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Some aspects of the growth and yield of Gracilaria tikvahiae in culture
Aquaculture, 15 (1978) o Elsevier Scientific
185-193 Publishing Company, Amsterdam - Printed in The Netherlands
SOME ASPECTS OF THE GROWTH TIKVAHIAE IN CULTURE
BRIAN E. LAPOINTE*
185
AND YIELD OF GRACILARIA
and JOHN H. RYTHER
Woods Hole Oceanographic
Institution,
Woods Hole, Mass. 02543
(U.S.A.)
*Present address: University of Florida, Department of Environmental Science, Gainesville, Fla. 32601 (U.S.A.) Contribution No. 4143 from the Woods Hole Oceanographic Institution, and Contribution No. 94 from the Harbor Branch Foundation (Received 15 May 1978; revised 24 July 1978) ABSTRACT Lapointe, B.E. and Ryther, J.H., 1978. Some aspects of the growth and yield of Grucilariu tihuakiae in culture. Aquaculture, 15: 185-193. A series of outdoor, continuous-flow seawater cultures (50 1; 0.23 m’) were used to investigate the effects of culture density (kg/m2), nutrient loading (total nitrogen input/day) with both NH,‘-N and NO,--N, and turnover rate (flow rate/culture volume) on the growth and yield of Gracilariu tikuakiae. Although specific growth rates as high as 60% per day were recorded for Gracilaria at low densities (0.4 kg wet wt/ml) in summer conditions, maximum year-round yields were obtained at densities of 2.0-3.0 kg wet wt/m*. Above a minimal daily nitrogen loading, the yield of Gracilaria was independent of (1) nutrient concentration, (2) nitrogen loading, or (3) whether nitrogen was in the form of NH,‘-N or NO,--N, but was (4) highly dependent upon flow rate. The time weighted mean annual production during 1976- 1977 was 34.8 g dry wt/m’l.day or 127 t/ha*yr based on la-months continuous operation at near optimal densities and flow rates in the non-nutrient limited culture system.
INTRODUCTION
Preliminary experiments on the growth and yield of vegetative clones of the macroscopic red algae Gradaria sp. and Hypnea musciformis (grown unattached in suspended culture) were described by Lapointe et al. (1976). The former has recently been assigned the species name Gracilariu tikuahiae (J.L. McLachlan, personal communication, 1978). Subsequent research involved the screening of some 42 species of seaweeds (including representation of the red, green, and brown algae) for their growth potential in suspended, outdoor, flow-through cultures. All were collected from the Indian River or the open Atlantic Ocean coastline of Central Florida. G. iikuahiue proved most successful in year-round viability and persistence in the vegetative (non-fruiting) condition, and the magnitude of its growth rate and dry weight yield. Subsequently, a series of experiments was designed to determine optimal conditions for its growth. These included investigation of the effects of nitrogen species, concentration, and daily loading, flow rate of
186
media through the culture, and density of the culture. Based on the results of those experiments, the alga was cultured for a year under the best growing conditions that could be achieved to determine its annual yield. Temperature, salinity and solar radiation, all recognized as important environmental factors affecting growth, were not regulated, partly because controls were difficult in the experimental system, and uneconomical or impractical in any largescale seaweed culture system. These environmental variables were, however, monitored so that annual growth could be correlated with the annual changes. MATERIALS
AND METHODS
The culture system designed for the screening program was used for the experiments with G. tikvahiae. The system consisted of four 6-m long 0.4-m diameter PVC pipes that were sectioned longitudinally and divided into 0.75-m (50-l) compartments by plywood partitions. Each section was provided with a calibrated flow of enriched sea water by a manifold fed from a headbox, and fitted with a non-clogging overflow drain. Compressed air was fed into the bottom of each compartment through holes drilled along the bottom of the pipe through an air line (a sectioned 2-in (ca. 5-cm) PVC pipe) cemented to the outside of the main pipe. Thirty-two individual growth assay chambers were fabricated (Figs 1 and 2). The growth chambers were located outdoors. Sea water was taken from a channel which connects to the Indian River, a shallow lagoon of the Atlantic Ocean. No attempt was made to control water temperature, which ranged from 12-34”C, or salinity, which ranged from 20-34°/oo. Sea water was pumped into a large reservoir tank. Before use, the stored sea water was enriched with the desired concentrations of nitrogen and phosphorus, provided