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Effects of light and pressure on photosynthesis in two seagrasses
Aquatic Botany, 13 (1982) 331--337 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
331
EFFECTS OF LIGHT AND PRESSURE ON PHOTOSYNTHESIS IN TWO SEAGRASSES
SVEN BEER and YOAV WAISEL
Department of Botany, The George S. Wise Faculty of Life Sciences, Tel Aviv University, Ramat Aviv (Israel) (Accepted for publication 3 December 1981)
ABSTRACT Beer, S. and Waisel, Y., 1982. Effects of light and pressure on photosynthesis in two seagrasses. Aquat. Bot., 13: 331--337. Photosynthetic responses to light and pressure (up to 4 atm) were measured for two seagrass species abundant in the Gulf of Eilat (Red Sea). In Halodule uninervis (Forssk.) Aschers. pressure decreased net photosynthetic rates, while in Halophila stipulacea (Forssk.) Aschers. pressure had no effect on net photosynthetic rates. In both species, light saturation was reached at 300 uE (400--700 nm) m -2 s -1 and the compensation point was at 20--40 , E (400--700 nm) m -2 s - I . Comparing these results to in situ light measurements, neither species should be light limited to a depth of about 15 m, and Halophila stipulacea should reach compensation light intensities at about 50 m. The latter depth corresponds well to the natural depth penetration of this species. Halodule uninervis is never found deeper than 5 m in the Gulf of Eilat, and it appears that pressure rather than light is one of the factors limiting the depth penetration of this species. The differential pressure response of the t w o species may be related to aspects of leaf morphology and gas diffusion.
INTRODUC~ON
Growth of freshwater angiosperms is restricted to only the upper few metres of the littoral zone. This is also true in very clear lakes where the light compensation point for both photosynthesis and growth should occur at a much greater depth than the observed occurrence of the plants. In such lakes the limiting factor for growth may be hydrostatic pressure rather than light {Hutchinson, 1975). Unlike these freshwater plants, m a n y marine angiosperms, or seagrasses, grow at depths exceeding 10 m; some are even f o u n d below 50 m. These seagrasses are apparently less affected by pressure than are freshwater angiosperms. In the southern part of the Gulf of Eilat (Red Sea), seven of the world's 50 seagrass species are distributed along a depth gradient ranging from the intertidal zone down to about 60 m (Lipkin, 1977). Halodule uninervis
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© 1982
Elsevier Scientific Publishing Company
332
(Forssk.) Aschers. is one of the species most abundant in the intertidal zone. It also grows somewhat deeper, b u t has never been observed below 5 m; Halophila stipulacea (Forssk.) Aschers. has a much wider distribution range. It grows f r o m the intertidal zone down to a depth of at least 50 m. The leaf size and chlorophyll concentration increases with depth in both species (Lipkin and Beer, field observations, 1981). These t w o seagrass species were investigated for photosynthetic responses to light and pressure in order to evaluate the possible contribution of these two factors to the plants' depth distribution pattern. METHODS
Mid-day light intensities (400--700 nm) at the growth sites were measured by lowering a light sensor (Lambda LiCor 185A, Lambda hst. Corp., Lincoln, NE) down to a depth of 30 m (limited by cable length) at 1 m intervals. Plants of Halodule uninervis and Halophila stipulacea were collected from 0.5 and 10 m depth, respectively, transported to the laboratory and maintained in natural seawater in a growth room at 20°C and at a light intensity of 200 pE (400--700 nm) m -2 s -1 provided by fluorescent lamps. Experiments were carried o u t within one week of collection. Net photosynthetic rates of leaves as a response to light intensity were measured by O2 evolution in a closed system as described by Beer et al. (1977). The experimental chamber was kept at 20°C, and the medium was stirred throughout the experiments. Light intensity was varied by placing neutral density filters between the plants and the light source. Changes in rates of 02 evolution were obtained within two minutes, following light in-
Fig. 1. Experimental apparatus used to measure photosynthesis under various pressure regimes. L, incandescent light source; MS, magnetic stirrer; M, magnetic stir-bar; S, rubber seal; G, pressure gauges; V, pressure release valve; N2, pressurized N 2 gas cylinder; W, narrow needle.
