l' , " c "t' .. \ THE ROLE OF SU13MERSED MAÇJWPHYTES IN PHOSPHORUS CYCLING / by @ Richard Carignan \ A thesis submltted to the Facult'Y_ of Graduate Studies and Research in partial fulfilment of the requirement for degree" of Doctor' of Phil9so phy. Department of Biology February 1980 McGill University _ t_ '" MontrEal t Canada !
103
Embed
THE ROLE CYCLING by - McGill Universitydigitool.library.mcgill.ca/thesisfile68546.pdf · THE ROLE OF SU13MERSED MAÇJWPHYTES IN PHOSPHORUS CYCLING / by @ ... Methods •• • ~
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
l' , "
c
"t' ..
\
THE ROLE OF SU13MERSED MAÇJWPHYTES IN PHOSPHORUS CYCLING /
by
@ Richard Carignan
\ A thesis submltted to the Facult'Y_ of Graduate
Studies and Research in partial fulfilment of
the requirement for t~e degree" of Doctor' of
Phil9sophy.
Department of Biology February 1980 McGill University _ t_
'" MontrEal t Canada
!
1
--.-/ , '
Ph. D. Biology
Richard Cari'gnan
The role' of submersed rnacrophytes in phosphQrus cycling. .. ~
ABSTRACT
The specifie aetivity of the 32p labeled sediment-P taken up by'submer-•
sed macraphytes was 'shown to be identical to the spedfi C' activi.ty of the
sediment mobile P, as measured by isato,pic dilution. The mobile P therefore . \
represents the sediment-P available to aquatic maerophytes .
. The abi 1 ity ta aecurately measure the specifie actlv,ity of the available
sedimeRt-P was applied to problems pertaining to the rolè of macrophytes in P \
cyeling. The relative contribution of water and sediments in supplying P to "
l' macrophytes WpS measured by growing macrophytes if1 situ, roçteq in 32p labeled "' -
sediments, and with the shoot in free contact with ~the .unlabeled overlying
water. Macrap~ltes grown. ,in mesotrophic and eûtrophie sites cterived"more than
95% of their P from the sediments alone. When grown in a hypertrophie site, , . 1
the sediment.s still supplied 70% of the P.
Rates of P release by macrophytes and;ignificance ... to their periphyt~n
and surrounding ph,ytoplankton were e~t;mat/d by using<tul:1Y labeled pla~ts. The periphyton derived only 6.5% o! its P form ~he macrophytes. Myrioehy11um
, released 0.32 Ug,g-~.h-~ P., ~ast,of whith. being read,ily'available to phyto
plankton.
The high vertical mObility of the available sediment-P was demonstrated
bath by exper.iment'al manipUlations ami 'direct observations.
, .
} ) 1. -
\
, . \ i
J
i ~ l 1 l
1
J
"
j \ 1
1 il
.1
,~ 1
ft ,i ;
1
c
O. Ph. Biologie
Richard Carignan
Le rôle des macrophytes sUbmergés dans le cycle du phosphore.
RESUME
On- montre que l'activité spécifique du P sédimentaire assimilable par
le:; macrophytes submergés et marqué a'u 32p , .est égale a l' ,activité spécifi
que- du P mobile, tel que mesuré par diluti,on isotopique. Le P mobile repré-
sente donc la fraction disponible du P sédimentaire.
, Cette possibili~é de mesurer de facon précise l'activité spécifique du
P sédimentaire disponible fut appliquée a des problèmes ayant trait au rO)e
("des macrophytes dans le cycle du P. On a mesuré la contribution relative de ,
(
l'eau et 'des séd'iments da~s 1 a nutri ti on des macrophytes en fa; sant cro1 tre
des macrophytes in situ, enracinés dans des sédiments marqués, et avec la par
tie verte en libre contac~ avec l'eau non marquée. Les plantes croissan,t dans
de~ milieux mésotrophes et 'eutrophes puisèr~nt plus de 95% de leur' P des sêdi
ments seuls. En milieu hypertrophe, les sédiments fournirent 70% du Po
L'utilisation de plantes marqu~es -de'facon homogène a permis d'estimer ,
des taux' di excrétion de P chez Myriophyllum ainsi qùê d'estimer l'importance \ ,
de ce P pour le périphyton et le phytoplancton. On-a trouvé que le p'ériphyton
ne tire que 6.5% de son P des macrophytes. Le taux moyen d' excr'ét i on de P pour
Myriophyllum fut de 0.32 U9.g- 1.h-1, do~t la/majeure portion Hait rapidement
di sponibl e au phytoplancton.