as sodium nitrate and monosodium (dibasic) phosphate at a ratio of 10: 1 by atoms of N:P. A second reservoir contained unenriched sea water. Both enriched and unenriched seawater supplies were pumped to headboxes from which they could be distributed to the experimental chambers at the desired rates of exchanges and nutrient concentrations. Weighed amounts of Grucilaria tikvahiae were stocked in the experimental chambers. At intervals of 5-10 days, depending upon growth rate, the algae were removed from the chamber, shaken vigorously to remove water, and weighed, The relationship between drained wet weight and dry weight was determined on replicate samples by oven drying at 90°C for 48 h. Water temperature, measured with a mercury thermometer, and salinity, measured with a Beckman induction-type salinometer, were taken each day at noon, at which time water samples were collected for analysis of PO4 3+-P (Murphy and Riley, 1962), NH4+-N (Solorzano, 1969) and N03--N (Wood et al., 1967). Incident solar radiation was monitored continuously with an Epply pyrheliometer mounted at the experimental site. Experiments were carried out to determine the optimum density to maintain G. tikuahiae in the experimental system to obtain maximum yields.
187
A
SEAWEED
Elevated
tteadboxes r_---_______’ ,__----_--
r----------
IT
CULTURE
SYSTEM
c-______-__--_---___________ ~________________________~ $_----_---__--___-_,
TO SCALE
B SEAWEED
medium
CULTURE
CHAMBER
se0w0te1 .._
Fig. 1. Diagrammatic representation of seaweed culture system.
,
188
Initial densities from 0.4 to 4.8 kg (wet wt) per square meter of culture vessel surface area were used, and the cultures harvested back to those starting densities at l-week intervals for a period of 2 months. The optimum density for maximum yield calculated in this way was then used in all subsequent experiments. It is difficult to separate experimentally the effects of nutrient supply and flow rate of water or medium through a seaweed culture. If nutrient concentration is held constant, the rate of input of nutrients to the culture will vary proportionately with the flow rate. If, on the other hand, nutrients are added separately at a constant input rate while seawater flows are independently varied, the instantaneous concentration of nutrients entering the culture will be inversely proportional to seawater flow rate. Consequently, both types of studies were conducted. In both series of experiments, the sea water was passed through the cultures at flow rates of 1, 7.5, 3 5, and 30 culture volumes per day, while simultaneously comparing growth with both NHI, ‘-N and NO3 - -N enriched sea water. In the first experiment, nutrients were added separately at a constant rate to each culture so that daily nutrient loading remained constant, but concentrations varied inversely with the flow rates of the diluting sea water. In the second experiment, the nutrients were mixed initially with the sea water so that all of the cultures received the same initial concentration, but the daily loading was directly proportional to the flow rate. The nutrients consisted of phosphorus as monosodium dibasic phosphate (NaH,P04*H,) and nitrogen as sodium nitrate (NaN03) or ammonium chloride (NH&I). In the first experiment, the input concentrations of N and P ranged from 300 and 30 pmole/l (at 1 exchange/day) to 10 and 1 pmole/l (at 30 exchanges/day), respectively. In the second experiment, the constant concentrations of N and P were 50 and 5 kmole/l, respectively. In both sets of experiments, Grucilaria was stocked at an initial density of 2 kg wet wt/m2 and weighed and harvested back to that starting density at l-week intervals over an experimental period of one month. Under the established conditions of optimum density, flow rate, nutrient concentration and form of nitrogen for best yield of G. tikvahiue, the alga was grown for a period of 1 year, and growth measured at l-week intervals between 20 July 1976 and 28 June 1977. RESULTS
AND DISCUSSION
The mean specific growth rates (% increase/day) and yields (g dry wt/ m2*day) of G. tikvahiae as a function of density are shown in Fig. 3 for both summer (August) and winter (January) conditions. At both times of year, growth rate decreased with increasing biomass, though both the absolute values and the range were much greater in summer than in winter. Yields (g dry wt/m2 - day) at both seasons peaked at densities of 2.0-3.0 kg wet wt/m2, but with a broad plateau at both seasons and some indication of a higher optimal density in winter than in summer. As expected, yields were
189
Fig. 2. Seaweed culture tanks showing headbox for medium supply.
