Isoetid Species Research Articles

Structural and functional changes in macrophyte species composition in softwater lakes after 60 years of land use

We compared structural and functional changes in macrophyte species composition in softwater lakes with isoetids located along the southern shore of the Baltic Sea (NW Poland) in two time periods (1955–1959 and 2015–2020). The research aimed to determine the trend of changes in macrophyte composition influenced by fields and/or urban fabric, as land use. The land-use pressure measure referred to the volume of land occupied around the lake. In the second time period, the number of plant species in the lakes increased twofold (20 vs. 39), compared to the first period. The average values of species richness were statistically higher (p < 0.001) in the second period (15.7 vs. 8.6). The functional diversity of plants in the lakes revealed statistically significant differences in both periods compared. The FD Rao values calculated for plant life span, growth forms, and FD multi-traits were statistically higher in the second period (p < 0.001). Our findings revealed that the anthropogenic pressure on lakes over a period of 60 years caused a decrease in the share of sensitive species in macrophyte species composition (isoetids and mosses), but an increase in common plants with a completely different set of species functional traits. This is related to the environmental changes that occurred between the two periods studied. First of all, we noticed significant changes in the transparency (visibility) of the water. In the second period, the value of this trait is used in each lake, which uses the transmission of photosynthetic active radiation (PAR) light transmittance to the plant and can affect the species composition. These findings show that an increase in biodiversity can relate to a decrease in freshwater ecosystem function, mainly via lost function of evergreen isoetid species.

The importance of catchment vegetation for alkalinity, phosphorus burial and macrophytes as revealed by a recent paleolimnological study in a soft water lake.

The land use within a catchment may markedly affect the environmental conditions in a lake and the storage capability of its sediments. This study investigated how changes in the dominant catchment vegetation (from local stands of deciduous trees over extensive heathland with some agriculture to mainly coniferous forest) occurring during the last ca. 200years were reflected in the sediments of a soft water lake and how these changes influenced the lake ecosystem. Pollen, macrofossils, metals, different phosphorus (P) forms, organic matter, carbon and nitrogen contents were determined in short sediment cores. This novel combination of proxies revealed that 1) the reduction of deciduous trees in the watershed seemingly reduced the calcium (Ca) supply to the lake and thereby its buffering capacity. This development was accompanied by decreased abundances of Ca-dependent species and subsequent increases in acidophilic species. 2) The sedimentary contents of organic matter, non-reactive P and humic-bound P were evidently higher in sediments deposited during the time when deciduous trees were abundant, which is probably linked to a stabilising effect by Ca. 3) An erosion event clearly reduced the amounts of macrofossils of isoetid species and characeans, indicating a reduction in their maximum distribution depth because of lower water transparency. Overall, the results of our paleolimnological study are of importance within lake management by convincingly showing how land use changes may (irreversibly) affect environmental conditions and species composition in soft water lakes and the storage of organic matter and P in their sediments.

Reduced root anchorage of freshwater plants in sandy sediments enriched with fine organic matter

