PLANKTONIC
ALGAE AS INDICATORS
WITH SPECIAL
REFERENCE
TO THE
OF LAKE TYPES,
DESMIDIACEAE
A. J. Brook
Department
of Botany,
University
of Minnesota
ABSTRACT
The biological
classification
of lakes is briefly rcvicwcd
and the value and limitations
of phytoplankton
quotients for the assessment of trophic status are discussed. Comparisons
are made bctwecn quotient determinations
and the characterization
of lakes in terms of
of algal plankton
to changes in
their dominant phytoplankton
spccics. The sensitivity
water chemistry is examined in the light of cxpcriments
with mineral fertilizers
and obSpecial attention
is paid to the use
servations of natural or seminatural
eutrophication.
plankton have
of desmids as indicators of trophic status. Studies of the British frcshwatcr
shown that many taxa of this supposedly
oligotrophic
algal group are most frequently
associated with eutrophic waters.
INTRODUCTION
ton types. Pearsall demonstrated a clear
progression from primitive
lakes with a
plankton dominated by desmids, through
an intermediate
stage in which diatoms
and desmids were most numerous, to
evolved, silted lakes dominated by diatoms
and blue-green algae.
The assessment of the trophic status of
a body of water is of practical and thcoretical importance, especially to the fishcrics
biologist. It would be of value to have an
index of characteristics or standards by
which the biological status of lakes could
be compared. Limnologists have, however,
PHYTOPLANKTON
QUOTIENTS
found it difficult to establish a generally
applicable
trophic index because of the
The relative dominance of the major
complex relationships of factors involved in groups of planktonic algae recognized by
freshwater organic production.
these earlier investigators
has more reAn early contribution
to the biological
cently been used as an indicator of the
classification of lakes resulted from extentrophic status of lakes. For example, assive studies of the British phytoplankton
suming that species of Chlorophyceae of
made by G. S. West and W. West (1909).
the Order Chlorococcales show eutrophic
They concluded that desmid-rich plankton
tendencies in contrast to desmids (the
corresponds geographically with precarbonChlorophyceae that are specifically most
iferous rocks and occurs in water that is numerous in oligotrophic
waters ) , Thunpoor in dissolved minerals. Although the mark (1945) suggested that the ratio of
Wests did not formulate definite biological
the number of species of Chlorococcales to
lake types, it was partly on the basis of Desmidiaccae in a body of water gives a
their results and those of Lemmcrmann
reliable indication of its trophic level and
( 1904) and Wesenberg-Lund
( 1905) that
found this quotient to be “eloquently exTeiling (1916) applied the terms Caledoprcssivc” of the trophic status of several
nian and Baltic, respectively, to lakes poor
Swedish lakes. Nygaard (1949) extended
an d rich in nutrients.
Naumann (1917, Thunmark’s
concept and attempted
to
1919) later substituted the ecological noclassify a number of Danish ponds and
menclature, oligotrophic and eutrophic, to
lakes using five different quotients, as folpoor and rich waters. Pcarsall ( 1921, 1924,
lows:
1930, 1932) introduced the concept of lake
evolution, and chemical investigations en- Myxophycean quotient = Myxophyceae
Desmidiaceac
abled him to give some insight into the
factors that probably were responsible for
Chlorococcales
Chlorophycean
quotient =
the appearance of distinctive phytoplankDesmidiaceac
403
404
A. J. BROOK
Diatom quotient
Euglenoid quotient
=
Centric diatoms
( Centrales )
Pennate diatoms
( Pennales )
Euglenineae
= Myxophyccae + Chlorococcales
Compound quotient
Myxophyceae + Chlorococcales + Centralcs +
Euglenineae
=
Desmidiaceae
Nygaard
interpreted
compound
quotients, which appeared to be the most reliable indicators of trophic status, of less
than 1.0 to indicate oligotrophic lakes, values of 0.0-0.3 suggesting dystrophy. Values greater than 1.0 probably indicate eutrophy and those between 5-20 a high degree of eutrophy with possible contamination by cattle excrement.