333 tensity alteration, and the linear rates obtained between 50--90% air saturated O2 concentrations were used for calculating photosynthetic rates. Previous experiments had confirmed that net photosynthesis in both species was n o t limited by O2 concentrations of up to 95% air saturation (ca. 230 pM at 20°C and 34%0 salinity). Chlorophyll was extracted by grinding leaves in 1% Triton-X, and adding acetone to a final concentration of 80%. Chlorophyll a and b concentrations were determined spectrophotometrically according to Arnon (1949). Because the O2 electrodes used in this work were pressure sensitive, the Winkler m e t h o d was used to determine concentrations of O2 in all pressure experiments. The device constructed for these rpeasurements (Fig. 1) consisted of a glass vial (50 ml) connected to a N~ pressure tank. Pressure was applied (up to 4 atm) to the contents of the vial through a narrow needle (W) w i t h o u t introducing an air phase. No N2 reached the vial by diffusion. Prior to each experiment, the vial was bubbled with N~ in order to reduce O2 concentration to a b o u t 50% of air saturation. The plant leaves were then inserted and allowed to photosynthesize for 1--1.5 h. Final O2 concentrations were not above 90% of air saturation. The experimental medium was stirred throughout the experiments. To terminate an experiment, pressure was released through a valve (V), and the O2-fixing reagents were injected through a rubber seal in the lid (S). The Winkler procedures for the determination of O2 were performed according to Drew and Robertson (1974). All pressures, as well as light experiments, were carried o u t in natural seawater (2.5 mM soluble inorganic carbon) at 20°C and at a light intensity of 500 ~E (400--700 nm) m -2 s -~ . RESULTS AND DISCUSSION Figure 2 shows the light intensities measured at the growth site of the plants. Figure 3 shows the photosynthetic responses of Halophila stipulacea and Halodule uninervis to various light intensities. The saturating light intensity for photosynthesis was a b o u t 300 gE (400--700 nm) m -2 s -1 for both species, and photosynthetic light compensation occurred at 20--40 pE (400--700 nm) m -~ s -1 . Comparing these data with light intensity values along the depth gradient where these t w o species grow, it is clear that the net photosynthesis of these plants should n o t be light limited from 0--15 m, and the compensation depth for net photosynthesis should occur at a b o u t 50 m. These correlations were made only considering mid-day light intensities at corresponding depths. Therefore, on a diurnal basis, or for growth, these critical depth values must occur s o m e w h a t higher up in the water column. Table I shows the effect of pressure on photosynthesis at light saturation. Whilst no effect could be observed in Halophila stipulacea, increased pressure substantially reduced n e t photosynthesis in Halodule uninervis. The
334 Light Intensity (~E m -2 s-1) 5OO IOO0 i
1500
l
i
i u///io.°,°°"°°"°"'°°" ,5o ... 2oo
o. /ii
.~ 15
3.5
..""
20
25
30
45,
/
:t"
Fig. 2. Light intensities ( 4 0 0 - - 7 0 0 nm) o f the depth gradient in the R e d Sea measured at mid;day in May with n o overcast. Full line, measured values; b r o k e n line, extrapolated values.
0.5
-=-_0.4
.~ ~ 0.3 l
|
100
200 300 Light Intensity (~E m-2 s -1 )
I
I
I
I
400
500
600
Fig. 3. Rates o f net p h o t o s y n t h e s i s as a f u n c t i o n of light intensity. U p p e r curve, Halophila stipulacea. L o w e r curve, Halodule uninervis. Data are average values of five experiments, s.d. was leas than 12% of the means.