Summary The decline of submerged macrophytes in nutrient‐enriched lakes has been explained by shading from phytoplankton. Reduced anchorage in softer organic sediments may, however, enhance the loss of submerged macrophytes; therefore, we tested whether increased water content results in a decline of anchorage of artificial root systems and isoetid species in sandy sediments enriched with fine organic particles, and compared anchorage and hydraulic drag for short isoetid and tall elodeid species to evaluate the risk of uprooting. Constructed and natural sandy sediments followed the same hyperbolic relationship of higher water content with increasing organic content. Anchorage of artificial root systems peaked in sediments containing small amounts of organic matter (0.7–1.2% dry weight) and water (20–27% WW) and was up to six times higher compared with more organic‐rich sediments. Anchorage of the isoetid, Lobelia dortmanna, in natural sandy sediments increased linearly with plant weight and root surface area and declined with higher organic content of sediments. Multiple regression models could account for the positive influence of plant size on root anchorage and the negative influence of organic matter and water in sediments. Hydraulic drag on rigid shoots of three isoetid species increased with the second power of water velocity. The species‐specific drag coefficient was consistently high for these isoetids and markedly lower and declining with increasing velocity for two elodeids with flexible tissue. Anchorage of two isoetid species in mineral and more organic sediments from shallow water markedly exceeded estimated hydraulic drag under storm events inducing 0.6 m s−1 currents, while elodeid species were only safely rooted in firm mineral sediments. These findings are in agreement with the growth of isoetids in wave‐exposed shallow waters, while elodeids prefer deeper or sheltered sites. The multiple regression models predicted that L. dortmanna could attain critically low anchorage on highly organic sediments in accordance with observations of plant‐free littoral sediments of colloidal organic texture. Because it is well established that sediment anoxia severely impairs root development of L. dortmanna, enrichment of sediments with labile organic matter may represent a double jeopardy by reducing root development and anchorage.

Organic enrichment of sediments reduces arbuscular mycorrhizal fungi in oligotrophic lake plants

Summary1. Arbuscular mycorrhizal fungi (AMF) commonly colonise isoetid species inhabiting oxygenated sediments in oligotrophic lakes but are usually absent in other submerged plants. We hypothesised that organic enrichment of oligotrophic lake sediments reduces AMF colonisation and hyphal growth because of sediment O2 depletion and low carbon supply from stressed host plants.2. We added organic matter to sediments inhabited by isoetids and measured pore‐water chemistry (dissolved O2, inorganic carbon, Fe2+ and ), colonisation intensity of roots and hyphal density after 135 days of exposure.3. Addition of organic matter reduced AMF colonisation of roots of both Lobelia dortmanna and Littorella uniflora, and high additions stressed the plants. Even small additions of organic matter almost stopped AMF colonisation of initially un‐colonised L. uniflora, though without reducing plant growth. Mean hyphal density in sediments was high (6 and 15 m cm−3) and comparable with that in terrestrial soils (2–40 m cm−3). Hyphal density was low in the upper 1 cm of isoetid sediments, high in the main root zone between 1 and 8 cm and positively related to root density. Hyphal surface area exceeded root surface area by 1.7–3.2 times.4. We conclude that AMF efficiently colonise isoetids in oligotrophic sediments and form extensive hyphal networks. Small additions of organic matter to sediments induce sediment anoxia and reduce AMF colonisation of roots but cause no apparent plant stress. High organic addition induces night‐time anoxia in both the sediment and the plant tissue. Tissue anoxia reduces root growth and AMF colonisation, probably because of restricted translocation of nutrient ions and organic solutes between roots and leaves. Isoetids should rely on AMF for P uptake on nutrient‐poor mineral sediments but are capable of growing without AMF on organic sediments.

Surprising spectra of root-associated fungi in submerged aquatic plants

Similarly to plants from terrestrial ecosystems, aquatic species harbour wide spectra of root-associated fungi (RAF). However, comparably less is known about fungal diversity in submerged roots. We assessed the incidence and diversity of RAF in submerged aquatic plants using microscopy, culture-dependent and culture-independent techniques. We studied RAF of five submerged isoetid species collected in four oligotrophic freshwater lakes in Norway. Levels of dark septate endophytes (DSE) colonization differed among the lakes and were positively related to the organic matter content and negatively related to pH. In total, we identified 41 fungal OTUs using culture-dependent and culture-independent techniques, belonging to Mucoromycotina, Chytridiomycota, Glomeromycota, Ascomycota as well as Basidiomycota. Sequences corresponding to aquatic hyphomycetes (e.g. Nectria lugdunensis, Tetracladium furcatum and Varicosporium elodeae) were obtained. Eight arbuscular mycorrhizal taxa belonging to the orders Archaeosporales, Diversisporales and Glomerales were also detected. However, the vast majority of the fungal species detected (e.g. Ceratobasidium sp., Cryptosporiopsis rhizophila, Leptodontidium orchidicola, and Tuber sp.) have previously been known only from roots of terrestrial plants. The abundance and phylogenetic distribution of mycorrhizal as well as nonmycorrhizal fungi in the roots of submerged plants have reshaped our views on the fungal diversity in aquatic environment.

