FWRI Technical Report TR-11 77

Introduction

Question 3—What regulates the onset, persistence, and fate

of planktonic algal blooms in Florida Bay?

From the 1950s to the mid 1980s, Florida Bay was characterized by expansive seagrass beds, mangrove islands, and crystal-clear water. In the past 16 years,

there have been some dramatic changes in the ecology

of Florida Bay; examples are the die-off of large areas

of seagrass and the appearance of intense blooms of

planktonic algae. There is a general consensus in the

scientific community that these phenomena are indicative of changes in key environmental conditions

within the bay.The shift in primary producers and the

alteration of the photic environment in Florida Bay

have been hypothesized to have major effects on the

flora and fauna of the bay (e.g., Boesch et al., 1993).

Fourqurean and Robblee (1999) have recently reviewed

many of the changes. In this paper, we examine the current state of knowledge about the character and causes

of planktonic algal blooms in the bay.

In reviewing the scope of our knowledge on algal

blooms in Florida Bay, it is important to keep in mind

that detailed monitoring of water quality in the bay has

been underway for just over a decade.This is a somewhat surprising fact considering the importance of

this aquatic resource to Florida and the surrounding

waters of the Atlantic Ocean and Gulf of Mexico. One

of the factors that triggered the initiation of extensive

multidisciplinary research in the bay was the on set of

massive cyanobacterial blooms over large areas of the

bay in the early 1990s.

The first quantitative indications of major increases

in phytoplankton densities in the interior regions of

Florida Bay came from chlorophyll a data collected by

Florida International University (FIU) (Boyer et al.,

1997, 1999). Boyer et al.’s (1999) research, which began

in 1988, indicated a significant step increase in chlorophyll levels in 1991–1992. The increase in algal biomass within the bay was corroborated by incidental

observations by individuals frequenting the bay for

other research and recreation activities. In 1993, a separate research group from the University of Florida

joined FIU in monitoring efforts that revealed high

concentrations of cyanobacteria in the central portion

of Florida Bay (Figure 6.1; Phlips and Badylak, 1996;

Phlips et al., 1999). In 1994, the phytoplankton research

efforts were expanded further to include the Florida

Marine Research Institute [now the Fish and Wildlife

Research Institute]. All three research teams observed

large cyanobacterial blooms in the central regions of

the bay, particularly during the summer and fall (Phlips

et al., 1999; Steidinger et al., 2001).

During the same period, algal blooms were also

recorded in the western region of the bay (Figure 6.1).

However, these blooms were typically dominated by diatoms, not by cyanobacteria. (Phlips and Badylak, 1996;

Phlips et al., 1999; Steidinger et al., 2001) Recent efforts

by researchers from the University of Miami focusing

on the western bay have provided further support for

the importance of diatom blooms in the western region.

Analysis of sediment cores from the western side of

Florida Bay indicate that diatom blooms along the western margin were a well-established feature by 1970,

significantly prior to the increases in cyanobacteria

blooms within the bay’s interior (Jurado, 2003). Most recently, chemotaxonomic studies of phytoplankton in the

bay have been initiated using full-spectrum photodiode array detectors and high-performance liquid chromatography analysis (Louda, 2001).

Regional Differences

The observed disparity in the quantity and quality

(i.e., composition) of phytoplankton biomass in different parts of the bay manifests the need to view phytoplankton dynamics by ecological zones. In contrast

to the numerous phytoplankton blooms encountered

in the western and central regions of Florida Bay, almost no blooms are located in the eastern portion of

the bay (Figure 6.1).There have been several efforts to

divide Florida Bay into ecologically distinct zones.

These zonal breakdowns have been based on a variety of parameters, including geology (Prager et al.,

1996), distributions of benthic plants (Zieman et al.,

Chapter 6

Plankton Blooms

Team Leaders Gary Hitchcock (University of Miami), Ed Phlips (University of Florida), 

Larry Brand (University of Miami), Douglas Morrison (ENP, PMC)
Florida Bay Synthesis Hunt and Nuttle

78 FWRI Technical Report TR-11

1989), faunal distributions (Thayer and Chester, 1989;

Holmquist et al., 1989), and water-quality characteristics (Phlips et al., 1995, Boyer et al., 1997, 1999; Burd and

Jackson, 2001).These efforts have yielded from 3 to 20

zones with different boundary designations.The lack

of consensus on the specific boundaries and number

of zones is largely attributable to differences in the

choice of parameters used in the discrimination of the

regions.

If the focus is limited to the frequency of phytoplankton blooms and the general composition of the

blooms, it is possible to define three general regions

that roughly follow longitudinal lines of demarcation

(Figure 6.2): (1) The eastern region of the bay, east of

80°39 ́W and west of the Florida Keys, where major phytoplankton blooms are seldom observed; (2) the central region of the bay, between 80°39 ́W and 80°49 ́W,

where bloom concentrations of cyanobacteria (i.e.,

greater than 1 million cells per ml) are regularly encountered; and (3) the western region of the bay, west

of 80°49 ́W, which experiences periodic blooms of diatoms.These regions provide a useful context for discussing both the character of blooms in the bay and the

factors that control them.

In the most general terms, phytoplankton standing

crops are dictated by factors that control gains and

losses of algal biomass (Figure 6.3). From a gain perspective, the most obvious contributor to the increase

in phytoplankton biomass is cell growth. Net growth

rates are under the control of resource availability, such

as nutrient and light, and of factors that affect the rates

of photosynthetic and respiratory activity, such as temperature, salinity, and oxygen level. Algal biomass can

also increase as a consequence of inputs to the water

column from the benthos through resuspension or inflows of water from adjacent environments.

