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LIMNOLOGY May 1974 AND OCEANOGRAPHY 1 Volume 19 Number 3 Depth distribution of photosynthetic activity in a Myriophyllum spicatum community in Lake Wingral Michael S. Adams and John Titus Department of Botany, University of Wisconsin, Madison 53706 Michael McCracken Department of Biology, Texas Christian University, Fort Worth 76109 Abstract Photosynthetic rates and productivity of Myriophyllum spicatum L. were examined in the littoral by incubating shoot sections at three rooting depths using a 14C technique. At a rooting depth of 240 cm, the interrelation of biomass and photosynthetic rates resulted in the production of 56% of the total photosynthetic productivity within 100 cm of the water surface in May. In August, 57% of the total productivity occurred within only 20 cm of the surface. Variations in the depth distribution of biomass, due to differing growth and sloughing patterns, resulted in different depth relations of photosynthesis at the other rooting depths. Light extinction and light adaptation of photosynthetic tissues were other factors determining the depth relations of photosynthesis. Myriophyllum spicatum L., the Eurasian watermilfoil, is a submergent aquatic macrophyte introduced into the United States in the 19th century, which has achieved nuisance levels of growth in lakes and slowmoving waterways of several states (Smith et al. 1967; Anderson 1972). We have studied its production dynamics in the littoral of Lake Wingra (Madison, Wisconsin), a shallow eutrophic lake with a surface area of 140 ha (Poff and Threinen 1962) and a bottom mainly composed of marl, with sand and organic matter in places (Nichols and Mori 1971). The University of Wisconsin Arboretum borders the lake on its southern shore, and the northern shore is dominated 1 Contribution No. 124 of the Eastern Deciduous Forest Biome, US-IBP, funded by the National Science Foundation under Interagency Agreement AG 199, 40-193-69 with the Atomic Energy Commission-oak Ridge National Laboratory. LIMNOLOGY AND OCEANOGRAPHY by recreational and residential areas. The littoral occupies about 43 ha, or 31% of the surface of the lake, most of it colonized by M. spicatum. Early in spring, Myriophyllum plants grow from established rootstocks and mainly short shoots more or less straight up to the surface, then grow along the surface to form a canopy in areas of dense growth. Meanwhile leaves are sloughed from the lower stems. Thus, by mid- and late summer most of the active photosynthetic tissues ( leaves) are located within the canopy near the water surface, whereas leafless stems predominate below the canopy. This growth form corresponds to the modified herb type (Ikusima 1965). The primary objectives of the work summarized here were to obtain measurements of the natural depth profiles of photosynthetic production and to examine factors in- 377 MAY 1974, V. 19(3) 378 Adams et al. fluencing differences in photosynthetic rates exhibited by those profiles. Photosynthetic profiles of plants incubated at depths of natural occurrence have been examined for both phytoplankton ( Ryther 1956; Talling 1966) and macrophytes (Ikusima 1965, 1966). All studies show pronounced differences in photosynthetic rates with depth and raise questions concerning the factors of greatest importance in controlling these differences. Historically, light has probably received more attention than any other single factor. Early workers on aquatic macrophytes (Ruttner 1926; Schemer 1934; Meyer et al. 1943) incubated plant samples of similar weight, age, and preconditioning regimes at different depths; resulting profiles of photosynthetic rates were considered to be a mcasurc of light extinction. Maximum photosynthetic rates for macrophytes were reported at water depths from O-5 m, apparently dependent on turbidity, incident light intensity, and the species under investigation. Stanley ( 1970) has examined the effccts of temperature on photosynthesis and respiration of Myriophyllum spicatum. Current flow (Westlake 1967), calcium carbonate encrustations (Wetzel 1960), and water chemistry ( Unni 1972) are additional potentially important factors that may vary with depth in macrophyte communities, but WC did not investigate these. One biological factor that obviously changes with depth is biomass distribution. This may not directly affect the rate of photosynthesis, but has a bearing on total photosynthctic productivity within a given stratum. Sorokin (see Koblenz-Mishke 1960) devised a system to investigate this factor with incubations of diff crent phytoplankton populations at different depths. We used a similar design in our experiments with macrophytes. Qualitative changes in biomass with depth may also be important. For macrophytes such as M. spicatum, tissue age and leaf/stem biomass vary with depth. Light adaptation occurs in both algal (Jorgensen ( Gessner 1938; 1969) and