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water
Review
Legacy Phosphorus in Lake Okeechobee (Florida, USA)
Sediments: A Review and New Perspective
Thomas M. Missimer 1, * , Serge Thomas 2and Barry H. Rosen 2
Citation: Missimer, T.M.; Thomas, S.;
Rosen, B.H. Legacy Phosphorus in
Lake Okeechobee (Florida, USA)
Sediments: A Review and New
Perspective. Water 2021,13, 39.
https://doi.org/10.3390/w13010039
Received: 21 October 2020
Accepted: 23 December 2020
Published: 28 December 2020
Publisher’s Note: MDPI stays neu-
tral with regard to jurisdictional claims
in published maps and institutional
affiliations.
Copyright: © 2020 by the authors. Li-
censeeMDPI, Basel, Switzerland. This
articleis an open accessarticle distributed
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Creative CommonsAttribution (CCBY)
license(https://creativecommons.org/
licenses/by/4.0/).
1Emergent Technologies Institute, U. A. Whitaker College of Engineering, Florida Gulf Coast University,
16301 Innovation Lane, Fort Myers, FL 33913, USA
2Department of Ecology and Environmental Studies, Florida Gulf Coast University, 10501 FGCU Boulevard
South, Fort Myers, FL 33965-6565, USA; sethomas@fgcu.edu (S.T.); brosen@fgcu.edu (B.H.R.)
*Correspondence: tmissimer@fgcu.edu
Abstract:
Lake Okeechobee is one of the largest freshwater lakes in the United States. As a eutrophic
lake, it has frequent algal blooms composed predominantly of the cyanobacterium genus Microcystis.
Many of the algal blooms are associated with the resuspension of a thixotropic benthic mud containing
legacy nutrients. Since Lake Okeechobee has an area of 1732 km
2
(40–50 km radius) and a mean
depth of only 2.7 m, there is sufficient fetch and shallow water depth to allow frequent wind, wave,
and current generated events, which cause sediment resuspension. Three types of mud exist in the
lake including an immobile dark-colored, consolidated mud, a brownish-colored mud, which is
poorly consolidated and mobile, and a dark-colored thixotropic, highly mobile mud that is a mixture
of organic matter and clay-sized minerals. Altogether, these muds contain an estimated 4.6
×
10
6
kg
of total phosphorus and commensurate high amounts of labile nitrogen. The thixotropic mud covers
most of the lakebed and contains the suitable nutrient ratios to trigger algal blooms. A bioassay
analysis of the thixotropic mud compared to the consolidated mud showed that it produced up
to 50% more nutrient mass compared to the consolidated mud. The thixotropic mud does not
consolidate, thus remains mobile. The mobility is maintained by the dynamics of the algal blooms
and bacterial decay of extracellular secretions (transparent exopolymer particles) that bind sediment,
transfer it to the bottom, and undergo bacterial digestion causing gas emissions, thus maintaining
the organic/sediment matrix in suspension. Despite major efforts to control external nutrient loading
into the lake, the high frequency of algal blooms will continue until the muds bearing legacy nutrients
are removed from the lake.
Keywords:
Lake Okeechobee; Florida; thixotropic mud; mud resuspension; legacy nutrients; harmful
algal blooms
1. Introduction
The occurrence and movement of labile forms of nutrients, mainly phosphorus (P)
and nitrogen (N), in the environment have considerable impacts on many natural ecosys-
tems. Transport and cycling of nutrients impact biological productivity, which in turn
can cause numerous complex and commonly negative effects to various surface–water
bodies including lakes, rivers, and wetlands [
1
]. It is therefore important to understand
the forms of anthropogenic P because it can be a limiting nutrient in inland environments,
from source to sink, to ascertain how to control and remove it before it severely impacts a
hydrosystem, sometimes leading to permanent damage.
Excessive nutrient loading, in some cases causing imbalances, has altered some of the
largest natural hydrosystems in southern Florida. The literature indeed shows that the
largest lakes, which are mostly located in north, central, and the northern portion of south
Florida (Lake Okeechobee, Lake George, Lake Seminole, Lake Kissimmee, Lake Apopka,
Lake Isokpoga, East and West Lake Tohopokaliga, Crescent Lake and Orange Lake) are
eutrophic to hypereutrophic [
2
,
3
]. Thus, this research reviews the sources and sinks of
Water 2021,13, 39. https://doi.org/10.3390/w13010039 https://www.mdpi.com/journal/water
Water 2021,13, 39 2 of 23
nutrients in Lake Okeechobee with emphasis on the accumulated legacy nutrients in muddy
sediments as well as the strategies to remove the problematic mud from the hydrosystem.
The Lake Okeechobee-Everglades natural system is one of the largest freshwater
lakes by surface area in the United Sates. It is hydraulically-connected to a very large
sedge and graminoid dominated moorland [
4
] (Figure 1). Historically, the Everglades
hydraulic continuum once constituted the world’s largest contiguous freshwater system [
5
].
The surface area of Lake Okeechobee covers an area of approximately 1732 km
2
with a
mean depth of 2.7 m and an estimated volume of 5.2 km
3
(Figure 2). Its deepest portion is
about 4.7 m when the lake stage stands at 4.2 m [6].
Water 2021, 13, x FOR PEER REVIEW 2 of 24
eutrophic to hypereutrophic [2,3]. Thus, this research reviews the sources and sinks of
nutrients in Lake Okeechobee with emphasis on the accumulated legacy nutrients in
muddy sediments as well as the strategies to remove the problematic mud from the hy-
drosystem.
The Lake Okeechobee-Everglades natural system is one of the largest freshwater
lakes by surface area in the United Sates. It is hydraulically-connected to a very large
sedge and graminoid dominated moorland [4] (Figure 1). Historically, the Everglades hy-
draulic continuum once constituted the world’s largest contiguous freshwater system [5].
The surface area of Lake Okeechobee covers an area of approximately 1732 km
2
with a
mean depth of 2.7 m and an estimated volume of 5.2 km
3
(Figure 2). Its deepest portion is
about 4.7 m when the lake stage stands at 4.2 m [6].
Figure 1. Lake Okeechobee and its associated watershed.
Figure 1. Lake Okeechobee and its associated watershed.
Water 2021,13, 39 3 of 23
Water 2021, 13, x FOR PEER REVIEW 3 of 24
Figure 2. Bathymetry map of Lake Okeechobee relative to a datum of 3.81 m above mean sea level
[7].
The hydrology of Lake Okeechobee was drastically altered with the channelization
of the Kissimmee River, coupled with the excavation of canals that connect it with the St.
Lucie River to the east and the Caloosahatchee River to the west, and the construction of
the Herbert Hoover Dike surrounding the lake [8]. These alterations prevented the natural
lateral expansion of the lake into the surrounding riparian wetlands, which used to se-
quester nutrient-bearing fine particulates issued from wind-driven lake resuspended sed-
iment [9]. While the Kissimmee River has undergone restoration by recreation some me-
anders, it still drains an urbanized and agropastoral watershed. Thus, the direct discharge
from the Kissimmee River at the north side of the lake, coupled with back-pumping of
water tied to the agriculture activities at the south end of the lake, other types of anthro-
pogenic water discharges, and natural system discharges (e.g., hurricanes) have altered
the lake hydrology and water quality [10]. Therefore, the P load to Lake Okeechobee has
been over 500 metric tons per year over most of the past 30 years [11]. Although a portion
of this P load has left the lake via the Caloosahatchee and St. Lucie rivers, a very large
quantity of P remains in the lake as a legacy P reservoir, which is periodically remobilized
by storm activity.
Bioavailable nitrogen is the limiting nutrient and, as such, it is an important nutrient
in Lake Okeechobee [12–14]. However, P is a key nutrient and contaminant within this
ecosystem since its excessive loading leads and amplifies the nitrogen limitation (i.e., sec-
ondary nitrogen limitation) and thus is the focus of this research. [15,16]. Phosphorus has
80º40'
Figure 2.
Bathymetry map of Lake Okeechobee relative to a datum of 3.81 m above mean sea level [
7
].
The hydrology of Lake Okeechobee was drastically altered with the channelization of
the Kissimmee River, coupled with the excavation of canals that connect it with the St. Lucie
River to the east and the Caloosahatchee River to the west, and the construction of the
Herbert Hoover Dike surrounding the lake [
8
]. These alterations prevented the natural lat-
eral expansion of the lake into the surrounding riparian wetlands, which used to sequester
nutrient-bearing fine particulates issued from wind-driven lake resuspended sediment [
9
].
While the Kissimmee River has undergone restoration by recreation some meanders, it still
drains an urbanized and agropastoral watershed. Thus, the direct discharge from the
Kissimmee River at the north side of the lake, coupled with back-pumping of water tied to
the agriculture activities at the south end of the lake, other types of anthropogenic water
discharges, and natural system discharges (e.g., hurricanes) have altered the lake hydrology
and water quality [
10
]. Therefore, the P load to Lake Okeechobee has been over 500 metric
tons per year over most of the past 30 years [
11
]. Although a portion of this P load has left
the lake via the Caloosahatchee and St. Lucie rivers, a very large quantity of P remains in
the lake as a legacy P reservoir, which is periodically remobilized by storm activity.
