Architectural adaptation in Myriophyllum spicatum L. in a lotic environment: is it caused by current velocity?

Little information is available for aquatic plants regarding their architectural response to strong environmental drivers like water flow. We examined architectural variability in Myriophyllum spicatum L. in the short terminal section of a small canal earlier used for inland navigation. This stretch is characterised by decreasing water depth towards a final spill-over construction, which causes increasing current velocity. Visibly different plant beds had developed at three sampling sites, located between the upstream end of the study reach and the end atthe spill-over. This situation bears some resemblance to an experimental flume due to regulated water flow and constant discharge, yet with aquatic plant beds still located in their permanent environment during the whole year. Following this precondition our hypothesis envisaged a close relationship between current velocity and realised plant architecture. Current velocity was measured with an electronic vane device, and representative architectural features of plants were recorded from plant samples at the sites of different flow. Characteristic and significant variation in the architecture of M. spicatum was demonstrated at the sites of different current impact. Regarding other environmental parameters like sediment composition, water chemistry or the effect of shading no influence seems likely expected, as samples were collected across the canal width at each site. The mean values of all architectural parameters of M. spicatum follow the same trend with high significance, regarding the increase in plant length, branching, and the overall dimension of the plant beds, which is in close relationship tothe current velocity at the sampling sites. The few other records available in literature cited in this paper point into the same direction, but these studies were also carried out in the field. In our opinion the clear results may not comply with a final and experimentally generalised relationship between aquatic plant architecture and water flow. But our contribution offers some statistical proof that our hypothesis is not too far from explaining the effects of current velocity, which is one of the main environmental parameters defining aquatic plant growth.