In terms of loss functions, one of the most important overriding considerations is water residence time.

In many coastal ecosystems, phytoplankton biomass is

limited by the flushing-rate of water that has lower phytoplankton content. Flushing rates are generally related

to a combination of rainfall inputs, tidal mixing, and

Figure 6.1 Biovolume of Synechococcus elongatus (Large Syn.), other cyanobacteria, dinoflagellates, diatoms and other phytoplankton at four sampling sites representative of different regions of Florida Bay (Phlips et al., 1999).

Plankton Blooms
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FWRI Technical Report TR-11 79

wind-driven circulation. Many environments with long

residence times tend to have higher phytoplankton

crops than do environments with short residence times,

assuming that nutrient and light availability are similar (Monbet, 1992).There are also biological processes

that contribute to the loss of biomass in many aquatic

ecosystems, such as grazing by benthic and planktonic filter-feeding organisms. The rates of loss are

closely tied to the structure and abundance of the

grazer community and to environmental factors that

affect the rate of activity, such as salinity, temperature, and phytoplankton composition (e.g., presence of

toxic species).

Summary of Research

Research to date indicates that there are three ecologically distinct regions within the bay from the standpoint of algal blooms: the northeastern, central, and

western regions.The nature and causes of blooms can

be most appropriately summarized within the context of these three regions (Table 6.1).

NORTHEASTERN REGION

• Algal blooms are largely absent from the northeastern region of Florida Bay. The phytoplankton

community of the northeastern region is a diverse

mixture of cyanobacteria, dinoflagellates, diatoms,

and microflagellates, none of which form blooms.

• The lack of algal blooms in the northeastern region

of Florida Bay is largely attributable to the severe

phosphorus limitation characteristic of the region.

Despite significant water inflows to the region from

the Everglades, the very low phosphorus levels in

these inputs combined with the calcium-carbonate

rich waters of the region, result in nutrient-limited

conditions.

CENTRAL REGION

• Large algal blooms have been a common feature of

the central region since 1992. The focal point for

blooms in the central region is in the north-central

region from Rankin Basin extending into Whipray

Basin.The greatest bloom activity is generally in the

summer and early fall, although blooms have been

Figure 6.2 Florida Bay basin configurations.

Plankton Blooms
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80 FWRI Technical Report TR-11

observed in other seasons. In the fall and early summer, blooms originating in the northern central region are pushed southward by prevailing wind-driven

circulation into the southern central region, where

they can spread out into the Atlantic reef tract through

cuts in the Florida Keys.The dominant bloom-forming alga in the central region is the picoplanktonic

cyanobacterium Synechococcus cf. elongatus, though

several species of diatoms and dinoflagellates occasionally occur in bloom proportions.

• Nutrient limitation of phytoplankton growth in the

central region appears to switch back and forth from

phosphorus to nitrogen.The nature of the sources of

nutrients for blooms in the central region has been

a subject of considerable debate, yielding several

hypotheses: (1) a significant supply of nutrients for

algal production is available from the flocculent

muddy sediments within the region; (2) a groundwater source of phosphorus is present within the region that, in combination with nitrogen from the

northeastern region, feeds algal blooms; (3) nutrients

from the shelf region west of the bay are available

to blooms in the central region via tidal exchange;

and (4) atmospheric deposition is a significant source

of nutrients (particularly N) for algal primary production. These hypotheses are not mutually exclusive, and several mechanisms may be operational at

the same time.

• The domination of blooms in the central region by

Synechococcus cf. elongatusis attributable to the unique

ecophysiological characteristics of this species of

cyanobacterium. These characteristics include (1)

wide tolerance to salinity, (2) superior ability to compete for phosphorus at low concentrations, (3) ability to regulate buoyancy and thereby take advantage

of nutrients available in the sediments, and (4) lower

susceptibility to grazing losses.

WESTERN REGION

• Algal blooms are also a common feature of the western region of Florida Bay. In contrast to the cyanobacFigure 6.3 Gain versus loss-functions for phytoplankton standing crops.

Table 6.1 Summary of research on plankton blooms by region.

West Central East

General Phlips and Badylak, 1996 Phlips and Badylak, 1996 Phlips and Badylak, 1996

Characteristics Boyer et al., 1997, 1999 Boyer et al., 1997, 1999 Boyer et al., 1997, 1999

Phlips et al., 1999 Phlips et al., 1999, 2002 Phlips et al., 1999

Steidinger et al., 2001 Steidinger et al., 2001 Steidinger et al., 2001

Jurado, 2003

Nutrients Lapointe et al., 1990, 1994

Robblee et al., 1991

Carlson et al., 1994

Durako, 1994

Zieman et al., 1994

Paul et al., 1995a,b

Tomas et al., 1999

Lapointe et al., 2002

Composition and Tomas et al., 1999 Richardson et al., 2001

Growth Rates Jurado, 2003

Grazing and Lynch and Phlips, 2000

Flushing Brenner et al., 2001

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terial blooms in the north-central region, these

blooms are dominated by diatoms.The diatom-dominated blooms in the western region are mainly composed of Rhizosolenia spp., although Chaetoceros spp.

and pennates are major genera in terms of cellular

biomass. Diatom blooms in the western region begin

in late summer and are advected into the bay from

shallow coastal waters off Cape Sable.

• The diatom-dominated blooms in western Florida

Bay are mainly limited in their growth by nitrogen,

either singly or in combination with phosphorus

and/or silicon supply. Temperature, salinity, and

light do not appear to be important factors in the initiation or maintenance of diatom blooms. Maximum

growth rates of the diatom community during bloom

initiation in the western region range from 1 to 2 day–1

(first-order rate constant) based on increases in netplankton (greater than 5 μm size fraction), chlorophyll

a, or biogenic silica.The onset of diatom blooms on

the western Florida Shelf appears to be associated

with enhanced riverine outflow, predominantly from

the Shark River.The onset of increased diatom biomass occurs within the riverine, low-salinity plume

near Cape Sable.

