Water hardness is a function of the concentration of dissolved carbonates (HCO3~— known as temporary hardness) and other salts (permanent hardness): the greater the concentration of such salts, the greater the ‘hardness’ of the water. In more acidic waters, dissolved carbonates are converted to CO2 and H2O; acidic water therefore tends to be less hard than alkaline water. The concentration of CO2 in hard water is generally very low, limiting the ability of submerged plants to absorb it for photosynthesis. Instead, plants typical of hard water tend to use HCO3~ as a carbon supplier. This is especially true for many undesirable filamentous algae. Therefore, the problem with this kind of weed increases with water hardness. On the other hand, several submerged perennials are not able to assimilate bicarbonate, and so need a higher amount of dissolved carbon dioxide, which is only available at low water hardness or in ‘soft water’. For this reason, species such as the highly demanding Hottonia palustris are not cultivatable in hard water. Figure 8.1 shows approximate values for trophic and hardness levels and their related chemical parameters.
Classification of water bodies according to trophic level and water hardness
Running and standing water bodies
In nature we find standing water bodies as ponds or lakes. Ponds are shallow enough to be populated completely by green plants whilst lakes have a deeper zone. Running water types include springs, streams or rivers. In running water, plants are more mechanically strained than in ponds or lakes. So typical plants of this vegetation do form no wide leaves, but narrow, thread-shaped bodies. Sometimes the same species produces quite different phenotypes in standing and running water.
In running water, warm layers will be continuously substituted with cooler ones through the action of the current. So standing water will reach higher temperatures than running water.
Oxygen and carbon dioxide are better incorporated in running waters than in standing ones because turbulence on the surface gives rise to a larger interface with the air. For that reason, some plants typical of streams grow poorly in ponds. The vegetation found in running water is also better supplied with nutrients. In still water, a nutrient depletion zone may form around plants because of their uptake. In comparison, in rivers and streams fresh supplies of nutrients are carried to roots and submerged shoots continuously. Typical species of running water, such as Veronica beccabunga, therefore should be cultivated under conditions rich in nutrients.
Flow rate also has a considerable influence on the range of plants that can be grown. Vegetation within the water body of swiftly running streams and rivers is likely to be limited, whilst the vegetation along the banks of rivers subjected to regular flooding and disturbance is likely to be composed of ruderal species able to tolerate periodic disturbance.