On demontre la mobilité verticale élevée du P sédimentaire disponible
par ~es observâtions directes et manipulations expérimentales. ,
GENERAL CONCLUSIONS. " ........................... i ........................ .
RF • - i'lIi» 1 [pt
4
5 6 Y
14 17
, 19
20 21 22 25
'30
32
33 34 36 39 56
59
60 61
,6 J 64 76
79
l-l'
\
/ /
1
1 ,1 j • 1 l
i i
, LIST OF FIGURES
PART 1
Figure J. Partitioned container used in growth experiments wlth Heteranthera rooted in 32p labeled sediments ............ 8
Figure 2. Specifie aetivity of ion exchange eftraetable P _ (mobiJe p) with tlme .•...................•. \ ..•..•.• , ..•.•.. 12
~~ 1J
F j gure 3. SpeC'l fie act i v 1 ty of j on exchange ext ractab leP (lllObile p) in sediments Incubated at '.and 20°C ...... -; ...... 13
PART '1
Figure 1. Experimental apparatus used in phosphorûs uptake measU(ements ....... Il' •• Il , Il Il Il •• Il Il Il Il •• Il • Il Il'.11 •• Il • Il Il ••• Il Il Il Il'' •• Il 23
l ~"-
PART 111
Fig,ure 1. Phosphoru~ release by the Hyriophyl lum-periphyton complex duri,ng a 24h cycle .. , ............................... ' .. 43
Figure 2., Phosphorus release by the Hyrloph'yl1um-periphyton . complex before and after addition of carrier P .... ,'" ......... 45
FI gure ). Remaining total, ,particulate and soluble 32p after addition of carrier-free 32P04 ln a chamber contalning
(
an unlabeled Hyrlophy'1um-periphyton cortlplex ................ 52
PART IV
Figure l,. SRp· and Its speclflc' activlty versus tlme in the anaerob je conta i ners used for the measurement of mobile P by Isotoplc di lutlon ............................... 63
Figure 2. Lead-210 and total P profiles in Central Bas in, Lake , Hemph remagog ••••.•.•.•••....••.....••••....•••.....• '" .•.••• 65
Figure 3. Total, mobile and, Interstl,tlall P profIles after 5 weeks 1 n an 1 nit i a lly homogenous mud •..•••••••••...••••...•. 68 ,
'Ù
(
i
J 1
î
1
\
i j
1 - "
Figure 4. Total and interstitlal Fe profiles after 5 weeks_ ln an Inltlal1y homogenous mud .................. ~ .......... 69
1 < Figure 5. Total, mobile and Interstitial P profiles at station A ••••. 72
Figure 6. Total, mobile and interstitlal P profiles at station 8 •..•. 73
Total and mobile P profî les at 5 ta t ton C .• 1 Il •••••••• " • " •••• 7.4 1 !.
\ J
J
" /
\
1
, '" [
\ 1
LIST OF TABLES
PART 1 Page>
Table 1. Correspondence between plant aval1able phosphorus (âP) and mobile sediment phosphorus •••••....••••••.••• " ••.••• ~ ••.• ,15
PART Il
Table 1. Water and sediment P charae teristics for the three Southern Québec sites imiest igated ••••••••••••••••• ',' ••••••••• 27
Table 2. Mean percent P uptake_ from t~ _ sedimen·ts for six maerophyte species grown in Central Lake Memphremagog •••••••• ,28
Table 3. Mean percent P uptake from, the sediments for three .' macrophyte species grown under various conditions of water and sediment P availabilities •••.••••••. , ••••••••••••••• 29
PART III
Table 1. Total P content of macrophytes, associated epiphyte-P and % epiphyte-P derived from su~porting macrophyte for . nine macrophyte speeies cOllected\ in Central Memphremagog •• " •. :41
Table 2. P release rate by macrophyte+periphyton, macrophyte-derived P release rate a~d % macrophyte-de'rived P particulate after Sb for Myriophyllum spica tum •• , , , , ••••• , , , •. " • , , •• , , , .......... 48
1 wish to acknowledge the National Sciences and Engineering Research
Council of Canada and the Di rection Général e des Etudes Supérieures du Québec
for postgraduate scholarships'. 1 also wish to thank Jaap 'l<alff, my thesis , -,
. . superv,; sor, for encouragement and support during this- t\çearch.