Eastern Florida Bay

Phytoplankton abundance is lower in the eastern part

of the bay than it is in the other two zones within the

bay (Figure 6.1). Chlorophyll a concentrations in the

eastern bay are typically less than 2 mg m–3 throughout the year. Phlips et al. (1999) reported that the maximum chlorophyll a concentrations in this region were

less than 5 mg m–3 between August 1993 and October

1997, with most values less than 1 mg m–3 (Figure 6.4).

Similarly, Boyer et al. (1997, 1999) observed low chlorophyll a concentrations in eastern Florida Bay for the period between 1989 and 1997 (mean = 0.85 mg m–3).

The phytoplankton communities in the eastern

bay are principally composed of centric diatoms (e.g.,

Thalassiosira sp.), dinoflagellates (e.g., Protoperidium

spp., Ceratium sp., Prorocentrum micans), and small

cyanobacteria that constitute the major fraction of cellular biovolume (Phlips and Badylak, 1996). Steidinger

Figure 6.4 Chlorophyll a concentrations and salinities at sampling sites representative of different regions in Florida Bay (Phlips et al.,

1999).

Plankton Blooms
et al. (2001) reported that the dominant phytoplankton

taxa in the region from fall 1994 to early summer 1995

were dinoflagellates, whereas cyanobacteria were the

most abundant taxa from late 1996 to spring 1997. Cellular biovolume serves as a useful index to phytoplankton biomass that is independent of physiologically

induced changes in pigment concentrations such as

those that occur in chlorophyll a.In eastern Florida Bay,

total phytoplankton biovolume is less than 1 μm3 ml–1,

a value much less than that in the western and central

bay (Figure 6.1) (Phlips et al., 1999).

Two factors result in strong phosphorus limitation of primary production in the region, helping to explain the low phytoplankton concentrations that typify

this region of the bay.The eastern region of Florida Bay

receives significant freshwater inputs from Taylor

Slough and the C-111 canal system. Salinity values in

the eastern basins are on average lower than in the central and western bay, with typical values from 20 practical salinity units (psu) to 30 psu (Figure 6.4) (Phlips

et al., 1999).The external surface-water inputs to eastern Florida Bay are very low in phosphorus (Boyer et

al., 1999). In addition, the high concentrations of calcium in the region tend to bind free phosphate.

Central Florida Bay

Over the past twelve years, the central region of Florida

Bay has been the site of intense and prolonged algal

blooms. In the eastern region of the bay, biovolumes of

phytoplankton seldom exceeded 1 million μm–1/ml;

however, in the central bay phytoplankton biovolumes

have regularly exceeded 10 million μm–1/ml (Phlips et

al., 1999). The intensity of blooms in the central bay is

somewhat masked by the fact that the chlorophyll content of phytoplankton in the central region is relatively

low when compared to the chlorophyll content in the

western bay and some other coastal ecosystems in

Florida (Phlips et al., 1999, 2002). It has been hypothesized that this is partially attributable to the very shallow depth of the central region of the bay (generally less

than 2 m), resulting in high light availability and sunadaptation by algal cells (Phlips et al., 1999). It is also possible that the domination of the plankton community

by the cyanobacterium Synechococcus cf. elongatus and

the nutrient limitation contribute to the low ratio. Irrespective of the causes for the low ratios, it is important to consider the ratio when evaluating the intensity

of blooms in the bay. For example, peak chlorophyll concentrations in the St. Johns River in northeast Florida

are three to four times those in central Florida Bay, but

the peak biovolumes of phytoplankton are very similar (Phlips et al., 2000). These considerations manifest

the fact that the 20–40 mg m–3 chlorophyll levels commonly encountered in central Florida Bay are representative of serious bloom conditions, whereas similar

chlorophyll levels in other ecosystems might be viewed

as moderate bloom levels.

Over the twelve years since the initiation of major

phytoplankton blooms in central Florida Bay, the most

intense blooms have occurred during the summer and

early fall, except during 1993 and 1994, when high phytoplankton standing crops persisted throughout the

year.Three of the past ten years (1993, 1994, and 1998)

had particularly intense blooms. In central Florida

Bay, phytoplankton blooms originate in the northern

part of the region. For most of the peak bloom season

(May–September), phytoplankton populations in the

south-central region are relatively low. However, in

the fall and early winter, passing cold fronts, along

with the concomitant shift in wind direction to the

north, displace phytoplankton-rich water southward.

This recurring pattern is apparent in the appearance

of bloom concentrations of phytoplankton as far south

as the Florida Keys reefs.

Cause of Blooms in the Central Region

The apparent rise of planktonic algae blooms in the

central region of Florida Bay in the early 1990s raises

two fundamental questions: (1) What changes occurred

in the region that facilitated the onset of the blooms,

and (2) What are the nutrient sources that have sustained blooms since that time? In relation to the first

question, the onset of blooms indicates one of two

possibilities: (1) more nutrients were made available

for phytoplankton production either through redirection of internal reserves and/or increases in external

loading, and/or (2) biomass loss terms were dramatically reduced.The presence of a thick, organic sediment

layer in the north-central region of Florida Bay indicates

that this region of the bay has historically been productive. Prior to the 1990s, seagrasses were most likely

the dominant primary producers in the central bay.

Even after 1990, seagrasses were still present in the region, although were not as prominently as in previous

years.

