Yes, it is phosphorus (P) that originates primarily from continuing man-generated imports:
urban wastwaters and agricultural activities in the Lake watershed.
Most of that P ends up in Lake Okeechobee which currently contains ~180 ppb of Phosphorus in its water -
40ppb is the recommended maximum.
The bottom muds of the Lake contain some 51,000 tons of P (called "legacy P") that keeps accumulating there as -
the Lake is receiving 656 tons of P per year (2009) - 110 t/year is the recommended maximum
(this TMDL, Total Maximum P Daily Load has never been attained). Most of the incoming excess Phosphorus ends up
being deposited in the Lake muds.
The Lake just cannot cope with the P overload that keeps pouring in !
This is why the lake water is not welcomed anywhere - creating havoc in the estuaries where it curremtly goes
(via the Caloosahatchee and St. Lucie River) - let alone in P-sensitive Everglades located downstream.

Some of the more pleasant facts and brief history of Lake Okeechobee can be seen, for instance, on this video clip provided by the US Army Corps of Engineers (CLICK picture or HERE). US-ACE are maintaining the Lake's flood-protection system dike (Hoover Dike). Yes, apart from supporting tourism and recreational activities, the Lake serves as a major and natural water reservoir to buffer Florida's seasonal weather extremes - freshwater is precious and flood protection is a must - - The South Florida civilization's wellbeing and life depends on both.

Lake Okeechobee Statistics

Natural freshwater lake - receives water from its 9 watersheds 1-foot rainfall = 3 feet lake rise Twice as large as NY City - Lake can fill 6-times faster than water can be released LO Surface Area 451,000 acres 780 sq miles   Shore Length 135 miles Average Depth 9 feet Maximum Depth 12 feet Minimum Elevation 9 feet Maximum Elevation 19 feet Water Volume 1,100,000 ac-feet Normal Elevation above sea 14 feet Catchment/Drainage Basin Area 4,785 sq. miles Water Residence Time 3.5 years 1 f of Lake water level difference = 450,000 acre-feet of water (approximately)   Water into the Lake 2.58x10exp10 m3 39% rainfall, 31% Kissimmee River + smaller inflows Water out of the Lake 2.20x10exp10 m3 66% evapotranspiration, 12% West + 4% East, 18% canals 5 gated outlets and 5 gated inlets 9 pump stations 33 primary & secondary culverts 9 nav. locks
Concentration of Phosphorus in LO is out of control	Expanding mud deposits in Lake Okeechobee
Useful Links for information on Lake Okeechobee

REPORTS	WEBPAGE-WEBSITE
EvergladesHUB - Blog with Bo-	ACE-CERP website
Sludge Residuals Application - Audubon Report-090620	Lake Okeechobee Brief - SFWMD
Lake Okeechobee Lake Stage - CERP Performance, 2007	Audubon Recipe for LO Recovery
2007 South Florida Environmental Report (SFER, SFWMD)	ACE - LO & Watershed
Lake Okeechobee: A Synthesis of Information and Recommendations for its Restoration - Audubon 2005	ACE - LO Herbert Hoover Dike Project, photos
Resources of Lake Okeechobee - Audubon, 2005	2010 South Florida Environmental Report (SFWMD), Chap.10: Lake Okeechobee
BioOne-2005: LAKE OKEECHOBEE CONCEPTUAL ECOLOGICAL MODEL (by K.E.Havens & D.E. Gawlik)	TMDL – TECHNOLOGIES& RESEARCH: LESSONS LEARNED Florida Dept. Agri. 2005

Why can't Engineers dredge Lake Okeechobee to remove the phosphorus loaded muck ?