Bioavailable nitrogen is the limiting nutrient and, as such, it is an important nutrient in
Lake Okeechobee [
12
–
14
]. However, P is a key nutrient and contaminant within this ecosys-
tem since its excessive loading leads and amplifies the nitrogen limitation (i.e., secondary
nitrogen limitation) and thus is the focus of this research [
15
,
16
]. Phosphorus has indeed
led to water quality issues within Lake Okeechobee, the Greater Everglades Ecosystem,
and the major discharge rivers to the east and west including the St. Lucie Canal/River
continuum and the Caloosahatchee River, respectively. It is also noteworthy to assert
that even prior to human intrusion, Lake Okeechobee suffered periods of eutrophication
and anoxia [
4
]. However, the anthropogenic colonization of the region and subsequent
Water 2021,13, 39 4 of 23
linearization of the Kissimmee River caused it to become a drainage canal and increased
the P (and other nutrients) loading into the lake. These high concentrations of nutrients
entering the lake led to its partial eutrophication and recurrent algal blooms. As such,
and despite its outlets, the lake has been converted into a large holding basin rather than a
natural lake emptying south into the Everglades [15].
Since 1980, a number of water management actions have been utilized to lessen the
degree of P loading of Lake Okeechobee, coastal ecosystems, and the Everglades ecosystem.
Phosphorus source-control measures were implemented in the basins affected by inflow
to streams from agriculture [
17
]. It was recognized that high concentrations of nutrients
occurred in Taylor Creek and were associated with the dairy farms in its basin, which led
to the purchase and removal of some dairy farms in this watershed [
18
]. In addition,
agricultural runoff best management practices were implemented to control soluble and
particulate P, and the meanders were placed back into the surface-water system of the
Kissimmee River partly to help assimilate P. Despite all of these control measures, there
has been a massive accumulation of P in the sediments of Lake Okeechobee and recycling
of P from these bottom sediments into its water column. These sediments are considered to
be legacy P sources [
16
,
19
]. The planned removal of organic sediments containing P inside
the Hoover Dike was also implemented during some drought periods [20].
Prior to human alteration, water moved continuously southward from the central
Florida area into Lake Okeechobee and then out of the southern part of the lake into
the Everglades. After the construction of the Hoover Dike, discharge of water to the
Everglades was redirected east and west. In the latest attempt to control P loading to the
remaining Everglades, large man-made wetlands were constructed to treat the stormwater
to remove P using submergent and emergent wetland plants in combination with epiphytic,
epipelic, epilithic, and metaphytic algae [
21
,
22
]. However, the runoff during the wet
season and occasional hurricane events continues to load the natural system with excess
phosphorus. Another potential problem looms as possible increases in the dry and wet P
fallout of African dust becomes a more common contribution [
23
]. Therefore, additional
P removal will be required in the future to improve and maintain the natural systems
associated with this ecosystem, particularly within the estuarine systems of the St. Lucie
and Caloosahatchee Rivers.
A detailed understanding of the forms of P, and how it cycles through the natural
system is necessary to develop new and innovative engineering methods to alleviate its
environmental impacts. A key issue is the removal of the legacy P in Lake Okeechobee, a
significant component of which is the fluid mud in the lake. The purpose of this research is
to provide a detailed assessment of the occurrence and impacts of muddy bottom sediments
in Lake Okeechobee.
2. History and Management of Lake Okeechobee with Its Associated
Environmental Impacts
2.1. Geology of Lake Okeechobee
Geologically, Lake Okeechobee began as a geographic landform about 6300 years ago,
but did not acquire its current morphometry until about 4000 years ago [
7
]. The lake likely
had a higher stage in the geological past with the maximum level being reached during
the Neo-Atlantic climatic interval, which was known for being hurricane prone [
4
,
7
].
The deepest part of the lake is currently located at or near mean sea level. Geologic
formations occurring under Lake Okeechobee and along the Caloosahatchee River consist
of the Tamiami Formation, the Caloosahatchee Formation, the Fort Thompson Formation,
and the Lake Flirt Marl. Therefore, it is likely that the area from Lake Okeechobee to the
west coast was a marine embayment with episodic sea level events that produced marine
parasequences separated by thin, freshwater, laminated limestones. All of the marine
flooding events in southern Florida from the Middle Pliocene to the late Pleistocene had
termination high stand elevations of at least 7.6 m above sea level. Therefore, the limestone
in Lake Okeechobee is of Middle Pliocene to Late Pleistocene in age. The presence of
Water 2021,13, 39 5 of 23
marine fauna in the lake bottom sediments indicates that the area was tidal during at least
part of the Pleistocene.
Heilprin [
24
] described the pre-alteration geology of Lake Okeechobee based on direct
observations made in 1886. The lake had an extensive sandy beach shoreline on the
northern side and a predominantly sand bottom devoid of mud. The only muddy area was
on the “immediate borders, where there was a considerable outwash of decomposed and
decomposing vegetable substances” [
24
]. The consolidated mud on the lake bottom was
not observed by Heilprin, because no systematic survey of the lake bottom was conducted.
The vegetation along the northern lake margin was typically upland varieties with the
south side being wetland varieties, which transitioned into the Everglades. Will [
25
]
made observations of the south side of the lake, where he described eight stream channels
oriented in a southerly direction and extending 1.6 to 3.2 km south into the Everglades [
25
].
The channels were up to 3 m deep and contained a mud bottom. The banks of the bayous
were elevated 0.4 to 0.8 m above the lake stage and consisted of mud populated by willow,
custard apple, and other wetland vegetation [
25
]. According to Will [
25
], who lived near
the lake beginning in 1913, at high water, the lake extended north up to 9.7 km from the
normal shoreline position.
The first systematic evaluation of the Lake Okeechobee bottom characteristics and
sediment was conducted by Gleason and Stone [
4
]. They ran numerous bottom depth
and seismic reflection profiles of the lake to help determine the bottom characteristics.
In addition, they collected grab samples of the bottom and 82 sediment cores.
The investigation of the bottom sediments of Lake Okeechobee showed that about
one-third of the lake bottom was covered with a stiff organic mud in 1975 [
4
]. However,
most of the western half of the lake had a sandy bottom and the southern part of the
lake bottom was rock or calcareous marl (Figure 3). The organic mud thickness ranged
from 0 to 80 cm [
4
]. The map compiled by Gleason and Stone [
4
] showed the general
bottom sediment pattern (Figure 3). A large sand area was found in the northwest area
that consisted of quartz sand containing marine shells, predominantly Chione cancellata
(currently termed Chione elevata as originally described by Say [
26
]). A lobe of mud was
found from the northernmost part of the lake to the south into the deepest water. The mud
appeared to be linked to the inflow areas of the Kissimmee River and Nubbin Slough.
The mud lobe was bounded by rock and marl on the east and southern boundaries. An area
of thin mud containing Rangia cuneata (Sowerby) was found. This mollusk species is a
marine bivalve associated with the last time the lake was tidal, sometime in the Pleistocene.
Freshwater mollusks were found along the southwest margin of the mud lobe in the lake
bottom. Both deposits suggest that storm activity deposited the shell material, which was
later covered by mud [
4
]. The southern margin of the lake contained marsh areas and peat
deposits of variable thickness.
2.2. Shell Occurrence in the Mud and Implications of Mud Transport
The occurrence of freshwater mollusk shells within the mud may indicate that at least
some of the mud deposits are relatively stable or was during the time prior to sampling
and coring in 1975. The upper part of the mud contains hydrobiids, which commonly form
layers in the cores and some are likely to be storm deposits buried by mud deposition
during calm periods [4].
Water 2021,13, 39 6 of 23
Water 2021, 13, x FOR PEER REVIEW 6 of 24
Figure 3. Map of the bottom sediment in Lake Okeechobee in 1975 [4].
2.2. Shell Occurrence in the Mud and Implications of Mud Transport
The occurrence of freshwater mollusk shells within the mud may indicate that at least
some of the mud deposits are relatively stable or was during the time prior to sampling
and coring in 1975. The upper part of the mud contains hydrobiids, which commonly form
layers in the cores and some are likely to be storm deposits buried by mud deposition
during calm periods [4].
3. Muddy Bottom Sediments in Lake Okeechobee
3.1. Thickness and Volume of the Mud Unit in Time
Three types of mud occur in Lake Okeechobee: consolidated mud, partially consoli-
dated mud, and a fluid mud layer that occurs in variable thicknesses in time [16]. The
consolidated mud is dark-colored, layered, contains shell lag deposits, and has heteroge-
neous properties [4]. The partially consolidated mud has a brownish color and occurs
along marginal wetland areas. The fluid or thixotropic mud occurs in some of the streams
entering the lake such as the Taylor Creek/Nubbin Slough and as a thin layer occurring
over a large percentage of the lake bottom [22]. It is unknown if the Taylor Creek/Nubbin
Slough source of the fluid mud is significant in the overall fluid mud accumulation in the
lake. The fluid mud occurs in some thickness throughout the lake, but has accumulated
to the greatest extent in the deeper parts of the lake as a result of periodic resuspension
and erosion of the partially-consolidated and fluid muds, influx of organic debris into the
lake during storms, and erosion of organic debris/sediment from the edges of the lake.
MUD
SAND
THIN MUD
ON RANGI A
SHELLS
ROCK AND MARL
MUD
PEAT
RITTA
ISLAND
LAKE HARBOR
KREAMER
ISLAND
TORR
ISLAND
BELLE GLADE
CLEWISTON
012345
NAUTICAL MILES
OBSERVATION
ISLAND
OBSERVATION
SHOAL
ROCK
AND
MARL
OKEECHOBEE
PAHOKEE
CANAL POINT
SAND CUT
POR T M AYAC A
NORTH
LAKE
SHOAL
80º55’
80º45’ 80º40’
27º10’
27º05’
27º00’
26º55’
26º50’
26º45’
80º50’
Figure 3. Map of the bottom sediment in Lake Okeechobee in 1975 [4].