According to Nygaard, quotients can be
determined only from samples collected
when the algal groups on which the quotient depends have their greatest development. In the case of Myxophyceae, Chlorococcales and Desmidiaceae in northern latitudes, this is usually between mid-May
and September, although the diatom quotient is probably most reliable in spring
and summer.
Some limitations in the u-se of
plankton quotients
In determining
quotients,
one must
know how many species of Myxophyceae,
Chlorococcales, Desmidiaceae, and so on
are present in the sample. This requires
either detailed identification
or knowledge
of the limits of the genera and higher taxa
in the freshwater community, coupled with
the ability at least. to delimit species. It
is particularly
important in assessing the
quotients of eutrophic waters to be able
to separate desmid species. For example,
two well defined, though superficially similar, species of Staurastrum (S. chaetoceras
and S. pingue) occur in Eye Brook Reservoir. Both are fairly common in other
British lakes, but until recently (Brook
1959a) neither had been recorded from
them. The only other plankton dcsmid OC-
curring in Eye Brook Reservoir is Closterium ceratium. Twenty-six species of Chlorococcales, Myxophyceae, and centric diatoms arc present, but without specialized
knowledge of the Dcsmidiaceae one would
probably determine the quotient, in view
of the similarity in form of the two Staurastrum species, to be 26/2 = 13 instead of
26/3 = 8.7. If only the two Staurastrum
species were present, the failure to separate them would indicate a quotient of 26
instead of 13. Similar problems exist with
the diatom genera Melosira and CycZotelZa,
some genera of Chlorococcales, and many
of the coccoid Myxophyceae.
It has been stressed elsewhere that only
euplankters
should be included
in the
species-counts (Brook 1959b).
It may,
however, be difficult to distinguish these
from littoral or attached and weed-living
algae carried into open water. There is less
chance of finding
such tychoplanktonic
species in the middle of large, deep, steepsided lakes than in smaller, shallower lakes
where much of the bottom may be covered
with weeds. Thus, the distance from the
shore or from weed beds may influence
the composition of a plankton sample and
the quotient. Associated with this problem
is the relationship between pelagic and littoral plankton.
Fritsch ( 1931) stressed
that the adulteration of the pelagic by littoral plankton will depend on the quantity
of the latter, which will be related to the
extent of the littoral shelf, and also on its
dispersal into open water by wind, wave
action, and currents. The successful establishment of littoral species in the open
water will dcpcnd on environmental conditions, especially the supply of mineral nutrients. Naumann (1927) states that the
green alga Scenedesmus quudricauda is not
a significant
constituent
of the pelagic
plankton of oligotrophic
waters, although
it is often common or even abundant in
eutrophic waters. Although
it may be
present in the littoral region of oligotrophic
lakes, the lack of adequate nutrients prevents
it from
establishing
itself
in deeper
regions. Thus, the morphometry of a lake
may determine the composition as well as
PLANKTONIC
TABLE
ALGAE
AS INDICATORS
1. Seasonal variation in the phytoplankton
quotient of Loch Lomond (1955 and 1956)
Tarbet
Luss
Creinch
May
June
July
Sept
Ott
Nov
Dee
Feb
1.0
0.7
1.0
0.7
0.8
0.7
1.2
0.9
1.2
0.9
0.9
0.8
0.8
0.7
0.7
0.7
0.1
1.0
0.65 0.8
1.0 0.9
1.2 1.0
the abundance of its phytoplankton
(Rawson 1955). This has been demonstrated for
the Desmidiaceae ( Brook 1959b). It could
be argued that the number of tychoplankters of the eutrophic algal groups should
balance out those with oligotrophic preferences when quotients are determined.
It
is more probable that, in eutrophic waters,
there will be a preponderance of tychoplanktonic
Chlorococcales,
Myxophyceae,
and centric diatoms, while in even moderately oligotrophic
waters as many as 80
tychoplanktonic
dcsmids have been found
in a single sample taken from the shore
(Round and Brook 1959). Such chance
plankters give rise to misleading quotients.
THE
CHARACI’ERIZATION
OF THEIR
DOMINANT
OF LAKES
PLANKTON
IN TERMS
SPECIES
Rawson (1956) emphasized the confusion arising from the two methods by
which trophic status has been characterized. Plankton quotients characterize
a
body of water on the basis of numbers of
species without regard to abundance. The
other approach is to characterize waters in
terms of their dominant plankton species
TABLE
2.
