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.