3. Muddy Bottom Sediments in Lake Okeechobee
3.1. Thickness and Volume of the Mud Unit in Time
Three types of mud occur in Lake Okeechobee: consolidated mud, partially con-
solidated mud, and a fluid mud layer that occurs in variable thicknesses in time [
16
].
The consolidated mud is dark-colored, layered, contains shell lag deposits, and has hetero-
geneous properties [
4
]. The partially consolidated mud has a brownish color and occurs
along marginal wetland areas. The fluid or thixotropic mud occurs in some of the streams
entering the lake such as the Taylor Creek/Nubbin Slough and as a thin layer occurring
over a large percentage of the lake bottom [
22
]. It is unknown if the Taylor Creek/Nubbin
Slough source of the fluid mud is significant in the overall fluid mud accumulation in the
lake. The fluid mud occurs in some thickness throughout the lake, but has accumulated to
the greatest extent in the deeper parts of the lake as a result of periodic resuspension and
erosion of the partially-consolidated and fluid muds, influx of organic debris into the lake
during storms, and erosion of organic debris/sediment from the edges of the lake.
The fluid mud is here termed “thixotropic”, because it exhibits some very specific
properties that separate it from the consolidated mud as defined by Mewis and Wagner
(2009) [
27
]. Thixotropy is a time-dependent shear thinning property. Certain gels or fluids
that are thick or viscous under static conditions will flow (become thinner, less viscous)
over time when shaken, agitated, shear-stressed, or otherwise stressed (time dependent
viscosity). They then take a fixed time to return to a more viscous state. Some non-
Newtonian pseudoplastic fluids show a time-dependent change in viscosity; the longer the
fluid undergoes shear stress, the lower its viscosity. A thixotropic fluid is a fluid that takes
Water 2021,13, 39 7 of 23
a finite time to attain equilibrium viscosity when introduced to a steep change in shear rate.
A thixotropic fluid occurring in a dynamic environment does not consolidate.
The thickness of the consolidated mud layer was initially mapped by Gleason and
Stone [
4
] and is shown in Figure 4. The sampling locations were originally estimated by
dead-reckoning and Loran C, and are not as accurate as modern positions determined
by GPS. Therefore, the exact boundaries have some error, but the general pattern is con-
sidered accurate. The thickness of the mud ranged from zero to 80+ cm with a general
correspondence between the highest thickness of mud and the greatest depth of the lake [
4
]
(Figure 3). The average thickness of the mud was estimated to be 0.46 m or 46 cm. The total
estimated volume was 201 ×106m3.
Water 2021, 13, x FOR PEER REVIEW 7 of 24
The fluid mud is here termed “thixotropic”, because it exhibits some very specific
properties that separate it from the consolidated mud as defined by Mewis and Wagner
(2009) [27]. Thixotropy is a time-dependent shear thinning property. Certain gels or fluids
that are thick or viscous under static conditions will flow (become thinner, less viscous)
over time when shaken, agitated, shear-stressed, or otherwise stressed (time dependent
viscosity). They then take a fixed time to return to a more viscous state. Some non-New-
tonian pseudoplastic fluids show a time-dependent change in viscosity; the longer the
fluid undergoes shear stress, the lower its viscosity. A thixotropic fluid is a fluid that takes
a finite time to attain equilibrium viscosity when introduced to a steep change in shear
rate. A thixotropic fluid occurring in a dynamic environment does not consolidate.
The thickness of the consolidated mud layer was initially mapped by Gleason and
Stone [4] and is shown in Figure 4. The sampling locations were originally estimated by
dead-reckoning and Loran C, and are not as accurate as modern positions determined by
GPS. Therefore, the exact boundaries have some error, but the general pattern is consid-
ered accurate. The thickness of the mud ranged from zero to 80+ cm with a general corre-
spondence between the highest thickness of mud and the greatest depth of the lake [4]
(Figure 3). The average thickness of the mud was estimated to be 0.46 m or 46 cm. The
total estimated volume was 201 × 106 m3.
Figure 4. Thickness of organic mud on the bottom of Lake Okeechobee in 1989 [16].
Figure 4. Thickness of organic mud on the bottom of Lake Okeechobee in 1989 [16].
Subsequent surveys of the mud thickness in Lake Okeechobee were conducted in
1988, 1998, and 2006 [
16
,
28
–
30
] (Figure 5). The estimated average mud thickness was 66,
74, and 51 cm, respectively [
20
]. The calculated mud volumes were 219
×
10
6
, 197
×
10
6
,
and 144
×
10
6
m
3
[
31
]. Based on the use of modern GPS, the subsequent surveys may
have a slightly greater accuracy. However, the mud volume thicknesses reported by Yan
and James [
31
] seem to be somewhat different than the actual studies made. Gleason and
Stone [
4
] estimated the mud volume to be 201
×
10
6
m
3
(Figure 4). Kirby et al. [
7
] estimated
the volume to be 193
×
10
6
m
3
. In addition, they also reported that a fluid mud layer
was found in several cores that ranged from 0 to 10 cm in thickness. However, only the
Water 2021,13, 39 8 of 23
consolidated mud was mapped, and no comprehensive mapping of the fluid mud was
made.
Water 2021, 13, x FOR PEER REVIEW 8 of 24
Subsequent surveys of the mud thickness in Lake Okeechobee were conducted in
1988, 1998, and 2006 [16,28–30]. (Figure 5). The estimated average mud thickness was 66,
74, and 51 cm, respectively [20]. The calculated mud volumes were 219 × 106, 197 × 106,
and 144 × 106 m3 [31]. Based on the use of modern GPS, the subsequent surveys may have
a slightly greater accuracy. However, the mud volume thicknesses reported by Yan and
James [31] seem to be somewhat different than the actual studies made. Gleason and Stone
[4] estimated the mud volume to be 201 × 106 m3 (Figure 4). Kirby et al. [7] estimated the
volume to be 193 × 106 m3. In addition, they also reported that a fluid mud layer was found
in several cores that ranged from 0 to 10 cm in thickness. However, only the consolidated
mud was mapped, and no comprehensive mapping of the fluid mud was made.
Figure 5. Sediment zone maps of Lake Okeechobee from 1988, 1998, and 2006 as compiled by Yan and James [31]. Note
that the positioning systems used during these studies changed from LORAN to GPS. Therefore, some inherent geo local-
ization errors occurred when comparing the maps.
The data from the last three surveys suggest that the total volume of mud in Lake
Okeechobee declined by 10% between 1988 and 1998 and by 27% between 1998 and 2006
with an overall decline of 52% between 1988 and 2006 [31]. An important consideration is
that the 2006 measurements were made after a very active hurricane year when two major
storms passed over the lake. This decline of mud volume in the lake is subject to consid-
erable debate based on subsequent scientific investigation on the stability of the consoli-
dated mud layer and the ability of storms to re-suspend it (see Section 4.1).
3.2. Composition of the Mud
3.2.1. Total and Organic Carbon Content of the Mud
The first measurements of the total and organic fractions of the mud were made by
Gleason and Stone [4]. The total carbon, consisting of both non-organic and organic car-
bon, was mapped using numerous samples across the lake bottom. The percentage of total
carbon in the mud ranged from 10.3 to 21.3% (Figure 6). Based on only six samples, the
mud contained between 17.3 and 25.1 % organic material. Gleason and Stone [4] con-
cluded that the likely origin of the mud was from transport into the lake and not from
precipitation from phytoplankton. However, the non-organic carbon fraction in the form
of fine calcite crystals could have originated in periphyton algal mats that can occur near
the lake banks.
Figure 5.
Sediment zone maps of Lake Okeechobee from 1988, 1998, and 2006 as compiled by Yan and James [
31
]. Note that
the positioning systems used during these studies changed from LORAN to GPS. Therefore, some inherent geo localization
errors occurred when comparing the maps.
The data from the last three surveys suggest that the total volume of mud in Lake
Okeechobee declined by 10% between 1988 and 1998 and by 27% between 1998 and 2006
with an overall decline of 52% between 1988 and 2006 [
31
]. An important consideration
is that the 2006 measurements were made after a very active hurricane year when two
major storms passed over the lake. This decline of mud volume in the lake is subject to
considerable debate based on subsequent scientific investigation on the stability of the
consolidated mud layer and the ability of storms to re-suspend it (see Section 4.1).
3.2. Composition of the Mud
3.2.1. Total and Organic Carbon Content of the Mud
The first measurements of the total and organic fractions of the mud were made by
Gleason and Stone [
4
]. The total carbon, consisting of both non-organic and organic carbon,
was mapped using numerous samples across the lake bottom. The percentage of total
carbon in the mud ranged from 10.3 to 21.3% (Figure 6). Based on only six samples, the
mud contained between 17.3 and 25.1% organic material. Gleason and Stone [
4
] concluded
that the likely origin of the mud was from transport into the lake and not from precipitation
from phytoplankton. However, the non-organic carbon fraction in the form of fine calcite
crystals could have originated in periphyton algal mats that can occur near the lake banks.
