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

Abstract

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.