3. RESPONSES OF SUBMERSED PLANTS TO LIGHT AND TURBIDITY

3.1 LIGHT ATTENUATION

The depth to which submersed aquatic vascular plants are distributed depends on the availability of light, if no other factors such as hydrostatic pressure, nutrient supply, substrate composition, and turbulence limit growth. three factors affect light attenuation: absorption by water itself, absorption by suspended particles, and absorption by dissolved substances.

Monochromatic light passing through chemically pure water is absorbed exponentially and thus decreases at a constant rate with increasing increments of depth. This relationship can be expressed as the extinction coefficient - n, which decreases from the red to the blue end of the visible spectrum. The extinction coefficient, n is a function of the light intensity at the surface (Io) and the intensity at depth z in meters

The extinction coefficients of natural waters deviate greatly from those of pure water due to the presence of dissolved and particulate substances which absorb and scatter light. The spectral quality of light is particularly affected by dissolved substances, since light scattering by particulate matter is relatively nonspecific in optical effects.

The extinction coefficient of natural waters is separated into three components such that

where nt is the total extinction coefficient, and the remaining terms are due to water, suspended particulates, and dissolved color, respectively. Thus the expression can be rearranged so that the intensity of light, Iz , at a depth of one meter below Io is

Figure 6, adapted from the data of James and Birge (1938) for Lake George in Wisconsin, clearly shows the effects of absorption by each of these components. (The data are graphed as percentile absorption, expressed by the formula, , where Iz is at 1 m depth.) Short wavelengths (violet and blue) are most strongly adsorbed by the dissolved material which mostly consists of dissolved organic compounds, absorption of long wavelengths (red and infrared) is due mostly to water, while the particulate matter is quite nonspecific in its absorption properties, at least when in low concentration. However, in using sediment concentrations between 50 and 5,000 ppm, Otto and Enger (1960) found that the red wavelengths penetrated somewhat further than blue. The spectral discrimination was greater for a commercial sodium base montmorillonite type bentonite than for a sediment obtained from a reservoir. This indicates that although suspended sediments may have a negligible effect on spectral quality at low concentrations (<=50 ppm), higher concentrations may cause significant shifts in the relative penetration of various wavelengths. Selective absorption by algal pigments may occur in the blue (400-500 nm) and red (640-680 nm) when phytoplankton are dense, but the amount is minor when compared with the attenuation by particulate matter throughout the visible spectrum (Westlake, 1966).

Figure 6. Total absorption spectrum of water from Lake George, Wisconsin, as compared with spectra for pure water, dissolved organic color, and suspended particulates. Modified from James and Birge (1938).

Vertical extinction coefficients then represent a composite of all wavelengths and vary considerably among natural waters depending on the contribution by suspended particulate and dissolved components. Values of range from about 0.2 for exceptionally clear lakes such as Lake Tahoe, California, to values in excess of 10 where turbidity is extremely high such as for reservoirs receiving inputs from flooding rivers (Wetzel, 1975; Westlake, 1966) Ice cover reduces light transmission to variable degrees depending on whether the ice is clear, contains bubbles, or is stained. Theoretically, clear ice transmits light better than natural water because the dissolved substances have been reduced. However, snow cover reduces transmission considerably. Self shading by submerged macrophytes may be large depending on biomass, and as little as 0.1 percent of the surface light may reach the bottom of a river weed bed, mostly in the green wavebands (Westlake, 1966).