3.2.2. Phosphorus Content of the Mud
Another key parameter measured by Gleason and Stone [
4
] was the concentration
of phosphorus in the mud, which ranged from 731 to 1353
µ
g/g (Figure 7). These values
were much higher than the values found in the sandy sediments on the lake bottom and
two to three times higher than in the peats found beneath the southern part of the lake.
The phosphorus concentration in the three sediment types showed a consistent pattern
with mud > peat > sand [
4
]. Gleason and Stone [
4
] calculated the total phosphorus stored in
the sediment to be 45.12
×
10
6
kg. They estimated that the time required to accumulate the
phosphorus was 180 years, based on an annual rate calculated by Joyner [
32
] for the period
1 January 1969 to 31 January 1970. If the data from Davis and Marshall [
33
] measured in
1973–1974 were used, instead, for the time required to accumulate the phosphorus, it would
take 72 years, which better matches the measured age of the mud (see Section 4.1). The total
phosphorus measured in the upper 0–10 cm of the organic sediment for 1988 and 1998
Water 2021,13, 39 9 of 23
was reported by Fisher et al. [
29
]. These measurements are considered more accurate than
the original measurements based on better station positioning and improved laboratory
techniques. Note that the pattern of the mud changed between the two measurements with
some change in estimated volume. However, the estimated concentration of total P in the
sediments varied little from 1034 µg/g in 1988 and 1035 µg/g in 1998.
Water 2021, 13, x FOR PEER REVIEW 9 of 24
Figure 6. Total carbon percent (organic and inorganic) in the bottom sediments of Lake Okeecho-
bee [4].
3.2.2. Phosphorus Content of the Mud
Another key parameter measured by Gleason and Stone [4] was the concentration of
phosphorus in the mud, which ranged from 731 to 1353 µg/g (Figure 7). These values were
much higher than the values found in the sandy sediments on the lake bottom and two to
three times higher than in the peats found beneath the southern part of the lake. The phos-
phorus concentration in the three sediment types showed a consistent pattern with mud
> peat > sand [4]. Gleason and Stone [4] calculated the total phosphorus stored in the sed-
iment to be 45.12 × 10
6
kg. They estimated that the time required to accumulate the phos-
phorus was 180 years, based on an annual rate calculated by Joyner [32] for the period 1
January 1969 to 31 January 1970. If the data from Davis and Marshall [33] measured in
1973–1974 were used, instead, for the time required to accumulate the phosphorus, it
would take 72 years, which better matches the measured age of the mud (see Section 4.1).
The total phosphorus measured in the upper 0–10 cm of the organic sediment for 1988
and 1998 was reported by Fisher et al. [29]. These measurements are considered more ac-
curate than the original measurements based on better station positioning and improved
laboratory techniques. Note that the pattern of the mud changed between the two meas-
urements with some change in estimated volume. However, the estimated concentration
of total P in the sediments varied little from 1034 µg/g in 1988 and 1035 µg/g in 1998.
Figure 6.
Total carbon percent (organic and inorganic) in the bottom sediments of Lake Okee-
chobee [4].
Water 2021, 13, x FOR PEER REVIEW 10 of 24
Figure 7. Total phosphorus in the bottom sediments of Lake Okeechobee (in mg/kg) [4,29].
3.2.3. Nitrogen Content of the Mud
The percent of nitrogen in the mud was also enriched compared to the sand. How-
ever, a comparison of the nitrogen in the sediments showed a different pattern compared
to the phosphate, which was peat > mud > sand. The estimated quantity of nitrogen in the
mud was 402 × 10
6
kg. An analysis of the annual rate of deposition based on the Joyner
[32] nutrient data suggests that it took 230 years to accumulate the nitrogen in the mud.
The Davis and Marshall [33] data suggest that the accumulation of the nitrogen in the mud
took 67 years.
3.2.4. Iron Content in the Mud
Another substance that is important in creating algal bloom activity in the water col-
umn is iron. The percentage of iron as Fe
2
O
3
in the mud ranged from 1.4 to 3.5%. There is
a question concerning the form of iron in the sediments and the mechanism of how it was
historically added to the lake (e.g., decay of aquatic vegetation, atmospheric fallout). How-
ever, since there is a significant presence of sulfur, the iron may occur within the mineral
pyrite (FeS
2
), and is not bio-available as long as the conditions are chemically reducing,
when the sediments are mobilized in the water column. However, the oxidation of the
pyrite could release some iron into the water column during storm activity.
3.2.5. Shell Occurrence in the Mud and Implications of Mud Transport
The occurrence of shell within the mud may indicate that at least some of the mud
deposit is relatively stable or was during the time prior to sampling and coring in 1975.
The upper part of the mud contains hydrobiids, which commonly form layers in the cores
and some are likely storm deposits buried by mud deposition during calm periods [4].
The interstitial water collected from the mud showed high concentrations of ortho-
phosphate and ammonium [4]. When the muddy sediments are mobilized, the interstitial
water is released into the water column and adds to the overall load of nutrients in the
lake water.
4. Dynamics of Sediment Transport and Movement
Based on the research done to date on Lake Okeechobee, there are conflicts in the
possible role of the muddy sediment in terms of impacts on lake water quality. Studies on
the thickness and volume of the sediments suggest they are dynamic and have potential
for remobilization during and shortly after storms. These remobilizations impact the wa-
Figure 7. Total phosphorus in the bottom sediments of Lake Okeechobee (in mg/kg) [4,29].
3.2.3. Nitrogen Content of the Mud
The percent of nitrogen in the mud was also enriched compared to the sand. However,
a comparison of the nitrogen in the sediments showed a different pattern compared to the
phosphate, which was peat > mud > sand. The estimated quantity of nitrogen in the mud
was 402
×
10
6
kg. An analysis of the annual rate of deposition based on the Joyner [
32
]
nutrient data suggests that it took 230 years to accumulate the nitrogen in the mud. The
Davis and Marshall [
33
] data suggest that the accumulation of the nitrogen in the mud
took 67 years.
Water 2021,13, 39 10 of 23
3.2.4. Iron Content in the Mud
Another substance that is important in creating algal bloom activity in the water
column is iron. The percentage of iron as Fe
2
O
3
in the mud ranged from 1.4 to 3.5%.
There is a question concerning the form of iron in the sediments and the mechanism of
how it was historically added to the lake (e.g., decay of aquatic vegetation, atmospheric
fallout). However, since there is a significant presence of sulfur, the iron may occur within
the mineral pyrite (FeS2), and is not bio-available as long as the conditions are chemically
reducing, when the sediments are mobilized in the water column. However, the oxidation
of the pyrite could release some iron into the water column during storm activity.
3.2.5. Shell Occurrence in the Mud and Implications of Mud Transport
The occurrence of shell within the mud may indicate that at least some of the mud
deposit is relatively stable or was during the time prior to sampling and coring in 1975.
The upper part of the mud contains hydrobiids, which commonly form layers in the cores
and some are likely storm deposits buried by mud deposition during calm periods [4].
The interstitial water collected from the mud showed high concentrations of orthophos-
phate and ammonium [
4
]. When the muddy sediments are mobilized, the interstitial water
is released into the water column and adds to the overall load of nutrients in the lake water.
4. Dynamics of Sediment Transport and Movement
Based on the research done to date on Lake Okeechobee, there are conflicts in the
possible role of the muddy sediment in terms of impacts on lake water quality. Studies on
the thickness and volume of the sediments suggest they are dynamic and have potential
for remobilization during and shortly after storms. These remobilizations impact the water
column and release nutrients, causing rather rapid triggering of algal blooms. However,
chronological investigations of the consolidated bottom mud seem to indicate a degree of
stability. In addition, Kirby et al. [
7
] concluded that “The microscopic primary fabric is clear
evidence that the deeper layers of the mud patch are not susceptible to frequent reworking.”
4.1. Chronological Assessment of the Consolidated Mud in Lake Okeechobee
Three cores were obtained from the mud “zone” of Lake Okeechobee and detailed
measurements of radioactive isotopes were performed to determine the age and stability
of the sediments [34]. The locations of the cores are shown in Figure 8.
The cores were 85, 42, and 41 cm in length corresponding to sites L3, K8, and L9,
respectively. All of the cores showed a clear delineation with the underlying sediment,
either marl or quartz sand. The cores contained layers of gastropods [
4
]. The activity of
210
Pb showed a steep decline from the surface to a depth of 10 cm with a lower slope of
decline below that depth. The calculated ages for the muds showed a range from 2006 to
about 1880 in L3, from 2006 to 1880 in L9, and 2006 to 1940 in K8. Calculated ages in the
cores produced similar results using
137
Cs activity. Based on the isotope dating analysis,
Schottler and Engstrom [
34
] concluded that the muddy sediment was not disturbed to any
significant degree since 1930. However, prior to this time, there is some indication that the
mud could have been disturbed. The lake did have a direct perturbation event in 1928 as a
category 4 hurricane passed over it and caused the historical muck dike surrounding the
lake to breach [
25
]. Their conclusion for the 75-year relatively stable period from 1930 to
2005 seems to contradict the changes in mud distribution found by comparing the mud
thickness between 1998 and 2006. Another contributing factor could be that two major
hurricanes impacted the lake in 2004.
Water 2021,13, 39 11 of 23
Water 2021, 13, x FOR PEER REVIEW 11 of 24
ter column and release nutrients, causing rather rapid triggering of algal blooms. How-
ever, chronological investigations of the consolidated bottom mud seem to indicate a de-
gree of stability. In addition, Kirby et al. [7] concluded that “The microscopic primary
fabric is clear evidence that the deeper layers of the mud patch are not susceptible to fre-
quent reworking.”
