Why do wetlands have a rich supply of nutrients?

Well, first off, the sediments get brought down from the mountains after eroding away. Those sediments combine with others as it passes through the creeks and streams, rivers etc. Whatever dies, for example lets say chum salmon since they have nutrients within them. They aren't good swimmers so they are normally at the mouth of the river or in the small streams nearby. When they die, their nutrients go into the soil, that soil or whatever nutrients in that, could be swept out to the wetlands and piled up. Thus, adding rich nutrients. Adding the sediments from high up, you have a rich supply of nutrients from the wildlife that dies there as well.