3.2 RELATIONSHIP OF ACTUAL LIGHT TRANSMISSION AND SECCHI DISC TRANSPARENCY ESTIMATES TO THE EUPHOTIC ZONE

The euphotic zone is the region from the surface to the depth at which 99 percent of the incident surface light has disappeared. Work based on response of phytoplankton suggests that the intensity of light at this level, i.e., 1 percent of the surface light, represents the compensation light intensity at which photosynthesis and plant respiration are in balance. For many of the studies we cite, only Secchi disc transparency depths are available. It would be valuable to be able to relate the Secchi depth to the more theoretically sound measurements of extinction coefficient or percent light penetration at which the Secchi disc disappears. Attempts have been made to do this (Poole and Atkins, 1929; Verduin, 1956; Cole and Barry, 1973). As Hutchinson (1957) pointed out, the Secchi depth measurement actually is based on a comparison of the brightness of the disc and the water surrounding it. Thus light reflected from the bottom in shallow water or scattered upward by silt-laden waters can introduce considerable error. Nevertheless, factors of 2.7 to 3.0 times the Secchi depth have been found to approximate the 1 percent level in many cases (Cole, 1975). Based on empirical evidence for coastal waters, Holmes (1970) suggested that a factor of 3.5 might be most appropriate in water with a Secchi depth of less than 5 m and a factor of 2.0 for water with a Secchi depth between 5 and 12 m. However, as will be discussed later, lower factors appear to be more appropriate for relatively clear waters.

Within a single lake, seston would be expected to have a greater correlation with Secchi depth transparency than dissolved colored compounds because of greater seasonal change in suspended material. However, between lakes, the confounding effects of varying color would weaken the relationship between Secchi depths and extinction coefficients.

3.3 DEPTH ZONATION AND TURBIDITY TOLERANCE OF SUBMERSED SPECIES

Species that become established and grow in the deeper regions of aquatic ecosystems where only a small fraction of the surface irradiation remains are better adapted to survival at low levels of light than those restricted to shallower and better illuminated zones. In shallow aquatic ecosystems where light is rapidly attenuated by high concentrations of suspended sediments, the same species tolerant to low levels of light might be expected to compete more effectively than those requiring high light intensities. To test this hypothesis, the depth distributions for a number of species from diverse systems are presented graphically with their Secchi depth estimates in Figure 7. Available North American depth distribution records of submersed angiosperms have been tabulated for aquatic systems where water depths were such that the maximum depth of submersed plants would likely be limited by irradiance rather than the shallownss of the system. Some data are given for areas other than North America but an extensive search of world literature was not made. Depth distributions for submersed macrophytes.

The variety of methods of collecting and reporting the data summarized in Figure 7 and the complexity and variations within and between the ecosystems put the comparative analyses to be made later within the realm of approximations. Normally for each species the shallowest and greatest depths reported are shown along the depth axis and the area of greatest frequency, density, cover, or standing crop is indicated by cross hatching. No absolute values are given; only the relative occurrence of a species along the depth gradient is graphed. These data were often given in the literature just as reported here, but in some cases we have given our best estimate. For example, depth distribution may have been reported within range classes such as percent frequency at 0-1, 1-3 and 3-8 m (Rickett, 1922, 1924; Wilson, 1935, 1941). In this specific case the minimum depth would be graphed as 0.5 m if the species occurred at 0-1 m while the maximum depth would be graphed as 5.5 m if the species occurred at 3-8 m.

Secchi disc depths as given in the literature or as estimated from submarine photometer readings are given when available. There are many problems associated with attempting to relate Secchi readings to the light environment of the plant. These range from the visual acuity of the observer to the necessity of using Secchi data taken at times other than the growing season for the plants in question. One possible source of error in Secchi readings which doesn't appear to be serious is the lack of standardization in disc size and contrast (white as compared with black and white) common in earlier studies. Baker and Magnuson (1976) found no significant differences in transparencies in Crystal Lake, Wisconsin, as measured with a 20 cm black and white disc and a 10 cm white disc.

To convert occasional clear water light transmission data taken with a submarine photometer to Secchi depth transparency estimates, the depth (m) at the 1 percent light level was divided by 1.7. This factor is consistent with observations by Wile (in preparation) for fresh waters in southern Canada and Wetzel (1975; Figures 5-9 and 5-16) for Lawrence Lake, Michigan. Based on studies of turbid waters in Back Bay, Virginia (Sincock, 1965), a conversion factor of 2.5 was used for the data of Bourn (1932).

Perhaps the maximum depths of plant growth recorded in Figure 7 are more reliable than other information given. However, as discussed by Hutchinson (1975) and Spence (1976), there can be problems in establishing the maximum depth in a water body where a species is rooted and growing. Accuracy includes judgment as to whether a small number of plants (or the plant) at a depth are living and are truly rooted. Ceratophyllum demersum is a special problem because it does not form roots although a portion of the shoot often becomes embedded in the sediment and thus 'rooted' (Arber, 1920). Other species such as Myriophyllum spicatum at times produce rooted floating shoot fragments which normally sink with time. Hence shoots of C. demersum and other species may be carried to the deeper areas and sink to the sediment where they may soon become moribund in the more stressful environment. Even though some shoots were found as deep as 9.2 m, Spence (1976) set the macrophyte limit for Loch of Lowes at 3.9 m which was close to the 1 percent cover line of 3.6 m. The increasing use of SCUBA divers in studies of submersed macrophytes should lead to more accurate data.

