Evaluation of seaweed mariculture potential on Guam

[From the U.S. Government Printing Office, www.gpo.gov]

EVALUATION OF
JUN 2 s 1297
SEAWEED MARICULTURE POTENTIAL ON GUAM.
1. AMMONIUM UPTAKE BY, AND GROWTH OF
TWO SPECIES OF GRACILARIA (RHODOPHYTA)

Stephen G. Nelson, Roy N. Tsutsui and.Bruce R. Best

Property of CSC Library

UNIVERSITY OF GUAM MARINE LABORATORY,

Techni.cal Report No. 61 U . S - DEPARTMENT OF COMMERCE NOAIA
QK COASTAL SERVICES CENTER
ZS
569 2234 SOUTH HOBSON AVENUE
* R4 January 1980 CHARLESTON, SC 294.05-2413
N45
1980

-This publication was printed under the auspices of NOAA,Office
of, Sea Grant, Department of Commerce, under Grant No. 04-8-MOl-178 to
the University,of Hawaii (Contract Number.11--07121 to the' University
of Guam). The U. S. Government is'authorized to produce and dis-
tribute repFints for governmental purposes notwithstanding any
copyright notation that may -appe ar herein.

UNIHI-SEAGRANT-CR-80-04
Project No. PM/M-1

TABLE OF CONTENTS

Page

,LIST OF TABLES . . . . . . . . . . . . . . . . ... . . .

LIST OF FIGURES . . . . . . . . . . . . . . . . . . . .

ABSTRACT . . . . . . . . ... . . . . . . . . . . . . . . iv

INTRODUCTION . . . . . . . . . . . . . . ... ... . . . .

MATERIALS AND METHODS . . . . I. . . . . ... . . . . 2

Ammonium Uptake . . . . . . . . . . . . . . . 2
Growth in Tank Culture . ... . . . . . . . .. . . . . . 3
Growtfi in Reef-Flat Culture System . . . . . . . . . . 3

RESULTS AND DISCUSSION . . . . . . . . . . . . . . . . . 7

Ammonium Uptake . . . . . . . . . . . . ... . . . . . 7
Growth . . . . . . . . . . . . . . . . . . . . . . . . 11

ACKNOWLEDGEMENTS . . . . . . . . . . . . . . . ... . . . 16

LITERATURE CITED . . . . . . . . . . . . . . . . . . . . 17

APPENDIX . . . . . . . . . . . . . . . . . . . . . . . . 19

LISTOF TABLES

Page

Table 1. Statistics.describing the regression of rate
of ammonium uptake, V (pg-at NH4 +-N*g`l.h-l)9
on'the mean substrate concentration, S
(jig-at NH4+-N-R.-l), for Gracilaria edulis and
Gracilaria arcuata at different salinities 10

Table 2. Means and standard deviations of nitrogen
content of Gracilaria edulis and Gracilaria
arcuata used in the uptake experiments,.' 12

Table 3. Specific growth rates (% increase in wet
weight per day) of Gracilaria edulis and
Gracilaria arcuata in both a tank culture
system and a raft culture system on Guam 13

Table 4. Statistics describing the regression of
specific growth rate (% increase in wet
weight per day) on initial size of thalli
of Gracilaria edulis and Gracilaria arcuata 15
7

LIS@ OF FIGURES

Page

Figure 1. Design of the bamboo rafts which were used
to culture Gracilaria edulis and Gracilaria
arcuata in Pago Bay, Guam . . . . . . . . . . . 4

Figure 2. Map of Pago Bay showing the locations of the
rafts . . . . . . . . . . . . . . . . . . . . . 5

Figure 3. Plot of amm6nium-Initrogen uptake rate (V)
on mean substrate concentration (S) for
Gracilaria arcuata at 340/oo 230/oo (o)
and 130/oo .8.

