Short-Term Changes in Suspended
Particulate Carbon and Nitrogen
Near the Bottom of a Marine Bay
by
B. T. Hargrne
FISHERIES AND MARINE SERVICE
SERVICE DES PECHES ET DES SC IENCES DE LA MER
TECHNICAL REPORT No.
RAPPORT T ECHNIQUE N"
1976
602
••
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me~
RAPPORT TECHNIQUE NO .602
602
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Research Board of Canada.
et des Sci0nces de la
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TECHNICAL REPORT No.
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report number 457).
rapport numero 457).
Short-Term Changes in Suspended Particulate Carbon and Nitrogen
Near the Bottom of a Marine Bay
by
B. T. Hargrave
This is the forty-eighth
Ceci est Ie quarante-huitiem"
Technical Report from the
Rapport Technique de la Direction de la
Research and Development Directorate
Recherche et Developpement
Marine Ecology Laboratory
Laboratoire d'Ecologie Marine
Bedford Institute of Oceanography
L'Institut Oceanographique de Bedford
Dartmouth, Nova Scotia
Dartmouth, Nouvel1e-Ecosse
1976
iii
TABLE OF CONTENTS
Page
ABSTRACT
v
LIST OF TABLES
ix
LIST OF FIGURES
xi
•
INTRODUCTION
1
METHODS
2
RESULTS
3
DISCUSSION
12
ACKNOWLEDGEMENTS
18
REFERENCES
19
v
ABSTRACT
Hargrave, B.T.
1975.
Short-Term Changes in Suspended
Particulate Carbon and Nitrogen Near the Bottom of
a Marine Bay.
Fish. Mar. Servo Res. Dev. Tech. Rep.
Water samples taken near the sediment surface at 70 m in
St. Margaret's Bay from a submersible resting on the bottom were
analyzed for total particulate weight, carbon and nitrogen
content and particle size.
Disturbance of bottom sediments
resuspended high amounts of material with a biomodal size
distribution and relatively low carbon and nitrogen content.
When surface sediment was not disturbed, water samples contained
variable amounts of particulate matter and the percentage of
carbon and nitrogen was inversely related to the total weight
of suspended material.
Differences in samples taken over
18 minutes implied short-term variation in suspended material
which may reflect heterogeneity of material carried
currents above the bottom.
b~
horizontal
vii
SOMMAlRE
Hargrave, B.T. 1975.
Short-Term Changes ir Suspended Particulate Carbon
and Nitrogen Near the Bottom of a Marine Bay.
Oev. Tech. Rep. 602:
Fish. Mar. Servo Res.
27 p.
On a analyse des echantillons preleves pres de la surface sedimentaire,
a
70 m de profondeur dans la Baie de St Margaret, depuis un submersible reposant
sur le fond, afin de determiner le po ids total de la particule, le contenu en
carbone et en azote, ainsi que la taille des particules.
L'agitation des sedi-
ments de fond a remis en suspension de grandes quantites de matieres
biomodale en taille et
a
a
repartition
contenu relativement faible en carbone et ' en azote.
Lorsque les sediments de surface etaient au repos, les echantillons d'eau contenaient des quantites variables de particules et le pourcentage de carbone et
d'azote etait inversement proportionnel au poids total des matieres en suspension.
Les differences entre les echantillons preleves au cours d'une periode de 18 minutes impliquaient une variation
a
court terme des matieres en suspension, ce qui
pourrait reveler l'heterogeneite des matieres transportees au-dessus du fond par
des courants horizontaux.
ix
LIST OF TABLES
•
Table 1 - Suspended particulate weight and
organic carbon and nitrogen content
50 cm above the sediment surface at
68 m depth in St. Margaret's Bay,
October 18, 1975. Surface sediment
disturbe d. Values are from single
determinations.
5
Table 2 - Suspended particulate weight and
organic carbon and nitrogen content
30 cm above the sediment surface at
70 m depth in St. Margaret's Bay,
October 19, 1975. Surface sediment
undisturbed. Values are means of
suplicate determinations.
8
•
xi
LIST OF FIGURES
Fig. 1 - Particle size spectra comparing the
equivalent spherical diameter of particle
volume to concentrations in parts per
million. Water samples weIe taken 15
and 20 minutes after stirring of surface
sediment.
6
Fig. 2 - Comparison of percentage carbon and
n i trogen and total weight of suspended
mate ri al s ampled at 70 m depth in St.