Though the ecological importance of straggling plants surviving in the lower depths is probably minimal, depth records suggest that there are physiological limits due to hydrostatic pressure as discussed previously. Hutchinson (1975), in summarizing depth records for a number of aquatic macrophytes, injected some of the published depths of colonization due to problems mentioned above. He suggested that the record depth for submersed freshwater angiosperms was for Potamogeton strictus Philippi, which is apparently not found in North America (Shetler and Skog, 1978). Tutin (1940) found this species at slightly deeper than 11 m in the high mountain Lake Titicaca, Peru-Bolivia. Recent data, especially those of Moeller (1975), Sheldon and Boylen (1977), and Wile (in preparation), all from the same geographical area of northeastern United States and southeastern Canada, extend the depth ranges for a number of species native to North America. Perhaps the most notable depth record is for Elodea canadensis which was found at one of the 12 m sampling stations in Lake George, New York (Sheldon and Boylen. 1977).

Relationship between Maximum Depth Distribution and Secchi Transparency

Figure 8 is a summary of depth records for 10 species in Figure 7 plotted as a function of Secchi depth. Some trends are discernible and factors which may affect depth distribution will be discussed. However, conclusions drawn from these graphs are necessarily tentative owing to the paucity of data points as well as problems already mentioned in interpretation of the original data. The resistance of these species to environmental changes will be discussed further [in a later section].

Figure 8. Maximum depth distribution of selected species from FIgure 7 plotted against Secchi disc transparencies of the waters where the distributions were observed: (a) Eleocharis acicularis, (b) Potamogeton praelongus, (c) Najas flexilis, (d) Potamogeton pectinatus, (e) Myriophyllum spicatum, (f) Vallisneria americana, (g) Najas guadalupensis, (h) Ceratophyllum demersum, (i) Potamogeton subsection Perfoliati, (j) Elodea canadensis.

Since Eleocharis acicularis is a sedge, one might judge a priori that it is a shallow water species. This is confirmed by depth records of 2 m or less for five of the seven data points (Figure 8a). The scattering of the points suggests that E. acicularis does not respond to water clarity in a predictable manner as do some other species that have increasing maximum depth distribution with increasing Secchi depth. In fact, Wilson (1935) placed it in an ecological group of species (mainly rosulate) which becomes totally submerged only in response to changing lake conditions. Other species placed m this group were Lobelia dortmanna, Juncus pelocarpus, and Gratiola aurea. These relatively clear water species with generally shallow maximum depth records (Figure 7) are probably limited in depth maxima by factors other than reduced irradiance. An affinity for sandy sediments which are normally characteristic of shallow areas subjected to fetch and water turbulence is one possible explanation for the depth distribution patterns. Laboratory and field experiments have shown that certain species grow best when rooted in specific types of aquasoils (Pond, 1903; Brown, 1913; Bourn, 1932; Misra, 1938).

The relationship between depth maxima and Secchi depths for Potamogeton praelongus (Figure 8b) can be considered representative of several species of this genus with a North American distribution, primarily in clear fresh waters in Canada and northern United States. In addition to P. praelongus these species include P. robbinsii, P. zosteriformis, P. amplifolius, and P. gramineus. Since depth maxima for this group are normally high, one might expect that these species would tend to survive under reduced light penetration due to suspended particles. However, some studies of long term changes in lakes where turbidity has increased show that these northern species tend to disappear or decrease in biomass (Volker and Smith, 1965; Lind and Cottam, 1969; Stuckey, 1971; Nichols and Mori, 1971; Crum and Bachmann, 1973; Baumann et al., 1974; Bumby, 1977). Especially striking was the virtual elimination of the group with increasing human activities over 70 years in the vicinity of Put-In-Bay Harbor, Lake Erie, Ohio (Stuckey, 1971). Though these species do well at the low light intensities of deeper waters, they appear to be restricted to rather narrow conditions which do not include highly turbid waters.