Figure 4. Plot of ammonium-nitrogen uptake rate (V)-on
mean substrate concentration (S) for
Gracilaria edulis at 340/oo (-), 230/oo (o)
and l3o/oo . . . . . . . . . . . . . 9

Figure A-1. Response of the Orion ammonia electrode.
The relation between dissolved ammonium-
nitrogen and electrical potential . . . . . . . 20

-ABSTRACT

We examined the specifi@ growth rates of and ammonium uptake.
by Gracilaria edulis and Gracilaria. arcuata from Guam. Ammonium
uptake kinetics were examined in the laboratory at salinities of
130/oo, 230/oo and 340/oo for each species. Ammonium uptake was.
examined over a wide range of substrate concentrations with the
highest concentration at 5 x 103 pg-at NH +-N-k-l. A strong dif-
4
fusion component was indicated in the ammonium uptake systems of
both species. The nitrogen content of@the thalli used,in the
experiments was variable but averaged 2.07% for G. edulis and
3.31% for G. arcuata. Specific growth rates for thalli of each
species were compared in both an outdoor tank culture system and.
a raft culture system in Pago Bay, Guam. Growth rates were
generally higher in the.raft culture system than in the tank
culture system. Mean specific growth rates ranged from 2.02%
per day for G. arcuata in tank culture in the fall to 5.11% per
day for G. Julis in tank culture in the summer.

INTRODUCTION

Recently there has been an increasing interest in seaweeds
and seaweed products on the world market (Michanek, 1978). Con-
comitant with this growing interest, efforts have been directed
toward the development of seaweed mariculture systems throughout
the world. The major purposes for seaweed mariculture include
phycocolloid production, wastewater purification and human con-
pumption. Some seaweed culture systems have proven so .productive
that they have also been suggested as a means of biomass produc-
tion for energy conversion (Doelling, 1978).

One genus of red algae (Rhodophyta) on Guam seems to have
potential in each of these-:categories. This is the genus
Gracilaria:which is represented by several species on the reef-
flat habitats of Guam.' This genu� is the object of commercial
harvest and culture in many areas of the world. In 1976 the
worldwide value ofthe agar from Gracilaria:' and its direct con-
sumption was valued at approximately US$2,7000,000 (Tsuda and
Doty, 1978). One species of this genus, Gracilaria edulis (Gmel.)'
Silva, is often found in the markets of Guam where it is sold as
a fresh vegetable for human consumption.

The two most common spe'eies'of Gracilaria on Guam are G.
edulis and G. arcuata Zanard. Our preliminary studies to eval-
uate the m'ariculture potential of seaweeds on Guam have focussed
.on these two species. The objectives of the present study were
1) to examine the kinetics of ammonium uptake by the two species,
and'2) to determine the growth of individual thalli of each
speIcies in outdoor tank culture and in reef-flat culture systems.
These objectives were chosen to aid us both in the design of
future experiments and in the development of a strong research
focus for evaluating the potential for the culture of Gracilaria
on Guam. The data on ammonium uptake will also be of value in
determining the potential value of these species for removal of
ammonium from seawater enriched with wastewater.,

MATERIALS AND METHODS

Specimens for the study were collected from two locations on
Guam. Thalli of Gracilaria edulis were collected.frgm the reef
margin near Talofofo Bay, while those of G. arcuata were collected
from the reef flat at Hilaan Point.

Ainmonium Uptake
Thalli were acclimated at 320C from four to six days at
salinities of either 13' 23 or 34 0 oo prior to determining uptake
rates. For the uptake experiments, 0.4 to 1.0 gram dry weight of
the seaweed were incubated in jars with 350 mZ of filtered (0. 45p)
seawater in an environmental chamber. The temperature was main-
tained at 320C �J0C and 200 foot candles of light were.supplied
from banks of incandescent and fluorescent lights. Ammonium I..
chloride (NH4C0 was added to the jars as an ammonium source. 3
Mean substrate concentrations ranged from 3.0 x 100 to 5.0 x 10
jig-at All jars were aerated to ensure adequate mixing.
Control jars without plants were also monitored under the same
conditions over the range of ammonium concentrations.

