Shorter hydroperiods of three or four months promote the growth of periphyton: algae and other microscopic organisms covered with calcium carbonate crystals.[15] Periphyton is the basic building block of marl, a calcitic mud. In areas with hydroperiods of longer than nine months, peat builds up over hundreds or thousands of years due to many generations of decaying plant matter. Peat and marl are considered nutrient-poor soils that foster the growth of specialized vegetation depending on the length of the regional hydroperiod.