G.S. Kleppel, Oceanographic Center, Nova Southeastern University, Dania, FL 33004; Carmelo Tomas, Florida Marine Research Institute, Department of Environmental Protection, St. Petersburg, FL; C.A. Burkart, E. Kerby, and L. Houchin, Oceanographic Center, Nova Southeastern University.

Studies of zooplankton feeding and production are underway in Florida Bay as part of an ongoing investigation of plankton dynamics by FMRI and its academic collaborators. The zooplankton component of the program involves measurement of phytoplankton pigments (chlorophylls and carotenoids), copepod and microzooplankton grazing rates and copepod egg production rates and egg viability in the Gulf, transitional and interior regions of the bay. The conceptual basis of the program is that changes in dissolved nutrients and hydrodynamics have shifted the distribution of primary production in the bay from the benthos (i.e. seagrass) to the water column (phytoplankton). The emphasis of the project is on phytoplankton dynamics and, particularly, on those conditions that result in blooms. Two hypotheses are being addressed as part of the zooplankton component: H1 -- phytoplankton blooms result from high rates of primary production, which exceed the ability of the zooplankton to graze off the biomass, and which, under the appropriate hydrodynamic conditions, result in the accumulation of this excess biomass; H2 -- certain phytoplankton species are not grazed or, when ingested, do not support the zooplankton production necessary to remove excess primary production. It is expected that both sets of processes will be operative at various times.

Measurements are being made on samples from six stations along two southwest to northeast lines (3 stations per line). These extend from approximately the Gulf of Mexico-Florida Bay interface to the inner bay, with stations located in the major basins. The northern line of stations (west to east) includes Sandy, Johnson and Rankin Basins. The southern line consists of Sprigger, Twin and Captain Basins. The first measurements for the zooplankton component of the program were made in June 1995; the study is scheduled to continue until November 1995.

Water samples for hplc-based phytoplankton pigment analysis are collected at each station, each month. The pigment compositions of the six basins are being compared to aircraft observations of bay water color to ascertain whether or not certain phytoplankton community compositions can be discerned from remotely sensed water color attributes.

In addition, each month, microzooplankton grazing rates are measured by the dilution method at Rankin and Captain Basins, and the egg production rates of the dominant copepod species are determined by the standard 24-h bottle incubation technique at Rankin, Captain, Sprigger and Sandy Basins. In four of the six monthly study periods the diets and ingestion rates of the dominant copepods are being determined, also by bottle incubations.

Results from the initial (June-July) experiments indicate a close coupling between taxon-specific phytoplankton biomass and copepod (Acartia tonsa) feeding and production. Energy flow is multivorous, incorporating both microbial [nanoplankton --> microzooplankton (ciliate) --> mesozooplankton (copepod)], and classical, herbivorus (netplankton --> mesozooplankton) pathways. Highest production seems to occur when copepod diets are composed of a combination of of phytoplankton (diatoms and dinoflagellates) and ciliates. Viability of Acartia tonsa eggs has been consistently high (>80%) during the summer and seems independent of the egg production rate, which is temporally and spatially variable. Thus, egg production may be a useful correlate to copepod secondary production (P/B). Rankin basin typifies most clearly the transformation from benthic to water column dominated trophic dynamics with high phytoplankton biomass, but also a productive zooplankton fauna.

Peter B. Ortner and Michael J. Dagg

The zooplankton of Florida Bay have received comparatively little attention; to date there is not a single published report quantitatively characterizing the resident population nor estimating their contribution to secondary production. One reason for this is that until recently the Bay was extremely clear and seagrasses (and their epiphytes) purportedly dominated primary production. To some this suggested that macroinvertebrates (and teleosts) grazing directly upon macrophytic plant production were the dominant trophic pathway between primary and secondary production. However, the Bay has historically supported substantial populations of teleost larvae (e.g., spotted sea trout), whose primary food (when they are small) are crustacean nauplii. Adjacent shallow water environments like Biscayne Bay support large populations of estuarine copepods like Acartia tonsa that supplement their phytoplankton diet with macrophytic plant detritus (Roman et al., 1983). Moreover, many macroinvertebrates (e.g. mollusks) have meroplanktonic stages that can be important food resources to larval fish. Last demersal zooplankton like amphipods or harpacticoids can be extremely abundant in shallow water marine systems. In short, zooplankton likely played a significant role in the Bay even when it was clear and phytoplankton blooms were rare. Given the decline in seagrass coverage and the increase in the areal extent and duration of phytoplankton blooms, the role of zooplankton both as consumers of phytoplankton and/or detritus and as food for ichthyoplankton may be changing.

An investigation was launched in the summer of 1994 to answer the following questions:

1) What is the importance of zooplankton consumption in Florida Bay and how does this vary within the Bay as the salinity and temperature distributions change throughout the seasonal cycle?

2) What is the relative abundance of micro-zooplankton and macrozoplankton and how does this vary within the Bay as the salinity and temperature distributions change throughout the seasonal cycle?