4.1. Chronological Assessment of the Consolidated Mud in Lake Okeechobee
Three cores were obtained from the mud “zone” of Lake Okeechobee and detailed
measurements of radioactive isotopes were performed to determine the age and stability
of the sediments [34]. The locations of the cores are shown in Figure 8.
Figure 8. Map of Lake Okeechobee showing the locations of cores studied by Schottler and Eng-
strom in 2003 [34].
The cores were 85, 42, and 41 cm in length corresponding to sites L3, K8, and L9,
respectively. All of the cores showed a clear delineation with the underlying sediment,
either marl or quartz sand. The cores contained layers of gastropods [4]. The activity of
210Pb showed a steep decline from the surface to a depth of 10 cm with a lower slope of
decline below that depth. The calculated ages for the muds showed a range from 2006 to
about 1880 in L3, from 2006 to 1880 in L9, and 2006 to 1940 in K8. Calculated ages in the
cores produced similar results using 137Cs activity. Based on the isotope dating analysis,
Schottler and Engstrom [34] concluded that the muddy sediment was not disturbed to any
significant degree since 1930. However, prior to this time, there is some indication that the
mud could have been disturbed. The lake did have a direct perturbation event in 1928 as
a category 4 hurricane passed over it and caused the historical muck dike surrounding the
lake to breach [25]. Their conclusion for the 75-year relatively stable period from 1930 to
2005 seems to contradict the changes in mud distribution found by comparing the mud
thickness between 1998 and 2006. Another contributing factor could be that two major
hurricanes impacted the lake in 2004.
4.2. Impact of Density and Shear-Strength on Mud Transport
The shear strength of mud is a key factor affecting potential remobilization by bottom
current velocity (erosion), regardless of whether it is a current or wave-orbital motion.
Figure 8.
Map of Lake Okeechobee showing the locations of cores studied by Schottler and Engstrom
in 2003 [34].
4.2. Impact of Density and Shear-Strength on Mud Transport
The shear strength of mud is a key factor affecting potential remobilization by bottom
current velocity (erosion), regardless of whether it is a current or wave-orbital motion.
Detailed measurements of the density and shear-strength of the mud units in Lake Okee-
chobee were made [
7
,
35
]. The upper 0 to 8 cm of the mud contained the thixotropic
unit (see Section 3.1). The in situ measurements yielded densities ranging from 1.01 to
1.03 g/cm
3
. The shear-strength was below the detection limits of the instrument. It has
been documented that at density values below 1.065 g/cm
3
, the shear-strength becomes
zero, which means the mud behaves as a fluid and can be entrained, resuspended, and
mixed quite easily. The mud densities in the consolidated unit ranged from 1.2 to 1.3 g/cm
3
.
These values had shear-strengths generally over three times the critical value for erosion.
Numerous measurements were made to determine the relationship between bulk density
and shear strength in the Lake Okeechobee sediments [35,36] (Figure 9).
Water 2021, 13, x FOR PEER REVIEW 12 of 24
Detailed measurements of the density and shear-strength of the mud units in Lake Okee-
chobee were made [7,35]. The upper 0 to 8 cm of the mud contained the thixotropic unit
(see Section 3.1). The in situ measurements yielded densities ranging from 1.01 to 1.03
g/cm3. The shear-strength was below the detection limits of the instrument. It has been
documented that at density values below 1.065 g/cm3, the shear-strength becomes zero,
which means the mud behaves as a fluid and can be entrained, resuspended, and mixed
quite easily. The mud densities in the consolidated unit ranged from 1.2 to 1.3 g/cm3. These
values had shear-strengths generally over three times the critical value for erosion. Nu-
merous measurements were made to determine the relationship between bulk density
and shear strength in the Lake Okeechobee sediments [35,36] (Figure 9).
Figure 9. Mud vane shear strength variation with density. Redrawn from Hwang [36].
4.3. Occurrence of a Thixotropic Organic Mud Unit (Organic Floc Layer) in Lake Okeechobee
Based on studies of the consolidated mud deposit on the bottom of Lake Okeechobee
and the difficulty in eroding and remobilizing it, there must be an additional source of
sediment that has more mobility [10]. In fluid density profiles, Kirby et al. [7] found that
a fluid mud ranging from 0 to 10 cm in thickness occurred at the top of several cores. They
also believed that this unit was more susceptible to resuspension. Hansen et al. [37] de-
scribed an unconsolidated “floc” layer above the consolidated mud in their study of sed-
iment resuspension in Lake Okeechobee. The layer was about 10 cm thick lying atop the
consolidated mud layer. Moore et al. [19] described the occurrence of an “organic floc”
layer that was 2 to 5 cm thick in the Kissimmee River and Taylor Creek in proximity to
Lake Okeechobee. They also found a 2 to 5 cm thick layer of “organic floc” at sampling
station 17, which was located about 10 km due south of the entrance of the Kissimmee
River into Lake Okeechobee. A core collected on 14 June 2020 by our research team,
showed an “organic floc layer” 10 to 20 cm thick above the consolidated mud (GPS loca-
tion-N 26°57.245′, W 080°45.138′). A shallow core and a water column sample collected by
the authors in July 2020 contained the dispersed floc material in the water column above
the bottom sediment. When the fluid material (organics and mud) settled, it was 5 to 10
cm in thickness.
The influx of thixotropic organic mud into the lake may be occurring as direct flow
from entering streams, especially the Taylor Creek/Nubbin Slough where this layer is
known to occur on the bottom. Vegetative storm debris carried into the lake during hur-
ricanes may also contribute to the layer. In-lake processes such as periodic erosion of wet-
land areas located inside of the Hoover Dike and mixing events that incorporate poorly
consolidated sediment into the organic floc could also contribute to this layer.
Figure 9. Mud vane shear strength variation with density. Redrawn from Hwang [36].
Water 2021,13, 39 12 of 23
4.3. Occurrence of a Thixotropic Organic Mud Unit (Organic Floc Layer) in Lake Okeechobee
Based on studies of the consolidated mud deposit on the bottom of Lake Okeechobee
and the difficulty in eroding and remobilizing it, there must be an additional source of
sediment that has more mobility [
10
]. In fluid density profiles, Kirby et al. [
7
] found that
a fluid mud ranging from 0 to 10 cm in thickness occurred at the top of several cores.
They also believed that this unit was more susceptible to resuspension. Hansen et al. [
37
]
described an unconsolidated “floc” layer above the consolidated mud in their study of
sediment resuspension in Lake Okeechobee. The layer was about 10 cm thick lying atop
the consolidated mud layer. Moore et al. [
19
] described the occurrence of an “organic floc”
layer that was 2 to 5 cm thick in the Kissimmee River and Taylor Creek in proximity to
Lake Okeechobee. They also found a 2 to 5 cm thick layer of “organic floc” at sampling
station 17, which was located about 10 km due south of the entrance of the Kissimmee River
into Lake Okeechobee. A core collected on 14 June 2020 by our research team, showed
an “organic floc layer” 10 to 20 cm thick above the consolidated mud (GPS location-N
26
◦
57.245
0
, W 080
◦
45.138
0
). A shallow core and a water column sample collected by the
authors in July 2020 contained the dispersed floc material in the water column above the
bottom sediment. When the fluid material (organics and mud) settled, it was 5 to 10 cm
in thickness.
The influx of thixotropic organic mud into the lake may be occurring as direct flow
from entering streams, especially the Taylor Creek/Nubbin Slough where this layer is
known to occur on the bottom. Vegetative storm debris carried into the lake during
hurricanes may also contribute to the layer. In-lake processes such as periodic erosion of
wetland areas located inside of the Hoover Dike and mixing events that incorporate poorly
consolidated sediment into the organic floc could also contribute to this layer.
4.4. Wave and Storm-Induced Impacts on the Muddy Sediment in Lake Okeechobee
Wind-induced re-suspension of sediments is a common global occurrence in shallow
and deep lakes [
37
–
53
] and Lake Okeechobee is no exception [
37
,
52
,
54
,
55
]. A series of
modeling efforts were used to assess the resuspension issue along with wind-related
variation in phosphorus concentrations in Lake Okeechobee [7,22,37,38,52,53,55–60].
The modeling work conducted by James et al., (1997) [
55
] concentrated primarily on
inorganic sediment resuspension. They had difficulty in determining the settling rate of the
sediments during the initial modeling efforts. They used the WASP model to compute the
sediment resuspension from bottom shear stress [
61
]. The resuspension part of the model
was used primarily to evaluate the inorganic suspended solids and not the thixotropic
organic/inorganic mud. During the initial run, the concentration of suspended sediments
was underestimated. A key finding was that the internal flux to external load ratio showed
that resuspension exceeded external loading by 123 times. This is consistent with the
presence of the thixotropic mud laying within the lake.
Bachmann et al. [
52
] modeled Lake Okeechobee and a large number of other shallow
lakes in Florida to research resuspension of bottom sediments. In their modeling efforts,
they calculated the following: (1) the surface area of the lake subject to resuspension
of sediments; (2) the percentage of the lake bottom disturbed 50% of the time; (3) the
cumulative percentage of time that 50% of the lake bottom was disturbed; and (4) the
percentage of time that 100% of the lake bottom was disturbed. The modeling exercise was
based on some common principles in wave dynamics. The first principle states that when
the water depth is less than half the wavelength, the horizontal water movement at the
lakebed may be sufficient to resuspend sediments [
40
]. Additionally, the dynamic ratio
of Håkanson [
39
], defined as the square root of the lake surface area in square kilometers
divided by the lake depth in meters, was found to impact the potential for resuspension.