One of the principal hypotheses about the cause

of phytoplankton blooms in the central bay focuses on

the role of seagrass die-off in making internal nutrient

reserves available for planktonic production (Robblee

et al., 1991; Carlson et al., 1994; Durako, 1994; Zieman

et al., 1994). There are a number of ways that the reduction of seagrass populations could enhance nutrient availability for planktonic production, including (1)

through the addition of recyclable nutrients from dead

seagrass, (2) by increasing access to sediment-bound

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nutrients via increased resuspension and diffusion,

and (3) by reducing competition for nutrients between

phytoplankton and epiphytic or benthic algae living in

seagrass communities. In addition to increasing nutrient availability, the reduction in seagrass communities can affect phytoplankton loss processes. The

populations of filter-feeding organisms associated with

seagrasses can influence phytoplankton densities, particularly in very shallow environments like Florida

Bay.The physical presence of dense seagrass beds also

increases the sedimentation of algae by reducing the

potential for resuspension of algae and sediments.

The importance of seagrass die-off in the stimulation of phytoplankton blooms is not a universally

accepted concept. Some researchers have suggested

that this line of reasoning is backwards and that increases in planktonic production have actually led to

the demise of seagrasses (Lapointe et al., 2002).The authors of this hypothesis cited observations by local

users of the bay of increases in turbidity prior to the

seagrass die-off in the late 1980s as evidence (DeMaria,

1996). However, these observations were principally restricted to the western region of the bay and therefore

do not provide substantial insight into what occurred

in the central bay during the 1980s.The north-central

region of the bay is very shallow (generally less than

2 m deep), and it remains to be demonstrated if blooms

could cause the demise of seagrass populations as a result of decreased light availability. It is possible that such

an argument might be more tenable in the western bay

and shelf environments, where average depths are

greater.

Irrespective of which came first, the seagrass dieoff or the algae bloom, the central questions remain the

same: what are the sources of nutrients for phytoplankton blooms and has the trophic state of the central bay increased over the past few decades. It is

important to consider the possibility that the overall

pool of nutrients may have remained stable but that the

distribution has changed, along with changes in loss

terms. This argument falls within the concept of “alternative stable states.”Under this scenario a major environmental disruption that resulted in seagrass die-off

could have ushered in environmental conditions conducive to the persistence of phytoplankton blooms.

Such disruptive events could include major storms, prolonged drought or flood periods, or a major shift in nutrient loading.

The first step in addressing these questions is to

identify what nutrients actually limit the production potential of phytoplankton in the central bay.There is little doubt that primary production in the eastern region

of the bay is strongly phosphorus-limited (Lavrentyev

et al., 1998; Tomas et al., 1999).There is also significant

evidence indicating that the western region of the bay

and adjacent shelf environment are commonly limited

by nitrogen or nitrogen-phosphorus co-limitation.This

information suggests that there is an excess supply of

phosphorus to this region.The nutrient limitation status of the central bay is more ambiguous. A number

of biogeochemical indicators, such as the ratios of total

nitrogen (TN): total phosphorus (TP) and inorganic

N:inorganic P, indicate that phosphorus should be the

most limiting nutrient (Fourqurean et al., 1993; Boyer

et al. 1997, 1999). However, it is well known that many

organic N molecules are not readily available to phytoplankton, whereas many organic P molecules are

bioavailable due to the activity of phosphatase enzymes (Vitousek and Howarth, 1991). The results of

nutrient-enrichment bioassays in the central region of

the bay suggest that the limiting status can alternate

between phosphorus and nitrogen (Tomas et al., 1999).

This observation implies that at different times, changes

in the loading rate of either nutrient could stimulate

increases in phytoplankton production. A number of

hypotheses have been forwarded regarding potential

sources of phosphorus and nitrogen for phytoplankton blooms in the central region of Florida Bay. Many

of these hypotheses deal specifically with the north-central region of the bay, where blooms appear to originate. It is important to recognize that these hypotheses

are not mutually exclusive. It is likely that various

pathways of nutrient supply operate simultaneously,

even though the relative importance of each varies

over time and space.

HYPOTHESIS 1—INTERNAL SUPPLY OF 

NUTRIENTS

There are a number of potential internal sources of nitrogen and phosphorus for phytoplankton production

(Rudnick et al., 1999). The thick layer of flocculent organic sediments found in the north-central region of

the bay represents a potential pool of bioavailable nitrogen and phosphorus. The recent die-off of seagrasses in the region has made this pool more

accessible via enhanced diffusion and resuspension

processes.There has been considerable debate within

the scientific community about whether the size of

this pool and the rate of supply from the sediments are

sufficient to sustain the blooms observed over the past

decade. The size and availability of the sediment nutrient pool is certainly an issue that warrants further

attention.The current information available is insufficient to either accept or reject this hypothesis.

There are a number of biological processes that

affect the internal supply rate of bioavailable nutrients. For example, nitrogen fixation by autotrophic and

heterotrophic bacteria and cyanobacteria is a potential

Plankton Blooms
source of nitrogen. In some aquatic ecosystems, nitrogen fixation can represent a major source of nitrogen

(Phlips et al., 1986). Conversely, denitrification processes

can result in a loss of bioavailable nitrogen. The relative roles of nitrogen fixation and denitrification are the

subject of ongoing research (Cornwell, 2001).

Other key elements of internal supply of nutrients for phytoplankton production are the rates of recycling of nutrients within the north-central region

and of export of nutrients from the region. Due to the

severe restriction of tidal exchange of water between

the north-central region and the waters of the Gulf of

Mexico and the Atlantic Ocean, water residence times

are long.The restriction should provide more time for

recycling and less opportunity for nutrient washout. Initial modeling efforts indicate that residence times in

the north-central region are on the order of months

(Jackson and Burd, 1999; Top et al. 2001) and could be

extended during drought periods.Very little is known

about the actual rate constants for recycling, sedimentation, or export of nutrients.

