John W. Barko

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.

Influence of sediment salinity and nutrients on the physiological ecology of selected salt marsh plants

Abstract An investigation of the influence of salinity and nutrients on the physiological ecology of Spartina alterniflora, S. foliosa, S. patens and Distichlis spicata was conducted under simulated tidal inundation. Growth differences with regard to sediment type were attributed to differences in sediment salinity and nutrients. Relative salinity tolerances of the four species were comparable to those obtained from field investigations performed by others. The species studied are known to be salt secreting, but were also shown to be capable of ion exclusion, which in some cases resulted in increased sediment salinity. Selective uptake of potassium was demonstrated, and tissue potassium was found to be linearly related to sediment salinity. The importance of salinity on plant growth in natural marshes is discussed in relation to other environmental factors.

Influence of potassium source (sediment vs. open water) and sediment composition on the growth and nutrition of a submersed freshwater macrophyte (Hydrilla verticillata (L. f.) Royle)

Abstract Potassium uptake by Hydrilla verticillata (L.f.) Royle from sediment versus overlying water was evaluated in relation to the K demands incurred by this species during an 8-week period of growth. The investigation was conducted on a heterogeneous assemblage of sediments and in two solutions differing fundamentally in the presence (2.3 mg l −1 ) and absence of K. Both biomass production and shoot morphology in Hydrilla varied significantly between solutions and among sediments. In contrast to N and P, which were readily mobilized from most sediments, K was mobilized from all sediments to only a minor extent by this species. Mobilization of K was proportional to interstitial water K concentration; yet on at least four of the six sediments examined, K supplied from sediments was insufficient to support the maximal growth of Hydrilla . The open water rather than the sediment appears to be the primary source of K supply to this species and perhaps to most other submersed freshwater macrophytes. Where K was supplied in solution, sediment-related differences in the growth of Hydrilla negatively correlated with sediment organic matter content over the range 0.2–56.2% dry sediment mass. It is tentatively suggested that the organic composition of sediments may influence the species composition of aquatic macrophyte communities.

The growth of Myriophyllum spicatum L. in relation to selected characteristics of sediment and solution

Abstract The growth of Myriophyllum spicatum L. was examined experimentally on a variety of sediments and in two ionically distinct solutions. The influence of solution on growth was greatest on sediments that provided optimum growth and least on sediments that retarded growth. In agreement with results of a related investigation involving another submersed macrophyte species, the growth of M. spicatum was relatively poor on highly organic sediments. The possibility that high-level accumulations of organic matter in sediments might be inhibitory to M. spicatum was substantiated in one experiment involving organic matter additions to sediment. In another experiment, the influence of ionic composition of solution on the growth of this species was demonstrated to involve differences in both exogenous calcium and dissolved inorganic carbon (DIC) concentrations. Through mechanisms similar to those investigated here, sediment and open water may interact in determining the growth of M. spicatum in natural systems. This interaction needs to be carefully considered in evaluating variations in the productivity and distribution of submersed macrophytes.

Influences of light and temperature on chlorophyll composition in submersed freshwater macrophytes

Abstract In three submersed freshwater macrophyte species grown in a greenhouse over broad experimental ranges of light and water temperature, total chlorophyll ( a + b ) increased with decreasing irradiance and, in two of the three species, with increasing temperature. In contrast, light and temperature had only minor and inconsistent influences on chlorophyll a : b in these species. From results of this and other investigations involving experimentally-controlled light conditions, it appears that total chlorophyll in submersed macrophytes is inversely related to irradiance above photon flux densities minimally required for plant growth. However, the general applicability of this statement to the species investigated here (or others) is uncertain, because thermal gradients in macrophyte dominated littoral zones may promote gradients in macrophyte total chlorophyll with depth in a direction opposite to that expected solely in response to light.

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.