Ennerdale
Thirlmere
Buttermere
Crummock
Haweswater
Derwentwater
Bassenthwaite
Coniston
Loughrigg
Ullswater
Windermere
Seasonal variations
OF LAKE
405
TYPES
and to classify them according to the communities
or associations they support.
Thus, the plankton of an oligotrophic lake
may be characterize,d by a large number
of desmid species although it may be dominated numerically by one or two species
of diatoms or Chrysophyceae, and a eutrophic lake may be characterized not by
a large number of species of Myxophyceae
but by very large numbers of filaments or
colonies of only one or two species, as during a waterbloom.
It cannot be assumed that eutrophic
waters always support only a few species.
Some distinctly eutrophic Irish loughs contain as many phytoplankton species as some
that are markedly oligotrophic (Round and
Brook 1959). The classification
of the
loughs in terms of the dominant taxa
agreed closely with the classification using
compound phytoplankton
quotients. However, the dominant species of a plankton
population can change rapidly so that comparisons in these terms can be valid only
when the lakes are sampled within a few
days of one another. This limitation does
not apply rigidly to the determination
of
quotients; a fact confirmed by sampling
several lakes at monthly intervals over an
extended period. Samples allowing the determination of month-by-month
variations
of the compound quotient have been provided from Loch Lomond by Dr. H. D.
Slack of the University of Glasgow, These
samples were collected from the deep,
in the phytoplankton
quotients
(1955)
of a number
of lakes in the Lake District
May
June
July
Aug
Sept
Ott
0.5
0.2
0.4
0.2
1.6
0.9
0.6
1.0
2.5
2.3
0.7
0.3
0.2
0.3
0.3
1.7
0.4
1.0
2.1
0.75
0.4
0.3
0.5
0.2
0.7
0.9
0.4
1.5
0.3
0.5
0.25
0.9
1.1
0.5
1.7
2.5
0.73
0.2
0.3
0.4
1.1
0.7
2.2
3.5
0.8
0.25
0.35
0.8
0.9
1.2
0.8
1.2
3.0
0.85
2:
0.6
Mean
nlknlinity
( ppm CaC03 )
3.7
3.4
2.5
3.0
E-i
713
6.0
12.6
14.3
8.3
406
A. J. BROOK
TABLE 3.
Enrichment
of Sutherland
lochs (1954 and 1955)
1954
1955
June
I,.
L.
L.
L.
L.
L.
Smuraich
Grosvenor
Mhullaich
Beiste Bricc
Daimh Beg
Daimh Mor
* NPK = nitrogen,
p
SP = Calcium
AW
Ca + NPK” + NaNOn
Ca + SPt
Control
NPK + NaNOn
Control
SP
NPK + NaN03
NPK + NaN03
phosphorus,
supcrphosphatc.
added
and
potassium
sp
in proportions
northern end (Tarbet), the middle region
and the comparatively
shallow
&W,
southern basin ( Creinch);
the northern
and southern stations are more than 10
miles (16 km) apart. Table 1 shows the
monthly quotients for each station between
May 1955 and February 1956.
Other series of monthly samples from
lakes in the English Lake District (Table
2) were provided for May to October 1955
by Dr. J. W. G. Lund of the Freshwater
Biological Association, Windermere. These
series indicate that the quotient determined
for a given loch at any time of the year
is reasonably constant.
SENSITIVITY
CIIEMKCAL
OF PHYTOPLANKTON
CHANGES
IN THE
SPECIES
TO
WATER
In recent years, mineral nutrients have
been added to comparatively
small lakes
( lochans ) in various parts of Scotland to
study effects on the growth of brown trout.
The first of these additions was to a 16.6hectare hill loch in northwest Perthshire
( Brook and Holden 1957). Calcium superphosphate ( 16.7% water soluble P205) was
added in the summer of 1952 to give a
phosphate phosphorus
concentration
of
Net samples of phytoabout 330 pg/liter.
plankton taken monthly from August 1949
until August 1958, and the compound quotients determined from them, indicated no
change in trophic status. There were more
than eightfold increases in the abundance
of phytoplankton in the first two years, but
the quotient varied only between 1.9 and
2.2 during the nine-year period.