HYPOTHESIS 2—GROUNDWATER SOURCES

OF PHOSPHOROUS

Phosphorite deposits were created during the Miocene

in northeastern Florida and subsequently transported

into south Florida by Appalachian erosion, resulting in

high phosphorus levels observed in geological formations in certain areas of south Florida (Brand, 1996).

Groundwater outflows from these formations can contain high phosphorus levels (Brand, 2002). The distribution of water-column phosphorus correlates well

with the phosphorite deposits (Brand, 2002).There has

been an effort, though unsuccessful to date, to discover direct phosphorus-rich groundwater inputs

within the north-central region of Florida Bay. In a

less direct manner, phosphate mining in central Florida

may have increased the input of phosphorus into the

southwestern coast of Florida. It has been hypothesized

that the phosphorite deposits are a persistent source

of phosphorus that has not changed significantly over

the past few thousand years (Brand, 2002). Although

phosphate mining over the past century may have increased the input of phosphorus into coastal waters,

the extent to which P from this source was transported

to Florida Bay and how this has changed over the past

century is unknown. The timing of phytoplankton

blooms (in the 1990s) does not seem to support a hypothesis that this P source has been an important factor driving changes in Florida Bay phytoplankton.

There is little evidence that phosphorus input to Florida

Bay has increased substantially in the past two decades.

Overall, many questions remain about the importance

of groundwater inputs of nutrients to algal blooms in

Florida Bay, but there is an active research effort underway (Corbett et al., 2000).

HYPOTHESIS 3—EXTERNAL LOADING OF 

NUTRIENTS

Current information on geographical gradients of critical nutrient elements in and around Florida Bay suggests that two of the major external surface-water

sources of bioavailable nitrogen for the north-central

region are the eastern half of the bay and the Everglades boundary. Conversely, phosphorus gradients in

the region indicate that the most likely source of

bioavailable phosphorus is from the western portion

of the bay. These nutrient gradients suggest that the

blooms in the north-central part of the bay could be fed

by nitrogen inputs from the east and by phosphorus

inputs from the west.

It has been hypothesized that phytoplankton

blooms in north-central Florida Bay are a result of increases in nitrogen loading related to recent changes

(since 1980) in water-management practices in southeast Florida (Brand, 2002).The following scenario is envisioned for explaining the increase in N load to Florida

Bay. In the early 1980s, agricultural runoff from the

Everglades Agricultural Area (EAA) was diverted south

via the South Dade Conveyance System (Light and

Dineen, 1994; South Florida Water Management District, 1992).Walker (1991) observed significant increases

in nitrate in water flowing south through the C-111

canal toward Florida Bay and through the S12A station

where water flows into the Shark River system. Unfortunately, there is little nitrogen concentration data

for Florida Bay before the beginning of the large regional monitoring program in 1989.Therefore, it is difficult to determine the degree to which nitrogen levels

in the north-central region of Florida Bay were affected by the later alterations in flow. This scenario

also depends on several assumptions for which the evidence is currently lacking or insufficient.The primary

assumption is that there is sufficient water exchange

between the western shelf, the putative source of phosphorus, and the north-central bay to sustain blooms.

Current hydrodynamic data are insufficient to establish this rate of exchange.There is also a need to more

carefully examine the nutrient-limitation status of phytoplankton in the central bay. As mentioned above,

chemical data indicate that the north-central region

should be phosphorus-limited, but bioassay data indicate a mixed limitation status.

HYPOTHESIS 4—SEWAGE AS A SOURCE OF

NUTRIENTS

Another potential source of nutrients is sewage and

other nutrient-rich freshwater runoff from land in

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FWRI Technical Report TR-11 85

nearby areas of high human activity. In the case of

Florida Bay, this would primarily be the heavily populated Florida Keys. Human activities in the Florida Keys

generate considerable amounts of N and P (U.S. EPA,

1992), and a fraction of this is clearly entering the local

waters (Lapointe et al., 1990, 1994; Shinn et al., 1994;

Paul et al., 1995a,b). The problem with this hypothesis

is that the source of nutrients is downstream of the

algal blooms, not upstream. Sewage may be causing

local eutrophication and leading to algal overgrowth of

the coral reefs downstream but cannot be the source of

the nutrients generating the algal blooms upstream to

the northwest. Concentrations of nitrogen, phosphorus,

and chlorophyll all decrease, not increase, along transects from northern Florida Bay to the Florida Keys.The

algal blooms in Florida Bay are, in fact, the farthest

from the Florida Keys and the human population.

HYPOTHESIS 5—ATMOSPHERIC INPUTS

Until recently, the potential role of atmospheric deposition of bioavailable nutrients in supporting algal

blooms in Florida Bay was largely discounted. However,

recent information indicates that such inputs may indeed be an important element to the broader nutrient

budget of the bay (Rudnick et al., 1999).

Domination by Cyanobacteria

One outstanding feature of algal blooms in the central

region of Florida Bay is the domination of the cyanobacterium Synechococcus cf. elongatus. A study of algal

blooms in the region from 1993–1997 showed that S. cf.

elongatus represented over 90% of total phytoplankton

biovolume in most instances when total phytoplankton biovolume exceeded 5 million μm3

/ml (Phlips et al.,

1999). Although cyanobacteria are the most abundant

phytoplankton in central Florida Bay, other taxonomic

groups can play important roles (Phlips et al., 1999;

Steidinger et al., 2001). For example, between May 1994

and April 1995, the diatoms Cyclotella choctawatcheeana

and Chaetoceros cf.salsugineus were frequently important elements of phytoplankton biomass (Steidinger et

al., 2001). Again, in July 1995, March 1996, and January

1997, diatoms were numerous.

