There are many ways that flooding can damage plants. Excessive moisture in soil decreases oxygen levels. This impedes respiration (where energy is released from sugars) in the roots leading to the build-up of carbon dioxide, methane and nitrogen gases. Ultimately, the roots can suffocate and die. Toxic compounds such as ethanol and hydrogen sulphide can also build up in the soil and damage plants. If leaves and stems are submerged, photosynthesis (absorption of energy from the sun to produce sugars) can be inhibited and plant growth can slow or even stop.