190
60
A,
NH i-N
45r
i 75 flow
30 rale
/culture
vofumes/doyl
Fig. 3. Effect of culture density on specific growth rate and yield of Gracilaria tikuahiae in summer (A) and winter (0). Fig. 4. Effect of turnover rate (culture volume exchanges/day) on yield of Gracilaria tikuahiae with both NH, ‘-N and NO, -N medium. A- - - -A Constant N and P loading, variable concentrations. a---• Constant N and P concentration, variable daily loading.
higher in summer than in winter. The difference is due primarily to solar radiation but temperature may have had second order effects. The effect of flow rate (culture volume exchanges per day), with both nutrient concentration and daily nutrient loading held constant, and with both ammonium and nitrate as nitrogen sources, are shown in Fig. 4. Initially, growth in all the experiments was similar, presumably because all the algae were from the same stock with the same previous nutritional history. A period of l-2 weeks is needed for plants to respond to new external nutrient conditions, reportedly because of their ability to store inorganic nutrients when they are available (Chapman and Craigie, 1977). By the third week differences between the different experimental treatments were apparent. The results of mean daily yields during the fourth week are shown in Fig. 4. The experiment was then terminated. It is clear from Fig. 4 that, of the variables tested in that experiment, flow
191
rate per se was the most important, with growth increasing markedly as the flow increased independently of nutrient (N,P) concentration or daily loading. At the lowest ammonia loading (50 ymole/l at one 50-l exchange per day or 35 mg N/day), growth did appear to have been nutrient-limited (i.e., compared to that at the same turnover rate of one volume per day at the constant N-loading of 210 mg N/day). And, for reasons that are not clear, growth in the nitrate-enriched media was about twice as great at exchanges of 7.5,15 and 30 volumes/day, when the nutrient concentrations were held constant at 50 pmole N and 5 pmole P/day, than when the daily loading was held constant and the nutrient, concentrations decreased with increasing flow rate. The latter was probably an experimental artifact since, at 7.5 exchanges/day, both cultures were receiving very nearly the same concentrations and daily loading of nutrients (40 and 4 vs. 50 and 5 pmole N and P/l, respectively). The experiment indicated that increased flow rate in every case increased seaweed production. The explanation may not be related to flow rate per se. All the cultures were mixed and agitated thoroughly by strong aeration to maintain them in suspension. This procedure is more effective in breaking down diffusion gradients for nutrients and gases, etc., than increased flow through the cultures. As the sea water was enriched only with N and P, it is possible that other nutrients were present in limiting concentrations in the sea water when it passed slowly through the cultures. Alternatively, the enhanced growth at high flow rates could have resulted from removal of toxic or growth-inhibiting metabolites excreted by the seaweeds. From the experiments the best growth of G. tikuahiae was obtained at a culture density of about 2 kg wet wt/m2, with a rapid exchange of enriched seawater medium, and at relatively low concentrations of nitrogen and phosphorus. Nitrate and ammonia were equally effective as nitrogen sources. Under these conditions, the annual growth of G. tikuahiue was monitored continuously from 20 July 1976 to 28 June 1977 in the same facilities, maintaining a culture of 2 kg/m2 (wet wt) by weighing the seaweed at approximately l-week intervals and removing the incremental growth. The flow rate was 22 culture volume exchanges per day of sea water enriched with ca. 15 pmole/ 1 N03=N and 4 lmole/l PO, 3+-P. Yield of the seaweed in g dry wt./m2 -day is shown in Fig. 5, together with incident solar radiation, mean noon seawater temperature and salinity of the culture. The continuous growth record shown