. , Acknowl edgements are al 50 due to Rob Peters, Bruce Lazerte and Bob
1
Flett, whose advices contributed si gnificantly to the- ful filment of t'hi 5
research. Special thanks are due to Bob Flett, who provided the lead-210
data. 1 wlsh to thank Tony Briza for skillful machining of the plexiglass
equip,ment and Robert Lamarche for photographie reptoduction of the figures. p , 1·
Most particularly, thank my wife, Susanne, who helped me more than
1 can tell.
t J
1 " 1
1
t
, '1
"
,1
" ' .' "
;". '
'.
" ,
" , , , , ,
o 'C
G'ENERAL 1 NTRODUCT ION
One of the 'most important achievéments'" of limnology in the last decade
is the observation, and subsequen.t 'gen~ralizatif)n, that phosphorus (P)' 1s a
l imiting nutrient in most aquatic systems. Thiso.finding si'gnificantly stimu-" , '
lated °research €'fforts on P 10ading pr,ocesses in lake!> and led to the deve-... ,IJ 7
lopment of predictive external nutrient loading models,that were used, in'
turn, to predict various biologicœl aspects of lake response'to P loading.
l
Sin~e internal nutrient loading proces'Ses were also observed'to parti- ' 1
cipate in the nutrient, dynamics of l'akes; our efforts to eval,uate ,the impor- ' , ,
tance of internal P loading agents, such as sediments 011 macrophytes, were J
also intensified. Unfortunately",<?ur accumulated Q~servatiQn~ ~re not",yet
amemlbl e to general ized predictive theories or mode) s for re,àsons tha,t appear , (l ~ -
o •
~rn9re rel ated to our present inabi 1 ity to acclJrately quanti f,y, the importance " .. Q ,
of int~rnal loding 'agents, rather than"to sorne ;ntrins,;c ,irreductibility
property of biologicaT systems.
As rooted
reservoi r;, they
aquatic macrophyt~s have access to a ~large se,dimenJary P -constitute a potentiallY important pa~hWay~ 'for' .int~;nal p'
o ) ~ ~ 1
\
",
loading through upward translocation of sediment~P," and subsequent ~
or sènescent P release. The po~ential importance of macrophytes ,in-P-cYéli~g .',
can easily be illustrated i'f one 'con~iders that in ,many l~kes hav.ing an--, , '
ex tens; ve Ji ttora l zone, the amount 0 f P Ued up in rnacrophyfes i S often n , (> 1
, ' , ,
~q~ivalent to, or greater than the ~nnual exterrial P 10ading to .~hese lakes.·1
Theo estimation o'f the importance of macrophY.tes a's internaI nutr1ent , . ,.,
. (
J
. "
• 1
l, r'
\ ~~, .
~.) ~ . , .~
,~~
." . v' : ''A è!.\ ~~.
" " h' r
~ ~" 1;' t!-~
t
loading agents includes three basic questions pertainin~ to:
a- the source (water OP sedi~ents) of macrophyte-P;
b- the in vivo relea~e and availability of macrophyte-P;
e- the senescent release and availability of macrophyfe-P.
The objective of this study was to develop and use techniques for the
accurate measurement of the relative contribution Pt. wat~r and sediments in
P uptal<e by aquati c macrophytes ànd to es tab 1; sh the s tgnif5 cance ot" macro
phyte released P to periphyton and phytoplankton. .,
" The general approach was to first investigate the feasibillty of labe-
2
1in9 the sediment-P fraction available to aquatic macrophytes with'32p, and of . measuring the degree ,nf label,ing of this fraction. This section has been
\ .published in the Journal of the Fisher,ies Researefi Board of Canada and
eonstitutes Part 1 of this th,esis.
The se~iment labeling technique developed in Part 1 was then applied , "
to a variety of ~pecies by grow;ng plants in sltu in labeled sediments of
known availabl~~P specifie aetivity, with the shoots in free contact with the .. , ... le "1 . ,
surr6undi~9 wate~, in o~der to quantify the relative importance of water and
sediments in nutrient ,supply to macrophytes. This section constitutes Part II
of }his thesis,and is being published in Science •
'Al' ~pe presently existing information on the rates of in vivo nutrient
release by macrophytes ând significance to their periphyton and surrounding
phytoplankton is questionable because of the highly artificial conditions
employed and because of non homogenous tracer distribution with1n the experi
mental plants. These two problems were "eliminated by producing. in situ.
ful1y labeled plants that were ,used to quantify P release by macrophytes and
'to evaluate its, significance to the periphyton and phytoplankton (Part III).
