FWRI Technical Report TR-11 77 

## Introduction

Question 3**—What regulates the onset, persistence, and fate of planktonic algal blooms in Florida Bay?** 

**F rom the 1950s to the mid 1980s, Florida Bay was char** **acterized by expansive seagrass beds, mangrove is lands, 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 in dicative 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).** 

**F ourqurean and Robblee (1999) have recently reviewed** **many of the changes. In this paper, we examine the cur rent 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 some what surprising fact considering the importance of**  **this aquatic resource to Florida and the surrounding**

**w aters 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.** 

**T**

**he 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 chloro** **phyll levels in 1991–1992. The increase in algal bio mass within the bay was corroborated by incidental**  **observations by individuals frequenting the bay for other research and recreation activities. In 1993, a sep**  **arate 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 di atoms, 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 west ern margin were a well-established feature by 1970,**  **significantly prior to the increases in cyanobacteria blooms within the bay’s interior (Jurado, 2003). Most re**  **cently, chemotaxonomic studies of phytoplankton in the bay have been initiated using full-spectrum photodi** **ode array detectors and high-performance liquid chro matography analysis (Louda, 2001).** 



### Regional Differences

**T he observed disparity in the quantity and quality**  **( i.e.,**  **composition) of phytoplankton biomass in dif ferent parts of the bay manifests the need to view phy**  **toplankton dynamics by ecological zones. In contrast to the numerous phytoplankton blooms encountered** **in the western and central regions of Florida Bay, al most 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.** 

**T hese zonal breakdowns have been based on a vari**  **ety of parameters, including geology (Prager et al.,**  **1996), distributions of benthic plants (Zieman et al.,**  

# Chapter 6

Plankton Blooms

T eam 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 characteris**  **tics (Phlips et al.,**  **1995, Boyer et al.,**  **1997, 1999; Burd and Jac**  **kson, 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** 

**c hoice of parameters used in the discrimination of the**  **regions. If the focus is limited to the frequency of phyto**  **plankton 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 phy toplankton blooms are seldom observed; (2) the cen**  **tral 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 en**  **countered; and (3) the western region of the bay, west of 80°49 ́W, which experiences periodic blooms of di**  **atoms.These regions provide a useful context for dis cussing 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 per**  **spective, 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 tem perature, salinity, and oxygen level. Algal biomass can**  **also increase as a consequence of inputs to the water column from the benthos through resuspension or in**  **flows of water from adjacent environments. In terms of loss functions, one of the most impor**  **tant overriding considerations is water residence time. In many coastal ecosystems, phytoplankton biomass is** **limited by the flushing-rate of water that has lower phy toplankton 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 phytoplank ton at four sampling sites representative of different regions of Florida Bay (Phlips**  **et al., 1999).**  Plankton BloomsHunt and Nuttle Florida Bay Synthesis

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 sim**  **ilar (Monbet, 1992).There are also biological processes that contribute to the loss of biomass in many aquatic** **ecosystems, such as grazing by benthic and plank tonic 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, tempera ture, and phytoplankton composition (**  **e.g., presence of**  **toxic species).**



### Summary of Research

**Research to date indicates that there are three eco logically distinct regions within the bay from the stand**  **point of algal blooms: the northeastern, central, and**

**w estern regions.The nature and causes of blooms can** **be most appropriately summarized within the con text of these three regions (Table 6.1).**  NORTHEASTERN REGION

**• Algal blooms are largely absent from the north** 

**eastern 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.** 

**• T he 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 BloomsFlorida Bay Synthesis Hunt and Nuttle

80 FWRI Technical Report TR-11

**observed in other seasons. In the fall and early sum mer, blooms originating in the northern central re**  **gion 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-form**  **ing alga in the central region is the picoplanktonic**

**cy anobacterium**  **Synechococcus cf.**  **elongatus, though** **several species of diatoms and dinoflagellates occa sionally occur in bloom proportions.** 

**• N utrient 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** 

**h**

**ypotheses: (1) a significant supply of nutrients for algal production is available from the flocculent** **muddy sediments within the region; (2) a ground**

**w ater source of phosphorus is present within the re**  **gion 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 pro** **duction. These hypotheses are not mutually exclu sive, and several mechanisms may be operational at** 

**the same time.**

**• T he domination of blooms in the central region by** 

**Synechococcus cf.**  **elongatus is attributable to the unique**  **ecophysiological characteristics of this species of**

