Sapropels are dark organic rich marine sediments that contain greater than 2% organic carbon by weight.

Increasing thickness of organic sediments resulted in the necessity to dredge the lake.

Nutrients, like nitrogen, as well as numerous organic sediments regularly feed into the coastal waters and "hug the coast" as they flush out, he said.

Other theory suggests that the organic sediment may have been deposited in a shallow, partially restricted marine environment.

The limestone was laid down 500 million years ago as organic sediments settled to the bottom of a shallow sea that covered much of the Midwest.

The Niger River in particular deposited organic sediments out to sea over millions of years which became crude oil.

Over long periods of time, lakes, or bays within them, may gradually become enriched by nutrients and slowly fill in with organic sediments, a process called succession.

These will then develop through the usual plant successions into areas of organic sediment rather like the backswamps in the interior of the delta.

They can also form in conditions such as oxygen rich bottom waters and organic poor sediments.

Where nutrients are plentiful, productivity is high, production far outstrips decomposition, and organic sediments accumulate on the lake beds.