Since Part II demonstrated the overwhelming~;mportance of-sediments
as a source of P to ,macrophytes, the d_istribution and dynamics of intersti
tial and available (mobile) P in lake sediments wer~ investigated. and are
presented as Part IV of this thesis.
\
./
.:,:::, '~1 ........ ________ • __ ~. ___ • __ /'
3
\
-, . \
,,' ~
, ; .
, . ,
1
•
1
\
"
PART 1
QUANTIFICATION OF THE SEDiMENT PHOSPHORUS AVAIlABlE
TO AQUATIC MACROP~YTES
\ \,
\::--
"
c,
." l' 'c
~, ~. "'" ~: 1,
il-ç: ~~~
t;~. '',If V-. ,'"
i ~it ,\;',
, , ~
1\
1
0
\
• "
"
\
ABSTRACT
The specifie activity of phosphorus taken up by three species of 'sub
merg~nt macrophytes. grown in partitioned containers and rooted in,32p labeled
sediments was shawn ta be identieal ta th~ specifie activity of the sediment
mobile phosphorus as measured\by two isotopie dilution techniques. Therefore,
the mobile phosphorus, as mè~sured by isotopie dilution, represents the total \
pool of sediment phosphorus avàjlable to macrophytes. Jhe ability to measure
and to specifically label this pool will allow the testing of hypotheses con
cerning the role of macrophytes in phosphorus cycling.
r /
.~
\
".
f
6
) INTRODUCTION
( \
\
Several studies have shdwn qualitatively that maerophytes are able to
take up phosphorus (P) via the roots (MeRoy and Barsdate, 1970; Brisfow and
Whitcombe, 1971; Denny, 1972; DeMarte and H~rtman, 1974; Twilley et al., 1977).
However, because of a laeK of appropriate techniques, it has been unc1ear
whether sediment-P controls macrophyte biomass in nature and whether macro
phytes act as nutrient pumps or sinks.
I~ situ measurements of the relative cont~butio~ ef water and sediments o
in the P nutrition of macrophytes are obtainable by growing plants in sediments
with their available P specifically labelled with a radiotracer. In addition,
·the potentially limiting role of sediment-P on macrophyte development could be, "- <~ 1
assessed by comparing available sediment-P to plant yield. However. to do . . . either or both requires accurate measurements of the sediment-P available to
fi. ' J )
macroph):'tes.
Studies on the short term 32p partitioning between the aqueous and par-,
ticulate ~hases of sediments (li et al., 1972) hafe shown that the interstitial
P of lake sediments ij in dynamic equilibrium with a variable amount of P
loosely held by the particulate phase. The sum of both fraction can be termed \ ~ ~""It--__ --=-/~} ...
IItotal exchangeable" or "mobile" P.
This paper reports 'a 'simple tedhnique that allows the quantification of
the amount and specifie activity of mobile P in 32p labelled sedillJents'. It [ ; -
also demonstrates that for one lakè at least, the sediment mobile, is the
'only sediment-P source available to aquatic macrophytes.
, -1
1 ,
t l
f j
l -1
(
1 1
J
1 •
{
/
MATERIALS AND METHODS
General approach
The experimental approach consisted in growing macrophyte species in
partitioned containers (Fig. 1) in whith the sediment compartment was filled'
with 32p equilibrated sediments (see belowf and the, upper compartment filled
with 32P_free lake water of known initial total P content (lO-15,ug/L).
Young macrophytes were collected from a shallow bay in mesotrophic lake Mem-.~
phremagog (Quebec-Vermont), which isthe subject of a long term study (e.g.
Ross and Kalff, 1975; Peters, 1978). ,
The plants were potted individually in the labelled sediments with the "
t
stems leaving the sediment compartment through a tightly fitted soft silicone
7
stopper which prevented al'l nutrient exchange between the two compartments.
The seal was sa efficient that à pressure difference, createa by ga~ production
in the sediments, was always evident at the end of each e~periment. The initial
plant P content was estimated by regressing total P on fresh weight for a .
, dozen similar plants collected in the same area. the resulting correlation
coefficients were always greater than 0.90.
The plants were grown,for periods sufficient (30-60 9).to reduce to
negligible (smaller than 1%) the'statistical error resulting from the estima
tion of the initial plant P content. Thus, any phosphorus, beyond the initial,
appearing in the plant or water com~artment.had to have been previously assi
milated by the rO,ot system and, therefore, belonged to the available sediment-P. , .