Other fertilization
experiments were initiated in 1954 in the county of Sutherland,
April
June
Aug
NPK
SP
NPK
SP
NPK
SP
NPK
SP
NFK
NFK
sp
SF
of 2.5
PH
Prefertilization
alkalinity
( ppm CaC0.s)
5.5-6.0
l-2
6.5-7.0
5-10
7.0-7.5
10-20
: 1 : 1 approximately.
where four lochans were enriched on five
occasions between June 1954 and August
1955 (Table 3). Two other lochs in the
area were untreated and studied as controls ( Holden 1959).
The fertilization
of these lochs, which
saturated their bottom muds with phosphate, led to a large increase in phytoplankton, especially in the two lochans
treated with the nitrogen-phosphorus-potassium fertilizers ( NPK); both developed
blooms of Chlorococcales and blue-green
algae.
The phytoplankton
populations
(Table 4) increased to as much as 60,000
cells/ml in the enriched waters compared
with maxima of 500 and 1,800 in the controls. The plankton quality in the treated
lochs also changed to become more typical
of markedly cutrophic waters. Chemical
analyses made to follow the conversion of
inorganic phosphate to soluble or particulate organic form, also reflected increases
in the phytoplankton.
In the two lochans
treated with superphosphate, Loch Grosvenor and Loch Daimh Mor, up to 50 pg/
liter of organic phosphorus were found; in
the two NPK treated lochs, values of up
to 150 ,g/liter of organic phosphorus were
obtained for a time. The untreated lochs
contained between 4 and 7 pug/liter of organic phosphorus during this period. Associated with these changes in the lochs
developing blooms were increases in pI1,
which rose at times to between 8 and 8.5,
in contrast to normal values of between
6 and 7.
The compound quotient of each loch
was determined before treatment and at
the ensuing
summer
intervals
during
PLANKTONIC
TABLE 4.
Minimum
and maximum
ALGAE
AS INDICATORS
OP LAKE
population
densities and dominant
ancl their controls
407
TYPEi
algal species of fertilized
Range of plankton
Treatment
Mhullaich
Control
Daimh Beg
Control
Daimh Mor
Smuraich
Ti?K
Beiste Brice
NPK
Grosvenor
Ca and SP
Dominnnt
organisms
Chroomonas acwta
Dinobryon so&ale wr. americanum
Cyclotella glomerata
Chroomonas acuta
Dinobryon sociale VR~. americanum
Cyclotella glomerata
Asterionella
formosa
Ankistrodesmus
falcatus
Dictyosphaerium
sp.
Synedra acus var. radians
Oscillatoria sp.
sp.
Dictyosphaerium
Diatoma elongatum
Chlorella sp.
An.abaena spp.
Anabaena inaequalis
Dictyosphaerium
sp.
Ankistrodesmus falcatus WI-. spiralis
months. No significant change was apparent in the quotients following
the 1954
fertilization
(Table 5)) but when enrichment was continued in 1955, marked increases became apparent, particularly
in
those lochs that developed blooms. On the
basis of Nygaard’s interpretation
of the
compound quotient, it is clear that the previously strongly oligotrophic lochs, Smuraich and Grosvcnor, temporarily
became
distinctly eutrophic, and the initially less
oligotrophic
Loch Bciste Brice became
rather eutrophic. However, there was no
significant change in the quotient of Loch
Daimh Mor, where only moderate increases in plankton populations followed
enrichment.
TABLE 5.
Dntc
1954
May
August
1955
April
May
June
August
Scptcmber
Compound
Smuraich
(NJ=)
0.3
0.2
2.0
3.0
6.0
1;
phytoplankton
Grosvenor
(Ca+SP)
quotients
lochs
density
Min
Max
140
500
110
1,800
2,700
2,800
11,000
52,000
13,700
60,000
8,300
41,000
These fertilization
experiments raise the
question of the sensitivity of phytoplankton
species to sudden changes in the chemical
composition of their environment.
There
are several examples in the literature of
the rapid evolution
( cu trophication ) of
lakes with correspondingly
rapid changes
in their phytoplankton.
