R. Michael Smart

Sediment‐Related Mechanisms of Growth Limitation in Submersed Macrophytes

Myriophyllum spicatum L. and Hydrilla verticillata (L.f.) Royle grown on 40 different sediments from 17 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.

Comparative Influences of Light and Temperature on the Growth and Metabolism of Selected Submersed Freshwater Macrophytes

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 CO2 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 CO2 compensation point did not reflect this condition. In the species examined, CO2 compensation points decreased with increasing temperature, suggesting adaptations to low free CO2 levels in the environ- ment. 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.

Laboratory culture of submersed freshwater macrophytes on natural sediments

Abstract Recent information on the relative roles of sediment and water as nutrient sources for rooted submersed freshwater macrophytes has facilitated the development of methods for culturing these plants. The use of natural sediments rather than culture solutions as the source of nitrogen, phosphorus and micronutrients largely prevents the occurrence of algal blooms and, for many purposes, obviates the need for axenic cultures. Growth requirements of submersed macrophytes are reviewed with regard to the provision of suitable culture conditions. Sediment substrate requirements are considered in relation to the role of sediment as a nutrient source. Two types of culture solution formulations are provided with procedures for establishing and maintaining submersed macrophyte cultures for experimental research.

Sediment-based nutrition of submersed macrophytes

The capabilities of four species of submersed freshwater macrophytes to mobilize nitrogen, phosphorus and potassium from three different sediments were comparatively examined in relation to their requirements for these nutrients during a 10-week period of growth under controlled environmental conditions. With all species, N and P were readily mobilized from each of the sediments and concentrated in plant shoots at levels well above those required for growth. However, the mobilization of K from all sediments was much less effective and may have limited the growth of the species considered here. Sediments represent a large and important source of N and P for rooted aquatic macrophytes, but K is probably supplied to these plants primarily from the water. Only small quantities, or none at all, of N and P were excreted from the species considered here during active growth. However, considerable quantities of these nutrients can be released to the water due to plant senescence and associated decay processes. Since a large fraction of the total nutrients, and in particular N and P, released during decay may derive from the sediment, this mechanism represents an important mode of sediment-nutrient recycling in aquatic systems.

Nitrogen Nutrition and Salinity Tolerance of Distichlis Spicata and Spartina Alterniflora

In a greenhouse investigation Distichlis spicata (L.) and Spartina alterniflora (Loisel.) were grown from seed on freshwater, brackish, and marine sediments. These species were also concurrently grown in sand culture under conditions of nitrogen or phosphorus limitation in order to establish critical tissue concentrations of these elements. These experimentally determined values were used to assess the limiting nutrient status of the plants grown on the sediments. Growth of both species was nitrogen limited on most sediments, and phosphorus was consumed luxuriously on all sediments. Aboveground biomass was correlated with interstitial water nitrogen concentration of the sediments. Under low nutrient conditions relatively more biomass was allocated to belowground rather than aboveground portions of the plants. Although the availability of nitrogen ultimately determined biomass accrual, growth rate was affected by the sediment salinity. Analysis of plant tissues and the sediment interstitial water at the end of the investigation revealed selective uptake of potassium and exclusion of sodium. These processes increased the ratio of sodium to potassium in the interstitial waters and also resulted in increased sediment salinities. The effect of salinity stress on plant growth in Spartina marshes is discussed in relation to nitrogen limitation.

Interrelationships between the growth of Hydrilla verticillata (L.f.) Royle and sediment nutrient availability

Abstract On sediment planted with Hydrilla verticillata (L.f.) Royle compared with control (unplanted) sediment we measured >90% and >30% reductions in concentrations of exchangeable N and extractable P following two consecutive 6-week periods of growth. Diminished N availability, owing to prior plant uptake, resulted in a 30% reduction in Hydrilla growth. Concomitant increases (>30%) in the concentration of exchangeable K suggest that this element, obtained via foliar uptake from overlying water, may be exchanged by roots of Hydrilla for ammonium in sediment. We postulate that under N-limiting conditions, K translocation in this species from shoots to roots potentially increases the availability of sediment N. An important implication of these findings is that under some circumstances the growth of Hydrilla may be self-limiting.