**cya nobacterium. These characteristics include (1)** **wide tolerance to salinity, (2) superior ability to com pete for phosphorus at low concentrations, (3) abil**  **ity to regulate buoyancy and thereby take advantage of nutrients available in the sediments, and (4) lower**  **susceptibility to grazing losses.**

WESTERN REGION

**• A lgal blooms are also a common feature of the west** 

**ern region of Florida Bay. In contrast to the cyanobac** Figure 6.3 **Gain versus loss-functions for phytoplankton stand ing crops.** 

T able 6.1 **Summary of research on plankton blooms by region.**

W

est 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**  **J urado, 2003** 

**Nutrients Lapointe**  **et al., 1990, 1994** 
**Robblee et al.,**  **1991**
**Carlson et al.,**  **1994 Durako, 1994** 

**Zieman et al.,**  **1994**

**P**

**aul et al.,**  **1995a,b**

**T**

**omas 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**  Plankton BloomsHunt and Nuttle Florida Bay Synthesis

FWRI Technical Report TR-11 81

**terial blooms in the north-central region, these blooms are dominated by diatoms.The diatom-dom** **inated blooms in the western region are mainly com posed 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.** 

**• T he 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 ini** 

**tiation 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 net plankton (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 bio mass 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 through**  **out the year. Phlips et al.**  **(1999) reported that the max imum 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 chloro phyll**  **a concentrations in eastern Florida Bay for the pe**  **riod between 1989 and 1997 (mean = 0.85 mg m –3**  **).**

**T he phytoplankton communities in the eastern**  **bay are principally composed of centric diatoms ( e.g.,**  **T**

**halassiosira sp.), dinoflagellates (**  **e.g., Protoperidium**

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

**cy anobacteria that constitute the major fraction of cel**  **lular 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**

**w ere dinoflagellates, whereas cyanobacteria were the** **most abundant taxa from late 1996 to spring 1997. Cel lular biovolume serves as a useful index to phyto**  **plankton 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 μm 3**  **ml –1** 

**, a value much less than that in the western and central**  **bay (Figure 6.1) (Phlips et al.,**  **1999).**

**Tw o factors result in strong phosphorus limita** **tion of primary production in the region, helping to ex plain 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 cen** **tral and western bay, with typical values from 20 prac tical salinity units (psu) to 30 psu (Figure 6.4) (Phlips**  **et al., 1999).The external surface-water inputs to east**  **ern Florida Bay are very low in phosphorus (Boyer et**  **al., 1999). In addition, the high concentrations of cal**  **cium 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 con**  **tent of phytoplankton in the central region is relatively low when compared to the chlorophyll content in the** 

**w estern bay and some other coastal ecosystems in**  **Florida (Phlips et al.,**  **1999, 2002). It has been hypothe sized that this is partially attributable to the very shal**  **low depth of the central region of the bay (generally less than 2 m), resulting in high light availability and sun**  **adaptation by algal cells (Phlips et al.,**  **1999). It is also pos sible that the domination of the plankton community**  **by the cyanobacterium**  **Synechococcus cf.**  **elongatus and** 

**the nutrient limitation contribute to the low ratio. Ir respective of the causes for the low ratios, it is impor**  **tant to consider the ratio when evaluating the intensity of blooms in the bay. For example, peak chlorophyll con**  **centrations 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 simi lar (Phlips**  **et al., 2000). These considerations manifest**  **the fact that the 20–40 mg m –3**  **c hlorophyll levels com** **monly encountered in central Florida Bay are repre sentative of serious bloom conditions, whereas similar** 

**c hlorophyll 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 phy toplankton standing crops persisted throughout the** 

**y ear.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.** 

**T his 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

**T he 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 sus** 

**tained 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 redirec** 

**tion of internal reserves and/or increases in external loading, and/or (2) biomass loss terms were dramati**  **cally 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 pro ductive. 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 re**  **gion, although were not as prominently as in previous**

**y ears.**  **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 re** **duction of seagrass populations could enhance nutri ent availability for planktonic production, including (1)**  **through the addition of recyclable nutrients from dead seagrass, (2) by increasing access to sediment-bound**  Florida Bay Synthesis Hunt and Nuttle

82 FWRI Technical Report TR-11 Plankton BloomsHunt and Nuttle Florida Bay Synthesis

FWRI Technical Report TR-11 83 **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 nu** **trient availability, the reduction in seagrass commu nities can affect phytoplankton loss processes. The**  **populations of filter-feeding organisms associated with seagrasses can influence phytoplankton densities, par**  **ticularly 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.** 