335 TABLE I Photosynthetic rates at light saturation at various pressures. (Data represent average values of four experiments, s.d. was less than 7% of the means)
Pressure (atm)
1 2 3 4
Rates of photosynthesis (umoles 0 2 min -~ mg -~ chlorophyll)
Halophila stipulacea
Halodule uninervis
0.41 0.42 0.40 0.41
0.35 0.30 0.23 0.12
negative effect of pressure on the latter species was linear between 1 and 3 atm and more pronounced at 4 atm. Some caution is advised when comparing the results of these pressure experiments with prevailing in situ conditions. Firstly, experiments were only of 1--1.5 h duration. Although linear rates of 02 production were obtained in our experimental setting, this short time period does n o t allow for possible morphological adaptations of gas lacunae (see below). Secondly, as in the light experiments, net photosynthetic rates of the leaves are only indications of net diurnal carbon budgets of wholeplant growth. However, w i t h o u t fully quantifying the results, the combined effects of light and pressure indicate the following: (1) Pressure corresponding to considerable depth has no significant effect on photosynthesis and, ultimately, growth of Halophila stipulacea; (2) light is probably the main factor limiting the depth penetration (ca. 50 m) of Halophila stipulacea; (3) light does not limit photosynthesis of either species until a depth of a b o u t 15 m; (4) pressure rather than light seems to limit net photosynthesis of Halodule uninervis in shallow water and may be a factor contributing to the restriction of this species to the shallow zone of the depth gradient. The reason for the differential pressure responses of these t w o seagrasses cannot y e t be assessed. It is known that photosynthesis of algae and other non-lacunated plants is little affected by even high hydrostatic pressures (Vidaver, 1969; Pope and Berger, 1973; Schreiber and Vidaver, 1973). Thus, it seems unlikely that the relatively low pressures (10--20 atm) which correspond to the maximal depths at which those plants grow should affect submerged plants on the biochemical level. On the other hand, gaseous systems such as the lacunar system of submerged angiosperms are far more susceptible to pressures in this range. A major difference between the two species investigated here is in the quantity and distribution of gas lacunae. While Halodule uninervis has many lacunar spaces distributed throughout the leaf, Halophila stipulacea has fewer gas lacunae which are concentrated mainly around the main vascular bundle, and the latter species has large peripheral leaf areas consisting of only t w o chloroplast-rich epidermal cell layers uninterrupted by lacunae.
336
Normally, gas exchange in Halodule uninervis is, at least potentially, partitioned between photosynthetic tissues and surrounding water as well as gas lacunae. Assuming that the lacunar system generally is of importance for photosynthetic gas exchange in aquatic macrophytes (Payne, 1980, personal communication), the pressure-induced decrease in photosynthetic performance of this species could be explained by a successive collapsing and/or flooding of lacunae until a condition where total net gas exchange of the photosynthetic tissue would not be sufficient to support high rates of photosynthesis. This argument may be supported by the observation that the 'deep' (5 m) growing plants ofHalodule uninervis have smaller gas lacunae relative to photosynthetic tissues than plants growing in shallower water. It is possible that the depth penetration of this species is restricted to the depth at which plants can develop gas lacunae of a minimum volume to support efficient gas exchange. The limited depth distribution of Halodule uninervis could be caused by other environmental factors as well. Nevertheless, it was one of the aims of the present article to show that hydrostatic pressure may be an important parameter to consider when evaluating the ecology of lacunated water plants. In the thin-leaved Halophila stipulacea, large portions of the leaves exchange gases only between the photosynthetic epidermis and surrounding water, and increasing pressure would not affect gas exchange significantly. Since it is apparent that such gas exchange is sufficient to support photosynthesis, gas exchange with the surrounding water must be more efficient in this species than it is in Halodule uninervis. The high rates of HCO~ uptake found in Halophila stipulacea (Beer et al., 1977) may contribute to such an efficiency. Ultimately, Halophila stipulacea may penetrate to greater depths through an ability to maintain efficient photosynthesis without involving lacunae to the same extent as does Halodule uninervis. However, for both species a certain minimal volume of lacunae may be of importance for other functions as well, such as supplying photosynthetic oxygen to the root systems of these plants.
ACKNOWLEDGEMENT
The authors thank Dr. F. Payne and Dr. R.G. Wetzel for discussions and comments which were of great help in writing this article.
REFERENCES Arnon, D.I., 1949. Copper enzymes in isolated chloroplasts. Plant Physiol., 24: 1--5. Beer, S., Eshel, A. and Waisel, Y., 1977. Carbon metabolism in seagrasses. J. Exp. Bot., 28: 1180--1189. Drew, E.A. and Robertson, W.A.A., 1974. A simple field version of the Winkler determination of dissolved oxygen. New Phytol., 73: 793--796.
337 Hutchinson, G.E., 1975. A Treatise on Limnology. John Wiley, New York, 660 pp. Lipkin, Y., 1977. Seagrass vegetation of Sinai and Israel. In: C.P. McRoy and C. Helfferich (Editors), Seagrass Ecosystems. Dekker, New York, pp. 263--293. Pope, D.H. and Berger, L.R., 1973. Algal photosynthesis at increased hydrostatic pressure and constant pCO 2. Arch. Mierobiol., 89: 321--325. Schreiber, U. and Vidaver, W., 1973. Hydrostatic pressure: A reversible inhibitor of primary photosynthetic processes. Z. Naturforsch., 28: 704--709. Vidaver, W., 1969. Hydrostatic pressure effects on photosynthesis. Int. Revue Gesamten Hydrobiol., 51: 697--747.