Thus, it was assumed that lakes with a higher dynamic ratio are subject to a greater degree
of sediment resuspension. Bachmann et al. [
52
] found the following: (1) the surface area of
Lake Okeechobee was 1292 km
3
; (2) the mean depth was 2.4 m; (3) the dynamic ratio was
14.98 km/m; (4) the percentage of area subject to sediment resuspension was 100%; (5) the
Water 2021,13, 39 13 of 23
percentage of area disturbed 50% of the time was 90%; (6) the percentage of time 50% of the
lake area had a disturbed bottom was 50%; and (7) the percentage of time 100% of the lake
bottom is disturbed was 39.5%. A critical aspect of this investigation was that the bottom
sediment was easily re-suspended based on the wave-generated orbital stress across the
bottom. In well-consolidated muds, the required water velocity to resuspend consolidated
clay-sized sediments is quite high and would not likely support the calculation estimates
following the fundamental wave dynamics evaluated [
7
,
62
]. However, if bottom sediments
are not well-consolidated with low density and high-water content, they can be readily
resuspended. A thixotropic sediment or organic floc unit can be easily resuspended at a
potential rate higher than calculated by Bachmann et al. [
52
]. Therefore, the wetland areas
located along the western margin and the thixotropic sediments on the bottom of Lake
Okeechobee are subject to resuspension in wind events occurring throughout the year,
likely occurring greater than 50% of the time. Based on the difficulty in resuspending the
consolidated sediments and the maps showing the changes in bottom sediment thicknesses
(Figure 5), there may be a third type of mud on the lake bottom (brown mud), which has
a density lying between the consolidated mud and the thixotropic mud. This material
may settle to the bottom, but not consolidate to a consistency that makes it resistant
to re-suspension.
4.5. Recycling of Organic Phosphorus from Bottom Sediments of Lake Okeechobee
Based on the modeling of sediments in Lake Okeechobee, there is no question that
resuspension is occurring and has done so for a prolonged period [
37
,
52
]. The key issue is
how much P and other nutrients are associated with the sediment during a resuspension
event, and how much of the nutrient mass is available for uptake by algae.
Algal assay research was conducted on Lake Okeechobee sediments (consolidated and
organic flocs) to determine if resuspension events caused the release or removal of P from
the water column [
37
]. It was found that a resuspension event could cause either a source
or sink for bioavailable P in the water column. Under reduced conditions, the resuspension
of sediments is a potential source of available sediments for algal growth. The conclusion
by Hansen et al. [
37
] is consistent with that found by Reddy [
58
,
59
], who concluded that
PO
4
release during resuspension events was dependent on both sediment composition and
the redox potential.
Depending on the sediment source that is in the water column, either inorganic
bottom sediment or the thixotropic sediment (referred to as organic floc sediment in some
publications), there are some differences in algal growth stimulation. The bioassay work by
Hansen et al. [
37
] suggested that when the biogeochemical conditions create algal blooms
from a resuspension event, the thixotropic or organic floc sediments cause a greater positive
change in growth, which can be up to 50% greater.
5. Lake Sediments as a Nutrient Source for Lake Okeechobee Cyanobacteria Blooms
Cyanobacterial (blue-green algae) blooms in Lake Okeechobee are frequent in late
spring and through the summer months. These blooms negatively impact the limnoecology
of the lake and downstream estuaries, causing declarations of State of Emergency for the
lake and estuaries [
63
,
64
]. It is essential to understand and ameliorate the nutrient sources
that feed these blooms, with emphasis on two essential macronutrients, nitrogen and
phosphorus. Microcystis is commonly the dominant organism in these blooms. It is a genus
that is reliant on the appropriate concentrations of both N and P to initiate and sustain
population growth and toxin production [
12
,
65
]. These nutrients must be available to the
phytoplankton community, which has a year-round cyanobacterial component, along with
other environmental cues, that allow Microcystis to be dominant and form massive blooms.
The sediments in Lake Okeechobee (fluid mud), which have accumulated from
decades of inflow from the watershed and in lake vegetation growth [
66
–
68
], serve as
a semi-permanent source of the key nutrients that can support and sustain these massive
blooms. This accumulation of nutrients in the sediments is common in many waterbodies
Water 2021,13, 39 14 of 23
and is commonly referred to as a “legacy” from prior watershed loading, enriching a
waterbody for decades [
69
,
70
]. In Lake Okeechobee, P in the sediments is estimated to
contain 2.87
×
10
7
kg [
71
] with different P fluxes as a function of sediment type: littoral,
peat, mud, and sand [
72
,
73
]. These studies have examined the flux from or to the water
column, which is mainly controlled by redox potential and pH through iron–P interactions.
In oxic sediments, ferric phosphorus controls P solubility. However, in anoxic sediments,
phosphorus release is an order of magnitude greater [
74
]. N in the forms of ammonium
and nitrate also efflux from the lake sediments [75].
The contribution of the sediment nutrients enriching the Lake Okeechobee phyto-
plankton community (Figure 10) remains a conundrum that has eluded the scientific
community. Many of the common practices used for reducing the nutrient flux into the
water column [
76
] have been piloted [
77
]. However, the P internal loading has not been
resolved. Until the nutrient flux from the massive reserves in Lake Okeechobee sediments
is curtailed, massive blooms of Microcystis will continue to plague this system, causing lake
and downstream deleterious impacts in the Caloosahatchee and St. Lucie Rivers.
Water 2021, 13, x FOR PEER REVIEW 15 of 24
Figure 10. Microcystis blooms are a function of the nutrient releases from the sediments.
In addition to the sediment-recycled nutrients, it is also acknowledged that both N
and P are transported to the lake via the tributary system. N
2
-fixing cyanobacteria also
add to N availability, thereby providing multiple sources that lead to the massive blooms
of Microcystis (Figure 11). There is also a recycling component associated with the die-off
of a bloom, sinking in the water column, accumulating in the sediments as organic matter
(added back to the thixotropic layer) and being mineralized by bacteria (Figure 12).
Figure 11. Nutrient sources that feed the algal blooms in Lake Okeechobee.
Figure 10. Microcystis blooms are a function of the nutrient releases from the sediments.
In addition to the sediment-recycled nutrients, it is also acknowledged that both N
and P are transported to the lake via the tributary system. N
2
-fixing cyanobacteria also
add to N availability, thereby providing multiple sources that lead to the massive blooms
of Microcystis (Figure 11). There is also a recycling component associated with the die-off
of a bloom, sinking in the water column, accumulating in the sediments as organic matter
(added back to the thixotropic layer) and being mineralized by bacteria (Figure 12).
The average chemical composition of cyanobacteria follows the atomic Redfield ratio:
106C:16N:1P, as the average cellular mole ratio of carbon, nitrogen, and phosphorus.
In freshwater systems, carbon is not limiting, leaving the ratio of 16:1 as the general
determinant of either N-limited or P-limited for a waterbody.
Cyanobacteria are only capable of utilizing orthophosphate (PO
4
) and its solubility
is a function of elements such as Ca
2+
, Mg
2+
, Fe
2+
, Fe
3+
, and Al
3+
and pH. Cyanobacteria
can also take advantage of organically bound P with the aid of phosphatase enzymes that
cleave P from the dissolved organic matter. This phenomenon happens when the cells
respond to nutrient depleted waters, allowing growth to continue when orthophosphate
becomes limiting. Lake Okeechobee peat zone sediment efflux ranges from 0.2–2.2 mg
soluble P/m2day, while the mud zone efflux ranges from 0.1–1.9 P/m2.day [11], creating
a pool of P readily available to the phytoplankton community. In addition, frequent
Water 2021,13, 39 15 of 23
sediment resuspension through wind, wave action, or periodically by hurricanes, creates
an abundance of P in the water column, which promotes cyanobacterial blooms when other
abiotic and biotic conditions are favorable.
Water 2021, 13, x FOR PEER REVIEW 15 of 24
Figure 10. Microcystis blooms are a function of the nutrient releases from the sediments.
In addition to the sediment-recycled nutrients, it is also acknowledged that both N
and P are transported to the lake via the tributary system. N
2
-fixing cyanobacteria also
add to N availability, thereby providing multiple sources that lead to the massive blooms
of Microcystis (Figure 11). There is also a recycling component associated with the die-off
of a bloom, sinking in the water column, accumulating in the sediments as organic matter
(added back to the thixotropic layer) and being mineralized by bacteria (Figure 12).
Figure 11. Nutrient sources that feed the algal blooms in Lake Okeechobee.
Figure 11. Nutrient sources that feed the algal blooms in Lake Okeechobee.
Water 2021, 13, x FOR PEER REVIEW 16 of 24
Figure 12. Post algal bloom “lake snow” deposition in Lake Okeechobee.
The average chemical composition of cyanobacteria follows the atomic Redfield ratio:
106C:16N:1P, as the average cellular mole ratio of carbon, nitrogen, and phosphorus. In
freshwater systems, carbon is not limiting, leaving the ratio of 16:1 as the general deter-
minant of either N-limited or P-limited for a waterbody.