Muck removal from a smaller Lake like Tohopekalinga is feasible upon draw-down - but this is hardly possible with LO

Although 30,000 tons of the phosphorus that plagues Lake Okeechobee is located in sediments at the Lake's bottom, it may not be economically, practically, or ecologically feasible to remove these sediments by dredging. Not only would the cost of that effort be astronomical ($3 billion according to a 2003 study commissioned for the SFWMD), but the effort could take an impractically long time given the volume of sediments to be removed. The SFWMD has estimated that 200 million cubic meters of sediment would need to be dredged to remove all internally loaded sediments from Lake Okeechobee. That is enough to fill 250,000 Olympic-sized swimming pools, and "ten times the amount of material removed from any lake in the world." If dredging techniques from another shallow Florida lake (Banana Lake) were used, such dredging this could take as long as 270 years! In addition, the process could pose unacceptable risks for the Lake's fish and wildlife: dredging would stir up more sediments and expose Lake life to increased turbidity and nutrient levels. The feasibility of dredging was recently studied by SFWMD and its partners as a part of the Lake Okeechobee Sediment Management Feasibility Study. This study concluded that "the no-action" alternative, i.e. no in-lake dredging or chemical treatment, is the preferred alternative. Instead, the study recommends concentrating control activities in the watershed versus the lake. (For more information on this study, see Blasland, Bouck & Lee, Inc., Evaluation of Alternatives for the Lake Okeechobee Sediment Management Feasibility Study C-11650: A report prepared for the South Florida Water Management District, April, 2003, here.

Brief History of Lake Okeechobee

6,000 years ago	Lake Okeechobee forms
1500's	Tequesta, Mayaimi and Calusa tribes inhabit territory around Lake Okeechobee
1700's	Seminole tribe moves to the lake and calls it Okeechobee, which means big water
1837	American troops discover Lake Okeechobee in territorial battle against the Seminoles
1839	Federal Armed Occupation Act encourages Americans to settle Florida
1845	Florida becomes a state
1881	Hamilton Disston buys 4 million acres of land around Lake Okeechobee and begins to drain the land and connect the lake to the Caloosahatchee River
1905	Gov. Napoleon Bonaparte Broward creates the Everglades Drainage District - vows to drain So. Florida swamps.
1915	First settlers arrive in Clewiston
1926	Hurricane kills 400 in Moore Haven
1928	Hurricane kills 2,000 to 3,000 in Belle Glade
1929	Okeechobee Flood Control District forms
1930	U.S. Army Corps of Engineers instructed to build a 68-mile levee on Lake Okeechobee's southern edge.
1947	Hurricanes cause major flooding south of Lake Okeechobee.
1948	Central and Southern Florida roject for Flood Control and Other Purposes instructs the Army Corps to extend levee around the lake and to dredge and straighten the Caloosahatchee, Kissimmee and St. Lucie rivers for navigation.
1971	Kissimmee River straightening complete
1975	Scientists begin to link blue-green algae blooms in Caloosahatchee River to phosphorus pollution coming from Lake Okeechobee
1986	A gigantic blue-green algae bloom, fed by phosphorus pollution, slimes Lake Okeechobee
1988	Federal Government sues state and South Florida Water Management District for channeling polluted lake to Everglades National Park.
1992	State and Federal government settle suit with a consent decree that instructs the state to reduce phosphorus pollution in Lake Okeechobee and to treat water before it enters the Everglades. Congress also instructs Army Corps to re-study the Central and Southern Project for Flood Control.
1994	State Legislature passes the Everglades Forever Act.
2000	Re-study results in 30-year, $8 billion Comprehensive Everglades Restoration Plan and Army Corps begins a new water regulation schedule meant to manage the lake at around 15 feet above sea level so that habitat can grow back.
2001	Billions of gallons of lake water flushed to the estuaries to bring the lake level down, followed by record drought that dropped the lake to about 9 feet.
2004	Hurricane rains muddy lake water and raise the water level to about 18 feet.
2005	More hurricanes churn up the lake and keep water levels too high. More than 965 billion gallons of water flushed from lake to Caloosahatchee River.
2006	Estimated cost of the restoration climbs to $10.8 billion. Half the land needed for the project has been acquired.
2009	Florida State purchase of 71,000 acres of agricultural (sugar cane) land in the EAA from US-Sugar is seen as instrumental for eventually reconnecting Lake Okeechobee and the Everglades south of it.