macrophytic Spence and Chrystal 1970; Ikusima 1966) species. We have considered changes in light, temperature, and quantity and quality of biomass with depth. The remaining factors we either controlled or regarded as less important. We thank W. H. Stone for his technical assistance in the field and laboratory and R. R. Kowal for his help in the statistical analyses. Methods During the summers of 1971 and 1972, we followed canopy and stem distribution of carbon uptake in the Myriophyllum community of Lake Wingra by a l”C technique modified slightly from that of Wetzel ( 1964). WC incubated sections of Myriophy2Zum in 510-ml glass bottles of lake water, to which 5 @i ‘“C-NaHCOn were added, clamped to aluminum poles within the macrophyte community. We have not tested the effects of sectioning Myriophyllum on carbon uptake rates, but sectioning EZoden canadensis had no significant effect on its photosynthetic rates (Ikusima 1965). We always collcctcd lake water from just below the surface, to avoid surface films and to ensure uniform chemical regimes for all sections in each experiment. To minimize nutrient and carbon depletion problems, incubation periods were limited to 2 hr, after which we first washed plant sections to remove periphyton and then quickfroze them in liquid nitrogen for transfer to the laboratory. We have not determined the exact significance of 14C uptake for M. spicatum, but assume it provides a measure of photosynthetic activity more closely approximating net than gross photosynthesis. Throughout this paper, we refer to I‘% uptake as carbon uptake, or simply photosynthesis, No dark bottles were incubated in the course of these experiments. In previous 14C experiments (Adams and Titus uapublished), the highest dark carbon fixation rates were always < 5% of the lowest carbon uptake rates observed in the light. In all field experiments, we measured irradiance with a recording pyranometer (13elfort) and temperature with a thermis( YSI ) . Water samples tor-thermometer were routinely collected for each experi- Myriophyllum photosynthesis with ment, and pH was measured with a portable meter (Sargent-Welch) equipped with a combination electrode. WC report three types of experiments : The construction of the natural profile of photosynthetic production with depthThree shoots from a common rootstock were collected, cut into lo-cm sections with attached leaves, placed in bottles to which carbon-14 was added, and incubated at their natural depths. The bottles contained differing quantities of plant material (0.14-1.44 g dry wt) to simulate the natural depth distribution of biomass-- and its accompanying - light environment. ‘l’hesc experiments were completed at three rooting depths (SO, 150, and 240 cm) at approximately 3-week intervals in the 1972 growing season. The sampling sites were chosen at the intersection of randomly selected parallel transects with the desired depth contours in the west end of the lake. Incubation of terminal sections at several depths within the water column--We designed these experiments to vary light and temperature conditions while holding other factors relatively constant, At a rooting depth of 180 cm, growing tips were sampled from O-IO-cm depth of water and incubated, three per bottle, at 15-cm intervals down to 180 cm. To minimize the effects of tissue variability, this experiment was replicated with three tips incubated and analyzed together; all bottles contained about the same plant weight (0.2-0.35 g) and had similar self-shading characteristics. A previous experiment (Titus and Adams unpublished) with six replicates of three tips each incubated under the same conditions yielded a coefficient of variability among carbon uptake rates of 14.5%. Ruttner (1926) and Golubic ( 1963) performed similar cxperiments on M. spicatum but not within natural communities. Incubation at the same depth of plant sections collected from different natural depths-Our intention here was to test the effects of light adaptation, leaf surface/biomass, and tissue aging on photosynthetic activity. Three Myriophyllum shoots rooted in 150 cm of water were collected, cut into 379 depth / NICHROME WIRE O2 OR N2 ._f’4C-C02 Fig. 1. The chamber used for plant samples containing 14C. After bustion in pure oxygen, the chamber with nitrogen. TRAP oxidation of sample comair is purged LO-cm stem sections with attached leaves, and incubated at the surface at the collection site. The experiment was repeated with bottles incubated 75 cm below the surface. Dry weights for the plant samples varied from 0.09 to 0.39 g. Plant samples were returned to the laboratory, lyophilized, and exposed to concentrated HCl fumes for 10 min to remove any l”C-monocarbonates precipitated on leaf surfaces during the experiments (Wetzel 1965), dried overnight at 95°C and weighed. For selected experiments, leaves and stems were weighed separately, then combined for further analysis. The dried Adams et al. 380 5 25.0 z 33 s $ 0 E F 24.0 23.0 22.0 I21.0 I20.0 I- - )- - - I- Percent of total plant photosynthesis Percent of total plant weight Temperature at incu bation depth ~-...