Cyanobacteria are only capable of utilizing orthophosphate (PO4) and its solubility is
a function of elements such as Ca2+, Mg2+, Fe2+, Fe3+, and Al3+ and pH. Cyanobacteria can
also take advantage of organically bound P with the aid of phosphatase enzymes that
cleave P from the dissolved organic matter. This phenomenon happens when the cells
respond to nutrient depleted waters, allowing growth to continue when orthophosphate
becomes limiting. Lake Okeechobee peat zone sediment efflux ranges from 0.2–2.2 mg
soluble P/m2 day, while the mud zone efflux ranges from 0.1–1.9 P/m2.day [11], creating a
pool of P readily available to the phytoplankton community. In addition, frequent sedi-
ment resuspension through wind, wave action, or periodically by hurricanes, creates an
abundance of P in the water column, which promotes cyanobacterial blooms when other
abiotic and biotic conditions are favorable.
Nitrogen sources for cyanobacteria incapable of fixing atmospheric nitrogen such as
Microcystis include inorganic forms, ammonium (H4N+), ammonia (H3N), nitrite (NO2−),
and nitrate (NO3−) [78]. The three amino acids, arginine, asparagine, and glutamine as well
as urea can all be used as organic sources of N for cyanobacteria [68]. In resuspended
sediments that are aerobic, ammonium can be oxidized to nitrite and nitrate by nitrifying
bacteria, while under anerobic conditions, fixed N is converted to N2 by denitrifying bac-
teria. Lake Okeechobee surface sediments are aerobic and the efflux of ammonium, nitrite,
and nitrate into the water column is highly conducive to cyanobacteria bloom formation
and expansion.
Microcystis colonies can also regulate their position in the water column using inter-
nal accumulation of gas in vesicles, which brings them to the surface for optimizing pho-
tosynthesis. The accumulation of photosynthate builds ballast during the day, and subse-
quently, colonies sink in the water column, typically into the nutrient-enriched waters
near the water-sediment interface. While at depth, cells absorb key nutrients, the photo-
synthate ballast is utilized for respiration, creating the gas that allows colonies to float up
to the water surface. This cycle of up–down migration is a mechanism that “pumps” sed-
iment nutrients directly into cyanobacteria blooms. In addition, cyanobacteria also accu-
mulate polymers to store excess nutrients internally, termed luxuriant, when concentra-
Figure 12. Post algal bloom “lake snow” deposition in Lake Okeechobee.
Nitrogen sources for cyanobacteria incapable of fixing atmospheric nitrogen such as
Microcystis include inorganic forms, ammonium (H
4
N
+
), ammonia (H
3
N), nitrite (NO
2−
),
and nitrate (NO
3−
) [
78
]. The three amino acids, arginine, asparagine, and glutamine as
well as urea can all be used as organic sources of N for cyanobacteria [
68
]. In resuspended
sediments that are aerobic, ammonium can be oxidized to nitrite and nitrate by nitrifying
bacteria, while under anerobic conditions, fixed N is converted to N
2
by denitrifying
bacteria. Lake Okeechobee surface sediments are aerobic and the efflux of ammonium,
nitrite, and nitrate into the water column is highly conducive to cyanobacteria bloom
formation and expansion.
Water 2021,13, 39 16 of 23
Microcystis colonies can also regulate their position in the water column using internal
accumulation of gas in vesicles, which brings them to the surface for optimizing photosyn-
thesis. The accumulation of photosynthate builds ballast during the day, and subsequently,
colonies sink in the water column, typically into the nutrient-enriched waters near the
water-sediment interface. While at depth, cells absorb key nutrients, the photosynthate bal-
last is utilized for respiration, creating the gas that allows colonies to float up to the water
surface. This cycle of up–down migration is a mechanism that “pumps” sediment nutrients
directly into cyanobacteria blooms. In addition, cyanobacteria also accumulate polymers
to store excess nutrients internally, termed luxuriant, when concentrations exceed cellular
metabolism requirements. For P, storage takes place in polyphosphate bodies and N is
stored as cyanophycin granules. Both storage polymers can be utilized by the cells when
needed due to a nutrient deficiency to continue growth. When a colony loses its buoyancy
and sinks into the sediment layer as “lake snow” [
79
], these storage compounds, other
cellular components, and a complex extracellular matrix of polysaccharides (mucilage) and
associated bacteria are carried with it (Figure 13). This mucilage can include transparent
exopolymer particles (TEP), which consist primarily of acidic polysaccharides.
Water 2021, 13, x FOR PEER REVIEW 17 of 24
tions exceed cellular metabolism requirements. For P, storage takes place in polyphos-
phate bodies and N is stored as cyanophycin granules. Both storage polymers can be uti-
lized by the cells when needed due to a nutrient deficiency to continue growth. When a
colony loses its buoyancy and sinks into the sediment layer as “lake snow” [79], these
storage compounds, other cellular components, and a complex extracellular matrix of pol-
ysaccharides (mucilage) and associated bacteria are carried with it (Figure 13). This muci-
lage can include transparent exopolymer particles (TEP), which consist primarily of acidic
polysaccharides.
Figure 13. Photomicrograph of Lake Okeechobee snow collected from 1.8 m below the surface,
June 2020. The image was obtained using a Zeiss AX10 microscope equipped with DIC and a digi-
tal imaging system.
At greater depths in the Lake, the snow includes more organic materials such as fecal
pellets from zooplankton (Figures 14 and 15).
Figure 13.
Photomicrograph of Lake Okeechobee snow collected from 1.8 m below the surface, June
2020. The image was obtained using a Zeiss AX10 microscope equipped with DIC and a digital
imaging system.
At greater depths in the Lake, the snow includes more organic materials such as fecal
pellets from zooplankton (Figures 14 and 15).
Therefore, the occurrence of the thixotropic mud in Lake Okeechobee can create the
nutrient source in the water column to initiate algal blooms. In addition, the persistence of
the thixotropic mud near the bottom after some settling can help maintain a bloom during
the sinking phases of the movement of Microcystis colonies.
Water 2021,13, 39 17 of 23
Water 2021, 13, x FOR PEER REVIEW 18 of 24
Figure 14. Lake Okeechobee snow from a 3.2 m depth, June 2020. Note the presence of filamentous
bacteria at arrows. The image was obtained using a Zeiss AX10 microscope equipped with DIC
and a digital imaging system.
Figure 15. Lake Okeechobee snow from 3.2 m depth, June 2020. These are fecal pellets from zoo-
plankton. The image was obtained using a Zeiss AX10 microscope equipped with DIC and a digi-
tal imaging system.
Therefore, the occurrence of the thixotropic mud in Lake Okeechobee can create the
nutrient source in the water column to initiate algal blooms. In addition, the persistence
of the thixotropic mud near the bottom after some settling can help maintain a bloom
during the sinking phases of the movement of Microcystis colonies.
6. Discussion
6.1. Necessity for Removal of the Mobile Mud from Lake Okeechobee
In reviewing the issue of legacy phosphorus in Lake Okeechobee, the occurrence and
dynamics of the bottom mud layer and its mobilization seems to be a key issue in the lake
Figure 14.
Lake Okeechobee snow from a 3.2 m depth, June 2020. Note the presence of filamentous
bacteria at arrows. The image was obtained using a Zeiss AX10 microscope equipped with DIC and
a digital imaging system.
Water 2021, 13, x FOR PEER REVIEW 18 of 24
Figure 14. Lake Okeechobee snow from a 3.2 m depth, June 2020. Note the presence of filamentous
bacteria at arrows. The image was obtained using a Zeiss AX10 microscope equipped with DIC
and a digital imaging system.
Figure 15. Lake Okeechobee snow from 3.2 m depth, June 2020. These are fecal pellets from zoo-
plankton. The image was obtained using a Zeiss AX10 microscope equipped with DIC and a digi-
tal imaging system.
Therefore, the occurrence of the thixotropic mud in Lake Okeechobee can create the
nutrient source in the water column to initiate algal blooms. In addition, the persistence
of the thixotropic mud near the bottom after some settling can help maintain a bloom
during the sinking phases of the movement of Microcystis colonies.
6. Discussion
6.1. Necessity for Removal of the Mobile Mud from Lake Okeechobee
In reviewing the issue of legacy phosphorus in Lake Okeechobee, the occurrence and
dynamics of the bottom mud layer and its mobilization seems to be a key issue in the lake
Figure 15.
Lake Okeechobee snow from 3.2 m depth, June 2020. These are fecal pellets from
zooplankton. The image was obtained using a Zeiss AX10 microscope equipped with DIC and a
digital imaging system.
6. Discussion
6.1. Necessity for Removal of the Mobile Mud from Lake Okeechobee
In reviewing the issue of legacy phosphorus in Lake Okeechobee, the occurrence and
dynamics of the bottom mud layer and its mobilization seems to be a key issue in the lake
ecology. There are major discrepancies in the data interpretations concerning the mobility
of the mud and how it releases phosphorus into the water column. Much has been written
on the mud occurrence, composition, and dynamics [
7
,
54
,
57
–
59
]. However, there are
two or three forms of the mud in Lake Okeechobee. These include consolidated mud on
Water 2021,13, 39 18 of 23
the bottom, semi-consolidated mud lying atop of the denser mud, and the thixotropic
mud that lies atop of the semi-consolidated and consolidated mud and in some of the
inflow streams. Kirby et al. [
7
] stated “A different scenario, however, envisages that,
notwithstanding present nutrient loads. A large portion of nutrients are sorbed onto
sediment particles, which are periodically resuspended leading to partial nutrient release.