331
EFFECTS OF LIGHT AND PRESSURE ON PHOTOSYNTHESIS IN TWO SEAGRASSES
SVEN BEER and YOAV WAISEL
Department of Botany, The George S. Wise Faculty of Life Sciences, Tel Aviv University, Ramat Aviv (Israel) (Accepted for publication 3 December 1981)
ABSTRACT Beer, S. and Waisel, Y., 1982. Effects of light and pressure on photosynthesis in two seagrasses. Aquat. Bot., 13: 331--337. Photosynthetic responses to light and pressure (up to 4 atm) were measured for two seagrass species abundant in the Gulf of Eilat (Red Sea). In Halodule uninervis (Forssk.) Aschers. pressure decreased net photosynthetic rates, while in Halophila stipulacea (Forssk.) Aschers. pressure had no effect on net photosynthetic rates. In both species, light saturation was reached at 300 uE (400--700 nm) m -2 s -1 and the compensation point was at 20--40 , E (400--700 nm) m -2 s - I . Comparing these results to in situ light measurements, neither species should be light limited to a depth of about 15 m, and Halophila stipulacea should reach compensation light intensities at about 50 m. The latter depth corresponds well to the natural depth penetration of this species. Halodule uninervis is never found deeper than 5 m in the Gulf of Eilat, and it appears that pressure rather than light is one of the factors limiting the depth penetration of this species. The differential pressure response of the t w o species may be related to aspects of leaf morphology and gas diffusion.
INTRODUC~ON
Growth of freshwater angiosperms is restricted to only the upper few metres of the littoral zone. This is also true in very clear lakes where the light compensation point for both photosynthesis and growth should occur at a much greater depth than the observed occurrence of the plants. In such lakes the limiting factor for growth may be hydrostatic pressure rather than light {Hutchinson, 1975). Unlike these freshwater plants, m a n y marine angiosperms, or seagrasses, grow at depths exceeding 10 m; some are even f o u n d below 50 m. These seagrasses are apparently less affected by pressure than are freshwater angiosperms. In the southern part of the Gulf of Eilat (Red Sea), seven of the world's 50 seagrass species are distributed along a depth gradient ranging from the intertidal zone down to about 60 m (Lipkin, 1977). Halodule uninervis
0304-3770/82/0000--0000/$02.75
© 1982
Elsevier Scientific Publishing Company
332
(Forssk.) Aschers. is one of the species most abundant in the intertidal zone. It also grows somewhat deeper, b u t has never been observed below 5 m; Halophila stipulacea (Forssk.) Aschers. has a much wider distribution range. It grows f r o m the intertidal zone down to a depth of at least 50 m. The leaf size and chlorophyll concentration increases with depth in both species (Lipkin and Beer, field observations, 1981). These t w o seagrass species were investigated for photosynthetic responses to light and pressure in order to evaluate the possible contribution of these two factors to the plants' depth distribution pattern. METHODS
Mid-day light intensities (400--700 nm) at the growth sites were measured by lowering a light sensor (Lambda LiCor 185A, Lambda hst. Corp., Lincoln, NE) down to a depth of 30 m (limited by cable length) at 1 m intervals. Plants of Halodule uninervis and Halophila stipulacea were collected from 0.5 and 10 m depth, respectively, transported to the laboratory and maintained in natural seawater in a growth room at 20°C and at a light intensity of 200 pE (400--700 nm) m -2 s -1 provided by fluorescent lamps. Experiments were carried o u t within one week of collection. Net photosynthetic rates of leaves as a response to light intensity were measured by O2 evolution in a closed system as described by Beer et al. (1977). The experimental chamber was kept at 20°C, and the medium was stirred throughout the experiments. Light intensity was varied by placing neutral density filters between the plants and the light source. Changes in rates of 02 evolution were obtained within two minutes, following light in-
Fig. 1. Experimental apparatus used to measure photosynthesis under various pressure regimes. L, incandescent light source; MS, magnetic stirrer; M, magnetic stir-bar; S, rubber seal; G, pressure gauges; V, pressure release valve; N2, pressurized N 2 gas cylinder; W, narrow needle.