The specifie activity of this assimilated P (~P) was subsequently compared
with the specifie activity of the mobile sediment-P (see below).
1 England Nuclear) was added'and well mixed with 20 kg (fresh weight) of sediment.
Before initiating the growth experiments, the sediments were a1lowed a minimum
of three weeks for the tracer to equilibrate with the mobile P pool. However, "
Fig. 2 shows that a true equilibrium had not been achieved even after three
months as there was always a slight decrease of the specifie activity of the \
motiile P. However. after three weeks, this decrease only ranged from 2 to 4% " -~-
per week between experiments and cou1d be approximated as linear over time for '. J
r--~~
the periods i~volved in the plant growth experiments (Fig. 2).
After initiating the growth experiments, the sp~cific activity of the
mobile P was followed by either or both the suspension or the ion exchange
techniques. Since Rre~iminary work had shown that the evolution over time of
the mobile P specifie activity was a1so temperature dependent (Fig. 3), care
was taken ta keep the labelled sediments used'for plant uptake and the sub-. samples used for specifie activity measurements at the same temperature (22°C).
Pl a'n~ growth
The plants were grown at 22°C in a temperature-controlled incubator with \
the light produced by a combination of fluorescent lights and incandescent
bulbs providing 15 w/m2 (PAR) at the container's surface. Filtered air was
continuously bubbled to provide COZ and ta prevent thermal stratification in ,
the vessels. At the end of the growth period, the shoots were sectioned at the
septum level and vigorously shaken in their own medium ta dis'lodge as many
epiphytes as possible. Wall grpwth was then suspended by means of a clean brush
and the water sampled for total P and 32p activity.
1 f ~ J t , l 1 î 1 !
'i
\
1
~\
Fig. 2. Specifie aetivity of ion éxchange extractable ~ (mobile,P) .
w;th time after addition of tracer10n day zero where the
mean mobil~ P specifie activity during growth is taken as
the estimated activîty at midgrowth period, as measured bj
linear regression.-
\
u
, , ,..,
\
()
1 1
1
i
l ,
l '
1-" ,
\ ," . . ~"i
: ' ,
,1 ' " " 1
L
L' 1
'" 1 1 1
;'1 1
"
(
1 () 1
(
,r" \
a o
.......... ..1
o Q)
,0 <O}
o v
o t\I
------~----~~----~~----~------~o ./ 0 CD CD, V ,,(\1
Total 32p (T32 p) and sQluble 32p (S32p ) were measured by
Cerenkov counting after concentration and wet oxidation of unfil
tered and filtered samples. Th'is was done byevaporating (90°C)'
on a hot plate 100 ml samples in 125 ml erlenmeyer flasks in pre-\
sence of l' ml of "HCl04. After çomplete evaporation of the, water,
the HCI04 was ,refluxed for 10 m~n. and allowed to cool. Thirty
ml of dlstilled water was then added and the solution gently boiled
for 10 min •. The solution was reduced to approximately 10 ml and was
then quantitatively transferred to a LSC vial with a fina~ volume of \ "
18 ± .2 ml. Recovery experiments performed by adding a known amount
of 32p-P04 to lake water samples conslstently gave recoveiies better
than 98% with no significant change in counting efficiency.
Prior to each sampling, the outside water was also sampled for
TP, SP and, occasionally, for CerenKov counting blanks. Particulate
3lp and 32p were obtained by difference. Finally, aIl the concentra
tions and release rates of 3l p and 32p were corrected for the simu~-/'
tane9us dilution of,chamber water by outside water during sampling
and for the progressive dilution effect of successive samplings.
ReSULTS AND DISCUSSION
a) Transfer of macrophyte-P to epiphytes
The epiphytes of nine species of fully labelled ma~rophytes
were collected between Ju1y and September 1977. Table 1 shows
39 ,r
,
1 1 1
l i 1
1 1 ,
l' 1
(
..
the mean,P content of the macrophytes investigated, the mean
amount of eplphyte-P present on them and the mean %
macrophyte-derived P found in the epiphyte-P. The tesu1ts show
that between 3.4 and 8.9% of the P present in the 1oose1y held
epiphytes was contributed by the macrophytes. Between species or
within species, no significant correlation could be detected
between the P content of macrophytes and the % ~piphyte-p derived
-from macrophytes.