in Fig. 5 implies that the same culture was maintained throughout the year. This was not the case. The cultures became overgrown with epiphytes during the experiments and were removed manually prior to each weighing. Eventually, epiphytes became so severe that entire cultures had to be discarded on several occasions and replaced with newly-collected material. Epiphytization, most commonly by Enteromorpha spp. in summer and Giffordia mitchellae in winter, is the single greatest problem and constraint to large-scale seaweed culture yet encountered. If allowed to develop unchecked, the epiphytes gradually smother and kill the host species and then themselves die. To determine the growth potential of G. tikvahiue
192
32 -
SALINITY
150-
-
SOLAR
RADIATION
6
~b
36-
$
24-
a i2i
b 9
, J
I 1 ASONDJ 1976
Fig. 5. Yield of Gracilaria 1976-1977.
,
I
,
fime tihuahiae,
I
GRACILARIA I I I FM AM 1977
I
YIELD , J
solar radiation, temperature, and salinity: July-June,
as represented in Fig. 5, it was therefore essential to use only clean, epiphytefree plants and to discard them when they became infected. A close correlation between Gracilaria yield and solar radiation between July and December is apparent. However, the seasonal increase in radiation early in the year was not immediately accompanied by an increase in algal yield, presumably due to the abnormally low (< 12°C) water temperature that occurred in January 1977. As both light and temperature increased in February, the algae responded with renewed acceleration of growth. The efficiency of utilization of solar energy by the alga, determined on a mean weekly basis, ranged from 0.5 to 3.5% of total incident radiation. The heat of combustion of G. tikuahiae was 4.5 calories/g ash-free dry weight, as determined with a Parr microbomb calorimeter. Mean efficiency for the entire year was approximately 2% of total solar radiation (about 4% of visible radiation). The time-weighted mean annual production of G. tikuahiae was 34.8 g (dry wt)/m*-day, a value equivalent to 12.7 t/ha*yr. However, the total dry weight of G. tikuahiae includes about 50% ash or salt (Ryther et al., 1977), so the total organic yield is only some 63.5 t/ha+yr. That is a rate of production which exceeds that of almost every known agricultural crop, including even the highly-productive C4 plants such as corn and sugar cane (Cooper, 1975).
193
However, the small-scale experiments reported here show only the potential productivity of the seaweed, and give no information concerning the kinds of yields that might be expected from any large-scale commercial operation. ACKNOWLEDGEMENTS
This research was supported by Contract EY-76-S-02-2948 A01 from the U.S. Department of Energy, the NOAA Office of Sea Grant, Department of Commerce Grant No, 04-6-158-44016, and the Jessie Smith Noyes Foundation, Inc.
REFERENCES Chapman, A.R. and Craigie, J.S., 1977. Seasonal growth in Laminaria longicruris: with dissolved inorganic nutrients and interval reserves of nitrogen. Mar. Biol.,
relations 40:
197-205. Cooper, J.P. (Editor), 1975. Photosynthesis and Productivity in Different Environments. Cambridge University Press, London, New York, N.Y., 715 pp. Lapointe, B.E., Williams, L.D., Goldman, J.C. and Ryther, J.H., 1976. The mass outdoor culture of macroscopic marine algae. Aquaculture, 8: 9-20. Murphy, J. and Riley, J.P., 1962. A modified single solution method for the determination of phosphate in natural waters. Anal. Chim. Acta, 26: 31-36. Ryther, J.H., Lapointe, B.E., Stenberg, R.W. and Williams, L.D., 1977. Cultivation of seaweeds as a biomass source for energy. In: J.T. Pfeffer and J.J. Stukel (Editors), Proc. Fuels from Biomass Symp. University of Illinois, Urbana, April, 1977, pp. 83-98. Solorzano, L., 1969. Determination of ammonia in natural waters by the phenolhypochlorite method. Limnol. Oceanogr., 14: 799-801. Wood, E.D., Armstrong, F.A.J. and Richards, F.A., 1967. Determination of nitrate and seawater by cadmium-copper reduction to nitrite. J. Mar. Biol. Assoc. U.K., 47: 23-31.