A cost-effective model for preliminary site evaluation for the reintroduction of a threatened quillwort

The choice of suitable sites for establishment of a new population of a species is the first step in a translocation programme. However, evaluation of a large number of sites can be demanding of time and money; practitioners need time- and cost-effective evaluation tools. A predictive model for the preliminary evaluation of potential reintroduction sites was developed using the endangered quillwort Isoetes malinverniana as a case study. The reliability of three water habitat variables (pH, conductivity, and clarity) as predictors of the presence/absence of I. malinverniana was tested. Three sets of logistic models were produced on the basis of the mean values of pH and conductivity measured over 3, 4 and 6 months to understand whether frequency of measurement affected the reliability of the models. Models of each set were ranked according to AIC and their reliability was then tested using data from the literature. The conductivity of water and pH were the most effective predictors of the suitability of sites for the reintroduction of I. malinverniana. In particular there was a negative relationship between the presence of I. malinverniana and these variables. Because of seasonal fluctuations in the variables considered, at least 4 months monitoring are required to obtain reliable results. Despite this, the method is still advantageous in terms of costs, compared with repeated measurements of a wide range of chemical characteristics at a large number of sites. The main goal of this method is to limit expensive chemical analysis to a few sites chosen after the exclusion of unsuitable sites, through application of the model. Considering the similar ecophysiological features of quillworts from oligotrophic waters, the same model or conceptual framework could be applied to other quillworts and isoetid species growing in degraded areas and needing active conservation. Copyright © 2012 John Wiley & Sons, Ltd.

Influence of quantity and lability of sediment organic matter on the biomass of two isoetids, Littorella uniflora and Echinodorus repens

SUMMARY 1. Despite real improvement in the water quality of many previously eutrophic lakes, the recovery of submerged vegetation has been poor. This lack of recovery is possibly caused by the accumulation of organic matter on the top layer of the sediment, which is produced under eutrophic conditions. Hence, our objective was to study the combined effects of quantity and lability of sediment organic matter on the biomass of Echinodorus repens and Littorella uniflora and on the force required to uproot plants of L. uniflora. 2. Lake sediments, rich in organic matter, were collected from four lakes, two with healthy populations of isoetids and two from which isoetids had disappeared. The four lake sediments were mixed with sand to prepare a range of experimental sediments that differed in quantity and lability of sediment organic matter. Two isoetid species, E. repens and L. uniflora, were grown in these sediments for 8 weeks. Sediment quality parameters, including elemental composition, nutrient availability and mineralisation rates, were determined on the raw sources of sediment from the lakes. Porewater and surface water were analysed for the chemical composition in all mixtures. At the end of the experiment, plants were harvested and their biomass, tissue nutrient concentration and (for L. uniflora) uprooting force were measured. 3. For both species, all plants survived and showed no signs of stress on all types of sediment. The biomass of E. repens increased as the fraction of organic matter was increased (from 6 to 39% of organic content, depending upon sediment type). However, in some of the sediment types, a higher fraction of organic matter led to a decline in biomass. The biomass of L. uniflora was less responsive to organic content and was decreased significantly only when the least labile sediment source was used to create the gradient of organic matter. The increase in shoot biomass for both species was closely related to higher CO2 concentrations in the porewater of the sediment. The force required to uproot L. uniflora plants over a range of sediment organic matter fitted a Gaussian model; it reached a maximum at around 15% organic matter and declined significantly above that.

