Nearly all plant material will remove oxygen from water as it decomposes. Living plants which photosynthesize take oxygen out of water and put carbon dioxide into water at night, but in much smaller quantities than the oxygen they produce during photosynthesis in sunlight. Nonliving plant material depletes oxygen as it rots, with green material like fresh-cut grass doing so at a much faster rate than dried material like brown leaves or dry straw.
Fish and other aquatic creatures can not utilize the oxygen which is bonded with hydrogen to form water molecules. They live on dissolved oxygen contained within the water. Aquatic plants which engage in photosynthesis convert sunlight into energy. In the process during daylight hours, they take in carbon dioxide from the water and release oxygen. At night, the process is inverted, and plants do remove some oxygen from the water, but much less than they put out during the day. Aquatic plants provide food for zooplankton and fish, but their most critical function is providing dissolved oxygen to the aquatic environment.
There are certain time frames when live photosynthezing aquatic plants take dissolved oxygen out of the water and produce carbon dioxide. While this happens to a small extent each night, cloudy days also severely diminish the plants' oxygen output. When several cloudy days occur in succession in a water body with significant plant and animal life, daytime plant oxygen production may drop below the total rate that oxygen is being used by fish and other animals day and night, and by plants at night.
Dead plants have the effect of taking oxygen out of the water, because oxygen is used in the decomposition process at the same time that the plant is no longer adding oxygen to the aquatic system through photosynthesis. Green plant matter, such as fresh grass clippings, can rapidly deplete oxygen to levels dangerous to fish and other aquatic animals. Brown plant matter like dead leaves takes oxygen out of water at a slower rate.
If an excessive amount of aquatic plants die off, decomposition creates a deoxygenated layer, or dead zone, at the bottom of the lake or pond. Nutrients released from decomposition cause another bloom of plant growth, which in turn adds to the decomposing layer and further depletes oxygen. Water bodies in an advancing state of deoxygenation are called "eutrophic." Eutrophic waters fill in with strains of algae with low oxygen requirements until nothing else can live in the vicinity. Eutrophic oxygen depletion creates a dead water body.
Pond owners can take advantage of the oxygen removing characteristics of some plants to reduce algae growth. Dried barley straw placed in water will begin to decompose. Lignens, the substance which makes up the woody walls of stem cells in barley, linen, and other similar plants, decompose slowly, combining with oxygen to create humic acids. When sun shines on water containing dissolved oxygen and humic acid, it creates low levels of hydrogen peroxide which kills off excess algal growth. This process provides the benefit of clean water without the addition of toxic aquatic herbicides.