
Th. oilhonoides
i n c r e a s e d r e l a t i v e t o Thermocyclops
s e e m e d t o o u t c o m p e t e M. leuckarii
oilhonoides
in f e r t i l i z e d e x p e r i m e n t a l e n c l o s u r e s ,
whereas
in n o n f e r t i l i z e d o n e s . T h e m o s t r e a s o n a b l e f a c t o r t o e x p l a i n t h e n u m e -
r i c a l r e l a t i o n s h i p b e t w e e n t h e t w o c l o s e l v - r e l a t e d s p e c i e s w a s a s e l e c t i v e p H e f f e c t : h i g h p H f a v o u r i n g M. leuckarii v i c e v e r s a . In the l a k e t h e t w o s p e c i e s s h o w e d a n h i s t o r i c a l d e v e l o p m e n t s u p p o r t i n g
t h e a b o v e h y p o t h e s i s . M.
and
leuckarii
w a s t h e d o m i n a t i n g s p e c i e s w h e n t h e l a k e e x p e r i e n c e d h i g h e s t p H d u r i n g the p e a k e u t r o p h i c p h a s e . F o l l o w i n g r e c o v e r y o f t h e l a k e , Th. oilhonoides
is a g a i n b e c o m i n g t h e m o r e d o m i n a n t s p e c i e s . T h i s c o n t r a s t i n g r e l a t i o n s h i p l o h i g h p H m a y
b e t h e r e a s o n f o r t h e v a r v i n g v e r t i c a l d i s t r i b u t i o n o f t h e t w o s p e c i e s in l a k e s . S i n c e Th. oilhonoides M. leuckarii,
with a lower p H . temperature
m o r e K-selected life
is r e c o r d e d b e l u w
and f o o d a b u n d a n c e , t h i s r e s u l t s in t h e o b s e r v e d s l o w e r d e v e l o p m e n t a n d a
history.
I n f l u e n c e d ' u n f o r t p H s u r l ' a b o n d a n c e de d e u x C o p é p o d e s c o e x i s t a n t d a n s les e a u x d o u c e s ( C o p e p o d a , C y c l o p o i d a ) .
L e s densités, d e p o p u l a t i o n d e Mesocyclops
leuckarti
a u g m e n t e n t p a r r a p p o r t à c e l l e s d e Thermucyclops
d e s e n c e i n t e s e x p é r i m e n t a l e s e n r i c h i e s , t a n d i s q u e Th. oilhonoides
s e m b l e d o m i n e r M. leuckarti
oilhonoides
dans
d a n s les e n c e i n t e s
non
e n r i c h i e s . L e f a c t e u r l e p l u s v r a i s e m b l a b l e q u i p u i s s e e x p l i q u e r la r e l a t i o n n u m é r i q u e e n t r e c e s d e u x e s p è c e s é t r o i t e m e n t l i é e s e s t l ' e f f e t s é l e c t i f d u p H ; l e s f o r t s p H a v a n t a g e a n t M. leuckarti l e l a c a p p u i e c e t t e h y p o t h è s e . M. leuckarti
était
et v i c e v e r s a . L a s u c c e s s i o n d e c e s deux e s p è c e s d a n s
l ' e s p è c e d o m i n a n t e d u r a n t la p h a s e e u t r o p h e l o r s q u e le l a c
u n p H é l e v é . A u c o u r s d e la r é g é n é r a t i o n d u l a c , 77t. oilhonoides
présentait
redevient l'espèce d o m i n a n t e . Cette situation
antagoniste
p a r r a p p o r t a u p H p e u t e x p l i q u e r la d i s t r i b u t i o n v e r t i c a l e d e c e s d e u x e s p è c e s d a n s l e s l a c s . L e fait q u e Th.
oilhonoides
s o i t d o m i n é e p a r M. leuckarti
dans des conditions de faibles p H , t e m p é r a t u r e
et a b o n d a n c e d e n o u r r i t u r e , e x p l i q u e r a i t
son plus lent d é v e l o p p e m e n t ainsi q u e son c y c l e b i o l o g i q u e de t y p e s t r a t è g e - K .