Margaret's Bay 30 cm above the bottom
over 18 minutes. Least squares regression
l i nes for carbon (y = 24.9 - 4.5x, r2 = 0.76)
and ni tro gen (y = 2.2 - 0.40x, r2 = 0.79)
are significant at p < 0.05).
9
Fig. 3 - Time-series changes in concentration of
suspended par ti culate carbon and nitrogen
in single samples. vertical lines ind i cate
standard deviation of replicate determinations.
10
Fig. 4 - Changes in suspended particulate carbon
concentrations expressed as deviations from
the mean. Numbers indicate carbon;nitrogen
ratios of particulate matter in single
samples.
11
Fig. 5 - Particle size spectra in water samples taken
from above undisturbed sediment at 70 m in
St. Ma rg aret's Bay.
13
- 1 -
INTRODUCTION
Submersibles have been used for underwater observation,
manipulation and experimentation in a variety of past studies.
•
In only a few cases, however, has the existance of water intake
valves permitted samples to be collected at closely s paced
intervals in space or time.
Such a sampling system exists on
the submersible PISCES IV and the availability of the ,craft and
its support vessel Pandora II to the Marine Ecology Laboratory
during October 1975 allowed the in-situ collection of water
samples from St. Margaret's Bay.
Several previous studies have provided information on the
amount and nature of suspended and sedimented material in
St. Margaret's Bay (Platt and Irwin 1968, Sutcliffe 1972,
Webster et al 1975).
--
Recent unpublished observations have
extended these studies and have confirmed the presence of a l a y er
of turbid water extending from 50 m to the bottom (70 m) at the
deepest point in the mouth of the bay.
No information exists
on the origin, persistance or variability of this suspended
material.
PISCES IV was used to take water samples immediately above
(30
50 cm) the bottom in the mouth of St. Margaret's Bay.
Total suspended weight, particle size spectra and carbon and
nitrogen content were measured in water samples taken during
dives on two consecutive days.
These data are compared with
similar measurements made previously for water and surface sediment samples taKen by conventional sampling methods.
-
2 -
ME THODS
Sampling was c a rried out d ur in g t wo 2-hr d i ves o n tr.e
aft ernoon of October 18 and the morni ng of October 1 9 1 1975
at a site ( 70 rn deep ) l o c a t ed i n t h e mouth o f St . Margaret's
Bay (6 3 'J 59
I,
44 0 32').
Wate r s amp l es were t ak
s ampling v alve inside t he s ubmersible.
~n
•
th r ough a
Interna l p re ssure is
main t ai ned at 1 atmos p he re dur ing a di v e and th us wa ter flows
by p re s sur e di f f e rential.
Tyg on t ub i ng, a ttached to t he
e xterna l wate r inta ke por t, exte nde d a l o n g the hyd r aulic arm
to p ermi t s a mp l in g fr om a po sition i n fr on t of t h e forwa r d
viewin g port.
Th e scope of movement of the hyd r aul i c a rm
perroi tte d samp l in g f rom th e se diment sur f ace to 1 50 cm off t .he
bo ttom.
Water samp les in St. Margaret's Bay we re al l taken
between 30 and 50 cm ab ove the sed ime nt .
The vo l ume of th e
t ub ing (500 ml) re q ui red that the in take system be flus hed
b e fore repli ca te ( u s ually two)
I -l iter samples were taken
i ~1.
pre-rin se p o l y ethy l ene bo tt les.
Samples were ke pt coo l
before analyses .
(5 C) and in dim light for 48 hr
Sub- samples
a sh e d pre -weighe d 0 .8
~
( 300- 5 00 ml ) were f i lte red through
si lver filt ers .
We ight of s u spended
material after rin s i n g wi t h d is t i lled wate r a n d jry i ng
( 6 0 C)
wa s de t ermine d by rewei ghi ng a nd then fi lters were combusted in
a Pe r k i n-Elmer 2 40-Elemen tal CHN An aly z e r for d e terminati on
of carbon
and ni tro gen.
Pre v jous mea sure ment s had shown that
pret r eatmen t wi th 4N HCL d id not d ecreas e e stimates of carbon
and thus carbon a tes were no t pre s e n t .
The coefficient of
-
3 -
(a as a percentage of the mean) for ten replicate 300 ml
subsamples taken from one homogeneous water sample was 8%
2% of this variation
of mean values of carbon and nitrogen.
was attributable to analytical error as determined in
standards of known carbon and nitrogen content.
Frequency distributions of particle size between 1 and
128
~
were determined on unscreened subsamples (200 ml) using
a model T Coulter Counter with
50
~
and 280
~
aperture
counting tubes (Sheldon et aI, 1972).
