Myriophyllum spicatum L. and Hydrilla verticillata (L.f.) Royle grown on 40 different sediments from 1 7 geographically widespread North American lakes indicated 10-and 20-fold declines in growth with increasing sediment organic matter up to a concentration of ""20% dry sediment mass. Poor growth was obtained also on inorganic sediments with a sand fraction exceeding 75% dry sediment mass. Diminished growth on organic sediments occurred at low values of sediment density, and on sands, at high values of sediment density. Differential centrifugation of organic sediment, facilitating an increase in sediment density with no change in organic matter content, stimulated Hydrilla growth, indicating that sediment density rather than organic matter content was most influential in regulating growth.Macrophyte growth and nutrient mass accumulation in shoots were closely correlated, but essentially unrelated to concentrations of nutrients in shoots. The magnitude (r value) and statistical significance of correlations between nutrients in macrophyte shoots and sediments varied appreciably, depending on the form of shoot nutrient data (concentration or accumulation), and the type (interstitial water or total), and basis (mass or volume) of sediment nutrient data. Growth and nutrient accumulation were highly correlated with sediment nutrient concentrations based on volume, indicating interrelationships among growth, nutrition, and sediment density.Additions of P and Fe in combination to organic sediments with N supplied in the overlying solution resulted in significant growth increases in Hydrilla, suggesting that diminished growth on unfavorable sediments was caused by multiple nutrient limitation. Nutrient uptake on low-density organic sediments was apparently limited by long diffusion distances. Limited rates of nutrient diffusion and exchange in coarse-textured sediments, in addition to low nutrient status, may have contributed to their poor ability to support macrophyte growth. Thus, mechanisms of growth limitation on both sands and organic sediments appear to involve nutrition.

Under controlled experimental conditions, the influences of light and temperature on the growth and metabolism of Egeria densa Planch, Hydrilla verticillata Royle, and Myriophyllum spicatum L. were comparatively examined. Light was controlled at six levels ranging between 5 and 75% of full sunlight at solar noon. Water temperature was controlled at five levels ranging between 16° and 32°C. Growth considerations included morphology, biomass, and nutrition. Photosynthesis, respiration, and C0 2 compensation points were determined to evaluate physiological differences in plant growth as affected by the experimental ranges of temperature and light.External morphology in these species was significantly affected by the different experimental light and temperature conditions. Both low light and high temperature promoted extensive shoot elongation and associated canopy formation. Biomass production and carbon metabolism in all species were influenced more by temperature than by light. Each of the species demonstrated metabolic acclimation to light over a broad range. Conversely, the macrophyte species considered here were not strictly capable of acclimating to temperature.Growth rate and the seasonal progression of senescence were interrelated in these species. Higher temperatures stimulated growth and promoted a compression of the growth cycle. The relationship between photosynthesis and respiration (P:R) was appreciably reduced by senescence, but the C0 2 compensation point did not reflect this condition. In the species examined, C0 2 compensation points decreased with increasing temperature, suggesting adaptations to low free C0 2 levels in the environment.Latitudinal differences in integral seasonal temperature, in relation to species-specific ranges of thermal tolerance, appear to be important in influencing the geographical distribution of the species considered here. Light may be the primary determinant of their depth distribution, but its importance in this regard could be somewhat diminished by their significant abilities to extend to the water surface under low light conditions.

The mobilization of sediment phosphorus (P) by three submersed freshwater macrophyte species was investigated on five different sediments. The study was conducted under controlled environmental conditions in lucite columns that enabled the separation of" sediment and plant roots from the overlying P-free 'complete* nutrient solution. The species investigated (Egeria densa. Hydrilla verticillata, and Myriophyllum spicatum) had minor root systems (on a biomass basis), but were demonstrated to be fully capable of deriving their P nutrition exclusively from the sediments. Phosphorus absorption and transloeation Into shoots (i.e., mobilization) was substantial, and in some cases suggested a greater than 1000-fold turnover of interstitial water PO4-P over a 3-month period. Sediment P mobilization, a function of both plant growth and tissue P concentration, differed considerably among plant species and sediments. Phosphorus release from the species investigated appears to be primarily dependent upon tissue decay rather than excretory processes. The mobilization of sediment P by submersed macrophytes represents an important aspect of the P cycle, and may affect the overall metabolism of lacustrine systems.

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