The dissolved ammonium levels were determined at the onset
and after five hours of incubation, with an Orion ammonia
electrode (Model 95-10) attached to a Beckman expanded-scale
pH meter (Model 55-2). Gilbert and Clay (1973) found that the
ammonia electrode compared favorably with the pheno1hypochlorite
method (Solorzano, 1969) for determining levels of ammonia in sea-
water. Since salinity is known to affect the probe, it was neces-
sary to calibrate it using standard solutions consisting of.NH 4 Ck
dissolved in each of the experimental salinities. Preparation of.
the appropriate salinities was accomplished by diluting seawater
with distilled'water. The response characteristics of the probe
are,discussed in.the Appendix.

After determination of the change in ammonium concentration
in the jars, the-thalli were rinsed in distilled water and dried
at 5 00C for 24 hours. Rates of uptake were calculated and expres-
sed as jig-at NN+@N-g-71-h7l- A least-squares analysis (Snedecor
and Cochran, 1967) was preformed for each regression of uptake
rate on substrate concentration.

Portions of.thalli from each experiment were analyzed for
organic nitrogen content by a microkjeldahl procedure with
replicate samples.

Growth in Tank Culture

Freshly collected thalli of G. edulis and G. arcuata were
drained to remove excess water an-d tagged with -a 1-5cm piece
of electrical wire attached to a mylar tag.. The thalli and
tags were weighed to the nearest O.-Ol g on a triple beam
balance. The thalli were then attached to the bottoms of
30 x 45 x 10 cm polypropylene mesh baskets'and placed in a
shallow wooden tank outdoors. Ten to fifteen thalli were
placed in each basket.@.The tank was supplied with a
constant and vigorous flow of seawater from Pago Bay. The
baskets floated in th tank and the thalli were maintained
at 10 cm below the surface. At approximatelyweekly intervals,
the tagged thalli were reweighed. At the end of the experi-
ments, the'tags and wires were removed and weighed and these
values were-substracted to obtain the actual wet weight of each
thallus.

The specific growth rate of each thallus was calculated
according to the following expression.

100 [In (Nt/No)]
t

where V specific growth rate as per cent increase in wet weight
per day.

Nt final weight on day t.

and
No initial weight

A least-squares analysis (Snedecor and Cochran, 1967) was
performed on:the regression of specific growth rate on the initial
size of thallus for each.species.

.Growth in Reef-Flat Culture System

To determine the growth of thalli in a reef-flat culture
system, two bamboo rafts were constructed and fastened in
position in Pago Bay. Weighed thalli were attached between the
strands of 3-ply polypropylene rope. The strands of rope were
weighted with pieces of iton rebar and suspended from the bamboo
rafts. The raft-culture system is depicted in Figure I and the
locations of the rafts in Pago Bay are shown in Figure 2.

To p View

f i oat

Side View
.7LM

Fig. 1. Design of the bamboo rafts which were used to culture
Gracilaria edulis and Gracilari a arcuata in Pago Bay, quam.
f I oat
/F

0
road

0 200
raft 1 m

Northeast
Reef Flat
< @raft

Pacific

Fig. 2. Map of Pago Bay showing the locations of the rafts.

One raft system and some thalli from the second raft system
were lost as a result of wave action when Typhoon Tip passed
south of Guam in October 1979. The remaining thalli were used to
calculate specific growth rates as described above.