A number of physical and biological changes, are likely to be associated with increasing turbidities from increasing suspended sediments. These include increasing inorganic nutrient levels and changes in biological components of the ecosystem. Of 19 lakes and ponds of Southern Ontario studied by McCombie and Wile (1971), Potamogeton amplifolius was present only in the most oligotrophic impoundment. Disappearance or reduced importance of northern species has also been associated with increased importance of other species, especially Myriophyllum spicatum (Lind and Cottam, 1969; Nichols and Mori, 1971; Steenis, 1970 and perhaps P. crispus (Fassett, 1957; Stuckey, 1971; McCombie and Wile, 1971; McIntosh et al., 1978). Both of these species have been naturalized from Europe.

Najas flexilis (Figure 8c) is found under a wider range of Secchi depths than the northern species of Potamogeton. This species principally has a northern distribution and does not range southward sufficiently to be considered cosmopolitan. There does not appear to be much tendency for an increase in maximum depth with increasing water transparency for the data points available.

Compared with the three species just discussed, the remaining species all show some degree of linearity between Secchi transparency and maximum depth distribution (Figures 8d-8j). All except Najas guadalupensis, which tends to have southern affinities, are strongly cosmopolitan. Potamogeton pectinatus grows well under a wide variety of conditions. This is consistent wit the wide range of Secchi transparencies and depth records for the plant. In their study of Canadian ponds and lakes, McCombie and Wile (1971) found P. pectinatus growing in waters of wide specific conductance range and spanning the complete range of Secchi transparencies from 0.9 to around 5.7 m. This species may be found in waters high in suspended sediment and organic pollution and is often rooted in silty sediments (Butcher, 1933; Hynes, 1970; Haslam, 1978; Ozimek, 1978). The essentially linear leaves have been observed to be relatively free of the silt blanket which tends to cover submersed macrophytes in waters high in suspended sediments as discussed previously (McCombie and Wile, 1971; Schiemer and Prosser, 1976).

The data points for Ceratophyllum demersum (Figure 8h) indicate a distribution at somewhat greater depths than other species for water bodies with Secchi depths less than 5 m. This suggests a degree of shade tolerance for this species which will be discussed in more detail in a later section. For Elodea canadensis, the linearity between maximum depth and Secchi transparency is remarkable. The depth record of 12 m in Lake George, New York (Sheldon and Hoylen, 1977) is the greatest depth reported for submersed angiosperms. Summer water temperature to 12 m ranged from 22-25ÉC in Lake George. Light transmission to 12 m during the summer was about 10 percent of incident and the water column and sediments were aerobic at least to 12 m water depth. Thus, these factors probably were not limiting the maximum depth of growth. Rather, hydrostatic pressure is important in limiting the depth of growth of deep water species in Lake George. Elodea canadensis was somewhat more resistant to excess pressure than two other species studied by Ferling (1957). A number of depth records in Figure 7 were for Lake George.

The linear pattern between maximum depth and Secchi transparency for Elodea canadensis differs from a tendency for some of the other species, such as Myriophyllum spicatum (Figure 8e), Vallisneria americana (Figure 8f), Najas guadalupensis (Figure 8g), and the Potamogeton subsection Perfoliati group Figure 8i , to reach a plateau at 6 to 7 m depth. This plateau indicates that hydrostatic pressure rather than light availability controls maximum depth distribution.

Turbidity Tolerance Index

When the maximum depths for submersed angiosperms of Figure 7 are plotted against their Secchi depths, a linear relationship is apparent at shallow depths for most species (Figure 8). This suggests that, in the range of around 2.5 m Secchi depth or less, turbidity is an important factor affecting maximum depths of growth. It follows that if a species is found in the deeper areas in water bodies with 2.5 m Secchi depths or less, the species would have a degree of turbidity tolerance. More specifically, the higher the depth maxima to Secchi depth ratio in the turbidity-stressed systems, the higher the turbidity tolerance of the species. This ratio, along with related information, is given in Table 1 for species of Figure 8. Species with higher turbidity tolerance indices are better adapted for surviva1 under conditions of low light transmission.