Based on empirical observations and experimental studies, a number of hypotheses have been forwarded to explain the domination of Synechococcus cf.

elongatus in the central region of Florida Bay. Several

of these hypotheses are based on some of the unique

ecophysiological characteristics of this cyanobacterium.

Several research groups have demonstrated the broad

range of salinities over which S. cf. elongatus can sustain high growth rates (5–50 psu; Phlips and Badylak,

1996; Richardson, 2001). Because of the isolation of

central Florida Bay from tidal mixing and the proximity of the north-central bay to the adjoining Everglades,

this region is subject to wide swings in salinity. For example, salinities exceeded 40 psu during the drought

period of 1992–1994 but dropped to as low as 11 psu

during subsequent flood periods, such as 1995 and

1997 (Figure 6.4) (Phlips et al., 1999; Boyer et al., 1999).

The euryhaline tolerances of S. cf. elongatus probably

provide the cyanobacterium with an advantage in

terms of long-term prominence. However, it does not

fully explain the fact that intense blooms of S. cf. elongatus have been observed during times when salinities

have remained in a moderate range (25–35 psu.)

Another factor that may contribute to the success

of S. cf. elongatus is its ability to compete for nutrients

at low concentrations (Richardson, 2001). As a picoplanktonic species, one might expect S. cf. elongatus

to have low half-saturation constants for nutrient uptake, strictly because of surface area to biovolume considerations. Experimental studies have provided

support for this hypothesis. A study of the kinetics of

phosphorus (PO4-P)-dependent growth compared four

important phytoplankton species found in the bay:

the cyanobacterium Synechococcus cf. elongatus, an

unidentified spherical picoplanktonic cyanobacterium,

and the diatoms Chaetoceros cf. salsugineus and Cyclotella choctawhatcheeana (Richardson et al., 2001).The

results of growth tests revealed maximal growth rates

for S. cf. elongatus, the spherical cyanobacterium,C. cf.

salsugineus, and C. choctawhatcheeana of 1.38, 1.37, 2.48,

and 3.37 divisions day–1, respectively.The half-saturation constant for growth, Ku, was determined to be

0.005–0.05 μmol liter–1 for the latter three species.The

Ku for S. elongatus could not be determined because the

growth rate was still maximal at the lowest test concentration of phosphate. A test of phytoplankton resource-based competition using Equilibrium Resource

Competition theory predicts that S. cf. elongatus is the

superior competitor under phosphorus-limitation over

the range of salinities examined (15–50 psu). The superior competitive ranking of S. cf. elongatus can be attributed to the exceptionally low Ku values.The model

ranked the spherical cyanobacterium second in terms

of phosphate competition, with the diatoms last. In

competition experiments under steady and varying

P-limitation, S. cf. elongatus also dominated in terms of

biovolume at the end of all experiments at all salinities.Thus, with sufficient time, S. elongatus should competitively displace all the other species under

phosphorus-limited conditions. Competition experiments further revealed that under nitrogen-limiting

conditions, the two species of cyanobacteria usually codominated in terms of biovolume under steady- and

non-steady-state conditions. In summary, under both

Plankton Blooms
steady-state and non-steady-state conditions, the

cyanobacterium S. cf. elongatus is the superior competitor for orthophosphate and can frequently outcompete diatoms under N-limiting conditions.

Related to the question of nutrient competition is

the ability of S. cf. elongatus to regulate its buoyancy by

altering the presence of gas vesicles (Phlips and Bledsoe, unpublished data). Under phosphorus-limited

conditions, the presence of gas vesicles diminishes

and S. cf. elongatus cells lose buoyancy. It has been

suggested that with the loss in buoyancy, nutrient-depleted cells sink to the sediment–water interface, where

they can take advantage of higher nutrient availability (Phlips et al., 1999).

Besides the characteristics that may enhance the

competitive advantage of S. cf. elongatus in acquiring

resources for growth, there may also be ways this

species is favored by lower magnitudes of certain loss

terms. Butler et al. (1995) proposed that S. cf. elongatus

was responsible for the observed demise of sponges in

Florida Bay because of its adverse effect on the filtration apparatus of sponges. Although there is evidence

that sponges can effectively filter out S. cf. elongatusfrom

the water column (Lynch and Phlips, 2000), the longterm effects of filtering the mucilage-producing

cyanobacterium are not known. Irrespective of the direct effects of S. cf. elongatus on sponge health, it is clear

that the diminishing sponge populations in the bay result in reduced grazing pressure on phytoplankton

and thereby increase the potential for blooms (Lynch

and Phlips, 2000).

As in the case of sponges, the relationship between S. cf. elongatus and other grazers, both benthic

(e.g., bivalves) and planktonic (e.g., zooplankton), are

not well defined. There is evidence that zooplankton

grazing rates are periodically high enough to affect phytoplankton population dynamics (Brenner et al., 2001).

In grazing experiments, daily metabolic carbon demand of the net zooplankton community ranged from

less than 2% to greater than 100% of the phytoplankton carbon stock. Microphytoplankton growth rates

ranged from 0.08 to 2.33 divisions d–1 at 50% available

light. Diatoms, dinoflagellates, and the cyanobacterium

Synechococcus typically dominated the microphytoplankton community, although chlorophytes and

prasinophytes were occasionally major constituents.