RESULTS AND DISCUSSION

Ammonium Uptake

The uptake of ammonium by both species of Gracilaria was
linearly related to the mean substrate concentration. Figures 3
and 4 indicate the relation between the rate of ammonium uptake
and the mean substrate concentration for each species. Statistics
describing the regression of uptake rate (V) on mean substrate
concentration (S) are shown in Table 1. The correlation coeffi-
cients for these regressions ranged from 0.94 to 0.98. This
suggests that a diffusion component in addition to a process of
active transport is responsible for ammonium uptake by these
species. The studies of D'Elia and DeBoer (1978) also indicated
a strong diffusion component in ammonium uptake by the red algae
Gracilaria foliifera (Forsskal) Borgesen and Neog.@rdhi6lla
baileyi (Harvey ex K;Rzing) Wynne and Taylor.
In studies of other marine macrophytes, NH +uptake could
4
be adequately described by the Michaelis-Menton equation. This
suggests an active transport mechanism is involved. Topinka
(1978) showed that the ammonium uptake kinetics of the brown
alga, Fucus spiralis L. could b'e described by the Michaelis-
Menton expression and that the rate of.uptake was influenced by
temperature. Studies of the green alga Codium. fragile subsp.
tomentosoides (Van Goor) Silva (Hanisak and Harlin, 1978)
indicated saturation kinetics of ammonium uptake which were
influenced by both light and temperature.

The data of D'Elia and DeBoer (1978) showed that-ammonium.
uptake kinetics for Gracilaria foliifera and Neogardhiella:
baileyi were probably the result of two uptake systems. The
first was characterized as a high affinity system which could be
described by the Michaelis-Menton expression. This system
predominated at substrate concentrations below 10 VM of NH4
(7.8 Vg-at NH4 +-N - Z-1.) .Above.this concentration, a strong.-
diffusive component predominated. A similar pattern of NR4
uptake was described for corals which contained.symbiotic algae
(Muscatine and D'Elia, 1978)., The mathematical expression derived
to describe the uptake kinetics'is as fQIIQws,:
V = Vmax S -(K'+S )-1 +(K D -S)

where V and S represent the rate of uptake.and the mean substrate
conceiitration)respectively;'Vmax represents the maximum rate
of uptake, K represents the substrate concentration when V 1/2
Vmax, and KD is equal to the slope of the linear portion of the
plot of V on S.

0
0.
0

000
U) o

+ +
4- TO
+
z

U)
++
+

10
100. + 00
S (Ag a t N H4- N C.I)O

Fig. 3. Plot of ammonium-nitrogen uptake rate (V) on mean
substrate concentration (S) for Gracilaria. arcuata
at 340/oo 230/oo (o) and 130/oo

0
0.
0

+++ CP

CP
0.

190

OD
T
0.
L--
z
4- + +
+0 +
08
0
U)

0 OqD
00

0
10 100 + 1000
S (Ag-at NH 4- N-I

Fig. Plot of anmonium-nitrogen uptake rate (V) on mean substrate
concentration (S) for Gracilaria edulis at 340/oo H,
230/00 (0) and 130/00

Table 1. Statistics describi @g the regression of rate of ammonium
uptake, V (pg-at NH4-N *g-l-h-1), on the mean substrate
concentration, S (pg-at NH +- N-k-1), for Gracilaria edulis
and Gracilaria arcuata at aifferent salinities.

Salinity Correlation
Species 0/00 Slape y-intercpt Coefficient N

G. edulis 13 0.089 8.529 0.9816 38

G. edulis 23 0.051 @3.754 0.9826 40

G. edulis 34 0.188 24.703 0.9818 24

'G. arcuata 13 0.089 -0.303 0.9448 20

G. arcuata 23 0.114 1.050 0.9849 16

G. arcuata 34 0.060 7.910 0.9395 60

At high substrate concentrations, the rate of uptake (V) is
essentially determined by the diffusion component (KD-S). This
was the prevailing situation in the present study and is evidenced
by the high degree of correlation between V and S for both G. edulis
and G. arcuata. Since the study of D'Elia and DeBoer (1978) was
limi7ted to substrate concentrations below 50 11M of NH4 (38.9.pg-at
+ . - I ) V
NH.4.-N Z we have demonstrated that non-saturation kinetics are
evident even+at very high substrate concentrations (up to 5.0 x
103 Vg-at NH4-N-X-l).