1. — Introduction
T h e f e w studies on this subject deal a l m o s t e x c l u s i v e l y w i t h c l a d o c e r a n s . B o g a t o v a (1961) ; I v a n o v a (1969) a n d W a l t e r (1969) all c l a i m e d that the
Several
investigations
concerning
population
pH
limit
for
dynamics of limnetic zooplankton have been carried
(1969) s h o w e d
o u t in w h i c h p a r a m e t e r s
mum
s u c h as t e m p e r a t u r e ,
oxy-
cladocerans
was
10.5-11.5.
upper
Ivanova
relatively l a r g e d i f f e r e n c e s in
p H values
(measured
as filtration
w e e n v a r i o u s c l a d o c e r a n s p e c i e s , but m o s t
tances and vertebrate/invertebrate
had an optimal physiological balance a r o u n d
predation
have
b e e n a n a l y s e d t o e x p l a i n d i f f e r e n c e s in c o m p e t i t i v e ability and p o p u l a t i o n density o f the various of
zooplankton.
The
ecological
lakes
and
ponds,
has
only
been
of
eutrophic
offered
bet-
species neu-
pH.
species
importance
e x t r e m e p H values, f r e q u e n t l y o b s e r v e d in
tral
opti-
rate)
g e n c o n c e n t r a t i o n , m e t a l ions, nutrients, toxic subs-
minor
interest.
A s f a r as w e a r e a w a r e , n o s t u d y has b e e n hed o f the investigation o f the relative
publis-
abundance
o f c y c l o p o i d c o p e p o d s in r e l a t i o n t o h i g h p H , e v e n though
a number
of investigations have been
f o r m e d in e u t r o p h i c
per-
localities w i t h fairly h i g h p H .
In this s t u d y , t h e r e s p o n s e o f t h e c y c l o p o i d c o p e 1. Zoological Institute, University of Oslo, P.O. Box 1050, Blindem, Oslo 3, N o r w a y .
pods
Thermocyclops
leukarti
Article available at http://www.limnology-journal.org or http://dx.doi.org/10.1051/limn/1983022
oilhonoides
and
to artificial eutrophication
Mesocyclops
was tested
by
D.O. H E S S E N ETJ.P. N I L S S E N
196
s h o r t - d u r a t i o n e x p e r i m e n t s in f e r t i l i z e d a n d tilized bags Both occur
in L a k e
unfer
Gjersjoen.
in o l i g o t r o p h i c ( S a r v a l a
1979),
mesotrophic
( P a t a l a s 1954 ; F a a f e n g a n d
N i l s s e n 1981) a n d
phic
Gliwicz
lakes
(Patalas
In
1954;
oligotrophic
l a k e s , Th.
oithonoides
and
1969;
slightly
hypeoccu-
in W e s t e r n P o m e r a n i a
Poland, including mesotrophic to strongly 1954). W h e n
M.
leuckarti
and
(Pata
l a s 1 9 5 4 ; G l i w i c z 1 9 6 9 ) . I n l a k e G j e r s j o e n Th. s t r o n g l y d o m i n a t e s o v e r M. leuckarti,
oitho and
f o u n d d e e p e r t h a n t h e l a t t e r , in a g r e e m e n t w i t h
is
the
prep).
L a k e G j e r s j o e n has b e e n m o n i t o r e d since the early 1950's, a n d
since
the
lake's
history
during
the
I960
1965
•
1970
Ml
1975
.
1980
interpretation
last t w o
decades.
T h e r e h a v e b e e n s t r o n g o s c i l l a t i o n s in the cal relationship
flu 111 M I IjI I I 1111 B 1955
FlG. 1. L a k e G j e r s j o e n , 1953-1980. T h e r e l a t i o n s h i p b e t w e e n Mesocyclops leuckarti ( v e r t i c a l l i n e s ) a n d Thermocyclops oithonoides, related to p H . Black dots : highest r e c o r d e d p H v a l u e f r o m i n t e g r a t e d s a m p l e s 0-10 m , d u r i n g s u m m e r . T r i a n g l e s : h i g h e s t r e c o r d e d p H in epilimnion d u r i n g s u m m e r . Data b a s e d on F a a f e n g and N i l s s e n (1981), a n d i n t e r n a l p u b l i c a t i o n s f r o m N o r w e gian Institute f o r W a t e r R e s e a r c h ( N I V A ) .