185-193 Publishing Company, Amsterdam - Printed in The Netherlands
SOME ASPECTS OF THE GROWTH TIKVAHIAE IN CULTURE
BRIAN E. LAPOINTE*
185
AND YIELD OF GRACILARIA
and JOHN H. RYTHER
Woods Hole Oceanographic
Institution,
Woods Hole, Mass. 02543
(U.S.A.)
*Present address: University of Florida, Department of Environmental Science, Gainesville, Fla. 32601 (U.S.A.) Contribution No. 4143 from the Woods Hole Oceanographic Institution, and Contribution No. 94 from the Harbor Branch Foundation (Received 15 May 1978; revised 24 July 1978) ABSTRACT Lapointe, B.E. and Ryther, J.H., 1978. Some aspects of the growth and yield of Grucilariu tihuakiae in culture. Aquaculture, 15: 185-193. A series of outdoor, continuous-flow seawater cultures (50 1; 0.23 m’) were used to investigate the effects of culture density (kg/m2), nutrient loading (total nitrogen input/day) with both NH,‘-N and NO,--N, and turnover rate (flow rate/culture volume) on the growth and yield of Gracilariu tikuakiae. Although specific growth rates as high as 60% per day were recorded for Gracilaria at low densities (0.4 kg wet wt/ml) in summer conditions, maximum year-round yields were obtained at densities of 2.0-3.0 kg wet wt/m*. Above a minimal daily nitrogen loading, the yield of Gracilaria was independent of (1) nutrient concentration, (2) nitrogen loading, or (3) whether nitrogen was in the form of NH,‘-N or NO,--N, but was (4) highly dependent upon flow rate. The time weighted mean annual production during 1976- 1977 was 34.8 g dry wt/m’l.day or 127 t/ha*yr based on la-months continuous operation at near optimal densities and flow rates in the non-nutrient limited culture system.
INTRODUCTION
Preliminary experiments on the growth and yield of vegetative clones of the macroscopic red algae Gradaria sp. and Hypnea musciformis (grown unattached in suspended culture) were described by Lapointe et al. (1976). The former has recently been assigned the species name Gracilariu tikuahiae (J.L. McLachlan, personal communication, 1978). Subsequent research involved the screening of some 42 species of seaweeds (including representation of the red, green, and brown algae) for their growth potential in suspended, outdoor, flow-through cultures. All were collected from the Indian River or the open Atlantic Ocean coastline of Central Florida. G. iikuahiue proved most successful in year-round viability and persistence in the vegetative (non-fruiting) condition, and the magnitude of its growth rate and dry weight yield. Subsequently, a series of experiments was designed to determine optimal conditions for its growth. These included investigation of the effects of nitrogen species, concentration, and daily loading, flow rate of
186
media through the culture, and density of the culture. Based on the results of those experiments, the alga was cultured for a year under the best growing conditions that could be achieved to determine its annual yield. Temperature, salinity and solar radiation, all recognized as important environmental factors affecting growth, were not regulated, partly because controls were difficult in the experimental system, and uneconomical or impractical in any largescale seaweed culture system. These environmental variables were, however, monitored so that annual growth could be correlated with the annual changes. MATERIALS
AND METHODS
The culture system designed for the screening program was used for the experiments with G. tikvahiae. The system consisted of four 6-m long 0.4-m diameter PVC pipes that were sectioned longitudinally and divided into 0.75-m (50-l) compartments by plywood partitions. Each section was provided with a calibrated flow of enriched sea water by a manifold fed from a headbox, and fitted with a non-clogging overflow drain. Compressed air was fed into the bottom of each compartment through holes drilled along the bottom of the pipe through an air line (a sectioned 2-in (ca. 5-cm) PVC pipe) cemented to the outside of the main pipe. Thirty-two individual growth assay chambers were fabricated (Figs 1 and 2). The growth