Since D'Elia and DeBo.er (1978) found a strong dependence of
ammonium uptake on.the C:N ratio of the thalli, we analyzed our
specimens for organic nitrogen content. The results ar e displayed
in Table 2. There was a considerable amount of variability in the
.nitrogen content of the thalli, which we suspect was the result of
individual variation. There were no apparent seasonal differences.
The percentage of organic nitrogen ranged from 2.20 to 4.23% for
G. arcuata and from 1.02 to 3.95% for G. edulis with means of 3.31%
and 3.07%, respectively.

Our data cover a wide range of substrate concentrations. We
were able to monitor uptake rates at very high substrate concentra-
tions through the use of an Orion ammonia electrode. Many studies
of ammonium uptake utilize the method of Solorzano (1969) for
determination of dissolved ammonium (Topinka, 1978; Hanisak and Harlin,
1978; D'Elia And DeBoer, 1978). This method is generally limited to
ammonium concentrations below 50 pg-at NH _'_ L-1. With the ammonia
4-
probe, we were able to monitor uptake rates at substrate concentra-
-1 of NH +_
tions greater than 103 vg-at-t 14 Of course.,
4 -
trations are much greater than those which.would be found in natural
Gracilaria. habitats, but the data are interesting in light of the
potential of Gracilaria for stripping ammonia from wastewater. It
appears, from our data, that both species of Gracilaria we studied
would be well suited for removal of NH@-N from seawater.

Growth

The specific growth rates of-G. edulis thalli in tank culture
during September and.October of 1977-9 ranged up to 6% per day with
a mean of 2.6% per day. This species had a higher growth rate
during Juneand July 1979 when specific growth rates averaged
5.1% per day. In tank culture, the mean specific growth rate of
G. arcuata during September and October of 1979 was 2.02% per day.
In field culture during September and October, mean specific growth
rates for G. edulis and G. arcuata were 4.8 and 3.5% per day, respec-
tively @Fhe -mean specitlic growth rates for each species and culture
system@are shown in Table 3.

Table 2. Means and standard deviations of nitr ogen content of Gracilaria
edulis and Gracilaria arcuata used in the uptake experimeints.

MeAn % Nitrogen
Species Date Standard Deviation Sample Size

G. edulis 9/13/79 1.02 0.00 2

G. edulis 9/28/79 2.87 0.22 2

G. edulis 10/26/79 .1.45 0.02 2

G.,edulis 10/26/79 1.88 0.116 2

.G. edulis ll/ 9/79 1.99 0.12 2

-G. edulis 12/ 9/79 3.95 0-00 2

G. edulis 12/16/79 1.33.� 0.00 2

G. arcuata 7/26/79 4.23 0.17 2

G. arcua@a 7/28/79 3.50 1.02 2

G. arcuata 7/29/79 2.20 0.06 2

G. edulis all data 2.07 0.95 14

G..arcuata all data 3.31 1.03 6

Table 3. Specific growth rates (% increase in wet weight per day) of
Gracilaria edulis and Gracilaria arcuata in both a tank
culture system and a raft culture system on Guam.

Mean Specific
Culture Growth Rate Standard
Species System Date W Deviation N

G. edulis tank 6/79 j.11 0.23 30

G. edulis tank 9/79 2.56 2.58 .60

G. arcuata tank 9/79 2.021- 1.16 19

G. edulis raft 9/79 4@. 80 3.53 15

G. arcuata raft 9/79 3.50 0.72 V

Table 4 displays the statistics which describe the regression
of specific growth rate on initial weight of the thallus for each
species, in both the tank culture and the reef-flat culture systems.
With the exception of G. arcuata in reef-flat culture, there was a
negative correlation (r ranged from -0.3075 to -0.4298)
between initial size and specific growth rate. In the
case of G. arcuata from:the reef-flat culture systein, there
was no significant correlation (r.= 0.2353, p < 0.05). However,
because of the small sample size (n = 8), we feel that the data,
in this case, are inconclusive. Most of the reef-flat-grown G.
arcuata thalli were lost as a result of storm damage from Typ7hoon
Tip.