According to this internal loading dependent scenario, decreasing the fresh nutrient input
will have little short-term impact, because nutrient releases will continue to be dominated
by fine-sediment entrainment and nutrient loading”.
There are differing opinions concerning the impact of the sediment resuspension in
Lake Okeechobee [
16
]. In 2012, Yan and James [
31
] concluded that the net solids load
into Lake Okeechobee is negative with more export of solids than the inflow of solids.
The data supporting this conclusion seems to be based on a combination of the bottom
mud mapping and the balance of flows and associated turbidity.
The resuspension of the mud tends to release nutrients and initiate algal blooms.
If the lake is experiencing a net loss of mud, then the impact on water quality should
be declining. This begs the question that if the mud balance shows that the solids and
associated legacy phosphorus is being “mined” from the lake, the inflow of dissolved and
particulate phosphorus is being controlled to a better degree, then why have the algal
blooms in Lake Okeechobee increased in frequency, duration, and intensity? It can be
concluded that P is continuing to accumulate in Lake Okeechobee with no net loss [16].
We assert that the mud budget of the lake has not adequately accounted for the
thixotropic component of the mud. This material cannot be cored and is known to oc-
cur in a bottom layer in the Taylor Creek/Nubbin Slough at a variable thickness [
7
,
21
].
It is documented that thixotropic mud can be mobilized by wind-induced currents, in-
creased wave orbital motion during hurricane events, and by “normal” wave activity in the
lake [42,52].
It is much easier to resuspend a fluid mud unit compared to consolidated inor-
ganic clay, which requires a high scouring or orbital velocity according to Hjulström [
63
].
The higher velocity wave orbital motion would produce a limited occurrence of resuspen-
sion events based on normal wind activity and associated wave generation. Resuspension
of the thixotropic unit would produce a much higher number of resuspension events and
associated algal blooms.
Sedimentation in Lake Okeechobee is likely much more complex than previously
described. While provisions have been made for the organic content of the clay-size
sediments in models of the lake [
55
], the dynamics may not allow bottom deposition of
these sediments within the consolidated mud, thus making them difficult to remobilize
(e.g., Hjulström velocity required for orbital wave resuspension). In addition, a less stable
layer may occur between the consolidated mud and the thixotropic mud [
7
]. This layer
also has a greater susceptibility to resuspension. However, it is known that the “flocculent
mud” or thixotropic sediments and the shallow depth of Lake Okeechobee allow frequent
resuspension events [56–59].
A key question is the residence time of the thixotropic mud in the water column before
it is deposited on the bottom and becomes part of either the consolidated mud deposit or
the overlying layer or thixotropic mud containing gas [
7
]. A mechanism to maintain the
thixotropic mud in a long-duration fluid state must be identified. Since the algal bloom fre-
quency in Lake Okeechobee has increased greatly based on both nutrient influx into the lake
and remobilization of legacy nutrients, the production of acidic polysaccharides and other
extracellular substances (EPS) has also increased. It has been experimentally demonstrated
that EPS interacts with clay minerals, aggregates these particles, and increases the settling
velocity [
80
]. However, this does not necessarily indicate that the flocs will settle to the
bottom and consolidate. The EPS, particularly the transparent exopolymer particles (TEP),
are a major food supply for bacteria, which will secrete carbon dioxide during feeding
and with breakdown of other organic molecules. The carbon dioxide release may keep the
sediment in suspension for extended periods, aided by normal wave orbital motion. The
gas issue in the mud has been raised in the past by Kirby et al. [
7
], who suggested that it
Water 2021,13, 39 19 of 23
may have an impact on the mobility of the mud. Therefore, the thixotropic mud layer may
be semi-stable, lingering above the consolidated mud bottom, which allows resuspension
of the sediment and nutrients at a lower wind velocity (smaller wave orbital motion).
In addition, the organic material breakdown will tend to reduce oxygen concentrations
based on combined chemical and biological oxygen demand. This reduction may make
phosphate more bioavailable during resuspension events.
If this process is truly active in Lake Okeechobee, the assertion by Yan and James [
31
]
that there has been approximately a 52% reduction in the mud within Lake Okeechobee
between 1998 and 2006 is unlikely. Only the form of the muddy sediment was changed from
consolidated to thixotropic with greater mobility. Hansen et al. [
38
] actually sampled the
unconsolidated “floc layer” (thixotropic mud) in their investigation of phosphate release
from this material and its relationship to algal blooms. The thixotropic or floc material tends
to have a greater positive impact on stimulating algal growth compared to the consolidated
mud. This conclusion is supported by Canfield et al. [
16
], who clearly stated that the
restoration of Lake Okeechobee was “mission impossible” without the removal of the
legacy nutrients that occur in the thixotropic mud.
6.2. Implications for Management of Other Complex Ecosystems and Future Research Required
The Lake Okeechobee Watershed including the Caloosahatchee and St. Lucie Rivers
and the Everglades is one of the largest and most complex freshwater ecosystems on Earth.
Management of the system is a daunting challenge, especially since it has been altered to
such a large degree by anthropogenic activities. A very large number of scientific investiga-
tions have been conducted on the ecosystem including water quality assessments, surface,
and groundwater modeling, assessments of nutrient budgets (including wet and dry fall-
out), wave and current dynamic models of the lake, engineering investigations on surface
water flooding and control, sediment dynamics and geochemical studies, and many others.
This is perhaps the most studied freshwater ecosystem in the world. However, perhaps
the greatest challenge is to integrate all of the studies and plans into a comprehensive,
system-wide restoration effort that achieves the key water quality goals. Elimination of the
thixotropic mud containing a massive quantity of legacy nutrients in Lake Okeechobee is a
key component that has not been achieved.
A research effort that cannot be ignored is the formulation of a plan to remove the
thixotropic mud from the lake. While many investigations have been conducted on map-
ping the mud unit in the lake, the primary effort has been directed at measuring a mud
type that is not mobile and does not really impact water quality. Future research must be
directed at comprehensively investigating the thixotropic mud that is mobile and directly
causes algal blooms. This impact needs to be modeled and quantified. In addition, engi-
neering plans need to be developed to remove the mobile mud from the ecosystem using a
cost-effective technology. The development of this method could be applied to many other
lake systems that have a similar mobile mud that contain legacy nutrients in numerous
global locations.
7. Conclusions
Legacy nutrients are a major issue that impacts the trophic state of Lake Okeechobee
and are clearly associated with the occurrence of thixotropic mud (fluid organic floc
material). The thixotropic mud has a much greater mobility compared to the consolidated
mud layer on the bottom of the lake. It is resuspended from the bottom into the water
column by currents, wind, and waves, particularly during strong storms and hurricanes.
The resuspended sediments contain nutrients, which tend to stimulate algal growth to a
high degree during favorable biogeochemical conditions.
There is no evidence that the volume of the thixotropic mud in Lake Okeechobee is
declining, when in fact it could be increasing based on its occurrence on the bottom of
streams feeding the lake, especially during hurricane events and major floods. In-lake
Water 2021,13, 39 20 of 23
processes may be adding to the volume of the thixotropic mud based on the transport of
vast quantities of particulate organic material.
There is a direct association between sediment resuspension events and blooms of
cyanobacteria in Lake Okeechobee. The thixotropic mud contains a greater concentration
of nutrients compared to the immobile consolidated mud. In addition, bioassay studies
have demonstrated that the resuspended thixotropic mud produces up to a 50% higher
rate of algal growth versus the consolidated mud.
The thixotropic mud is an important component of the legacy P and N in Lake Okee-
chobee and as long as it remains in a mobile state in the lake, the frequency of algal blooms
will be high. A method must be developed to effectively remove the thixotropic mud
(organic floc material) from Lake Okeechobee and prevent it from continuing to damage
the lake and the ecosystems of outflow streams (i.e., Caloosahatchee and St. Lucie Rivers).
Since the thixotropic mud containing legacy nutrients is a major factor in controlling
algal blooms in the lake, it must be considered within the framework of total ecosystem
management. A combination of nutrient influx management, water budget management,
and elimination of the sources of legacy nutrients, must be used to manage not only
Lake Okeechobee, but also the surface water bodies that receive discharge from the lake
including the Caloosahatchee and St Lucie Rivers and the Everglades. The algal blooms
starting in the lake have damaged the riverine and estuarine ecosystems from the lake
the entire distance to the east and west coasts of Florida. This is a classic case of the
need to manage a vast, regional ecosystem within a framework of all contributing factors
including the problem of legacy nutrient management, which has been largely ignored
because of the difficulty of the task and costs involved. Without removal of the thixotropic
mud, cleanup and restoration of Lake Okeechobee and the associated estuarine system is
“mission impossible”.
Author Contributions:
T.M.M. wrote the sediments part of the manuscript and obtained project
funding. S.T. wrote parts of the sediments section of the manuscript and drew the graphics. B.H.R.
wrote the algae parts of the manuscript. All authors have read and agreed to the published version
of the manuscript.
Funding:
The funding for this project was provided by Hedrick Brothers Construction Company,
Inc., West Palm Beach, Florida, USA.
Data Availability Statement: All data used in this paper are contained in the references provided.
Acknowledgments:
Most of the authors used the facilities at the Emergent Technologies Institute,
U. A. Whitaker College of Engineering at Florida Gulf Coast University for the performance of
this research.
Conflicts of Interest: The authors declare no conflict of interest.
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