The absence of Potamogeton praelongus in the systems with turbidity stress typifies potamogetons that are mainly restricted to northern areas as discussed previously. The low ratio for Eleocharis acicularis is not surprising; the depth distribution of this species does not correlate with Secchi transparency. Elodea canadensis is apparently sensitive to turbidity, even though it may grow at great depths where turbidity is low. Perhaps the effects of suspended particles on light quality reaching the plants is especially important for this species. Of the remaining species Ceratophyllum demersum, Vallisneria americana, Najas guadalupensis, and P. pectinatus have the highest turbidity tolerances. The mean depth of Ceratophyllum demersum is greater than that of other species (Figure 8).

Thus, it appears that not all species tolerant to low levels of light and growing at great depths in clear lakes will be successful in colonizing aquatic ecosystems of high turbidity. These exceptions may be species that are sensitive to factors such as eutrophication, substrate type, siltation of leaves, or light quality, rather than just the quantity of light. Nevertheless, the turbidity tolerance index provides an approximation of the relative resilience of several species to turbidity stress and their rank is supported by observations on distribution in nature. Additional data are needed to firmly establish the relationship between submersed species and turbidity.

Table 1. Turbidity tolerance index for selected species of figure 7 expressed as the ratio of depth maxima to secchi depth where secchi depth is 2.5 m or less.

(a)From data for lakes of southern Ontario, Canada. Twenty-three sampling stations were in Georgian Bay of Lake Huron with the rest from various other lakes (I. Wile, in preparation).

3.4 PHOTOSYNTHESIS AND GROWTH IN RESPONSE TO LIGHT

It is apparent from the foregoing discussion that some macrophyte species have an affinity for deeper or more turbid waters while others tend to be restricted to conditions of higher light intensities. It would be impossible from the data given above to classify all species as being either shade tolerant or high light requiring although the approach used in Table 1 is promising. Rather we will examine the experimental evidence available for a few species to see if it provides further insight to the possible light control of macrophyte zonation and turbidity tolerance.

Many of the experiments reported in the literature on apparent photosynthetic rates of submersed macrophytes relate to the light intensity at which the photosynthetic system is saturated. The lowest irradiance necessary to achieve the maximum rate of photosynthesis (saturation) provides a valuable point of reference for comparing species. Since the subject of this review is concerned more with the response of macrophytes to reduced levels of light, these experiments might appear irrelevant. However, where light is only occasionally limiting due to shading by high turbidity waters, the competitive advantage of species with a high photosynthetic efficiency may help to explain their occurrence. Such conditions might occur in shallow zones of lakes or normally clear rivers with pulses in turbidity due to storms, dredging, high runoff, etc. Moreover, it might be proposed that species with high light saturation correspondingly have lower photosynthetic rates at low light levels and higher compensation points than shade adapted species.

The compensation point of light, i.e., where photosynthesis and respiration are in balance, should limit the lower depth distribution of a species. In laboratory and field experiments where compensation points are measured, they can only approximate depth distributions in nature. This is partly because experiments are normally of short duration, while under natural conditions plants respond to a seasonal range of light conditions (turbidity, day length, solar angle, etc.). Furthermore, experiments are normally with active apical portions of plants and do not reflect respiratory utilization of photosynthate by older stem portions and underground parts. There is another problem with extrapolating low irradiance experiments in the laboratory to deep water conditions. In deep clear lakes, selective light absorption by water (red region) and by organic compounds (blue region) may be as great a factor in photosynthesis as reduction in total irradiance (Figure 6). Research appears to be lacking on this problem for aquatic macrophytes.

A further problem with interpretation of the experiments discussed below is the lack of consistency among experiments. For example, some workers report light values as illuminescence (lux or foot candles) while others more appropriately use irradiance (cal/sq cm /hr) or microEinsteins/sq m /sec). The two expressions are not interconvertable because illuminescence does not take into account the variation in energy distribution of different spectral regions. Moreover, there are problems with differences in temperature, light source, enclosed biomass, and inorganic carbon availability both within and among experiments. Finally problems are associated with accumulation and utilization of oxygen in intercellular spaces (lacunae) of submersed plants (Hartman and Brown, 1967). However, to the extent possible, these experiments will be discussed as they may relate to macrophyte depth zonation and turbidity tolerance.

