Water connectivity

In front of you, there is a channel which is in direct contact with the river. In the area there are also lakes that are situated further away from the river. These waterbody’s differ in their ‘connectivity’ to the river and this has consequences for the ecology and functioning of these lake ecosystems.

Lakes directly connected with the river will have approximately the same chemical water quality as the river itself. A lake in the floodplain that is not directly connected to the river, also receives wet deposition (which is a difficult word for rain and snow). The chemical composition of rain water is quite different from river water with usually lower concentrations of most substances. With a rain fall of 800 mm per year this means that those lakes receive per surface area 80 cm of relatively clear water, thus dilution of chemicals may take place. This in nicely visible if one measure the electric conductivity (EC), which is a measure of all positive and negative ions. In most not-connected floodplain lakes EC is much lower. For example, measurements near Wageningen showed that the Rhine was around 800 and the lakes around 450-500. Furthermore, seepage entering these lakes may also affect the water quality, usually leading to a better quality (lower EC).

Those differences in chemical composition may also be reflected in differences in phytoplankton and zooplankton communities; as well as in the vegetation that can be found. The connectivity of lakes with the river is also essential for a number of fishes and thus the fish communities present in the lakes. Some fish use the rivers and their connected floodplain lakes, but are rare/absent in the isolated lakes.

Restoration efforts and successes will be different for lakes that are isolated compared to lakes that have a strong connection with the river. For example, management of nutrients for connected lakes might be more difficult than for isolated lakes. This is also the case for biomanipulation. Removal of fish from a turbid lake that is connected to the river is doable, but efforts are usually in vain. Due to the connectivity, those lakes are quickly recolonized by the unwanted fish, since there are many of them in the rivers.

In short: The closer to the river, the higher the connectivity, the more the ecosystem is influenced by the river.