Historical changes in the macrophyte community of a Norwegian softwater lake

Changes in macrophyte communities have occurred over the past decades in many oligotrophic softwater lakes with low carbon availability. Slow-growing isoetid species have been replaced by faster-growing elodeid species. Commonly, these changes are explained by anthropogenic nutrient enrichment or acidification of the lake water. Here we present a multi-proxy study in which we analysed plant macrofossils, pollen and spores, as well as sedimentological data from several cores taken from a SW Norwegian softwater lake. Our results indicate that the elodeid macrophyte Callitriche hamulata first appeared in this lake in the 1970s. Proliferation of C. hamulata occurred in the 1990s, replacing the hitherto dominant submerged Isoëtes macrophyte vegetation. Independent lines of evidence, such as diatom-inferred TP and pH reconstructions, showed no change during the past 200 years, therefore ruling out both acidification and phosphorus enrichment of the lake as possible causes for the observed change in the macrophyte community. Alternatively, expansion of Callitriche at the expense of Isoëtes may have been related to increased aquatic carbon availability, although nitrogen enrichment may also have been important.

Competition between isoetids and invading elodeids at different concentrations of aquatic carbon dioxide

Summary1. The growth of submerged macrophytes in softwater lakes is often assumed to be carbon limited. Isoetid species are well adapted to grow at low carbon availability and therefore commonly dominate the submerged macrophyte vegetation in softwater lakes. In many such lakes, however, large‐scale invasions of fast‐growing elodeid species, replacing the isoetid vegetation, have been observed.2. In a laboratory experiment, we tested how rising aquatic carbon availability, in interaction with different densities of the isoetid Littorella uniflora, affected the growth (and thereby the potential invasion success) of the elodeid Myriophyllum alterniflorum. For this purpose, the growth of M. alterniflorum was determined at a combination of three concentrations of dissolved CO2 (15, 90, 200 μmol L−1) and three densities of L. uniflora (0, 553, 1775 plants m−2).3. At an ambient CO2 of 15 μmol L−1, M. alterniflorum could not sustain itself, whereas at raised CO2 concentrations, growth became positive and increased with higher CO2 availability.4. The presence of L. uniflora, independent of its density, reduced the growth of M. alterniflorum by 50%. Whether this is related to nutrient availability or other factors is not clear.5. Despite the growth reduction of M. alterniflorum by L. uniflora, at CO2 ≥90 μmol L−1, L. uniflora was still overgrown by M. alterniflorum. This may imply that, in field situations, M. alterniflorum can invade softwater systems with relatively high CO2 availability, even in the presence of dense stands of L. uniflora.

The effects of species immigrations and changing conditions on isoetid communities

Historical data from the 1930s were compared with new data gathered during the 2000s to evaluate the effects of increased numbers of larger stature submersed species (both elodeids and characeans) on resident isoetid communities. The cover and species richness of submersed species were assessed in 23 seepage lakes in northwestern Wisconsin, USA, using randomly located 1 m × 1 m plots. Water clarity, conductivity and residential land use were determined on a whole-lake basis and the sediment type and water depth were recorded at each plot. The probability of elodeids or characeans occurring in isoetid plots increased with the number of elodeids and characeans gained by a lake since the 1930s, with additions ranging from two to 15 species per lake. However, not all species were equally likely to co-occur with isoetids. Six elodeid species ( Najas flexilis, Najas gracillima, Potamogeton gramineus, Potamogeton pusillus, Potamogeton spirillus and Vallisneria americana) along with Chara spp. were the most frequent isoetid associates, while other species that were common in the lakes, such as Elodea canadensis and Potamogeton robbinsii, were less frequent in isoetid plots. The lake-wide proportion of isoetid plots colonized by elodeids or characeans ranged from 5% to 100%, with increasing conductivity and total elodeids (plus Chara spp. and Nitella spp.) the strongest predictors of this colonization. Approximately half (49.6%) of all isoetid plots sampled had elodeids or characeans present (39.2% elodeids, 7.4% elodeids and characeans, 3.0% characeans), and isoetid cover and species richness were lower when these larger stature species were present. The risks this colonization poses for the long-term viability of isoetid species appeared to depend on multiple factors, including whole-lake characteristics, opportunities for refuge, and connections among regional isoetid populations. There was evidence of a time lag between the introduction of elodeid or characean species to a lake and invasion of isoetid plots within the lake, a process that deserves further study.