**T he importance of seagrass die-off in the stimu**  **lation of phytoplankton blooms is not a universally accepted concept. Some researchers have suggested** **that this line of reasoning is backwards and that in creases in planktonic production have actually led to**  **the demise of seagrasses (Lapointe et al.,**  **2002).The au thors 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 re stricted 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 re sult 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 die**  **off or the algae bloom, the central questions remain the same: what are the sources of nutrients for phyto** **plankton blooms and has the trophic state of the cen tral 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 “al** **ternative stable states.”Under this scenario a major en vironmental disruption that resulted in seagrass die-off** 

**could have ushered in environmental conditions con ducive to the persistence of phytoplankton blooms.** **Such disruptive events could include major storms, pro longed drought or flood periods, or a major shift in nu**  **trient loading.**

**T he first step in addressing these questions is to** **identify what nutrients actually limit the production po tential of phytoplankton in the central bay.There is lit**  **tle 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**  **e vidence 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 sta tus 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 phy toplankton, whereas many organic P molecules are** **bioavailable due to the activity of phosphatase en zymes (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).**

**T**

**his observation implies that at different times, changes in the loading rate of either nutrient could stimulate**  **increases in phytoplankton production. A number of**

**h**

**ypotheses have been forwarded regarding potential sources of phosphorus and nitrogen for phytoplank**  **ton blooms in the central region of Florida Bay. Many of these hypotheses deal specifically with the north-cen** **tral region of the bay, where blooms appear to origi nate. It is important to recognize that these hypotheses**  **are not mutually exclusive. It is likely that various pathways of nutrient supply operate simultaneously,** 

**ev en though the relative importance of each varies** 

**ov er time and space.** 

HYPOTHESIS 1—INTERNAL SUPPLY OF

NUTRIENTS

**T here are a number of potential internal sources of ni**  **trogen and phosphorus for phytoplankton production (Rudnick**  **et al., 1999). The thick layer of flocculent or**  **ganic sediments found in the north-central region of the bay represents a potential pool of bioavailable ni** **trogen and phosphorus. The recent die-off of sea grasses 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 nu**  **trient pool is certainly an issue that warrants further**

**attention.The current information available is insuffi cient to either accept or reject this hypothesis.** 

**T here are a number of biological processes that** **affect the internal supply rate of bioavailable nutri ents. For example, nitrogen fixation by autotrophic and**  **heterotrophic bacteria and cyanobacteria is a potential** Plankton Blooms**source of nitrogen. In some aquatic ecosystems, nitro gen fixation can represent a major source of nitrogen**  **(Phlips et al.,**  **1986). Conversely, denitrification processes can result in a loss of bioavailable nitrogen. The rela**  **tive roles of nitrogen fixation and denitrification are the subject of ongoing research (Cornwell, 2001).** **Other key elements of internal supply of nutri ents for phytoplankton production are the rates of re** 

**c ycling 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. Ini**  **tial 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, sedi mentation, 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 for**  **mations in certain areas of south Florida (Brand, 1996).**

**Groundwater outflows from these formations can con tain high phosphorus levels (Brand, 2002). The distri**  **bution of water-column phosphorus correlates well with the phosphorite deposits (Brand, 2002).There has** **been an effort, though unsuccessful to date, to dis cover 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 in**  **creased 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 hy pothesis that this P source has been an important fac**  **tor driving changes in Florida Bay phytoplankton.**

**T**

**here 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 un derway (Corbett**  **et al., 2000).** 

HYPOTHESIS 3—EXTERNAL LOADING OF

NUTRIENTS**Current information on geographical gradients of crit ical nutrient elements in and around Florida Bay sug**  **gests 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 Ever glades 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 in creases in nitrogen loading related to recent changes** **(since 1980) in water-management practices in south east Florida (Brand, 2002).The following scenario is en**  **visioned 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 Dis trict, 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. Un fortunately, there is little nitrogen concentration data** **for Florida Bay before the beginning of the large re gional monitoring program in 1989.Therefore, it is dif**  **ficult to determine the degree to which nitrogen levels in the north-central region of Florida Bay were af**  **fected by the later alterations in flow. This scenario also depends on several assumptions for which the ev**  **idence is currently lacking or insufficient.The primary assumption is that there is sufficient water exchange** **between the western shelf, the putative source of phos phorus, and the north-central bay to sustain blooms.** **Current hydrodynamic data are insufficient to estab lish this rate of exchange.There is also a need to more** **carefully examine the nutrient-limitation status of phy toplankton in the central bay. As mentioned above,** 