-..,a - 1 )- )- - - - I 1 IO 70 00 100 90 110 Distance from growing tip (cm) 50 70 60 00 90 100 110 120 130 150 140 160 170 I60 I90 200 220 210 230 240 Water depth (cm) 30.0 -..jy -... o--o -... -a- . .._.., q - . ..-...- q - ,,__... 0 -...-... -... -o-. - .,,_.., !O.O -0 10.0 __--- o”- //- o-----o-----o----- t d--o L0 4 0.0- ‘; 2 0.6-p B O------o o----...-o i -5 L‘ 0 m 0.4. Percent of total plant photosynthesis Percent of total plant weight Temperature at incubation depth B 0 CT 0.2E 60 0 00 100 120 Distance 0 20 40 60 00 I40 from 100 Water I , t , I , I , I60 180 200 220 growing 120 depth I40 (cm) I 240 tip (cm) 160 I80 200 220 240 + c al 0 L 2 Myriophyllum photosynthesis samples were ground to powder, and IO-mg subsamples were wrapped in ashless filter paper and burned in oxygen in a chamber developed here ( Fig. 1). Evolved CO2 and 14C-C02 was trapped in 5 ml of ethanolamine, A 0.2-ml aliquot of the ethanolaminc was then pipetted into a modified Bray’s solution (Bray 1960) and counted in a scintillation counter (Packard Tri-Carb) . Water samples were titrated to determine total alkalinity (Am. Public Health Assoc. 1965), from which total carbon was calculated using the table of conversion factors in Saunders et al. ( 1962). This enabled calculation of carbon uptake rates according to their formula, which includes an isotope correction factor of 1.06. Linear regression and correlation ( Steel and Torrie 1960) was used for analysis of several treatments and for comparisons between treatments. For some statistical comparisons we had to express data as percent of maximum photosynthesis because rates were expressed in different units. Because assumptions of linearity and independence were not always appropriate, the Kendall rank correlation test (Wilcoxon and Wilcox 1964) was also used for the nonparametric comparison of trends observed in the data. Unpaired t-tests (Steel and Torrie 1960) were also used for group comparisons. Results and discussion Photosynthesis profiles Two natural profiles of photosynthesis with depth for the 240-cm water depth class are shown in Fig. 2, representing stages in the growth form of Myriophyllum. Figure 2A shows the photosynthetic profile on 26 May 1972, when the tips of plants rooted in 240 cm of water were 50 cm from the surface. Photosynthetic rates, temperature, percent of total plant photosynthesis, and percent of total plant weight are all shown for each of the lo-cm depth classes. Aver- with 381 depth age surface total irradiance for the period of incubation was 1.15 langleys min-‘. Temperature changed about 4°C with depth through the water column occupied by the plants. The photosynthetic rate of the top depth class in which plants are found is slightly greater than in the third and fourth depth classes; the overall pattern seems to bc a plateau in the uppermost 4 or 5 depth classes and a gradual decline in photosynthetic rates with increasing depth. Greatest biomass concentration in the depth class 80-90 cm below the water surface coupled with high photosynthetic rates made that class the most productive. Of the total photosynthetic productivity, 56% occurred in the top 100 cm of the water column. Figure 2B shows the profile on 14 August 1972, also for shoots rooted at 240 cm. Photosynthetic rates, temperature, percent total plant weight, and percent photosynthesis are shown for all depth classes. At this time, maximum canopy formation, which occurred in July for the 240-cm depth class, was reduced by some sloughing of tips. However, terminal sections were still growing along the surface. The mean surface irradiance was 0.92 ly min-l. Temperature decreased quite rapidly to about 50 cm, then only slightly down to the bottom. Again there was an apparent plateau of photosynthetic rates near the surface, followed by a much more rapid reduction than earlier in the year. This effect is magnified by the depth distribution of biomass, resulting in the contribution of 57% of the total photosynthetic production by plants within only 20 cm of the surface. Biomass changes follow a pattern similar to that of temperature, except for the increases in the two lowest depth classes. Even in this shallow littoral, dense subsurface concentrations of macrophytes, by hindering circulation, may have induced stratification of tempcraturc and possibly of other factors. WC usually found maximum photosynthetic rates in the top 10 cm, although occasionally pcnk photo- Fig. 2. Carbon uptake of lo-cm shoot sections of Myriophyllum spicatum incubated depths. Rooting depth of shoots was 240 cm. A. 26 May 1972; B. 14 August 1972. at natural 382 Adams et al. 100 - 60 A ------A A-A 0 ------O 60- ()-3Ocm 30 - 60cm 60-80cm 40 - ct7 CL 20- I / I I I“0------------- n