Laboratory Experiments

In a series of laboratory experiments, Gessner (1938) determined light saturation for photosynthesis in six submersed species. Four of the species, Ceratophyllum demersum, Cabomba aquatica, Hottonia palustris, and Ranunculus aquatilis, appeared to saturate at approximately 10,000 to 40,000 lux. This low light saturation of C. demersum is in agreement with its high turbidity tolerance index (depth maximum to Secchi depth ratio) reported in Table l.

The other two species, Elodea crispa and Potamogeton perfoliatus, did not saturate within the range of light intensities use . It would appear that particularly for P. perfoliatus light saturation was somewhat above 80,000 lux. This concurs somewhat with the low turbidity tolerance index of Potamogeton subsection Perfoliati of Table 1. The photosynthetic curves for C. demersum and C. aquatica appear to increase quite rapidly at the lower light intensities relative to the other species.

Boyd (1975) reported changes in net photosynthesis of 1 g samples with increasing light intensity. Photosynthesis in three of the species reached light saturation at about 10,000 lux (Eleocharis acicularis, Elodea densa, Najas flexilis), while light saturation occurred at 15,000 lux for Potamogeton sp. and 20,000 lux for Ceratophyllum demersum. None of the species required more than 9,000 lux for 50 percent of maximum photosynthesis. These results are in general agreement with those of Gessner (1938) except for the two species that he reported which did not reach maximal photosynthesis.

Comparative photosynthetic rates for four species of submersed plants reported by Van et al. (1976) didn't differ greatly in irradiance required for saturation (600-700 microEinstein/sq m/ sec), but light compensation points differed substantially. The compensation point for Hydrilla verticillata was lowest, at 15 microE/sq m/sec, Cabomba caroliniana was highest at 55 microE/sq m /sec, while Myriophyllum spicatum and Ceratophyllum demersum were intermediate at 35 microE/sq m/sec. Even though H. verticillata and C. demersum had high to medium tolerances, respectively, to low light levels, their maximal photosynthetic rates were higher than the other two species per unit of chlorophyll content. It would appear then that some species are adapted to a wide range of light conditions, being able to tolerate quite low levels of light and have high photosynthetic capacities at high light levels. Since H. verticillata exhibits this capability, this would explain its rapid biomass production under favorable light conditions as well as its large standing crops where self shading is high (Nall and Schardt, 1978). Carr (1969a, b) also characterized C. demersum as a shade plant, being adapted to low levels and saturating at about 40,000 lux in flasks and about 15,000 lux under artifical stream conditions. She observed maximum photosynthesis in plants collected from 5 m depth in Lake Ohakuri, New Zealand, where light intensity was about 2 percent of surface. However, Meyer and Heritage (1941) found this species to have maximum photosynthesis at the surface of Lake Erie. As Carr (1969b) pointed out, this may have been a result of using only plants collected from the surface rather than incubating them at light intensities at the depth from which they were collected. Further evidence for shade adaptation of Ceratophyllum demersum relative to other species was reported by Guilizzon (1977) for lake Wingra, Wisconsin. Saturation of photosynthesis occurred at 250 microE/sq m/sec compared with 800 microE/sq m /sec for Myriophyllum spicatum.

It is uncertain to what extent these short term (usually 1 hour) laboratory experiments, in which dissolved oxygen production rates are measured, are comparable to longer term studies in which growth is measured by increase in length or biomass. For example, Blackburn et al. (1961) reported that Elodea densa (Planch) Caspary has a low light requirement and that longterm growth (12 weeks under fluorescent lamps) was optimum at about 100 foot candles. Above 125 foot candles rapid chlorosis and death occurred. In comparison, Heteranthera dubia had a high light requirement with optimum intensity at 590 foot candles. Long-term studies would appear to take into account differing abilities of species to adapt to light intensities.

A fundamental difference in response to light saturation by sun leaves (acclimated under high light intensity) and shade leaves was reported by Gessner (1938) for several species of submersed macrophytes. Proserpinaca palustris, Elodea crispa, and Potamogeton densus all showed light saturation at about 40,000 lux or less, while the sun leaves continued to show a near linear photosynthetic response to increasing light above that level. Similarly, the emergent leaf form of the heterophyllous Proserpinaca palustris saturated at higher light levels than the submersed form. is suggests that not only are there inherent differences in light saturation levels among species, but that there is considerable plasticity within a species depending on exposure to light conditions prior to experimentation.