Microzooplankton grazing rates ranged from 0.00 to

5.28 d–1, and their average ingestion rates ranged from

0.67 to 3.42 mg C m–3 d–1. These ingestion rates correspond to a daily ingestion demand ranging from less

than 1% to greater than 300% of the initially available

carbon, indicating that microzooplankton are capable

of exerting a controlling influence on the phytoplankton community. Despite the observed grazing rates, the

frequency of cyanobacterial blooms in the north-central region of the bay demonstrates that grazing activity

is frequently not sufficient to prevent blooms, although

it may play a role in their ultimate demise.There is no

information on whether the domination of S. cf. elongatus in the north-central region is in any way related

to selective avoidance by grazers, either benthic or

planktonic.

Western Florida Bay

The highest phytoplankton biovolume in western

Florida Bay generally occurs in the late summer to

winter, when the annual diatom bloom reaches its seasonal peak. Phytoplankton composition in the western

bay is distinct from composition in the central bay because a higher proportion of total biovolume occurs in

diatoms. Phlips and Badylak (1996) were the first to

identify the species composition of bloom components

in western Florida Bay. They found that centric (predominantly Rhizosolenia) and pennate (Cocconeis, Navicula, Surirella) diatoms were dominant species in terms

of cell abundance in December 1993. Steidinger et al.

(2001) subsequently reported that other taxa, especially cyanobacteria, may also account for a large fraction of total biovolume in western Florida Bay and

could equal the biovolume attributed to diatoms. In

2000 and 2001, Jurado (2003) conducted a study of netphytoplankton (diatom) silica demand at Carl Ross

Key in western Florida Bay. She found that centric diatoms (predominantly Rhizosolenia spp., Chaetoceros

spp., and Thalassiosira nitzschiodes) and pennate diatoms (Nistchia, Navicula, Gyrosigma spp.) dominated

the netphytoplankton fractions (greater than 5 μm).

These observations are in general agreement with

Phlips and Badylak’s (1996) results. Peak concentrations

of chlorophyll a (2–9 mg m–3) were higher than values

observed in the eastern bay but less than concentrations in the north-central basins. Netplankton chlorophyll a accounted for more than 50% of total pigment

biomass.

Although relatively few, if any, observations of

phytoplankton-community species composition in

western Florida Bay exist prior to 1990, biogenic silica

in the sedimentary record from Carl Ross Key suggest

that an increase in the abundance of diatoms has occurred since 1960 (Figure 6.5). Biogenic silica is a distinct marker for the presence of diatoms. Although

only two values exist from the sedimentary record before 1960, the vertical profile suggests that biogenic silica concentrations increased from less than 1 mg silicon

(Si) to greater than 1 mg Si (g sediment)–1 sometime

Florida Bay Synthesis Hunt and Nuttle

86 FWRI Technical Report TR-11

Plankton Blooms
Hunt and Nuttle Florida Bay Synthesis

FWRI Technical Report TR-11 87

after 1950, although values in the last decade have

been highly variable.This trend suggests that enhanced

diatom biomass in western Florida Bay has developed

within the past four or five decades.

Cellular division rates determine the success of a

species in the natural environment. Net growth rates

equal the difference between the cellular division rate

and the sum of all processes that represent losses, including grazing, sinking, advection, and death. Net

increases in phytoplankton communities of as little as

0.1 day–1 result in an increase of less than 1 to greater

than 20 mg m–3 chlorophyll a in less than a month. Although we have no direct measures of netphytoplankton growth rates in western Florida Bay,

community growth rates for netplankton have been estimated from increases in chlorophyll a and biogenic

silica in dilution gradient experiments. This method,

principally developed by Landry and Hassett (1982) and

their colleagues, provides a measure of maximum

growth. In March 2001, when phytoplankton abundance was low, netplankton chlorophyll a exhibited

growth rates of 1.5 day–1, increasing to 2.9 day–1 in May,

July, and September, as netplankton biomass increased.

By November 2001, netplankton biomass had reached

a stationary phase, and maximum growth rates were

again ca. 1 day–1. Grazing losses ranged from 0.6 to

1.75 day–1 in these experiments, yielding net growth

rates of 0.5 (November) to greater than 1.3 day–1 in

May and September. Although these experiments represent only the potential for diatom growth in situ,

they suggest that conditions in summer-fall provide diatoms with a setting for growth rates that exceed the

sum of losses.

Exchange of surface waters in western Florida Bay

with the West Florida Shelf is evident in the nature of

the phytoplankton community and property distributions. Salinity values in western Florida Bay show less

temporal variability than in the central or eastern

basins. On average, basins in the western bay have a

higher average salinity (mean = 35.2 psu) and lower average TN (30.5 μmol kg–1) and dissolved inorganic nitrogen (DIN) (<2 μmol kg–1) than do those in the eastern

or central bay (Boyer et al., 1999). Chlorophyll a concentrations from the long-term FIU monitoring program averaged 1.9 mg m–3, which, as suggested by

Phlips et al. (2000), is less than the average values in the

central bay but higher than in the eastern bay.

Ratios of TN:TP (x = 55.6) and DIN:SRP (soluble reactive phosphorus) (x = 51.8) in the western bay are

higher than Redfield values but lower than those in either the eastern or central basins.These characteristics

suggest that the western bay is strongly influenced by

the surface waters of the West Florida Shelf that exchange along the boundary of Everglades National

Park. The influence of coastal processes is evident in

tidal exchange, which is much greater in the western

bay than in the interior or eastern basins because the

amplitude of the tides is damped from west-to-east

across the bay (Wang et al., 1994).