**c hemical data indicate that the north-central region** **should be phosphorus-limited, but bioassay data in**

**dicate 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** Florida Bay Synthesis Hunt and Nuttle

84 FWRI Technical Report TR-11 Plankton BloomsHunt and Nuttle Florida Bay Synthesis

FWRI Technical Report TR-11 85 **nearby areas of high human activity. In the case of Florida Bay, this would primarily be the heavily popu**  **lated 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**

**w aters (Lapointe**  **et al., 1990, 1994; Shinn**  **et al., 1994;** 

**P**

**aul 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 tran**  **sects 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 de position of bioavailable nutrients in supporting algal**  **blooms in Florida Bay was largely discounted. However, recent information indicates that such inputs may in**  **deed 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 cyanobac** 

**terium 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 phytoplank ton biovolume exceeded 5 million μm**  **3 /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**  **we re frequently impor**  **tant 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 experimen** **tal studies, a number of hypotheses have been for**

**war ded 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 sus**  **tain 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 proxim ity of the north-central bay to the adjoining Everglades,** **this region is subject to wide swings in salinity. For ex ample, 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).**

**T he 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. elon**  **gatus 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 pi**  **coplanktonic species, one might expect S.**  **cf. elongatus** **to have low half-saturation constants for nutrient up take, strictly because of surface area to biovolume con**  **siderations. Experimental studies have provided support for this hypothesis. A study of the kinetics of**  **phosphorus (PO 4**  **-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 Cy** 
**clotella 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. c** **hoctawhatcheeana of 1.38, 1.37, 2.48,**  **and 3.37 divisions day –1** 

**, respectively.The half-satura**  **tion constant for growth, K**

**u , w**  **as determined to be 0.005–0.05 μmol liter** **–1 for the latter three species.The** 

**K**

**u for**  **S. elongatus could not be determined because the** **growth rate was still maximal at the lowest test con centration of phosphate. A test of phytoplankton re**  **source-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 su**  **perior competitive ranking of S.**  **cf. elongatus**  **can be at tributed to the exceptionally low K** 

**u v** **alues.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 salini**  **ties.Thus, with sufficient time, S. elongatus**  **should com petitively displace all the other species under** **phosphorus-limited conditions. Competition experi ments further revealed that under nitrogen-limiting** **conditions, the two species of cyanobacteria usually co dominated 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**

**cy anobacterium**  **S. cf.**  **elongatus is the superior com** **petitor for orthophosphate and can frequently out compete 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 Bled soe, 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-de**  **pleted cells sink to the sediment–water interface, where they can take advantage of higher nutrient availabil**  **ity (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** 

**w as responsible for the observed demise of sponges in** **Florida Bay because of its adverse effect on the filtra tion apparatus of sponges. Although there is evidence**  **that sponges can effectively filter out S.**  **cf. elongatus**  **from the water column (Lynch and Phlips, 2000), the long**  **term effects of filtering the mucilage-producing**

**cy anobacterium are not known. Irrespective of the di** 

**rect effects of S.**  **cf. elongatus**  **on sponge health, it is clear that the diminishing sponge populations in the bay re**  **sult 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 be** 

**tween 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 phy**  **toplankton population dynamics (Brenner et al.,**  **2001). In grazing experiments, daily metabolic carbon de**  **mand of the net zooplankton community ranged from less than 2% to greater than 100% of the phytoplank**  **ton 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 microphyto plankton 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**  **. T hese ingestion rates corre**  **spond 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 phytoplank**  **ton community. Despite the observed grazing rates, the frequency of cyanobacterial blooms in the north-cen**  **tral 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. elon**  **gatus in the north-central region is in any way related**  **to selective avoidance by grazers, either benthic or planktonic.** 



## W estern Florida Bay

**T he highest phytoplankton biovolume in western**  **Florida Bay generally occurs in the late summer to winter, when the annual diatom bloom reaches its sea**  **sonal peak. Phytoplankton composition in the western bay is distinct from composition in the central bay be**  **cause 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 (pre**  **dominantly Rhizosolenia**  **) and pennate ( Cocconeis, Nav**  **icula, Surirella ) diatoms were dominant species in terms**  **of cell abundance in December 1993. Steidinger et al.** **(2001) subsequently reported that other taxa, espe cially cyanobacteria, may also account for a large frac**  **tion 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 net phytoplankton (diatom) silica demand at Carl Ross** 