1958 b y the N o r w e g i a n I n s t i t u t e
for W a t e r Research, thus permitting of
d e v e l o p m e n t i n t h e s a m e p e r i o d ( f i g . 1).
Th.
w a s m o s t f r e q u e n t l y f o u n d in d e e p e r w a t e r s
et al. in
pH
in
latter
above findings (Brabrand
the
e u t r o p h i c peak. T h e s e c h a n g e s m a y be related to the
eutrophic
c o - o c c u r e d in eutrophic lakes the
noides
subdominants.
domi
l a k e s i s m o r e o b s c u r e . M. leuckarti
(Patalas
as
Nilssen
1 9 8 1 ) , w h i l e t h e r e l a t i o n s h i p in e u t r o p h i c a n d
lakes
scutifer
eutro-
reutrophic
oithonoides
Cyclops
Karabin
s e e m s to b e the m o r e
of lakes
and
mesotrophic
n a n t o f the t w o ( S a r v a l a 1979 ; F a a f e n g a n d
r e d in all kinds
leuckarti
The opposite relationship was observed during
species have similar ecology and often co-
1978).
(2)
3. — Materials and methods
numeri
b e t w e e n the c l o s e l y - r e l a t e d c y c l o -
p o i d c o p e p o d s M. leuckarti
a n d Th. oithonoides,
pro
T h e e x p e r i m e n t s w e r e c a r r i e d o u t in p o l y e t h y l e n e
b a b l y d u e t o c h a n g e s in t h e t r o p h i c status o f the lake
b a g s , 135 j * m t h i c k . W e u s e d
(cf. F a a f e n g a n d
t e r o f 1.5 m , d e p t h 4 . 2 m a n d a v o l u m e o f a b o u t
Nilssen
1981).
m \
experiments
were
zone. performed
in the
meso-
eutrophic L a k e Gjersjoen. S E - N o r w a y (59° 47' N , 10° 47' E ) (area = increased from
an
2
2.7 k m , m a x . d e p t h
sewage
deposition,
oligotrophic
T h e b a g s w e r e c l o s e d at t h e b o t t o m a n d
in a w o o d e n f r a m e w o r k , a n c h o r e d
2. — Study area The
12 b a g s w i t h a d i a m e
lake in
=
62 m ) . D u e to
the
lake
the
early
developed 1950's
to
The
experiment
lasted
7
placed
in t h e
pelagic
four
weeks,
for
25.7-28.8.1980. T h e b a g s w e r e filled b y m e a n s o f a water p u m p and vertical net-samples
from
w e r e a d d e d using a 45 ^ m plankton-net m e t e r o f 27
15-0
m
w i t h a dia
cm.
Twice a week zooplankton samples were
taken,
h i g h l y e u t r o p h i c c o n d i t i o n s in 1972, w h e n a s e w a g e
while phytoplankton-samples w e r e taken three limes
treatment
d u r i n g the e x p e r i m e n t . A l l s a m p l e s w e r e taken f r o m
plant
was
built
1981). S i n c e then, there
(Faafeng
and
Nilssen
has b e e n a s l o w i m p r o v e
0 , 1, 2 a n d 4 m d e p t h s u s i n g a m o d i f i e d v a n
Dorn
m e n t a n d a s l i g h t l y l o w e r p H , b u t the l a k e still e x h i
s a m p l e r as d e s c r i b e d b y B l a k a r (1978). W a t e r f r o m
bits p h y t o p l a n k t o n b l o o m s being mainly
all d e p t h s w a s m i x e d , a n d t w o 5 1 s a m p l e s w e r e fil
by
dominated
tered
blue-greens. The zooplankton community
nated
by small species, caused
is a t p r e s e n t
domi
b y the intense
dation of a huge stock of planktivorous roach lus
rutilus)
poid
pre(Ruti-
( B r a b r a n d e t al. 1979). A m o n g t h e c y c l o -
copepods,
Th.
oithonoides
is at
d o m i n a n t s p e c i e s , as in t h e 1950's, w i t h
present
the
Mesocyclops
through
plankton. taken
for
a 45
M
m
net
for collecting the
100 m l w a t e r f r o m t h e m i x e d s a m p l e phytoplankton-analysis.