O-30cm 30-60cm 60-90cm 90-120cm 120-150cm 150 -180cm 210-240cm expressed as percent of total photosynthesis of photosynthesis, Fig. 3. Seasonal depth distribution (PS) within 30-cm depth classes. A. Water depth = 80 cm; 13. water depth = 240 cm. Values less than 2% are not shown. synthesis occurred down to 50 surface (Fig. 213 and data not approximate correspondence photosynthetic depth profiles cm below the shown). The of these to for both Pot- amogeton crispus and Vallisneria asiatica (Ikusima 1965), the latter with a growth form different from that of M. spicatum, indicates caution in assigning “typical” pho- Myriophyllum photosynthesis with 383 depth 6.0 - - - - - 0 IO 20 30 40 50 60 70 80 Distance 50 60 70 80 90 100 110 120 130 90 100 from 140 Water 110 growing 150 depth 160 130 140 150 160 170 160 190 190 200 210 220 230 240 tip (cm) I70 180 (cm) Fig. 4. Net photosynthesis of lo-cm sections of Myriophyllum depths, expressed on a leaf weight basis. Data of 26 May 1972. tosynthetic depth profiles to macrophyte species. A comparison of maximum photosynthetic rates in May and August (Fig. 2), both at high light intensities, indicates pronounced seasonal variation. Figure 3 shows seasonal patterns in the depth distribution of photosynthetic production, regardless of such rate diffcrcnces. The percentage of total photosynthesis was calculated for all depth classes, each representing the consolidation of three lo-cm classes. For plants rooted at a water depth of 80 cm, seasonal trends based on four field experiments are clear ( Fig. 3A). Near the beginning of the growing season, in early May, the shoot apices lie at 30 cm and, accordingly, tissues in the lower 50 cm of the water column (represented by the lower two depth classes) contribute all the photosynthetic activity, The top depth class rapidly increased in importance on succeeding dates, almost to the exclusion of contributions by the lower two. 120 spicatum incubated at natural The results for rooting depths of 240 cm ( Fig. 3B) are in many ways similar to those at 80 cm. All depth classes below 60 cm are unimportant early in the season. Plant parts in the second depth class first increase in percentage of photosynthesis as lower depths become shaded out, then decrease in July as the top class attains nearly 90% of the total photosynthesis. Late summer sloughing of plant parts, particularly tips, and the concomitant partial opening of the canopy, may explain the depth distribution on the last date, which is the same as that in Fig. 213. The percentage of photosynthesis in the top depth class has noticeably dcclincd by 14 August, whereas the second depth class has increased. The sprouting of new growing tips from rootstocks and lower stems is evidenced in the slight increase contributed by the lowest depth class ( Fig. 2B and 3B ) . This late season sprouting may produce the shoots which we have observed to persist beneath winter ice cover 384 Adam Table 1. Statistics of linear regression and correlation on nonrelativixed and relativixed data. Treatments: A-surface incubation, photosynthetic rates expressed on a total weight basis; Bsurface incubation, leaf weight basis; C--75cm incubation, total weight basis; D--75cm incubation, leaf weight basis; E-natural depth incubation, total weight basis; F-natural depth incubation, leaf weight basis. et al. Table 2. Summary of statistical comparisons of treatments by linear regression and Kendall’s rank correlation test. Treatments described in Table 1. Treatments compared Linear regression Form of data A C A B E and and and and and B D C D F Relativized Relativized Raw Raw Relativized Kendall Results Assoc. * t * * ? + z z 2 Id p: C -.0057 -.a5 D +.0030 +.60 E -.0061 -.98 F -a0032 -.84 and to develop rapidly after ice-out in spring. The same phenomenon may account for the slight increase in percent contribution of the bottom depth class at a rooting depth of 80 cm (Fig. 3A), but the production of new tips at that rooting depth was not as striking as at 150 and 240 cm. In addition, sloughing of plant parts was not as evident in the 80 as in the 240-cm class. The results of experiments at the 150-cm class are less readily interpreted and are not shown here. They indicate two important sloughing periods, rather than the one shown for 240 cm in Fig. 313. Patten (1954) and Nichols ( 1971) documented two postflowering sloughing periods for M. spi- T NS + i + *No overlap of 95% confidence intervals slopes. tNo overlap of 99% confidence intervals. fsignificant (0.05 level) association shapes. §Highly significant (0.01 level). E Signif. of of curve catum; we observed two such periods for the 1970 growing season in Lake Wingra, but they were less distinct in 1971 and 1972. A combination of the depth relations of photosynthesis reported here with diurnal variations in photosynthesis of growing tips and seasonal variations in tip photosynthesis have yielded a calculation of total