chambers were located outdoors. Sea water was taken from a channel which connects to the Indian River, a shallow lagoon of the Atlantic Ocean. No attempt was made to control water temperature, which ranged from 12-34”C, or salinity, which ranged from 20-34°/oo. Sea water was pumped into a large reservoir tank. Before use, the stored sea water was enriched with the desired concentrations of nitrogen and phosphorus, provided as sodium nitrate and monosodium (dibasic) phosphate at a ratio of 10: 1 by atoms of N:P. A second reservoir contained unenriched sea water. Both enriched and unenriched seawater supplies were pumped to headboxes from which they could be distributed to the experimental chambers at the desired rates of exchanges and nutrient concentrations. Weighed amounts of Grucilaria tikvahiae were stocked in the experimental chambers. At intervals of 5-10 days, depending upon growth rate, the algae were removed from the chamber, shaken vigorously to remove water, and weighed, The relationship between drained wet weight and dry weight was determined on replicate samples by oven drying at 90°C for 48 h. Water temperature, measured with a mercury thermometer, and salinity, measured with a Beckman induction-type salinometer, were taken each day at noon, at which time water samples were collected for analysis of PO4 3+-P (Murphy and Riley, 1962), NH4+-N (Solorzano, 1969) and N03--N (Wood et al., 1967). Incident solar radiation was monitored continuously with an Epply pyrheliometer mounted at the experimental site. Experiments were carried out to determine the optimum density to maintain G. tikuahiae in the experimental system to obtain maximum yields.
187
A
SEAWEED
Elevated
tteadboxes r_---_______’ ,__----_--
r----------
IT
CULTURE
SYSTEM
c-______-__--_---___________ ~________________________~ $_----_---__--___-_,
TO SCALE
B SEAWEED
medium
CULTURE
CHAMBER
se0w0te1 .._
Fig. 1. Diagrammatic representation of seaweed culture system.
,
188
Initial densities from 0.4 to 4.8 kg (wet wt) per square meter of culture vessel surface area were used, and the cultures harvested back to those starting densities at l-week intervals for a period of 2 months. The optimum density for maximum yield calculated in this way was then used in all subsequent experiments. It is difficult to separate experimentally the effects of nutrient supply and flow rate of water or medium through a seaweed culture. If nutrient concentration is held constant, the rate of input of nutrients to the culture will vary proportionately with the flow rate. If, on the other hand, nutrients are added separately at a constant input rate while seawater flows are independently varied, the instantaneous concentration of nutrients entering the culture will be inversely proportional to seawater flow rate. Consequently, both types of studies were conducted. In both series of experiments, the sea water was passed through the cultures at flow rates of 1, 7.5, 3 5, and 30 culture volumes per day, while simultaneously comparing growth with both NHI, ‘-N and NO3 - -N enriched sea water. In the first experiment, nutrients were added separately at a constant rate to each culture so that daily nutrient loading remained constant, but concentrations varied inversely with the flow rates of the diluting sea water. In the second experiment, the nutrients were mixed initially with the sea water so that all of the cultures received the same initial concentration, but the daily loading was directly proportional to the flow rate. The nutrients consisted of phosphorus as monosodium dibasic phosphate (NaH,P04*H,) and nitrogen as sodium nitrate (NaN03) or ammonium chloride (NH&I). In the first experiment, the input concentrations of N and P ranged from 300 and 30 pmole/l (at 1 exchange/day) to 10 and 1 pmole/l (at 30 exchanges/day), respectively. In the second experiment, the constant concentrations of N and P were 50 and 5 kmole/l, respectively. In both sets of experiments, Grucilaria was stocked at an initial density of 2 kg wet wt/m2 and weighed and harvested back to that starting density at l-week intervals over an experimental period of one month. Under the established conditions of optimum density, flow rate, nutrient concentration and form of nitrogen for best yield of G. tikvahiue, the alga was grown for a period of 1 year, and growth measured at l-week intervals between 20 July 1976 and 28 June 1977. RESULTS