3) What species and types of zooplankton and/or microzooplankton are the primary food of larval and near juvenile fishes?

a) What is the distribution and abundance of the prey of larval fish within the Bay and how does this vary within the Bay as the salinity and temperature distributions change throughout the seasonal cycle?

To date bi-monthly samples have been collected over one annual cycle (September 1994 to September 1995) at eight stations in conjunction with Florida DEP juvenile fish sampling. We jointly sample stations at Murray Key, Whipray Basin, Eagle Key Basin, Twin Key Basin, Johnson Key Basin, Duck Key and the south end of the Shell Key Channel. At each we obtained 64um zooplankton net tows as well as 20um deck filtered microzooplankton bucket samples. For the first four bimonthly samples we obtained replicate 150um samples as well at each location. In addition one Night-Day comparison series of twelve 150um net tows was made at Twin Key basin in the South Central region. All samples obtained to date have been enumerated. The effort has been expanded somewhat and now incorporates the following elements:

There are four basic elements in the present research effort.

1) We are continuing our sampling in conjunction with the Florida DEP project (Florida Marine Fisheries Independent Monitoring Program) at the same eight sites on a bimonthly basis. Bimonthly sampling includes 64um mesh net tows for macrozooplankton, 20um sieved bucket samples for nauplii, whole water samples for protozoan microzooplankton.

2) Day/Night intercomparisons will be repeated at Twin Key Basin later in the Fall. Not only abundance will be determined but also gut pigment analysis will be performed to evaluate the consumption of chlorophyll derivative - containing plant materials.

3) Gut contents analyses are being made upon larvae sorted from 150um mesh net tows and upon small-mouthed juveniles and adults sorted from Fla DEP trawl samples taken at the same stations and seine net samples taken nearby.

4) Whether small copepod nauplii in fish guts are predominantly Acartia tonsa will be determined using a species-specific genetic probe. Using Acartia tonsa collected in Biscayne Bay a graduate student of own of us (Ortner) amplified a partial region of the large subunit of ribosomal DNA using the PCR (polymerase chain reaction) method. She then sequenced this region on a LiCor automated sequencer and isolated a unique species-specific area of the subunit. A non-isotopic DNA probe was then prepared for that oligonucleotide sequence signature. The efficiency and utility of this probje is being currently tested using a three primer competitive PCR detection technique. The approach is not only qualitative but potentially quantifiable.

While this limited effort suffices for initial system characterization and could be continued as restoration (and water flow modification) it is missing an important component. It should be expanded in scope to accommodate microzooplankton grazing activity. In studies we previously conducted in the shallow waters of the northern Gulf of Mexico, microzooplankton grazing has typically equalled or exceeded mesozooplankton grazing and this is characteristic of other subtropical coastal lagoonal environments.

Edward J. Phlips, Susan Badylak, Tammy Lynch, and Phyllis Hansen, Department of Fisheries and Aquatic Sciences, University of Florida/IFAS, Gainesville, Florida 32653.

Florida Bay and the surrounding reefs of the Florida Keys are among the nation's most productive and biologically diverse coastal environments. As the only tropical marine habitat in the continental United States this region is a focal point for sportfishing, diving and tourism, bringing over two billion dollars to the economy of Florida. Recent proliferation of blue-green algal blooms in the bay have raised serious concerns among both commercial and environmental interests about the ecological stability of the bay. Blue-green algal blooms have been implicated in fish kills, sponge die-offs, reductions in seagrass communities and potential alteration of the food webs. State and federal agencies have proposed a major effort to increase freshwater flow to Florida Bay as a means of mitigating these adverse effects, and restoring the character of this vital ecosystem. The potential efficacy of this management strategy must, however, be judged in the context of a clear understanding of the factors which control blue-green algal blooms and the consequences of blue-green algal dominance on the bays ecology.

In August of 1993 we began a Sea Grant funded study to characterize spatial and temporal patterns in the relative dominance of blue-green algae within the planktonic algal assemblage in Florida Bay. In the summer of 1995 this study was expanded with additional Sea Grant funding including, 1) Characterization of spatial and temporal patterns of nutrient and light limitation for planktonic primary production, 2) Determination of the role of salinity in defining the growth and composition of phytoplankton, 3) Determination of the grazing efficiency of zooplankton assemblages on blue-green algal dominated phytoplankton populations, and 4) Characterization of the consequences of blue-green algal dominance for the structure of the food web. This report focuses on the results of the initial phase of this research effort dealing with spatial and temporal variation in the composition and abundance of phytoplankton.

Seventeen sites within Florida Bay (Figure 1) were sampled monthly. The composition and abundance of phytoplankton were characterized using light microscopy of Lugol's preserved samples (Utermohl method) and fluorescence microscopy of live samples. Biovolume estimates were based on the closest geometric shape method. Sub-samples of water were also analyzed for chlorophyll a, TP, TN, Si, suspended solids and color. On-site determinations were made of salinity, temperature, O2, pH, and light extinction.