community productivity (Adams and McCracken unpublished). We should caution that the depth relations which we report are based largely on experiments performed in late morning to midafternoon: a 24-hr cycle, with changing angles of incoming radiation, doubtless would change the depth relations. Considering the properties of low angle irradiance and light penetration into lakes, in both early morning and evening the upper depth classes would probably contribute an even greater proportion of the total photosynthesis. Factors controlling the photosynthetic profiles We recalculated the data of 26 May (Fig. 2A) on a leaf weight rather than a total weight basis, to eliminate variation due to changes in quantities of photosynthetic tissue with depth. This transformation reduccd the differences among the carbon uptake rates (Fig. 4). After the data of Figs. 2A and 4 had been expressed in relative terms, linear regression and correlation ( treatments E and F: Table 1) indicated Myriophyllum photosynthesis with - 0 90 105 17 I 60 75 385 depth 120 135 n m - 150 165 I60 _- fncubation depth (cm) Fig. 5. Net photosynthesis phyllum spicatum community. (Ps) of growing tips incubated high negative correlation coefficients, of -0.98 and -0.84, for the relations C uptake : unit shoot dry wt and C uptake : unit leaf dry wt. The 99% confidence intervals of the slopes of the two relations do not overlap (Table 2). Kendall’s rank correlation test (Table 2)) however, indicated that the shapes of the two figures show highly significant positive correlation. Thus the expression of carbon uptake rates on a leaf weight basis showed that changes in quantities of photosynthetic tissue with depth accounted for a significant amount of the variation observed in Fig. 2A, but did not alter the general decreasing trend in the relation of carbon uptake versus depth, Most of the remaining variation was probably due to differing light intensities and light adaptation, as temperature changes during this experiment were not great. The effects of light adaptation would be at different depths within the Myrio- expected to counter rather than to accentuate those of light attenuation. Figure 5 shows results representative of the experiments designed to examine the effects of light alone, in which growing tips collected at a rooting depth of 180 cm were incubated at different depths. Again, temperature changes with depth were slight. Mean surface light intensity during the course of the experiment was 0.50 ly min-l. The effects of light are dramatic with a nearly exponential decline in tip photosynthetic rates with depth. In comparable studies with growing tips of a number of macrophyte species, Manning et al. ( 1938) and Meyer and Heritage ( 1941) also found decreasing photosynthetic rates with depth. However, others found maximum photosynthetic rates at 1 to 5 m below the surface (Ruttner 1926; Schemer 1934) ; Ruttner’s ( 1926) study included M. spicatum. et al. 6.0 El 5.0 40 - 30 -i 2 5 20 IO 0 - a, 1 - :I m 0 0 20 40 60 80 r-n JO- r 2.0. 1.0. 0 - 20 40 DISTANCE 60 80 -r 100 120 FROM GROWING TIP (cm) Fig. 6. Net photosynthesis of lo-cm sections of Myriophyllum spicatzcm from different depths incubated at two common depths. A-Incubation at the surface, expressed on a total weight basis; B-incubation at the surface, cxpresscd on a leaf weight basis; C-incubation at 75 cm, expressed on a total weight basis; D-incubation at 75 cm, expressed on a leaf weight basis. A natural concomitant of the effect of light on photosynthesis is its effect on macrophyte establishment and growth, discussed by Peltier and Welch (1969, 1970) and Spencc and Chrystal ( 1970). Incubation of sections from different natural depths at a common depth provided uniform physical and chemical conditions during incubations. Attention is focused on the effects of light adaptation, and again on quantities of photosynthetic tissue, in the experiments summarized in Fig. 6. Figure 6A shows photosynthetic rates of MyriophyZZum sections from different natural depths incubated at the surface; mean irradiance level was 0.46 ly min-l. Photosynthesis decreased with distance from the growing tips, which approximately corresponded to the natural depth of the section before incubation. Transformation of the same data to a leaf weight basis nearly climinated the variation ( Fig. 6B), though the three highest photosynthetic rates are still found in the top half of the plant. Linear regression on relativized data for each figure ( Tables 1 and 2) revealed that the 95% confidence intervals of the slopes do not overlap. Kendall’s rank correlation test indicated that the general trends of the two curves are significantly positively associatcd. Thus, again, the transformation of photosynthetic rates to a leaf area basis removed most, but not all, of the variation. The photosynthetic activity of plant sections incubated at 75 cm, approximately half the plant