333 tensity alteration, and the linear rates obtained between 50--90% air saturated O2 concentrations were used for calculating photosynthetic rates. Previous experiments had confirmed that net photosynthesis in both species was n o t limited by O2 concentrations of up to 95% air saturation (ca. 230 pM at 20°C and 34%0 salinity). Chlorophyll was extracted by grinding leaves in 1% Triton-X, and adding acetone to a final concentration of 80%. Chlorophyll a and b concentrations were determined spectrophotometrically according to Arnon (1949). Because the O2 electrodes used in this work were pressure sensitive, the Winkler m e t h o d was used to determine concentrations of O2 in all pressure experiments. The device constructed for these rpeasurements (Fig. 1) consisted of a glass vial (50 ml) connected to a N~ pressure tank. Pressure was applied (up to 4 atm) to the contents of the vial through a narrow needle (W) w i t h o u t introducing an air phase. No N2 reached the vial by diffusion. Prior to each experiment, the vial was bubbled with N~ in order to reduce O2 concentration to a b o u t 50% of air saturation. The plant leaves were then inserted and allowed to photosynthesize for 1--1.5 h. Final O2 concentrations were not above 90% of air saturation. The experimental medium was stirred throughout the experiments. To terminate an experiment, pressure was released through a valve (V), and the O2-fixing reagents were injected through a rubber seal in the lid (S). The Winkler procedures for the determination of O2 were performed according to Drew and Robertson (1974). All pressures, as well as light experiments, were carried o u t in natural seawater (2.5 mM soluble inorganic carbon) at 20°C and at a light intensity of 500 ~E (400--700 nm) m -2 s -~ . RESULTS AND DISCUSSION Figure 2 shows the light intensities measured at the growth site of the plants. Figure 3 shows the photosynthetic responses of Halophila stipulacea and Halodule uninervis to various light intensities. The saturating light intensity for photosynthesis was a b o u t 300 gE (400--700 nm) m -2 s -1 for both species, and photosynthetic light compensation occurred at 20--40 pE (400--700 nm) m -~ s -1 . Comparing these data with light intensity values along the depth gradient where these t w o species grow, it is clear that the net photosynthesis of these plants should n o t be light limited from 0--15 m, and the compensation depth for net photosynthesis should occur at a b o u t 50 m. These correlations were made only considering mid-day light intensities at corresponding depths. Therefore, on a diurnal basis, or for growth, these critical depth values must occur s o m e w h a t higher up in the water column. Table I shows the effect of pressure on photosynthesis at light saturation. Whilst no effect could be observed in Halophila stipulacea, increased pressure substantially reduced n e t photosynthesis in Halodule uninervis. The
334 Light Intensity (~E m -2 s-1) 5OO IOO0 i
1500
l
i
i u///io.°,°°"°°"°"'°°" ,5o ... 2oo
o. /ii
.~ 15
3.5
..""
20
25
30
45,
/
:t"
Fig. 2. Light intensities ( 4 0 0 - - 7 0 0 nm) o f the depth gradient in the R e d Sea measured at mid;day in May with n o overcast. Full line, measured values; b r o k e n line, extrapolated values.
0.5
-=-_0.4
.~ ~ 0.3 l
|
100
200 300 Light Intensity (~E m-2 s -1 )
I
I
I
I
400
500
600
Fig. 3. Rates o f net p h o t o s y n t h e s i s as a f u n c t i o n of light intensity. U p p e r curve, Halophila stipulacea. L o w e r curve, Halodule uninervis. Data are average values of five experiments, s.d. was leas than 12% of the means.