With a mean TP as 10was 10.5 ,.g. Liter- l in the water co1umn,
the phytoplankton (Carignan, unpublished data)' and, presumably, the
epiphytic algae of the Lake Memphremagog alfe P 1 imited. Cattaneo &
Kalff (1980) have found a linear relationship between epiphytic
biomass or primary productiçn and TP in the water column a10ng the
troghic gradient of Lake Memp~remagog, suggesting that epiphytes
respond to increased P av~ilability by a proportional increase in r
biomass qnd primary production. If we assume that the
. macrophyte-deriv~d P found \n epiphytes (Table 1) constitutes an
extra source of P to which epiphytes respond by proportionately
increasing thèir biomass and/or primary productioQ, the epiphytes
should then, be, at best, only slightly advantaged as compared to
algae occurr ing on non-1 iving substrates. This suggestion is
supported by Cattaneo & Kalff (1979) who found that epiphytes
growing on living macrophytes cou1d not be distinguished from those
growing on plastic plants 'in terms of biomass and primary 0
produc tion.
The minor importance of P transfer from macrophytes to
epiphytes found' in this study does not support the hypothesized \
1 . \
1 1
, , i
(
TABLE: 1: Total P content of macrophytes, assoéiated epiphyte-P and % epiphyte-P derived from macrophyte for nine macrophyte species collected in Central Memphremagog. AlI values expressed as mean ± standard error.
Species
Myriophyllum spicatum
Date Collected
77-08-20
Myrioehyllum 77-09-13 spicatum
Myriophyllum 77-08-25 al terniflorum
Potamogeton* . 77-07-28 zosteriformis
Potamogeton 77-07-19 fol iosus
Heteranthera 77-08-16 dllbia
Vallisneria americana
Bidens Beckii
Elodea canadensis
Potamogeton P i'char!3son i i
77-08-22
77-08-28
77-08-28
77-06-28
'senescent on sampling
P conte'n~ . of
macroPh1te (pg~g- )
2500 :t 130
2210 ± 150
3790 ± 290
3160 ± 170
2570 :t 180
2650' 'j: 300
2730 :t 60
3230 ± 240
2870 :t 250
5020
Epiphjtic-P on
macroph1te (}'g.g
macrophyte)
303 ± 32
252 :t 43
222 :t B
291 ± 28
14
± 61
124 :t 22
142 :t 21
164
% epiphytic-P derived frDIO macrophyte
6.82 ± 0.81
6.34 :t 0.36
4.28 ± 0.50
8.54 :t 1.31
B.BS :t 1.43
6.03:t1.77
4.73 ± 0.96
3.40 ± 0.95
8.99 ± 1. 30
7.3
n
9
2
4
4
6
4
4
4
6
1
~ 1 r
! 1
1 , 1 ~ -1 X· '/
1 1 1
1
1 l' 1
t _ 'symbiosis (Wetzel, 1975) between macrophytes and eplphytes for
nutrlent exchange.
1 ~
, \ \ f ,
f c: t
_l
1
\
b) P release rates in Myriophyllurn
Ten P release exper iments were performed on fully labelled
Myriophyllum between August 15 and Septernber 10, 197B. The
resul ts of two typical experiments are presenteQ in Figures 1 &
2 and will be used to illustrate severai aspects of P release
Myriophyllumn - periphyton complexe
Periodicity of P release
AlI three forms of P measured in the chambers (total,
particulate and soluble 3lp and 32p~ exhibited marked changes
with time. The apparent release of T31p or T32p, s3Ip or S32p
was maximum during daytime and minimum to negati ve duril1g
night. Although rates of TP release in the chambers were
usually high during day, net changes were small (,(1 p9. Li ter- l )
over a full 24 hr cycle (Fig. land 2). These observations
indicate that measurements of P re1ease rates by Myriophyllum
\ periphyton complexes are highly dependent on time of day.
Rates and sources of teleased P
The use of fully labelled macrophytes of known P specifie
activi:y allowed the distinction of the P released by,the
macrophyte + periphyton (Fig. la li 2a) from the 32p labelled
macrophyte-derived P (Fig. lb li 2b). For each plànt, the
diurnal macrophyte + periphy~on TP release rate was calcula,ted
42
, 1 • 1
1 1
i
1 1
l ! l l
(
\ .
( )
\
/
Fig. 1. Phosphorus release by a Hyriophyllum-periphyton community during
a 24h cycle. A: accumulation of 31p forms released by the macro-
phyte+pèr-i-phyton; symbols: A-total P,. e-particulate P, .-soluble P.