Contrasting Oxygen Dynamics in the Freshwater Isoetid Lobelia dortmanna and the Marine Seagrass Zostera marina

and Aims Submerged plants possess well-developed aerenchyma facilitating intra-plant gas-phase diffusion of O2 to below-ground tissues, which are usually buried in anoxic sediments. However, aquatic habitats differ in terms of O2 fluctuations in the water column and in O2 consumption of the sediment, and aquatic plants differ in aerenchymal volume and resistance to O2 diffusion through the plant and across leaf and root surfaces. The hypothesis that the freshwater isoetid Lobelia dortmanna and the marine seagrass Zostera marina should display pronounced contrasts in intra-plant O2 dynamics because of differences in morphology/anatomy, physiology and growth habitat was tested. In order to determine the O2 dynamics and relate this to the anatomy and morphology of the two species, O2 microelectrodes were inserted in the aerenchyma of leaves and roots, the sediment pore-water, and the water column in the field. Manipulation of water column O2 in the laboratory was also carried out. It was found that intra-plant transport of O2 between leaf and root tips takes place more readily in L. dortmanna than in Z. marina due to shorter distances and greater cross-sections of the aerenchyma. The major exchange of O2 across roots of L. dortmanna can be accounted for by small intra-plant resistances to diffusion, larger root than leaf surfaces, and greater radial diffusive resistance of leaves than roots. In contrast, the major O2 exchange across leaves than roots of Z. marina can be accounted for by the opposite anatomical-morphological features. The larger aerenchymal volume and the smaller metabolic rates of L. dortmanna compared to Z. marina imply that turnover of O2 is slower in the aerenchyma of L. dortmanna and O2 fluctuations are more dampened following changes in irradiance. Also, O2 accumulated in the aerenchyma can theoretically support dark respiration for a few hours in L. dortmanna but for only a few minutes in Z. marina. The build-up of O2 in the pore-water of L. dortmanna sediments during the day as a result of high release of photosynthetic O2 from roots and low O2 consumption of sediments means that sediment, aerenchyma and water are important O2 sources for respiration during the following night, while Z. marina relies on the water column as the sole source of O2 because its sediments are anoxic. These differences between L. dortmanna and Z. marina appear to represent a general difference between the isoetid species mainly inhabiting sediments of low reducing capacity of oligotrophic lakes and the elodeid freshwater species and marine seagrasses mainly inhabiting sediments of higher reducing capacity in more nutrient-rich habitats.

The isoetid environment: biogeochemistry and threats

Isoetid species are small, slow-growing, evergreen water plants with thick, stiff leaves or stems that form basal rosettes and have a proportionally large below ground biomass. Isoetids often dominate carbonate poor (weakly buffered) and nutrient poor (oligotrophic) water and are characterized by a (very) slow growth rate. The special adaptations to oligotrophic conditions enable them to grow where other plants are unable to thrive. The high porosity of the plants as a whole and the permeability of the roots in combination with the very low permeability of the leaves enable the efficient use of carbon dioxide from the sediment and also the oxidation of the sediment. The oxidation of the sediments helps them to create and maintain oligotrophic conditions. The processes involved are discussed in this review. Acidification and eutrophication, which are also discussed, are the most important threats for these systems.