This problem was addressed by Spence and Chrystal (1970a,b) who showed that some Potamogeton species had a greater reduction in photosynthesis with reduced irradiance than others. There was a tendency for these species to be restricted to the more shallow zones of lakes, and thus be less shade tolerant, than the ones that underwent less reduction in photosynthesis. The mechanism for the adaptation of shade species appeared to be related to lower leaf respiration and reduced thickness under low light conditions. These features allow net photosynthesis to continue under low irradiance. In contrast, the thicker leaves of sun species were more efficient at higher irradiances.

Otto and Enger (1960) maintained submersed plants (Potamogeton pectinatus, P. nodosus, Elodea canadensis) in tanks with varying concentrations of suspended sediment. Growth reductions relative to controls were 20 to 40 percent for sediment concentrations of 50 ppm. The amount of growth reduction was approximately linear with increasing sediment concentrations up to about 1,250 ppm. Abnormalities in growth at higher concentrations over the 4-week growth period included elongation of stem internodes and chlorosis of stems and basal submersed leaves. Potamogeton pectinatus appeared to be less tolerant to suspended sediments than the other two species and the authors attribute this to its lower leaf area. This does not agree with field observations (Table 1, Figure 7) which show it to be reasonably turbidity tolerant.

This research is of interest because it is the only known study in which suspended sediment concentrations have been manipulated to observe growth responses of submersed vascular plants. However, the authors conclude that 'Sediment concentrations greater than 1,250 ppm would be necessary to cause plant growth reductions that might be considered critical to the plants' ability to survive.' This statement should not be taken out of the context of the experiments conducted and applied to field conditions. First, the plants were growing in only about 60 cm of water and the results may apply only to shallow water conditions. Secondly, and probably more importantly, many natural stands of submersed vascular plants may be living near their limit of tolerance owng to physical, chemical, and biological factors discussed previously. Additional light reduction due to turbidity, even at the lowest levels of suspended sediment used in these experiments (50 ppm), may exceed the threshold of tolerance for plant communities already subjected to other stress factors.

Field Studies

Few field studies have been conducted to determine photosynthetic and growth responses to light. Because of the problems discussed above as well as changing light conditions within the course of a day or season, such experiments are difficult to interpret and extrapolate to field observations of plant distribution. However, they may have some value 1n a relative sense when several species are compared. For example, Meyer et al. (1943) determined the compensation depth for shoot tips of several submersed species in Lake Erie. Compensation light intensity was approximately 1 percent of the intensity measured at 5 cm depth for Elodea canadensis, Potamogeton richardsonii, Vallisneria americana and Heteranthera dubia. The value for Najas was somewhat higher at 2.6 percent.

Photosynthesis and respiration in submersed species were studied in detail by Ikusima (1965, 1966, 1967). He showed that photosynthesis decreased progressively from the upper part to the basal parts of community of Potamogeton crispus and Vallisneria asiatica Miki, but that respiration was fairly constant throughout (Ikusima, 1965). Photosynthesis in Vallisneria denseserrulata Makino beds varied considerably from day to day depending on whether the weather was clear, cloudy, or rainy (Ikusima, 1966). As expected, the compensation depth may vary from hour to hour during the day even though the light compensation point of the plants may remain constant. Monthly differences occurred as well (Ikusima, 1967). Interception of light by apical portions of shoots, which reduces considerably the rate of photosynthesis of lower portions, results in a large respiratory demand where biomass is general ly largest. Communities during overcast or rainy days may even have a negative net organic matter budget.

The results of the laboratory experiments and field studies cited above demonstrate a number of problems in interpreting instantaneous measurements of photosynthesis. Species can be ranked according to their capacity for photosynthesis at light saturation intensities, which may be indicative of their success in competition if light saturation intensities persist under field conditions. However, distribution and abundance of aquatic-macrophytes in nature reflects the totality of the forcing functions that affect macrophyte growth, of which light is just one.