One of the most
striking of these is the Zurichsee in Switzerland consisting of two basins, the Obcrsec
and Untersee, that are separated by a narrow channel. In less than 100 years the
oligoUntersee, a deep and originally
trophic lake, has become strongly eutrophic
owing to domestic pollution,
while the
Obcrsee has remained oligotrophic.
The
eutrophication of the Unterscc is reflected
of enriched
Sutherland
lochs (1954 and 1955)
Mhulltlich
( control )
Bciste Rrice
(NPK)
Daimh Beg
(control)
Daimh Mor
(SP)
0.3
0.17
0.5
0.5
0.7
0.5
0.8
0.9
1.6
1.0
0.z
0.5
0.5
0.6
0.9
1.0
1.0
5;
3.0
0.8
1.0
2.7
3.2
6.0
11.0
0;
0.8
2.0
1.7
-
07
408
A. J. BROOK
in changes in the type and quantity of
phytoplankton;
blooms of blue-green algae
are now common. There is similar evidence of rapid eutrophication
of Windermere in the English Lake District accompanying increasing urban development in
the lake’s catchment areas during the 19th
century. Evidence for this is the change
in the species of planktonic diatoms deposited in the lake’s bottom sediments
( Pennington 1943). Loch Leven, in Kinross-shire, Scotland, has been enriched by
agricultural
and urban development over
a similar period; there is evidence of significant biological changes occurring in the
loch during the past 60 years. Net samples of phytoplankton
collected in August
1904 during the bathymetrical
survey of
the Scottish freshwater lochs are preserved
and have been examined and compared
with samples taken in August 1954 and
1955. Seven species of planktonic desmids
were present in the 1904 samples; the compound quotient then was 11/7 = 1.6. In
the recent samples, only four desmid species have been found, while the number
of species of Chlorococcales has increased
considerably, and the quotient for both the
1954 and 1955 samples is 29/4 = 7.2. This
clearly indicates a significant
change in
trophic status. The desmid species rccorded in 1904 and in recent years in Loch
Leven are:
1904
Closterium aciculure
Cosmarium depressurn var. plunctonicum
Spondylosium plunum
Staurastrum cingulum var. o&sum
S. lunutum var. plunct~icum
S. sebaldi var. ornutum f. plunctonicum
S. pingue
1954 and 1955
Cosmarium depressurn var. plunctonicum
Staurastrum cingulum forma
S. pingue
S. chaetoceras
Like
Leven
blooms
Almost
the lower basin of Zurichsee, Loch
has occasionally
produced large
of blue-green algae in recent years.
10,000 filaments/ml
of Oscilkztoria
bornefii were recorded in the summer of
1937 (Rosenberg 1938), 20,OW of 0. limnetica in the early summer of 1954, and
17,000 of Aphanixomenon
flos-aquae in
1963.
Fertilization
experiments in Loch Kinardochy (Brook and Holden 1957) and the
Sutherland lochs indicate that increases in
nutrients alone do not necessarily bring
about changes in the composition of the
phytoplankton
community.
In Loch Kinardochy, and after two applications
of
fertilizer in the Sutherland lochs in 1954,
there were no significant changes in the
specific composition of the plankton, even
though it had increased numerically.
Only
after blooms developed did significant increases in the compound quotient become
apparent. The maximum concentrations of
added nutrients were no greater in 1955
than in the previous year, so it appears that
the abundance of algae was the decisive
factor in altering the specific composition
of the phytoplankton,
The pI-I increases
may have been responsible for the disappearance of desmids and the marked increases in quotients.
The production
of
antibiotics by freshwater algae may have
played a role in changing the specific composition, and hence in increasing the quotients, in the enriched lochans that developed blooms. This possibility is suggested
by Lefcvre, Jakob, and Nisbet ( 1952) in
their demonstration of antagonistic action
by abundant growths of planktonic blucgreen algae towards desmids. It appears
that many phytoplankton
species can tolerate sudden increases in the concentration
of certain nutrients, but they may be unable to survive changes resulting from unusually large populations of other planktonic algae. The latter may cause changes
in the species composition of the community and, in turn, the phytoplankton
quotient. The eutrophication
of Windermere
is especially interesting in this respect, for
despite its known enrichment by urban effluents, the composition of its plankton still
indica tcs a moderately oligotrophic
lake.