335 TABLE I Photosynthetic rates at light saturation at various pressures. (Data represent average values of four experiments, s.d. was less than 7% of the means)
Pressure (atm)
1 2 3 4
Rates of photosynthesis (umoles 0 2 min -~ mg -~ chlorophyll)
Halophila stipulacea
Halodule uninervis
0.41 0.42 0.40 0.41
0.35 0.30 0.23 0.12
negative effect of pressure on the latter species was linear between 1 and 3 atm and more pronounced at 4 atm. Some caution is advised when comparing the results of these pressure experiments with prevailing in situ conditions. Firstly, experiments were only of 1--1.5 h duration. Although linear rates of 02 production were obtained in our experimental setting, this short time period does n o t allow for possible morphological adaptations of gas lacunae (see below). Secondly, as in the light experiments, net photosynthetic rates of the leaves are only indications of net diurnal carbon budgets of wholeplant growth. However, w i t h o u t fully quantifying the results, the combined effects of light and pressure indicate the following: (1) Pressure corresponding to considerable depth has no significant effect on photosynthesis and, ultimately, growth of Halophila stipulacea; (2) light is probably the main factor limiting the depth penetration (ca. 50 m) of Halophila stipulacea; (3) light does not limit photosynthesis of either species until a depth of a b o u t 15 m; (4) pressure rather than light seems to limit net photosynthesis of Halodule uninervis in shallow water and may be a factor contributing to the restriction of this species to the shallow zone of the depth gradient. The reason for the differential pressure responses of these t w o seagrasses cannot y e t be assessed. It is known that photosynthesis of algae and other non-lacunated plants is little affected by even high hydrostatic pressures (Vidaver, 1969; Pope and Berger, 1973; Schreiber and Vidaver, 1973). Thus, it seems unlikely that the relatively low pressures (10--20 atm) which correspond to the maximal depths at which those plants grow should affect submerged plants on the biochemical level. On the other hand, gaseous systems such as the lacunar system of submerged angiosperms are far more susceptible to pressures in this range. A major difference between the two species investigated here is in the quantity and distribution of gas lacunae. While Halodule uninervis has many lacunar spaces distributed throughout the leaf, Halophila stipulacea has fewer gas lacunae which are concentrated mainly around the main vascular bundle, and the latter species has large peripheral leaf areas consisting of only t w o chloroplast-rich epidermal cell layers uninterrupted by lacunae.
336
Normally, gas exchange in Halodule uninervis is, at least potentially, partitioned between photosynthetic tissues and surrounding water as well as gas lacunae. Assuming that the lacunar system generally is of importance for photosynthetic gas exchange in aquatic macrophytes (Payne, 1980, personal communication), the pressure-induced decrease in photosynthetic performance of this species could be explained by a successive collapsing and/or flooding of lacunae until a condition where total net gas exchange of the photosynthetic tissue would not be sufficient to support high rates of photosynthesis. This argument may be supported by the observation that the 'deep' (5 m) growing plants ofHalodule uninervis have smaller gas lacunae relative to photosynthetic tissues than plants growing in shallower water. It is possible that the depth penetration of this species is restricted to the depth at which plants can develop gas lacunae of a minimum volume to support efficient gas exchange. The limited depth distribution of Halodule uninervis could be caused by other environmental factors as well. Nevertheless, it was one of the aims of the present article to show that hydrostatic pressure may be an important parameter to consider when evaluating the ecology of lacunated water plants. In the thin-leaved Halophila stipulacea, large portions of the leaves exchange gases only between the photosynthetic epidermis and surrounding water, and increasing pressure would not affect gas exchange significantly. Since it is apparent that such gas exchange is sufficient to support photosynthesis, gas exchange with the surrounding water must be more efficient in this species than it is in Halodule uninervis. The high rates of HCO~ uptake found in Halophila stipulacea (Beer et al., 1977) may contribute to such an efficiency. Ultimately, Halophila stipulacea may penetrate to greater depths through an ability to maintain efficient photosynthesis without involving lacunae to the same extent as does Halodule uninervis. However, for both species a certain minimal volume of lacunae may be of importance for other functions as well, such as supplying photosynthetic oxygen to the root systems of these plants.
ACKNOWLEDGEMENT
The authors thank Dr. F. Payne and Dr. R.G. Wetzel for discussions and comments which were of great help in writing this article.
REFERENCES Arnon, D.I., 1949. Copper enzymes in isolated chloroplasts. Plant Physiol., 24: 1--5. Beer, S., Eshel, A. and Waisel, Y., 1977. Carbon metabolism in seagrasses. J. Exp. Bot., 28: 1180--1189. Drew, E.A. and Robertson, W.A.A., 1974. A simple field version of the Winkler determination of dissolved oxygen. New Phytol., 73: 793--796.
337 Hutchinson, G.E., 1975. A Treatise on Limnology. John Wiley, New York, 660 pp. Lipkin, Y., 1977. Seagrass vegetation of Sinai and Israel. In: C.P. McRoy and C. Helfferich (Editors), Seagrass Ecosystems. Dekker, New York, pp. 263--293. Pope, D.H. and Berger, L.R., 1973. Algal photosynthesis at increased hydrostatic pressure and constant pCO 2. Arch. Mierobiol., 89: 321--325. Schreiber, U. and Vidaver, W., 1973. Hydrostatic pressure: A reversible inhibitor of primary photosynthetic processes. Z. Naturforsch., 28: 704--709. Vidaver, W., 1969. Hydrostatic pressure effects on photosynthesis. Int. Revue Gesamten Hydrobiol., 51: 697--747.