The nutritional ecology of Cyperus esculentus, an emergent aquatic plant, grown on different sediments

Abstract In a greenhouse investigation, Cyperus esculentus L. was grown from tubers on sediments obtained from different shoreline regions of the Great Lakes, U.S.A. The sediments were predominantly fine textures, but differed greatly in organic and nutrient contents. A wide range of biomass was obtained after 3 months of growth. Concurrent sand culture experiments with Cyperus under nitrogen- and phosphorus-limiting conditions provided critical concentrations of nitrogen and phosphorus, which were used to evaluate the supply of these elements to Cyperus grown on the different sediments. Differences in plant growth and biomass distribution were related to sediment fertility. Nitrogen was demonstrated to limit the growth of Cyperus on 10 of the 11 sediments examined. Ratios of belowground to aboveground biomass were negatively correlated with plant growth. The allocation of biomass to both aboveground and belowground portions of Cyperus is discussed in relation to the degree of nutrient limitation and other metabolic stresses.

The growth and biomass distribution of two emergent freshwater plants, Cyperus esculentus and Scirpus validus, on different sediments

Abstract The growth of two emergent freshwater plants, Cyperus esculentus L. and Scirpus validus Vahl., was investigated under greenhouse control in a simulated freshwater marsh environment. Plants were grown on one coarse and two fine-textured sediments differing significantly in phosphorus and nitrogen contents. Growth of both species, measured as total biomass accrual during a two-month period, was greatest on silty clay, intermediate on clay, and least on sand. Both shoot density (number of shoots per area) and the specific mass of individual shoots with regard to sediment type demonstrated patterns identical to that of total biomass. Below- to aboveground biomass ratios were inversely related to plant growth. The fine-textured sediments provided proportionately greater aboveground growth than the sand. Analysis of plant shoots indicated possible growth limitation by nitrogen on clay. Growth on silty clay was limited by neither nitrogen nor phosphorus, which were both present in the highest concentrations in this sediment. The biomass of both species on silty clay was within the range of most published estimates of the biomass of emergent freshwater plants in temperate regions of the world.

Establishment of Native Aquatic Plants for Fish Habitat: Test Plantings in Two North Texas Reservoirs

ABSTRACT Test plantings of native aquatic plant species were made in two Texas reservoirs. Founder populations of three native submersed or floating-leaved species were established within small (2- × 2-m) exclosures utilizing actively growing transplants. Herbivory and excessive sedimentation proved to be deterrents to plant survival and expansion. Nine small founder populations of Vallisneria americana were established within North Lake, a small reservoir with limited water level fluctuations and a developing community of native pioneer aquatic plants. Plants within all nine exclosures successfully established and began vegetative growth. In intact exclosures, the plants rapidly covered the sediment surface within the exclosures. Expansion beyond the exclosures was variable and occurred primarily during the cooler portions of the year when herbivory was assumed to be low. During the spring of the second growing season, herbivores cropped most of the previous expansion leaving only a narrow fringe of plan...

Impacts of Water Column Turbidity on the Survival and Growth of Vallisneria americana Winterbuds and Seedlings

ABSRACT Survival and growth of Vallisneria americana winterbuds was significantly related to both initial winterbud size and to the water column turbidity under which the plants were grown. Larger winter buds showed better survival and better growth than did smaller ones. Turbidity likewise significantly impacted the survival and growth of the plants. Over the turbidity range of 0.2–45 NTU (53–7% total incident light), the plants were shown to have progressively poorer survival and to produce fewer rosettes and total number of leaves. Vallisneria americana seedlings were likewise influenced by turbidity. Under high turbidity conditions the seedlings had significantly higher mortality, while surviving plants produced fewer rosettes and accumulated less biomass than seedlings grown under low turbidity conditions. In addition, under turbid conditions the seedlings had to invest proportionally more energy into above-ground tissues.