Bioassays at two locations along the western margin of Florida Bay (Sprigger Bank and Sandy Key) indicate that nitrogen is a primary nutrient that limits

phytoplankton growth (Tomas et al., 1999). In more

than 70% of the bioassays between March 1994 and May

1997, Tomas found that nitrogen in combination with

silica stimulated growth more than did individual additions of phosphorus or silica. When individual nutrients were excluded from otherwise complete

enrichments (all nutrients – N, or all nutrients – P), the

lack of nitrogen yielded a final biomass that was similar to controls. Phosphorus limitation of phytoplankton growth rates was detected in about 20% of all

experiments, and iron was even less frequently identified as a nutrient potentially limiting growth.

The responses to N and Si enrichments are consistent with the marine waters that flow into the western margin of Florida Bay and the fact that diatoms

frequently dominate these waters.The observations of

Figure 6.5 Vertical profile of biogenic silica in a sediment core from

Carl Ross Key.The sediment samples with corresponding dates were

kindly provided by Dr. J. Trefrey, Florida Institute of Technology

(Jurado, 2003).

Plankton Blooms
Florida Bay Synthesis Hunt and Nuttle

88 FWRI Technical Report TR-11

Tomas et al. (1999) were confirmed by Jurado (2003), who

found that nitrogen was the principal nutrient limiting both total phytoplankton and netplankton (diatom) growth in western Florida Bay during the

2000–2001 diatom bloom (Figure 6.6). Bioassays were

made in September and November 2000, as diatom

abundance increased, and in January and March 2001,

when diatom biomass declined. The results consistently showed that enrichments with nitrogen, and

secondarily silica or phosphorus, stimulated growth of

natural populations incubated under simulated in situ

conditions. An assessment of temperature and irradiance during the course of the year suggests that these

two parameters did not effectively limit the growth of

diatoms in western Florida Bay. Thus nutrient supply

may be the main factor that accounts for the enhanced

growth of diatoms in summer and fall in western

Florida Bay.

Seasonal distributions of salinity, nutrients, chlorophyll a, and biogenic silica on the West Florida inner

shelf, south from the Ten Thousand Islands to the western boundary of Everglades National Park, indicate

that diatom blooms observed in western Florida Bay

originate in coastal waters off the southwestern Florida

Shelf.These features develop in the shallow coastal waters west of Cape Sable and are associated with the lowsalinity plume emanating from the Shark River and

other riverine sources along the southwestern Florida

coast. A chlorophyll a “plume” spreads south from

Cape Sable to the western margin of Florida Bay during the fall (Figure 6.7). Bimonthly cruises of the NOAA

SFERPM Program between 1998 and 2003 have mapped

surface-water properties (salinity, nutrients, chlorophyll a, and particulate matter) in this region. Spatial

distributions of these properties indicate that the Shark

River is a major source of nutrients, especially silicate,

supporting the diatom bloom. Concentrations of dissolved inorganic nutrients in the Shark River plume at

bimonthly intervals indicate that the average concentrations of silicon, inorganic nitrogen, and phosphorus

in the plume of the Shark River indicate excess silicate

relative to inorganic nitrogen and phosphorus. Little

seasonal variation exists in nutrient concentrations,

so the volume of freshwater flow appears to be the primary determinant of nutrient flux to the inner shelf.

On the southwestern Florida Shelf, exogenous nutrients that support diatom growth are derived from the

Shark River plume and along-shore flow. Increased

freshwater flow from rivers often occurs in June at the

onset of the rainy season and provides silicates to the

inner shelf that contribute to development of the diatom bloom. Between 1998 and 2000, the diatom-dominated bloom on the inner shelf began between August

and October. Estimates of the fluxes of nitrate, phosphate, and silicate from along-shore advection (based

on bimonthly nutrient measurements and alongshore

currents from ADCP currents made by T. Lee and colleagues) and that from the Shark River (derived from

river discharge and bimonthly nutrient measurements)

were for the period of bloom inception in 1999 (Jurado,

Figure 6.6 Bioassay results from September 2000 at Carl Ross Key

(Jurado, 2003). Values correspond to increases in chlorophyll a

biomass relative to a control in an incubation of 24 hours.The total

phytoplankton community was filtered on Whatman GF/F filter,

and the netplankton fraction on a 5-μm Nuclepore filter.The combination of nitrogen plus phosphorus or silicon typically results

in the greatest stimulation of phytoplankton growth.

Figure 6.7 Contours of total chlorophyll a concentrations (top)

and surface salinity (bottom) in October 2000 during the peak of

the annual diatom bloom. Maximum chlorophyll a concentration

was 17 mg l–1 off middle Cape Sable (Jurado, 2003).

Plankton Blooms
2003).This is the latest year in which Shark River flow

data were available at the time the estimates were

made.The resulting flux estimates suggest that during

bloom inception, exogenous nutrient sources that support diatom bloom development are largely met by the

southerly flow along the southwestern Florida Shelf

(Figure 6.8). When the bloom is at its maximum, after

the onset of the rainy season, Shark River is a major

source of nitrate, nitrite, and phosphate. Alongshore advection of silicate appears to be the primary source of

this essential nutrient for diatoms, at least in that year.

In summary, diatom blooms in the western basins

of Florida Bay are a seasonal extension of blooms that

initiate on the southwestern Florida Shelf. Various

species of Rhizosolenia and Chaetoceros are the dominant members of this assemblage, in combination with

pennates that are likely resuspended from the bottom. Diatoms begin net growth (i.e., growth rates exceed losses) with the onset of the rainy season.

Bioassays suggest that nitrogen, often in combination

with phosphorus and/or silica, potentially “limit”

growth. Nutrients that support diatom growth are derived from riverine sources, principally Shark River, and

along-shelf (southerly) advection.The relative contribution of inorganic nitrogen, phosphorus, and silicon

varies with the stage of the bloom, and likely varies

yearly with variations of river discharge and alongshore

flow.

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