AND DISCUSSION
The mean specific growth rates (% increase/day) and yields (g dry wt/ m2*day) of G. tikvahiae as a function of density are shown in Fig. 3 for both summer (August) and winter (January) conditions. At both times of year, growth rate decreased with increasing biomass, though both the absolute values and the range were much greater in summer than in winter. Yields (g dry wt/m2 - day) at both seasons peaked at densities of 2.0-3.0 kg wet wt/m2, but with a broad plateau at both seasons and some indication of a higher optimal density in winter than in summer. As expected, yields were
189
Fig. 2. Seaweed culture tanks showing headbox for medium supply.
190
60
A,
NH i-N
45r
i 75 flow
30 rale
/culture
vofumes/doyl
Fig. 3. Effect of culture density on specific growth rate and yield of Gracilaria tikuahiae in summer (A) and winter (0). Fig. 4. Effect of turnover rate (culture volume exchanges/day) on yield of Gracilaria tikuahiae with both NH, ‘-N and NO, -N medium. A- - - -A Constant N and P loading, variable concentrations. a---• Constant N and P concentration, variable daily loading.
higher in summer than in winter. The difference is due primarily to solar radiation but temperature may have had second order effects. The effect of flow rate (culture volume exchanges per day), with both nutrient concentration and daily nutrient loading held constant, and with both ammonium and nitrate as nitrogen sources, are shown in Fig. 4. Initially, growth in all the experiments was similar, presumably because all the algae were from the same stock with the same previous nutritional history. A period of l-2 weeks is needed for plants to respond to new external nutrient conditions, reportedly because of their ability to store inorganic nutrients when they are available (Chapman and Craigie, 1977). By the third week differences between the different experimental treatments were apparent. The results of mean daily yields during the fourth week are shown in Fig. 4. The experiment was then terminated. It is clear from Fig. 4 that, of the variables tested in that experiment, flow
191
rate per se was the most important, with growth increasing markedly as the flow increased independently of nutrient (N,P) concentration or daily loading. At the lowest ammonia loading (50 ymole/l at one 50-l exchange per day or 35 mg N/day), growth did appear to have been nutrient-limited (i.e., compared to that at the same turnover rate of one volume per day at the constant N-loading of 210 mg N/day). And, for reasons that are not clear, growth in the nitrate-enriched media was about twice as great at exchanges of 7.5,15 and 30 volumes/day, when the nutrient concentrations were held constant at 50 pmole N and 5 pmole P/day, than when the daily loading was held constant and the nutrient, concentrations decreased with increasing flow rate. The latter was probably an experimental artifact since, at 7.5 exchanges/day, both cultures were receiving very nearly the same concentrations and daily loading of nutrients (40 and 4 vs. 50 and 5 pmole N and P/l, respectively). The experiment indicated that increased flow rate in every case increased seaweed production. The explanation may not be related to flow rate per se. All the cultures were mixed and agitated thoroughly by strong aeration to maintain them in suspension. This procedure is more effective in breaking down diffusion gradients for nutrients and gases, etc., than increased flow through the cultures. As the sea water was enriched only with N and P, it is possible that other nutrients were present in limiting concentrations in the sea water when it passed slowly through the cultures. Alternatively, the enhanced growth at high flow rates could