,Érror bars, when larger than symbols, indicate the range of dupli-
cate values. B: accumulation of macrophyte derived P forms (right
axis) as calculated from 32p evolution in chambers (left axis) and
specifie actlvity of ~~_macrôPhyte P; symbols: A-total 32p •• _
----partlc~li]ite 32P--:--;:solubl'e 32p. Error bars represent • one standard
macrophytes grow ~ng in Lake Memphremagog have been shown to' take
their P exclusively from the sediments, the macrophyte-derived P
re!eased constitutes a net P loading ta the littoral zone. The'
remaining 2.92 JI gP g_lhr- l released i5 due to periphyton alone
and comes from the surrounding water since it is not labelled.
The P release due to periphyton can thus be characterized as a P
recycling effect of this corrununity instead of a net P loading.
The s imilar i ty observed in Figure 1 and 2 between the
accumulation of macrophyte + periphyton derived SP and,
macrophyte derived SP suqgest that both fractions have the same
periphytic origin. This i5 not surprising since most of t!1~
observed changes in P concentration are due to the periphyton
and that 6-7% of the epiphyte-P found on lo1yriophyllum cons~sted \
of labelled P of macrophytic orig in (Table 1). Therefore, very 1
little P would be transferred directly from the ,macrophytes Jo
l,
J 47 ,
, ,
\ 1
i
l' t
: ! 1
, , \
1 < ,
1 (} 1
48
, 1 1
1
TABLE II: P release rate by macrophyte + per.-iphyton, macrophyte-derived P release rate and % macrophyte-derived P particu1ate after 5 h for ~yriophyllum spicatum. 1
P 1a/lt Dry weight P content Diurnal Diurnal % macrophyte 4 enclosed in (pg. g-1 L P re1ease P release derived P
chamber rate by rate by particulate (g) macrophyte+ macrophyte after 5 h.
1. SRP <e) and its specifie activ~ty (0) versus tlme ln the
anaerobic containers used for the measurement 'of mobile P
by Isotopie di Jutlon.
! (
1 1
o
1 )
r i
\
. 1 \ ~~,
, (
. \
l
o
SPECIFie ACTIVITY (cpm·~g·103). .....
'0 1\)'
o vs o
0..---..----.....---....---....------.
-f -s: mw o -a. '-'"
~'
~ 0 7.
01 o
. \
00 , ••
'/
(
:
j
/ '
snap-cap, thereby preyentiio,g any atmospheric 02 di ffus~on' into the
containers.' PreVious tes~s had shawn that under these conditions, the - . l' specifie .activity of the d1issolved P ,inside the dialysis vials reached
a nea~ly constant value wAhin 30 days (Fig. 1). The vials were thus
, incubated for this period lin darkriess at 2~oC and manlJally shaken every ,
two days. At the end of t e incubation period. the .dialysate was sampled,
and assayed for total.l an for 32p activity by Cerenkov counting. The
conten~s 0) the glass coJn ainers were then aried at 65°C and the sediments
weighed. lhe-amount of bile P was calculated from the amount of 32p "
- 1
initiallyadded and the sp dfic activity of the dissolved -P using standard
isotopie exchange equation (Wang et al. 1975).'
Results and discussion
Lead-2l0 and total~P of; es
During a study'of edimentation rates in Lake Memphremagog (Flett
and Marshall in prep.), 21 cores were collected at depths ranging fr?m
2 to 100 'm and analyzed fot 2l0Pb; stable Pb, 137Cs and .TP. Sixteen cores
,~hQwed 210Pb profiles withlsurfici.l zones (2-6 co) of'constant 2l'Pb
a~tivity fol1owed by expon ntial decreases with depth in the sediments
, ta a supported base level tfi9. "2). The remaini.ng cores showed either
~ very' low 210Pb activity 9r lack.ed a surficial zone of,constant activity
and came from either sandy~ compacted clay or from sediments overlain . \ ' . ,
by Mn nodul es.