The specific growth rates found in the present study for
G. edulis and G. arcuata are similar to those described by other
investigators for other species of Gracilaria. Causey et al.
(1946) showed that growth rates of Gracilaria confervoides (L.)
.Greville [now known as G. verrucosa (Hudson) Papenf.] in North
_i@t -
Carolina.were greatest depths of less than four feet (1.2 m).
They found also that growth rates were most rapid at temperatures
between 250 and 300 and were unaffected by salinity within the
normal range of fluctuation. At depths of less than 1.2 m,
plants doubled in weight within 10 days.,

In raceway culture systems at Woods Hole, Massachusetts,
DeBoer et al. (1978) reported specific growth rates of 3-5% for
Gracilaria foliifera in unenriched seawater. In the same studies,
specific growth rates of over 12% were obtained by enrichment of
the media with ammonia. DeBoer and Ryther (1977) reported potential
yields of Gracilaria foliifera from tank culture in Massachusetts of
28.0 tons (dry weight) ha-1--yr-1 while production of up to 46 tons
(dry weight) -ha-1-yr-1 were reported from culture systems for
Cracilaria in Florida (Lapointe et al., 1976). Hoyle (1978)
reported mean growth rates of 12.24% and 6.41% per day for G.
bursapastoris (Cmelin) Silva and G. coronopifolia J. Ag., respec-
tively, in tank culture in Hawaii.

It is most likely that the growth rates of G. edulis and'
G. arcuata on Guam can be enhanced by-nutrient enrichment. This,
could be accomplished by the addition of nutrients to tank cul-
ture systems or by the suspension of bags of nutrients near raft
culture systems.

Table 4. SEatistics describing the regression of specific growth rate
(% increase.in wet weight per day) on initial size of thalli
of Gracilaria edulis and Gracilaria arcuata.

Culture Correlation
Species System Date Slope y-intercept Coefficient N

G. edulis tank 6/79 -0.047 5.77 -0.368 30

G. edulis tank 9/79 -0.069 2.56 -0.303 60

G., arcuata tar)k 9/79 -0.048 2.29 -0.365 19

@.G. edulis raft 9/79 -0@.)377 7.572 -0.430 15

G. arcuata raft 9/79 0.057 3.035 0.2353 8

ACKNOWLEDGEMENTS

We would like to thank'Dr. Roy T. Tsuda and Dr. Jack R.
Davidson, of the Universities of Guam and Hawaii, respectively,
for arranging the funding for this preliminary study. We are
also grateful to Mr. Steve Hedlund who allowed us to use his
data on growth rates of Gracilaria edulis for the summer of
1979. Thanks go to Dr. James A. Marsh, Jr. and Dr. R. T.
Tsuda for reviewing the manuscript, and to Mrs. Teresita C.
Balajadia and Mrs. Maria C.*Flores for typing the draft and
final report, respectively.

LITERATURE CITED

Causey, N. B., J. P. Prytherch, J. McCaskill, H. J. Humm and
F. A. Wolf. 1946. Influence of environmental factors
upon the growth of Gracilaria confervoides. Duke
Univ. Marine Lab. Bull. 3:19-24.

DeBoer, J. A.$ H. J. Guigli, T. L. Israel and C. F. D'Elia.
1978. Nutritionalstudies of two red algae. I. Growth
rate as a function of nitrogen source and concentration.
J. Phycol. 14:261-266.

DeBoer, J. A.,and J. H. Ryther. 1977. Potential yields trom
a waste-recycling algal mariculture system. pp. 231-249.
In: R. Krauss ted.], The Marine Plant Biomass of the
Pacific Northwest Coast. Oregon State University Press.
D'Elia,- C. F.,and J. @. DeBoer. 1978. Nutritional studies of
two red algae. II. Kinetics of ammonium and nitrate uptake.
J. Phycol. 14:266-272.