RESULTS
October 18, 1975
Visibility during decent changed abruptly below the
thermocline at 35 m.
The number of particles and their si ze
increased below this depth.
The material appeared as amorphous
flocs of various dimensions ranging to several rom's diameter.
Distortion through the view port made size estimates inacc u r a te.
Large amounts of sediment 'were stirred on landing.
Th is
material was darker in colour and composed of more discre et
particles than material suspended above the bottom.
Grey-
black particles in the resuspended material settled rapidly.
Other suspended particles showed no clear downward movement and
northward moving current (up to 5 cm sec
-1
) were estimated
by the rate at which this material moved across the viewport.
The submersible remained in one position on the bqttom a t
70 m for 2 hr.
The hydraulic arm was used to take surface
- 4 sediment samples by means of rubber bulb pipern and to place
mackerel carcasses on the bottom.
These manipulations
continually stirred surface sediment but resuspended material
cleared rapidly through direct sedimentation and lateral
remo v al by c u rrent fl ow .
Water s amples tak en throughout the time on the bottom
co nta ined variab le amoun ts of suspended material.
The carbon
a nd nitrogen content in all samples was more variable than
wou l d be e xpected o n t he bas is of analyti ca l error alone
(Table 1).
pe a ks at 12
Particle s i ze distributions were bimodal with
~
a nd 12 8
~
(Fig. 1).
When no further manipulation
of the hydraulic arm was required, s u s p e nded material was
allowed to settle and a final series of water samples was taken.
All sizes of particles were removed in equal proportion to their
con centration (Fi g. 1).
Large particles did not disap2ear
more rapidly as might have been expected if settling was the
main process of removal.
-
5 -
Table 1 - Suspended particulate weight and organic carbon
and nitrogen content 50 cm above the sediment
surface at 68 m depth in St. Margaret's Bay,
October 18, 1975. Surface sediment disturbed.
Values are from single determinations.
Total dry weight
mg liter- 1
Percent
carbon nitrogen
C/N
Sample
Time
1
3:02
40.9
4;53
0.58
7.9
2
3:45
60.9
3.10
0.35
8.9
3
3:50
18.0
4.00
0.38
10.5
4
3:55
13.0
3.40
0.33
10.4
Mean
33.2
3.76
0.41
9.43
a
22.1
0.64
0.12
1. 25
Coefficient of variation
as percent
66.6
16.9
28.1
13.3
- 6 -
z
o
1.6
I;t
0::
!z E 1.2
w a.
o a.
z-
°
°EI..J
O
w
..J>
.6
o >-
i=
CD
.4
0::
<{
0..
o
I
2
4
6
16
32
PARTICLE DIAMETER
64
126
256
(fL)
Figure 1 - Particle size spectra comparing the equivalent
spherical diameter of particle volume to concentrations in parts per million. Water samples were
taken 15 and 20 minutes after stirring of surface
sediment.
- 7 October 19, 1975
The sampling station (70m) was 50 m south of the October
18th site.
Only the proximate part of the hydraulic arm was
functional and thus no external manipulation was carried out.
The first hour of the dive was used for observation and photography of the bottom immediately in front of the sutmersible.
Sediment stirred on landing cleared rapidly (5 min) and the
sediment was not disturbed further.
Seven replicate water samples tdken over 18 minutes wi t h the
hydraulic arm held in one position 30 cm above the b ottom
contained variable amounts of suspended material which differed
ln carbon and nitrogen content (Table 2) .
The total weight
of suspended material was an order of magnitude les8 than that
observed when surface sediment was disturbed and it was inver se l y
related to the percentage of carbon and nitrogen present (Fig. 2).
During the 18 minute sampling period, particulate carbon
content ranged from 0.24 to 0.41 mg liter-
l
(coefficient
of variation as percent = 22.4) and particulate nitrogen ra nged
from 0.024 to 0.037 mg liter
-1
(c.v
16.3).
The greatest
differences in carbon content were in samples at 4 and 12 min ute s
when suspended weight changed slightly but the percentage of
carbon increased above initial levels by 49% (Table 2).
Nitrogen
content in these samples was not significantly increased (Fig. 3).
Nitrogen content was greatest in the sample taken at 14 minutes
which contained the lowest weight of particulate material but t he
highest percentage of nitrogen (Table 2, Fig. 3).
Variation in
percentage carbon and nitrogen was approximately equal and was
75% greater than variation in total suspended weight (Table 2).