The lack of eVi~enœ ;o~ Pb mobility in sediments ha. led mo.\
authors ta interpret tAe surficial zone of constant 21DPb act1v1ty as
,
.' \
:; ,1
64
_ -1 t!;
. f )
1
J
i 1
1 : 1 ! <
l t !
t l
'~
1
1
, ,
\
\
,"
Fig. 2. lead-210 (e) and total phosphorus (0) profi les in Central
by a dissol ution-migration-preeipitation cyele of Mn or Fe with, P
co-precipitating or cu-dissolving with Mn and/or Fe. As hypothesized
by Lynn and Bonatti (1965) for Mn, the interstitial SRP maximum
observed aro,und 10 cm (Fig. 5 and 6) could be explained by the existence
of a redox houndary moving upward as new sediment is being accumulated j rI
and below which P-Fe or P-Mn compounds are being reduce1.f, thus producing
~ anfi~terstitial P maximum. However, the transport of oxidized sediment
\
to the reduced zone by burrowing animals could be a second explanation
for the presence of this interstitial P maximum. This possibility is ~ 4
supported by the work of Robf:jins et al. (1979) who found that tubificids
redistributed an added thin surface layer of 137Cs labelled clay to a depth
of 8 cm after 69 days.,
/ There is a striking similarity between the upward inerease in 1 , ,
; TP' and the increase in mobile P for the two pelagie stàtions (~igs. 6 \ \ and 7). The detennination of mobile P by isotopie dilution (see methods) .
presumably measures the sum of the reactive P in solution and of the P
loosely held to the particulate phase, the dissolved and particulate
phases being in dynamic equilibrium (Li et al. 1972). The partieulate
phase would include labile o,rganic P, adsorbed P and amorphous or
cryptocrystalline P minerals capable of exchange with the solution P.
, The fact that the increase in mobi leP i s almost equa1 to the inerease
in TP observed in the 0-10 cm zone shows that this P fraction could be
a good estimator of the surplus P whieh has migrated into this zone.
This ob?èrvation a1so shows that the reaction leading to the precipi~ation
and accumulation of P in the 0-10 cm zone is fully reversible under
70 '1
1 \ , ~ 't l
1 ! 1
.; , , 1 1
1
f !
1 1 1 1
1
1 ! ! " l ~ 1
. 1
1 1
1 1
~
1
1 j
l '\
,
' ,
•
,
\ 1
j
In situ profiles of total, mobile ,and interstitial P
To explore whether the inverse relationship betwe~n the
interstiti al SRP concentration and surface increase in TP observed 1
with homogenized mud could a~so be observed in undistrubed sediments, ~ 1
r14nterstitial water samplers w,re placed in JU,lY at two stations in
the lake (A: littoral, 2.5 m, s'R,~rsely vegetated; B: pelagie, 10 m,
macrophyte free) and cores were obtained from the inmediate vic'inity
of the samplers. Mobile P (isotopically exchangeable) and TP were
determined from each 1 em section on companion cores for station A,
and from a single core for station B; the results a're shawn in figures
5 and 6. Total and mobile P were also measured on a single core obtained . t \
from a third station (C: 100 m) and are shown in figure 7. Station A
and B ~ielded interstitia1 SRP profiles with well defined maxima at 8 . . and 12 cm, respectively. As in the aquarium expedment, the sharp
interstitial SRP gradient observed over the upper 10 cm corresponds to
an increase in sediment TP particularly'evident in figure 6. ,This
strongly suggests that'dissolution of P in the reduced zone of the sedi
ment, fol1owed by upward diffusio~ and finally by a precipitation in
the oxidized zone occurs in undisturbed sediments, as well as in h6mogenized ,1
sediments. Simi,lar profiles for manganese, also suggesting a post-
depositional mobil1ty, have been reported (Lynn and Bonatti 1965; Li et
al. 1969; Robbins and Callender 1975). Robbins and Callender (1975) , .
;n particular show a total and interstitial Mn profile from Lake Michigan
that, is remarkably simi1ar to the P profUes presented here. This suggests
that the upward movèment 'of P observed in Lake M~mphremagog 1s controlled
71
1 j
! 1 i 1 , 1 , '
~ ~ 1 :
1
1 . , 1· l
1
, J
l , 1
:1 "
!
- ! _-J"
\
Fig. 5. Total (e), mobile (O)'and interstitiaT\(.) P profiles from
station A (z=2.5m) J ,
\
1
\ 1
.. ;~. , r " 1.
~ ,'~
1 1 '1-,, __ , !'
i ( i
, i ,
, ) ,
1 î
l , , , ,
\ 1 , 1 ! 1
) J 1
i !
...-.
INTERSTITIAL P (~g·Liter-1) a 100 200, 300 400" 500
• t ~ o .~~~~------------------------O~.~ .~. l' 0 ___ '0 i