Alkalinity and trophic state regulate aquatic plant distribution in Danish lakes

Main distribution patterns of submerged macrophytes in a large number of Danish lakes were determined and relationships to environmental variables evaluated by different multivariate analysis techniques. The lakes varied greatly in location, size, depth, alkalinity and trophic status. There were distinct differences in the distribution of species and growth forms among the lakes. The lakes separated into five groups of characteristic species compositions. Alkalinity was the main factor responsible for the species distribution. Lakes of high alkalinity were dominated by vascular plants of the elodeid growth form, lakes of intermediate alkalinity contained a variety of elodeids and vascular plants of the isoetid growth form, while lakes of low alkalinity and low pH had several isoetids and bryophytes, but very few elodeids. Alkalinity is a close descriptor of the bicarbonate concentration, which is an important source of inorganic carbon in the photosynthesis of many elodeids. The species distribution was related to their ability to use bicarbonate and extract inorganic carbon, implying that the observed distribution has an eco-physiological foundation, though a substantial variation suggests an influence of phenotypic plasticity and local environmental heterogeneity. Trophic state also influenced the distribution of species, with very eutrophic lakes having only a few robust elodeid species able to compensate for turbid conditions, while small elodeids and slow-growing isoetid species were absent. The distance separating the lakes did not influence similarity in species composition among them.

Development of vegetation and environmental conditions in an oligotrophic Danish lake over 40 years

1. Oligotrophic ‘Lobelia‐lakes’ are rare in Denmark. The few that have survived widespread eutrophication are in uncultivated sandy regions and are threatened by acidification and increased nutrient deposition from the atmosphere. Grane Langsø was considered the clearest lake in Denmark in the 1950s, when isoetids grew to 5.5 m depth and characeans covered the deepest part of the lake bottom at 11.5 m. The goal of the study reported here was to examine whether the lake has maintained this high quality by studying the development of macrophyte distribution and composition in relation to the changes in pH, nutrient conditions and transparency over the past 40 years.2. Aquatic mosses now dominate the vegetation rather than rooted isoetids, probably as a result of acidification (summer median pH was 5.60 in 1958–1963 compared to 5.07 in 1976–1994). Along with a reduced median pH, annual pH fluctuations have increased from about 0.3 units 40 years ago to 0.9 units today.3. The depth limit of isoetid species has declined markedly and Nitella flexilis, which previously dominated the dense bottom vegetation in the hypolimnion, has disappeared. These changes can be accounted for by reduced light penetration and an increased cover of filamentous green algae and mosses on the isoetid species.4. The decline in light penetration was mainly caused by increased humic content following storm falls in 1981–84, but increased nutrient deposition may also have stimulated the growth of phytoplankton and filamentous green algae. The restricted macrophyte growth in the hypolimnion during summer can account for the change from oxygen supersaturation 40 years ago to anoxia today. The reduced light penetration has resulted in cooling of the hypolimnion, thereby increasing the stability and duration of thermal stratification. This increases the likelihood of anoxia in August–September. Profound changes in the plant community and in oxygen conditions have occurred, even though the decline of pH has been small and long‐term changes in nutrient concentration have been difficult to detect.

An individual's status in populations of isoetid species

Individual size and fecundity of Isoetes lacustris L., Lobelia dortmanna L. and Littorella uniflora (L.) Aschers., were studied in relation to the depth gradient of an oligotrophic lake. The plants examined included individuals growing in the central part of the population habitat and in the deep and shallow border zones. Zone-related variation in population habitat and structure over the depth gradient has been demonstrated. The Isoetes population is strcturally divided into central and shallow-water zones; the Lobelia population into central and deep zones; the Littorella population into shallow and wave-disturbed, as well as into deep border zones. Isoetes individuals in the cental zone of the habitat have a rosette size of 87.1±37.8 cm 2, root length of 10.9±3.6 m and 839±422.6 macrospores. Corresponding figures in the shallow, non-disturbed zone are 116.8±47.1 cm 2, 17.5±6.6 m and 2289.3±1167. Lobelia individuals in the central zone of the habitat have a rosette size of 10.6±4.1 Cm 2, root length of 5±1.9 m and 1065.3±24.6 seeds. Corresponding figures in the deepest zone are 17.2±6.8 cm 2, 5±2.1 m and 1213.5±245.6. A small group of large and fecund individuals whose growth cycle has taken place in the border zones play a significant role in the population regeneration process: Lobelia in the deep zone and Isoetes in the shallow area not disturbed by waves. In every population (and habitat) zone, fecundity is a function of individual size, and variation in size is always greater than variation in fecundity. There are more small individuals in a population than indicated by the normal distribution (Gauss curve). Distributions of individual size- and fecundity-defining characters are always positively skewed.