Its quotient is, on the average, about 0.75
and it is always possible to find from 12
PLANKTONIC
ALGAE
AS INDICATORS
to 14 euplanktonic desmids in net samples.
Windermere phytoplankton
is nearly identical in specific composition to that of the
unpolluted
and truly oligotrophic
Loch
Lomond ( Table 1). Although Windermere
now supports an abundant plankton, it
seems that it never attains a population
comparable to the blooms found in Loch
Leven or the fertilized Sutherland lochs.
This adds weight to the above conclusion
that the specific composition of the plankton may be influenced by comparatively
rapid changes in nutrient status only if
these first bring about well-marked biological changes, such as water blooms. In
lakes like Windermere, the compound quoticnt gives no indication of a change in
trophic status despite evidence of it from
other sources. Indeed, the Windermcrc observations suggest that desmids, which are
usually considered as reliable indicators of
oligotrophy, are remarkably adaptable and
can survive and even flourish in a lake
where there has been considerable enrichment. Their adaptability
is illustrated to
a lesser extent by the Loch Lomond observations ( Table 1). There is evidence,
in addition to the loch’s morphometry, to
suggest that the shallow southern basin,
Creinch, is considerably more eutrophic
than the deep northern end, Tarbet (Slack
1957)) but there is no indication of this difference in the phytoplankton
quotients. If
this tolerance and adaptability,
especially
of oligotrophic desmids, to increases in nutrient concentration is real, it is difficult to
explain the reasonably good agreement bctween the ,degree of evolution and the
phytoplankton
quotients of the principal
lakes in the English Lake District (Table
2). It can only be assumed that when cnrichment proceeds naturally, and presumably very slowly, the composition of the
phytoplankton
changes accordingly, without the intervention of blooms.
PLANKTONIC
DESMIDS
OF TROPHIC
AS INDICATORS
STATUS
Nygaard (1949) stressed that although
the majority of Chlorococcales, Myxophyceae, and centric diatoms occur most often
Ol? LAKE
409
TYPES
TABLE 6. Range of a2kalinity and compound
phytoplankton
quotients of lake waters in which
planktonic desmids occur
Compound
Species
Average
Staurastrum grncile
S. chaetoceras
S. planctonicum
S. pingue
S . furcigerum
S. cingulum var.
obesum
S . arctiscon
S. lunntum var. planctonicum
S. pseudopelagicum
S. boreale
S . cingulum
S. clenticulatum
S. anatinum
S . longispinum
S . brasiliense
S. longipes
S. ophiura
Cosmarium granatum
C. humile
C. botrytis
C. impressulum
C. depressum
C. abbreviatum
C. contractum
Sponclylotium planum
Cosmocladium saxonicum
Eunstrum verrucosum
Xanthiclium subhastiferum
X. antilopeum
X. controversum
Range
Alkalinity
range
(ppm CaCOs)