- 8 -
Table 2
Sample
-
Time
(min)
Suspended particulate weight and organic carbon
and nitrogen content 30 cm above the sediment
surface at 70 m depth in St. Margaret's Bay,
October 19, 1975. Surface sediment undisturbed.
Values are means of duplicate determinations.
Total dry weight
mg liter- 1
Percent
carbon nitrogen
C/N
1
11:11 ( 0 )
3.13
9.90
1. 00
9.9
2
11:13
(2 )
3.35
7.8 6
0.84
8.4
3
11:15 ( 4 )
2.95
13.94
0.84
16.5
4
11:23
(12 )
3 .2 3
12.26
0.92 .
13.3
5
11:25 (14)
2.60
11. 88
1. 41
8.4
6
11:27 (16 )
4.33
5.55
0.54
10.2
7
11:29
(18 )
4.14
6.03
0.63
9.6
3.39
9.63
0.88
10.9
0.63
3.25
0.28
3.0
Mean
6
Coe fficient of variation
as percent
18.5
33.7
32.0
27.2
- 9 -
14
o
1.4
,,
Ie
:2
,,
0
,, 0
,,
0
(I)
0: 10
,,
NITROGEN'"
<t
U
,
f-
:2
w
U 8
0:
W
•
a.
6
4
2
1.2
CARBON
,
:2
w
'"0
0
0:
',.,
,
.' ),. ,
•
f-
1.0
f-
,,
:2
W
,,
u
,,
0.8 ~
,,
a.
,,
,
',.,
4.0
3
3.5
TOTAL DRY WEIGHT LITER -I
2.:)
:2
.
,,
0.6
4.5
Figure 2 - Comparison of percentage carbon and nitrogen and
total weight of suspended material sampled at 70 m
depth in St.' Margaret's Bay 30 cm above the bottom
over 18 minutes.
Least squares regression lines
for carbon (y = 24.9 - 4.5x, r2 = 0.76) and nitrogen
(y = 2.2 - 0.40x, r2 = 0.79) are significant at
p < 0.05)
- 10 -
CARBON
_ .4
"">-
.04,
a:
>-'
~p
.03 ~
I
a:
""
::::i
z
CD
0
a:
>-
0::
<l
U
z
0'
::::E.2
.02
0
5
10
~
15
TIME (min)
Figure 3 - Time-series changes in concentration of suspended
particulate carbon and nitrogen in single samples.
Vertical lines indicate standard deviation of
replicate determinations.
- 11 -
•
•
16.5
.10
z
0
.08
I-
<1
a:::
I-
z
.06
w
u
Z
0
u
Z
<1
w
~
~
I
a:::
W
I-.J
04
02
U
Ol
E
a
9.9
0
a:::
IJ...
z
0
- . 02
-.04
I-
<1
>
w
0
-.06
a
5
10
TIME (min)
Figure 4 - Changes in suspended particulate carbon concentrations
expressed as deviations from the mean. Numbers
indicate carbon:nitrogen ratios of particulate
matter in single samples.
- 13 -
•
.5
z
o
~
0::
IZ
W
E .4
Cl.
Cl.
~E~
o 0
u >
W >..J CD
.3
.2
u
i=
.I
0::
~
O~~~2--~4--~8~~16~~3~2--~6~4--~1~28
PARTICLE DIAMETER
{fL 1
Figure 5 - Particle size spectra in water samples taken from
above undi s turbed sediment at 70 m in St. Margaret's
Bay.
- 14 -
These values are similar to those in material sedimented at (0 and
65 m (Webster et aI, 1975).
Carbon and nitrogen in deeper
samples (to 10 cm depth), however, are re duced by 40 to 60% of
these values.
Carbon:nitrogen ratios in resuspended material
•
(Table 1) are als o higher than those i n s urf ace sediment (7.5)
and sedimenting material (7.0) and mo re comparable to higher
ratios in subsurface sedimen t.
Th u s, r es uspe nsion of at least
the top few centimeters of sedimen t must h a v e oc cur re d .
A di v er s ity of p a rti cle si ze s i s fo u n d wi thin surface
sediments (Johnson 1974) and loose aggregations
w~i c h
exist
in situ are not like ly to r emain inta c t dur i n g sample collection.
The bimodal distribu ti on of parti cle concen t ration a t various
particle sizes in resuspended sEdiment (Fi g. 1) was stable,
however, and must reflect two d i st inct types of partic u late
material.
Microscopic examination would have been useful in
describing particles associated with peaks in concentration at
10
~
and 128
~.