Both
zoo was
zoo-and
phytoplankton w e r e fixed w i t h L u g o l ' s solution. Che mical analyses o f N H wegian
Institute
4
w a s c a r r i e d o u t at t h e
for W a t e r Research
by
w i t h h y p o c h l o r i t e in a n a l k a l i n e s o l u t i o n , a n d sured by a Technicon
autoanalyzer.
Nor
reaction mea
(3)
HIGH
A solution of a m m o n i u m hydrogenphosphate,
PH A N D T H E A B U N D A N C E S OF
nitrate and
ammonium
w h i c h g a v e a final
concentra I
tion
of
added
3200
Pre-adult bags
and
to half the
660
^g
1
I "
respectively,
D I N O P H Y C E A E
was
bags.
J C R Y P T O -
r o a c h (5-7 c m ) w e r e a d d e d t o t w o o f t h e
in o r d e r
197
COPEPODS
to detect
a possible selective
:
:] C H R Y S O
-
préda
t i o n o n t h e t w o s p e c i e s . 12 f i s h w e r e a d d e d t o
each
j B A C I LLAR10
-
bag. 1CHLORO
HIM
4. — Results Phytoplankton lume
:
of algae
throughout
was
the
O A
relatively
observed
stable
in
experimental
total
unfertilized
soon
collapsed.
Then
increase of chlorophyceans, ciliatum,
Golenkia
there
,
vo-
the
increase (fig.
2 ) . A t f i r s t t h e r e w a s a b l o o m o f Synedra this
] C YA N O P H Y C E A E
bags
period, whereas
fertilized bags s h o w e d a considerable
-
spp.,
was
a
but
sudden
s u c h a s Collodictyon
s p p . , Scenedesmus
28.8
tri-
quadricauda
a n d s m a l l , e l l i p s o i d , u n i d e n t i f i e d g r e e n a l g a e ( c a . 4-5 x
3 ^m). There w a s also a marked
increase
t h e c r y p t o p h y c e a n s , e s p e c i a l l y Cryptomonas noidifera.
cf.
T h e C y a n o p h y c e a e o c c u r e d in s i m i l a r
of
pyreden
s i t i e s a n d s p e c i e s c o m p o s i t i o n in b o t h f e r t i l i z e d a n d F E R T I L I Z E D
U N F E R T I L I Z E D
unfertilized
bags,
Chrysophyceae
as
did
the
Dinophyceae
and F i g . 2. D e v e l o p m e n t o f t h e d i f f e r e n t a l g a l g r o u p s in f e r t i lized and u n f e r t i l i z e d bags. M e a n values o f 5 b a g s f o r each group.
(Tab. I).
T a b l e 1. T o t a l c e l l v o l u m e ( m m
3
m
J
) o f t h e d i f f e r e n t a l g a l g r o u p s in u n f e r t i l i z e d ( U 1 - U 5 ) a n d f e r t i l i z e d ( F 1 - F 5 ) U2
Ul
U3
U4
bags.