6.25
6.0
3.2
2.6
1.8
2.3-12.0 46.0-195
0.9-20.0 4.0-194,
0.7- 9.5 2.0- 78
0.4-11.5 7.0-200
0.47 5.5 3.0- 78
1.2
1.2
0.2- 9.5
0.5- 2.0
2.0- 42
0.96
0.27
0.20%
0%
0.20%
0.27
OS0.20.2-
4.0
2.3
1.5
3.0
2.0
2.0
1.1
1.1
2.0
0.8
1.5- 38
1.6- 45
7.04.0l.Ol.O4.08.03.0-
89
38
38
12
8.4
45
38
0%
0.20.85 0.20.7 0.20.7 OS-
3.0
3.5
3.4
2.0
1.1
4.0l.O4.01.84.0-
45
38
78
41
9.4
0.67
0.66
0.64
0.6
0.58
1.5
1.1
1.5
1.3
1.3
1.1
0.9
0.87
0.85
0.83
0.79
0.75
0.71
0.70
0.59
0.55
Stauroclesmus deiectus 1.0
S. brevispinus
s. cuspidatus
S. curvatus
S. glabrus
S. megacanthus var.
scoticus
S. iaculiferus
S. sellatus
S. megacanthus
S. subtriangularis
S. triangularis
S. aversus
quotient
0.9
0.53
0.4
0%
O.30.20%
O.l0.20.z
1.8- 38
1.6- 16
1.c 10
l.O-
9.0
1.1-11.5 12.0- 76
0.5- 9.0 5.0-135
0.7- 6.3 3.0-135
l.O- 3.5
0.5- 7.0 3.6-194
0.95 O.% 2.0
1.6- 26,
0.67 0.2- 1.3 O.O- 26
4.8
3.4
2.75
2.1
1.8
0..2- 2.3
3.0- 33
0.71 0.2- 1.1
1.O 0% 5.0
1.6- 16
l.O- 20
0.86 OS- 1.1
0.76 0.2- 2.0
0.61 0% 1.0
SO- 38
O.O- 38
1.6- 16
0.88
Closterium ceratizcm
C. aciculare var.
subpronum
C. setaceum
5.75 5.0- 6.5 114.0-194
Micrasterias
0.7
sol
3.5
0.79
0.8-10.0 12-O-135
0s 2.0 l.O- 45
0.2- 5.0 l.O- 20
410
TABLE 7.
A.J.BROOK
Trophic
Genus
association
genera
desmid
Number of taxa associated with conditions
cutrophy
Cosmarium
Closterium
Staurastrum
Euastrum
Staurodesmus
Micrasterias
Xanthidium
Cosmocladium
Spondylosium
Total
of principal
mcsotrophy
5
2
4
0
0
0
0
0
0
11(240/o)
1
0
4
1
2
0
0
0
0
8 (17%)
of:
oligotrophy
1
1
9
0
9
2
3
1
1
27 (59%)
in eutrophic waters, there are several important exceptions.
Several pennate diatoms and a number
of desmid species occur in lakes that are
cutrophic and contain many centric species, while many blue-green algae occur in
nutrient-poor
waters. On the other hand,
many species within these major taxonomic
groups occur almost throughout the trophic
range (Round and Brook 1959). Rawson
( 1956) questions whether thcrc are, in fact,
many reliable indicators of oligotrophy,
quoting
Jarnefclt’s
( 1952) investigation
of more than 300 Finnish lakes. Jarncfelt
listed only 6 species found exclusively in
oligotrophic lakes and 30 found only in eutrophic lakes. Another aspect of species
tolerance is the possibility of the development of physiological races adapted to different trophic conditions but morphologically inseparable.
Rawson (1956) believed that this may explain the conflicting
conclusions several algologists have reached
concerning the trophic conditions in lakes
containing certain algal species.
In the present investigation,
particular
attention has been paid to planktonic Dcsmidiaceae and the trophic status of the
lakes in which they occur. Satisfied that
quotients can be applied with a fair degree of reliability if the limitations outlined
above are observed, determinations
were
made for some 300 lakes of a wide range
of types in various parts of the British
Isles. From these, the average compound
quotient has been calculated for each of
the most commonly occurring desmid species, along with the range of trophic levels
over which they have been found and the
alkalinity range in ppm of CaCO, (Table
6).
Table 6 shows that there is a strikingly
high number of taxa in this supposedly
oligotrophic algal group that are most frequently associated with eutrophic waters.
This is demonstrated in Table 7 where the
numbers of species of each dcsmid genus
occurring in eutrophic, mesotrophic, and
oligotrophic
lakes are listed. The table
shows that 24% of the planktonic desmids
common in British lakes are most frequently associated with waters having a
compound quotient of more than 2.0 and
arc thus distinctly eutrophic. Although the
analysis confirms the generally accepted
conclusion that desmid species are most
frequent (59% ) in oligotrophic waters, it
also indicates that the inclusion of all the
desmid species present in a sample when
determining compound quotients decreases
the reliability
of this method of assessing
trophic status. These observations
add
weight to the view ( Brook 1959b) that
the aim must be the formulation of a quotient based on a limited number of planktonic species, whose status in the plankton
and whose nutritional
requirements have
been adequately investigated.
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