Flocculation of organic-mineral matter can produce stable
s i ze distr ib u tions with modes determined by the inorganic
grain size (Kranck, 1973).
If size spectra are characteristic
of the inorganic nucleii around whi ch floc cu l ation occurs,
resuspended and naturally occurring suspende d matter flocculate
in a similar manner with a mode at 10
~
(Figs. 1&5).
Resuspended
material contains larger particles, presumably fine sand grains,
which are not normally suspende d .
Both size components were
removed from the vicinity of the s ubmersible in equal proportion
•
- 16 nitrogen content) than to differences in total suspendej
weight (Table 2).
Samples containing more suspended matter,
however, also c o n tained less carbon and ni trogen (Fig. 2)
implying a higher inorganic content.
These differences and
t he association of ma x imum carbon:n i t r oge n ratios with maximum
suspended particulate c a r bon
(F i g. 4 ) d e mon s trate tha t material
sampled over as b rief a per i o d a s 18 mi nu t e s is not h o mogeneously
distributed above the bottom.
Hor i zonta l ly movin g currents appear
to tran s p ort part i cu l ate matter but heterogeneity in composition
still occ u r s .
Resuspension of bottom sedime n ts a nd d e t r itus and lateral
transport to produce turbidity layers have been recogn i z ed as
widespread phenomena in lakes, estuaries, coastal waters and
the deep ocean.
Turbid near-bottom water may eithe::- remain
above the sediment as a nephaloid layer or become detached
where currents flow across steep gradie n t s in bottom relief.
Turbidity currents can also arise from submarine slumps
(Heezen et al 1 95 5) and the influx of water across a sill
--
(Sho k ovitz and Souta r 19 7 5 ).
I n all cases suspended matter
is stirred up from the bot tom by t u rbulent flow.
The thickness
of the turbid layer and its horizontal extent will depend upon
me a n c u r rent flow , t urb ulence and the settling rate of the
suspended material.
Nishizawa and Inoue (1958) summarize results of early
studies wh ich demonstrated that distribution of turbidity and
susp e nded mate rial in coastal waters were greatly influenced
- 18 horizontal and vertical directions.
Currents
co~ld
transport
such aggregations at a particular depth thus altering the
composition of suspended material over time at any fixed location.
Such transport would explain the variability observed In
•
•
suspended material above the bot t om in St. Margaret ' s Bay and
in many previous studies (see Go rdo n 1 9 71) .
A submersible or permane nt ly mo o red system which
continuously monitors turb i dity and makes fr equent collections
of water samples at one location c oul d be used to quantify
this temporal variability.
Scales of hor i zontal variation
could also be measur e d by s i multaneo u s determination of current
flow.
Such measurements of heterogene ity in particulate
distribution and its relation to physical structure in the
water are prerequisite for future studies of the transport
of suspended matter through the water column and supply to the
bottom.
ACKNOWLEDGMENTS
I thank the c r ew of PISCES IV and PANDORA II for their
suggestions and help in making the submersible operations
successful.
samples.
Dr. Trevor Hughes assisted in taking water
Dr. K. Kranck analyzed particle size distributions
,
and G. A. Phillips carried out carbon and nitrogen determinations.
I am grateful for their help.
•
- 20 Platt, T. and B. Irwin.
1968.
Primary productivity
in St. Margaret's Bay, 1967.
Tech. Rept. 77.
measura~ents
Fish. Res. Bd. Can add
123 pp.
Sheldon, R.W., A. Prakash and W.H. Sutcliffe, Jr.
1972.
size distribution of particles in the ocean.
Oceanogr.
ll:
•
The
Limnol.
327-340.
Sholkovitz, E. and A. Soutar.
1975.
Changes in the composition
of the bottom water of the Santa Barbara Basin: effect
of turbidity currents .
Sutcliffe, W.H., Jr.
1972.
Deep-Sea Res. 22: 13-21.
Some relations of land drainage,
nutrients, particulate material, and fish catch in
two eastern Canadian bays.
J. Fish. Res. Bd. Canada
29: 357-362.
Wangersky, P.J.
1975.
Heterogeneous distribution of organic
matter in the ocean and its ecological significance.
Presented at the International symposium of interactions
between water and living matter, October 6-10, 1975,
Odessa, USSR.
Webster, T.J.M., M.A. Paran jape and K.H. Mann.
1975.
Sedimentation
of organic matter in st. Margaret's Bay, Nova Scotia.
J. Fish. Res. Bd. Canada 32 : 1399-1407.
•