US
Date
1.8
14.8
28.8
1.8
14.8
28.8
1.8
14.8
28.8
1.8
14.8
28.8
1.8
14.8
21.8
Dinophyceae Cryptophyceae Chrysophyceae Bacillariophyceae Chlorophyceae Cyanophyceae
699 57 242 886 108 527
402 33 371 140 99 346
113 23 1005 76 187 28
410 427 347 740 67 747
875 258 1493 391 208 384
715 137 603 191 218 1117
815 542 197 795 63 820
530 204 162 366 166 231
330 363 450 186 203 2066
18 55 296 184 69 663
652 24 795 395 168 539
164 98 317 22 294 209
695 643 34 = 994 194 991
947 31 720 512 80 500
105 25 1718 195 242 239
F2
FI Date Dinophyceae Cryptophyceae Chrysophyceae Bacillariophyceae Chlorophyceae Cyanophyceae
F4
F3
1.8
14.8
28.8
1.8
14.8
28.8
1.8
14.8
28.8
49 320 208 1742 191 763
18 1277 454 8 2832 4684
86 3650 52 94 3115 574
41 134 142 541 159 674
2497 252 3 5 4092 967
87 891 76 33 2309 770
414 392 176 1386 257 1 174
1029 804 190 819 2292
2554 208 7037 2709
1.8 416 26 480 586 101 380
F5
14.8
28.8
1.8
14.8
28.8
288 77 1 1 63 3173 1 049 1730
978 35 549 9394 576
105 392 184 1762 366 1071
1386 1394 75 3764 875
1526 359 77 4142 319
DO.
198
Species composition was almost the hags, but of
H E S S E N ET J.P. N I L S S E N
identical
a great v a r i a n c e in the total
leuckarti
in all
e x p e r i e n c e h i g h e s t p H , Th. oithonoides
bags w a s the small ellipsoid g r e e n algae. T h e y
w i t h a r e c o r d e d m a x i m u m of 46 X 10 cells 1~ 9394 m m pH
3
1
The
relation
shown
or
relation
m .
between
disappeared.
77i. oithonoides
and
pH
i n F i g . 5, w h e r e i t i s e v i d e n t t h a t a h i g h
favours
3
: Simultaneously
copepods
s h i p b e t w e e n t h e t w o w a s 1 : 1. I n b a g s F 4 a n d F 5 , w h i c h
w e r e r e c o r d e d in h u g e n u m b e r s in all f e r t i l i z e d b a g s , 7
for 90-100 % o f the
f e r t i l i z e d b a g h a v i n g l o w e s t p H ( f i g . 3), t h e
spe
c i e s o f a l g a e w h i c h o c c u r e d a l m o s t s o l e l y in f e r t i l i zed
accounted
in 4 o f the 5 f e r t i l i z e d b a g s ( T a b . 2). In b a g F2, the
volume
the various species w a s observed, T h e only
(4)
M. leuckarti
and
vice
is pH
versa.
with the increased algal bioT a b l e 2. P e r c e n t a g e o f Th. oithonoides in unfertilized ( U 1 - U 5 ) a n d fertilized ( F 1 - F 5 ) b a g s at start ( S ) a n d e n d (E) of the e x p e r i m e n t .
m a s s in f e r t i l i z e d b a g s , t h e r e w a s a n i n c r e a s e in p H . W h e n the s t u d y w a s initiated all the b a g s had a p H o f a b o u t 9.5-9.7, w h i c h a g r e e d w i t h t h e p H in t h e l a k e e p i l i m n i o n at t h a t t i m e . In u n f e r t i l i z e d b a g s a s l i g h t
U3
U2
Ul
U4
U5
i n c r e a s e in p H w a s f o u n d d u r i n g the first w e e k ( u p to
10.0), b e f o r e the p H b e c a m e s t a b l e
(fig.
at a b o u t
increase
in p H t o o k p l a c e a n d w i t h i n a w e e k the p H
became
s t a b l e at a b o u t 10.6-10.7. T h e h i g h e s t r e c o r d e d
s
E
S
E
95.0
98.8
91.6
71.4
S
E
S
E
S
E
S
E
S
E
71.4
6.9
94.0
50.5
71.1
2.2
70.1
0
58.1
0
9.5
3). In f e r t i l i z e d b a g s , h o w e v e r , a s u d d e n
and
10.9 r e s p e c t i v e l y . A l s o b e t w e e n f e r t i l i z e d b a g s ,
there was fairly good agreement
between pH
dings. The development of p H and
F3
equally eliminated
These bags are therefore res a n d
:The only crustaceans which s h o w e d
a s t r o n g r e s p o n s e d u r i n g this s h o r t - t e r m e x p e r i m e n t a n d M. leuckarti.