Species richness – standing crop relationships along four lakeshore gradients: constraints on the general model

The applicability of a model describing the relationship between species richness and standing crop and litter was tested in each of four lakeshore habitats within Wilsons Lake, Nova Scotia. The shapes of the curves describing the species richness – standing crop relationships in the four habitats were significantly different. Standing crop and litter values were positively correlated with measured indices of soil fertility (loss on ignition and silt and clay content) and negatively correlated with disturbance as determined by the location of the shrub zone and overwinter damage to wooden pegs. Sites with high standing crop had a lower proportion of biomass composed of evergreen and isoetid species than the proportion in sites where standing crop was low. The among habitat variation observed in this study, and the variation in published values of standing crop values yielding maximum species richness, limit the accuracy of predictions from this model. Steps for refinement are suggested.

The effect of different growth conditions on dark and light carbon assimilation in Littorella uniflora

The effect of long-term exposure to different inorganic carbon, nutrient and light regimes on CAM activity and photosynthetic performance in the submerged aquatic plant, Littorella uniflora (L.) Aschers was investigated. The potential CAM activity of Littorella was highly plastic and was reduced upon exposure to low light intensities (43 μmol m−2 s−1), high CO2 concentrations (5.5 mM, pH 6.0) or low levels of inorganic nutrients, which caused a 25–80% decline in the potential maximum CAM activity relative to the activity in the control experiments (light: 450 μmol m−2 s−1; free CO2: 1.5 mM). The CAM activity was regulated more by light than by CO2, while nutrient levels only affected the activity to a minor extent. The minor effect of low nutrient regimes may be due to a general adaptation of isoetid species to low nutrient levels. The photosynthetic capacity and CO2 affinity was unaffected or increased by exposure to low CO2, irrespective of nutrient levels. High CO2, low nutrient and low light, however, reduced the capacity by 22–40% and the CO2 affinity by 35-45%, relative to control. The parallel effect of growth conditions on CAM activity and photosynthetic performance of Littorella suggest that light and dark carbon assimilation are interrelated and constitute an integrated part of the carbon assimilation physiology of the plant. The results are consistent with the hypothesis that CAM is a carbon-conserving mechanism in certain aquatic plants. The investment in the CAM enzyme system is beneficial to the plants during growth at high light and low CO2 conditions.

Distribution and quantitative development of aquatic macrophytes in relation to sediment characteristics in oligotrophic Lake Kalgaard, Denmark

SUMMARY. The distribution and quantitative development of aquatic macrophytes have been studied in oligotrophic Lake Kalgaard, Denmark. The vegetation is dominated by isoetid species, which are widely distributed (about 40% of the lake bottom) compared to emergent and floating‐leaved macrophytes (about 4%). Littorella uniflora dominates at depths of 0–2 m and Isoetes lacustris from 2.0 to 4.5m. Within the colonization area the mean midsummer biomass of Littorella is 112g organic dry weight m−2 and that of Isoetes, 66 gm−2. The total biomass of these two species constitutes 99% of the biomass of submerged macrophytes.The perennial Littorella shows only small seasonal biomass variations. The vegetational biomass, the above‐ground fraction of the biomass, and the weight of individual plants all increased with the organic content of the sediment at water depths from 0 to 0.75 m. At the same time the interstitial concentrations of carbon dioxide, extractable inorganic nitrogen, and exchangeable inorganic phosphorus increased, thus supporting the hypothesis that an increasing organic content of the sediments at this low level creates a physiologically richer medium for the plants.The isoetid growth form is discussed in relation to the chemical environment of oligotrophic, softwater lakes.