Flow-on effects of water movement

Posted on March 23, 2016

Although aquatic plants risk being uprooted in fast-flowing streams, if they’re capable of varying their growth form to suit the conditions they may be able to reduce the chance of dislodgement. To see if water plants can adapt to fast-moving water by increasing their strength and reducing their drag, Swedish researchers compared the growth of Eurasian watermilfoil (Myriophyllum spicatum), a submerged species with whorled leaves, in different flow velocities. Twelve 100 mm shoots were cut from several mother plants growing in the same stream under similar conditions of flow and substrate. Each shoot was planted in sand in a pot, and six pots were placed on the stream bed at two sites with contrasting flows (mean velocities of 0.04 and 0.20 m/sec). The experiment lasted for 78 days (June – September). The investigators found that water flow had a number of significant impacts on plant structure and ecology. Specifically, at the high-velocity site: (1) the mean root biomass (and therefore the anchoring strength) was higher; (2) lateral shoots increased in number and formed a shield canopy to protect against currents; (3) there were more leaf whorls per shoot length and the shorter internode distance increased stem flexibility; (4) the leaf whorls were more compact, which reduced drag; (5) the diameter of the stem base was greater, which reduced the chance of breakage near the sediment surface; (6) shoot biomass was higher, probably because faster flows and the associated sparseness of attached periphyton increased nutrient uptake and photosynthetic efficiency; and (7) the abundance of macroinvertebrates (especially midge and mayfly larvae) living on the more complex plant surfaces was nine times higher than in low velocity flows. In summary, the study provided good evidence that the flow regime can have significant effects on the morphology of aquatic plants, with implications for trophic cascades and the structure of stream ecosystems.

Reference: Choudhury, M.I. et al. 2014. Stream flow velocity alters submerged macrophyte morphology and cascading interactions among associated invertebrate and periphyton assemblages. Aquatic Botany 120, 333–337.