In the
tilized b a g s the f o r m e r o f these species
a b o u t 70-90 % o f the c y c l o p o i d n u m b e r s , as
the
lake. In fertilized bags, h o w e v e r , there w a s
increase. wed karti
After the first
f o l l o w i n g the
l o w d e n s i t i e s , t h e n a s u d d e n i n c r e a s e o f M.
leuc-
w a s o b s e r v e d . A t the end o f the e x p e r i m e n t
.
U
produces few clutches
with
bags w i t h water-temperatures ment within
from
late
Lake
h a s a l a r g e r c l u t c h - s i z e (20leuckarti
h a d a n e x t r e m e l y s h o r t d e v e l o p m e n t t i m e , in
M.
nauplii
to
some
16-20° C the d e v e l o p adult
female
occured
two weeks.
U3
U2
7
figu
3 0 e g g s ) , w i t h s m a l l e g g s . I n f e r t i l i z e d b a g s M.
pH sho
predation. in t h e
t i m e (ca. 2 m o n t h s in
G j e r s j o e n ) , M. leuckarti
a
to fish
a n d l a r g e e g g s (8-15 p e r c l u t c h ) a n d has a r e l a
tively long generation
in
10 d a y s , b o t h s p e c i e s
705
few
due
not included
tables.
W h i l e Th. oithonoides
unfer
accounted
for
s t r o n g d e c r e a s e o f Th. oithonoides
F5
F4
In t h e t w o b a g s c o n t a i n i n g f i s h , b o t h s p e c i e s w e r e almost
oithonoides
83.0
species c o m p o
in f i g . 4.
w e r e Th.
E
S
rea
s i t i o n o f c y c l o p o i d c o p e p o d s ( m e a n v a l u e s ) is s h o w n
Zooplankton
E
S
80.7 99.0 71.5 8 1.0 81.8
F2
Fl
mean
v a l u e w a s 10.7, w i t h u p p e r a n d l o w e r v a l u e s o f 10.5
E
S
U5
U4
70.0
95
pH 705 700
F2
95
14.8
28.B1.8
14. B
F
. 2B.81.B
14.B
2B.81.8
14.8
F 5
4
28.81.8
F i g . 3. V a r i a t i o n s in p H in unfertilized ( U 1 - U 5 ) a n d fertilized ( F 1 - F 5 ) b a g s .
14.8
28.8
199
HIGH PH A N D T H E A B U N D A N C E S OF C O P E P O D S
(5)
—
30-
O
20-
."X
z
o E
5
°f"
V)
MÉMT[!WmTTTTTM: 1.8
¿8
•
7.8
118
U S
188
21.8
25.8
28.8
T oithonoides
F i g . 5. R e l a t i o n b e t w e e n p H a n d Th. oithonoides (as p e r cent of total n u m b e r o f c y c l o p o i d c o p e p o d s ) . O p e n s q u a res : unfertilized bags. Filled squares : f e r t i l i z e d bags.
a) Release of toxic substances from certain
blue-
g r e e n s is k n o w n f r o m t h e e a r l y 1950's ( L e f e v r e
1950)
and
later v e r i f i e d by several a u t h o r s ( S h i l o 1964 ;
G e n t i l e a n d M a l o n e y 1969 ; A r n o l d 1971). T h e b l u e 1.8
¿ 1
11.8
7«
U.6
16.8
21.8
25.8
g r e e n s p e c i e s f o u n d i n t h e b a g s w e r e Anabaena
28.8
tenericaulis, F i g . 4 . R e l a t i o n s h i p b e t w e e n M. leuckarii ( v e r t i c a l l i n e s ) a n d Th. oithonoides in u n f e r t i l i z e d ( u p p e r ) a n d f e r t i l i z e s b a g s ( l o w e r ) . M e a n v a l u e s . p H i n d i c a t e d as b l a c k dots.