**Ke y in western Florida Bay. She found that centric di**  **atoms (predominantly Rhizosolenia**  **spp., Chaetoceros**  **spp., and T**  **halassiosira nitzschiodes ) and pennate di**  **atoms ( Nistchia, Navicula, Gyrosigma**  **spp.) dominated the netphytoplankton fractions (greater than 5 μm).** 

**T hese 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 concentra tions in the north-central basins. Netplankton chloro**  **phyll a**  **accounted for more than 50% of total pigment**

**biomass. Although relatively few, if any, observations of**  **phytoplankton-community species composition in**

**w estern 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 oc curred since 1960 (Figure 6.5). Biogenic silica is a dis**  **tinct marker for the presence of diatoms. Although only two values exist from the sedimentary record be** **fore 1960, the vertical profile suggests that biogenic sil ica 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 BloomsHunt 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, in**  **cluding 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 c**  **hlorophyll a**  **in less than a month. Al though we have no direct measures of netphyto**  **plankton growth rates in western Florida Bay, community growth rates for netplankton have been es**  **timated 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 abun dance was low, netplankton chlorophyll**  **a exhibited**  **growth rates of 1.5 day –1** 

**, increasing to 2.9 day** **–1 in May,**  **Ju ly,**  **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 rep resent only the potential for diatom growth**  **in situ, they suggest that conditions in summer-fall provide di**  **atoms 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 distribu**  **tions. 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 av**  **erage TN (30.5 μmol kg –1**  **) and dissolved inorganic ni trogen (DIN) (<2 μmol kg**  **–1 ) than do those in the eastern**  **or central bay (Boyer et al.,**  **1999). Chlorophyll a**  **con centrations from the long-term FIU monitoring pro**  **gram 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 re active phosphorus) (x = 51.8) in the western bay are** **higher than Redfield values but lower than those in ei**

**ther 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 ex**

**c hange along the boundary of Everglades National** 

**P**

**ark. 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 mar** **gin of Florida Bay (Sprigger Bank and Sandy Key) in dicate 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 ad** **ditions of phosphorus or silica. When individual nu trients were excluded from otherwise complete**  **enrichments (all nutrients – N, or all nutrients – P), the lack of nitrogen yielded a final biomass that was sim** **ilar to controls. Phosphorus limitation of phytoplank ton growth rates was detected in about 20% of all** **experiments, and iron was even less frequently iden tified as a nutrient potentially limiting growth.** 

**T he responses to N and Si enrichments are con** 

**sistent with the marine waters that flow into the west ern margin of Florida Bay and the fact that diatoms**  **frequently dominate these waters.The observations of** Figure 6.5 **V ertical 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 BloomsFlorida Bay Synthesis Hunt and Nuttle

88 FWRI Technical Report TR-11 **T**

**omas et al.**  **(1999) were confirmed by Jurado (2003), who found that nitrogen was the principal nutrient limit** **ing both total phytoplankton and netplankton (di atom) 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 consis tently 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 irradi ance 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, chloro**  **phyll a** **, and biogenic silica on the West Florida inner** **shelf, south from the Ten Thousand Islands to the west ern 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 wa ters west of Cape Sable and are associated with the low**  **salinity 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 dur**  **ing the fall (Figure 6.7). Bimonthly cruises of the NOAA SFERPM Program between 1998 and 2003 have mapped** **surface-water properties (salinity, nutrients, chloro phyll**  **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 dis**  **solved inorganic nutrients in the Shark River plume at bimonthly intervals indicate that the average concen**  **trations 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 pri mary determinant of nutrient flux to the inner shelf.** **On the southwestern Florida Shelf, exogenous nu trients 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 di** **atom bloom. Between 1998 and 2000, the diatom-dom inated bloom on the inner shelf began between August** **and October. Estimates of the fluxes of nitrate, phos phate, and silicate from along-shore advection (based**  **on bimonthly nutrient measurements and alongshore currents from ADCP currents made by T. Lee and col**  **leagues) and that from the Shark River (derived from river discharge and bimonthly nutrient measurements)** 

**w ere 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 com**  **bination 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 sup port 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 ad** 

**v ection 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 domi**  **nant members of this assemblage, in combination with pennates that are likely resuspended from the bot**  **tom. Diatoms begin net growth ( i.e.,**  **growth rates ex ceed 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 de rived from riverine sources, principally Shark River, and** **along-shelf (southerly) advection.The relative contri bution of inorganic nitrogen, phosphorus, and silicon** 

**v aries with the stage of the bloom, and likely varies** 

**ye arly with variations of river discharge and alongshore** 

**flow.**



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