length and half the water depth, is presented similarly (Figs. 6C and I3 ) . Irradiance was 0.06 ly min-l at 75 cm during the experiment. The nearly linear (r = -0.85) decline of photosynthesis with Myriophyllum photosynthesis with 387 depth Table 3. Matrix of statistics of F-tests (upper increasing distance from the tip in Fig. 6C right) to compare within group variances and unis similar to that in Fig. 6A, but differences paired t-tests (lower left) to compare group means. in magnitudes of photosynthetic rates are Bl-Surface incubation, first 7 sections from the quite pronounced. We suggest that these growing tip; B2-surface incubation, second 7 sections: Dl-75cm incubation. first 7 sections: 02 differences are due to differing light levels. ’ incubation, second ? sections. Expressing the 75-cm incubation results on --7&m a leaf weight basis reverses the downward trend, and photosynthetic rates actually increase with increasing distance from the tip (Fig. SD). We conclude that short-term tissue aging has little effect on photosynthetic potential in this case. Linear regression on relativized data indicated no overlap of the 99% confidence intervals of the slopes of the data in Figs. 6C and D. Kendall’s rank correlation test indicated a nonsignificant but ncgativc association between the trends of the two curves. Variability remaining after * Significant at 0.05 level. conversion to a leaf weight basis (Fig. 61) t Significant at 0.01 level. and particularly in 6D) we attribute to the effects of light adaptation. To clarify these results, we used further statistical tests. test indicated a significant difference bePlant sections for each incubation depth tween the within-group variances. were arbitrarily divided into two groups : The comparison between Bl and Dl one in which sections naturally occurred in yielded a highly significant difference bethe upper half and another for sections in tween group means. This is interpreted to the lower half of the plant length. The mean that the upper leaves utilize relatively means and within-group variances of sev- high light intensities more efficiently than eral combinations of four groups of obser- low light intensities. Thus, differences in vations were compared (Table 3). Com- photosynthetic response of upper and lower parison of the means of the photosynthetic lcavcs are apparently substantiated. By dirates of groups Bl and Dl (see Table 3) rect laboratory methods, Gessner ( 1938), with those of groups B2 and D2, rcspec- Ikusima ( 1966)) and Spencc and Chrystal tivcly, revealed no significant differences (1970) also found evidence of light adapin within-group variances or means, al- tation in a variety of aquatic macrophytes. though plant parts farther from the growing Whether light adaptation in MyriophyZZum corresponds to one of Jorgcnscn’s (1969) tip ( Fig. 6D) seemed better able to utilize the low light levels at 75 cm. Thcrc is also two types of light adaptation in algae is not no significant difference between means of known. B2 and D2; indicating that the tissues from Conclusions greater natural depths did not respond sigVariations of MyriophyZZum photosynthenificantly differently to the two light levels. sis with depth are very pronounced even in It should be emphasized that even the surthe relatively shallow littoral of Lake Winface irradiance was low on this particular gra. These depth relations arc variable for day, however. The possibility of photoindifferent water depths within the macrohibition of photosynthesis in lower leaves phytc community, possibly because of difby much higher light intensities has not fcrences in hormonal control of flowering been ruled out. The lack of a significant and mortality. Temporal variations in difference between means of 132 and D2 depth relations of photosynthesis are quite must be accepted with caution, as the F- apparent from the seasonal study, and diurL. rl Adams et al. 388 E. I. CLAY. 1943. Effect of depth of imnal change should also be considered in the mersion on apparent photosynthesis in subdevelopment of models of primary producEcology 24: 393merged vascular aquatics tivity. 399. The factors apparently most important AND A. C. HE~UTAGE. 1941 Effect of tuibidity and depth of immersion on apparent in causing variations in M. spicatum photophotosynthesis in Ceratophyllum demersum. synthesis with depth are light and depth Ecology 22: 17-22. distribution of photosynthetic tissue, in NICHOLS, S. A. 1971. The distribution and conagreement with Sorokin’s experimental aptrol of macrophyte biomass in Lake Wingra. proach ( Koblenz-Mishke 1960). Light Univ. Wis. Water Rcsour. Center Rep., OWRR B-019-Wis. 111 p. adaptation is also a factor for Myriophyllum. p, AND S. Mom. 1971. The littoral macroTissue aging with depth does not seem of phyte vegetation of Lake Wingra. Trans. prime importance. Temperature, current Wis. Acad. 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