Anabaena
a n d Oscillatoria
solitaria,
limnetica.
agardhii
The occurrence of these
s p e c i e s w a s , h o w e v e r , v e r y s i m i l a r in b o t h and
unfertilized
cf.
Oscillatoria
bags. T h e b i o m a s s of
fertilized
bluegreens
w a s in fact h i g h e r in s o m e o f the u n f e r t i l i z e d s o m e o f the fertilized
T h e only algal groups
5. — Discussion
greater
numbers
in
which always occured
fertilized
Cryptophyceae (represented a n d Rhodomonas Species d o m i n a n c e within the c y c l o p o i d c o m m u nity w a s obviously related to nutrient perturbations, both
regarding
longterm
short-duration experiments
changes
in
the
lake
and
in t h e b a g s . S e v e r a l fac-
tors may have caused this :
predation.
lacustris)
bags,
in
were
b y Cryptomonas and the
the spp.
Chlorophyceae
mainly by smalt ellipsoid and
species). W i t h i n the c h l o r o p h y c e a n s ,
coccoid
some
species
might be suspected to cause toxic effects ( S k u l b e r g p e r s . c o m m . ) . N e v e r t h e l e s s , an a l g a l toxin Th.
a) A selective toxic effect caused b y algal b l o o m s . b) Selective vertebrate or invertabrate
(represented
than
bags.
oithonoides
M. leuckarti
but
affecting
not the c l o s e l v - r e l a t e d
species
w o u l d be an interesting observation
i t s e l f a n d w i l l b e p u r s u e d in f u r t h e r s t u d i e s .
in
During
c) S e l e c t i v e toxic effects f r o m the fertilizers added.
the
d ) A c h a n g e in a v a i l a b l e f o o d , t h u s f a v o u r i n g
i n c r e a s e in the a b o v e a l g a e w a s o b s e r v e d h o w e v e r ,
of the
species.
e) An increase species.
one
in p H , a f f e c t i n g o n l y o n e o f
historical
something the
development
which
may
of Lake Gjersjoen
indicate
that a h i g h
m o r e important for the numerical w e e n the t w o
species.
no
pH
relationship
is
bet-
D O . H E S S E N ET J.P N I L S S E N
200
b) Fish karii,
predators
could
have favoured
M.
leuc-
the l a r g e r o f the t w o species. In the t w o b a g s
The
use some of her unpublis hed results. We should also like to thank Age Braband and Björn Faa Ieng for helpful assistance, discussion and comments. The work was partly supported by the Norwegian Council for Technical and Natural Research, being a research program on eutrophication of inland waters.
decline
d u r i n g t h e f i r s t w e e k , w h i l e i n c r e a s e o f M.
leuckarti
References
w a s f i r s t o b s e r v e d in t h e last h a l f o f t h e e x p e r i m e n tal p e r i o d (cf. f i g . 3 a n d 4 ) . A t h e o r v b a s e d o n c o m p e t i t i v e e x c l u s i o n t h e r e f o r e h a s t o b e r e j e c t e d in t h i s case. e ) O f all p o s s i b l e e x p l a n a t i o n s ,
the p H s e e m s
to
b e the m o s t r e a s o n a b l e to account for the o b s e r v e d p a t t e r n s in t h e b a g s . M o r e o v e r , a s f a r a s t h e
long-
t e r m c h a n g e s in L a k e G j e r s j o e n a r e c o n c e r n e d , and algal biomass are the onlv parameters
p H
which are
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the
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importance,
the p H - e f f e c t s act d i r e c t l y o r
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it is c l e a r t h a t t h e i o n - e x c h a n g e
m a n y z o o p l a n k t o n s p e c i e s is s t r i c t l y p H
The
straightfor among
dependent
( P o t t s & F r y e r 1979 ; N i l s s e n & al. 1983). M a n y c o p e p o d s s h o w e d a g o o d s o d i u m balance up to about = sen
pH
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H I G H PH A N D T H E A B U N D A N C E S OF C O P E P O D S
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