have resulted from removal of toxic or growth-inhibiting metabolites excreted by the seaweeds. From the experiments the best growth of G. tikuahiae was obtained at a culture density of about 2 kg wet wt/m2, with a rapid exchange of enriched seawater medium, and at relatively low concentrations of nitrogen and phosphorus. Nitrate and ammonia were equally effective as nitrogen sources. Under these conditions, the annual growth of G. tikuahiue was monitored continuously from 20 July 1976 to 28 June 1977 in the same facilities, maintaining a culture of 2 kg/m2 (wet wt) by weighing the seaweed at approximately l-week intervals and removing the incremental growth. The flow rate was 22 culture volume exchanges per day of sea water enriched with ca. 15 pmole/ 1 N03=N and 4 lmole/l PO, 3+-P. Yield of the seaweed in g dry wt./m2 -day is shown in Fig. 5, together with incident solar radiation, mean noon seawater temperature and salinity of the culture. The continuous growth record shown in Fig. 5 implies that the same culture was maintained throughout the year. This was not the case. The cultures became overgrown with epiphytes during the experiments and were removed manually prior to each weighing. Eventually, epiphytes became so severe that entire cultures had to be discarded on several occasions and replaced with newly-collected material. Epiphytization, most commonly by Enteromorpha spp. in summer and Giffordia mitchellae in winter, is the single greatest problem and constraint to large-scale seaweed culture yet encountered. If allowed to develop unchecked, the epiphytes gradually smother and kill the host species and then themselves die. To determine the growth potential of G. tikvahiue
192
32 -
SALINITY
150-
-
SOLAR
RADIATION
6
~b
36-
$
24-
a i2i
b 9
, J
I 1 ASONDJ 1976
Fig. 5. Yield of Gracilaria 1976-1977.
,
I
,
fime tihuahiae,
I
GRACILARIA I I I FM AM 1977
I
YIELD , J
solar radiation, temperature, and salinity: July-June,
as represented in Fig. 5, it was therefore essential to use only clean, epiphytefree plants and to discard them when they became infected. A close correlation between Gracilaria yield and solar radiation between July and December is apparent. However, the seasonal increase in radiation early in the year was not immediately accompanied by an increase in algal yield, presumably due to the abnormally low (< 12°C) water temperature that occurred in January 1977. As both light and temperature increased in February, the algae responded with renewed acceleration of growth. The efficiency of utilization of solar energy by the alga, determined on a mean weekly basis, ranged from 0.5 to 3.5% of total incident radiation. The heat of combustion of G. tikuahiae was 4.5 calories/g ash-free dry weight, as determined with a Parr microbomb calorimeter. Mean efficiency for the entire year was approximately 2% of total solar radiation (about 4% of visible radiation). The time-weighted mean annual production of G. tikuahiae was 34.8 g (dry wt)/m*-day, a value equivalent to 12.7 t/ha*yr. However, the total dry weight of G. tikuahiae includes about 50% ash or salt (Ryther et al., 1977), so the total organic yield is only some 63.5 t/ha+yr. That is a rate of production which exceeds that of almost every known agricultural crop, including even the highly-productive C4 plants such as corn and sugar cane (Cooper, 1975).
193
However, the small-scale experiments reported here show only the potential productivity of the seaweed, and give no information concerning the kinds of yields that might be expected from any large-scale commercial operation. ACKNOWLEDGEMENTS
This research was supported by Contract EY-76-S-02-2948 A01 from the U.S. Department of Energy, the NOAA Office of Sea Grant, Department of Commerce Grant No, 04-6-158-44016, and the Jessie Smith Noyes Foundation, Inc.
REFERENCES Chapman, A.R. and Craigie, J.S., 1977. Seasonal growth in Laminaria longicruris: with dissolved inorganic nutrients and interval reserves of nitrogen. Mar. Biol.,
relations 40:
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