Doelling, N. 1978. The economics and engineering of large-scale
algae biomass energy systems. MIT/Marine Industry Opportunity
Brief No. 11, Mass-lust. Technol. Sea Grant Program.
Cambridge, Mass. 20 pp.

Gilbert, T. R.,and A. M. Clay. 1973. Determination of ammonia
in aquaria and sea water using the ammonia electrode.
Analyt. Chem. 45:1757-1759.

Hanisak, M. D.,and M. Harlin. 1978. Uptake of.inorganic nitrogen
by Codium fra&ile subsp. tomentosoldes (Chlorophyta). J.
Phycol. 14:450-454.

Hoyle, M. D. 1978. Reproductive phenology and growth rates in
two species of Gracilaria.from Hawaii. J. Exp. Mar. Biol.
Ecol. 35:273-283.

Lapointe, B. E., L. D. Williams, j. C. Goldman and J. H. Ryther.
1976. The mass outdoor culture of macgoscopic marine algae.
Aquaculture 8:@-24.

Michanek, G. 1978. Trends in applied phycology with a literature
,review: Seaweed farming on an industrial scale. Bot. Mar.
21:469-475.

Muscatine, L.,and C. K. D'Elia. 1978. The uptake and retention
of ammonium-N by reef corals. Limnol. Oceanogr. 23:725-734.

Snedecor, G. W.,and W. G. Cochran. 1967. Statistical methods.
Iowa State Univ. Press, Ames. Iowa. 539 pp.

Solorzano, L. 1969. Determination of ammonia in natural waters
by the pheno1hypochlorite method. Limnol. Oceanogr. 14:799-
801.

Topinka, J. A. 1978. Nitrogen uptake by Fucus spiralis
(Phaeophyceae). J. Phycol. 14:241-247.

Tsuda, R. T.,and M. S. Doty. 1978. Summary report of the advisory
seminar for the Republic of China-United States of America
Cooperative Science program on cultivation and utilization
of economic algae (Guam Reef Hotel, June 5-7, 1978).
Unpublished report to the National Science Foundation.
Re Project FCS-022.

APPENDIX

Ammonia is present in solu'tion Ln seawater in two forms,
ionized (NH +)and un-ionized (NH The equilibrium equation
. 4.
for ammonia in aqueous solution is:

NH + + H 0 NH + H 0
4 2 3 3
The relative proportions of NH +to NH are strongly dependent
4 3
on pH. Increased pH shifts the equilibrium curve to the right.
The Orion ammonia probe was used to determine the amount
of total ammonia dissolved in seawater by treating the samples
with NaOH to convert the NH +
4 to the un..4ionized form (NH3). The
hydrophobic membrane on the ammonia probe is permeable to NH3.
When NH3 diffuses from.the sample across the membrane the pH
of the internal solution of the probe is changed. This pH
change was read as millivolts on a Beclanan expanded scale pH
meter. The change in millivolts can be converted to Vg-at-
9,-l of NH +-N or other convenient units by recording the probe's
4 31
response to a series of standard solutions. A typical calibra-
tion curve is shown in Figure A-1.

The probe response is very precise. The response curve is
linear at substrate concentrations greater than approximately
5 pg-at NH4 +-N- Cl. At.lower concentrations:thexesponse is
curvilinear. The response of the probe is affected by temperature
and salinity so the calibration curve should be constructed prior
to each group of samples.

We found that the ammonia probe was a convenient method for
monitoring ammonium uptake by seaweeds.

0
0.
0

0
0-

+
m 0.
z

CT)

150 9 .30
POTENTIAL M V

Fig. A-1. Response of the Orion ammonia electrode. The
relation between dissolved ammonium-nitrogen
and electrical potential.