The spatial and temporal differences observed in phytoplankton standing crops and composition supported the hypothesis that the bay is composed of at least four ecologically distinct regions (Figure 2). The highest standing crops of planktonic algae and cyanobacteria were generally found in the north-central region of the bay, where the small unicellular cyanobacterium Synechococcus dominated the planktonic assemblage. Chlorophyll a concentrations ranged up to 40 mg m-3 in this region, and Synechococcus was responsible for over 90% of total phytoplankton biovolume in most of the samples analyzed. In other regions of the Bay total chlorophyll a concentrations were generally lower and the relative importance of diatoms and dinoflagellates increased in relationship to total phytoplankton biovolume. The north-eastern region of the bay exhibited the lowest standing crops, i.e. chlorophyll a concentrations consistently less than 2 mg m-3. In the south-central region phytoplankton standing crops were low accept in the late fall and early winter when elevated levels of Synechococcus were observed and chlorophyll a values reached 20 mg m-3. Gradients of phytoplankton abundance and composition indicate that these elevated standing crops may result more from the influx of water from the north-central region than in-situ production. In the western region of the bay phytoplankton standing crops were variable but averaged around 5 mg m-3 and the phytoplankton community was typically dominated by diatoms. The significant variability of chlorophyll a concentration and taxonomic composition in the western region may be a consequence of temporal variation in the character and amount of intruding Gulf waters.

The consistently high planktonic chlorophyll a concentrations observed in the central- interior region of Florida Bay may be related to spatial patterns of water circulation and inflow. The impact of tidal water exchange in this region of the bay is reduced by the shallow mudbanks around its periphery (Figure 1). Turnover rates for water and nutrients are, therefore, low compared to the western side of the bay. It is also a region of the bay impacted by inflows of water from the mainland through Taylor Slough (Figure 1). It may be hypothesized that the combined effects of periodic inputs of nutrients from the mainland, nitrogen fixation and low turnover rates heighten the potential for elevated chlorophyll a levels. At the beginning of the second year of the study period, fall '94, extended periods of high rainfall resulted in high levels of freshwater input to the north-central region. Salinities were observed to drop from over 40 ppt to less than 20 ppt and phytoplankton standing crops also declined temporarily. These observations point to a significant role for freshwater inflow in the dynamics of phytoplankton populations.

Phytoplankton standing crops may be further accentuated by the shallow polymictic character of the bay which increases the potential for nutrient recycling through sediment resuspension, i.e. internal nutrient loading. The relatively deep layer of muddy surface sediments in the north-central region present the potential for significant sediment resuspension. The role of internal nutrient loading in the maintenance of high phytoplankton standing crops is well established for in a number of freshwater and marine environments.

References

Phlips, E. J. and S. Badylak. 1996. Spatial Variability in phytoplankton standing crop and composition in a shallow inner-shelf lagoon, Florida Bay, Florida, USA. Bulletin of Marine Science 58(1).

Phlips, E. J., T. C. Lynch and S. Badylak. 1995. Chlorophyll a, tripton, color and light availability in a shallow tropical inner-shelf lagoon, Florida Bay, USA. Marine Ecology Progress Series. In Press.

Laurie L. Richardson, Department of Biological Sciences and Drinking Water Research Center, Florida International University, Miami, Florida 33199 .

Background

A quantitative study of phytoplankton bloom dynamics in Florida Bay was initiated in 1994, and is currently funded through August 1997. This research effort was originally part of a project supported by the National Aeronautics and Space Administration (P.I. L.L. Richardson) to determine the use of remote sensing to detect algal accessory pigments and estimate water quality. Field research includes an investigation into the biology of the system, and we routinely collect surface water samples for HPLC analysis of algal pigments (accessory pigments, chlorophyll a, and chlorophyll a degradation products) of different areas of the phytoplankton bloom, as well as measurements of optical properties (surface reflectance), salinity, and temperature. Florida Bay was chosen as a research site due to the optically dense nature of the phytoplankton bloom and the fact that the bloom is not uniform in terms of algal population composition thus is a good field site for discriminating between different algal populations.

An outcome of this project has been documentation of phytoplankton bloom composition, spatial and temporal bloom dynamics, and analysis of the bloom over time and space in terms of correlation with salinity, nutrients (data from Ron Jones), temperature, and dissolved oxygen.

Objectives

The principal objective of the biological component of this research effort is to document the nature of the phytoplankton bloom in terms of population composition, spatial distribution, seasonal variability, and correlation with water quality. The overall goal of this research (not reported here) is to demonstrate the use of hyperspectral remote sensing to detect algal accessory pigments for the study of phytoplankton dynamics.

Methods

I. Approach. Nine diagnostic algal pigments are quantitatively measured, whose presence indicates overall phytoplankton biomass (healthy and dead) as well as information about specific algal groups. The pigments measured are: chlorophylls, a, b, c; chlorophyllide a; fucoxanthin; diadinoxanthin; zeaxanthin; lutein; B-carotene; and myxoxanthopyll. The presence of these pigments are interpreted as follows: The distribution of chlorophyll a (present in all algae) reflects the overall distribution (and biomass present) of healthy phytoplankton. Chlorophyllide a, a natural degradation product of chlorophyll a, indicates the presence of dead phytoplankton material. Chlorophyll b is present only in Green algae (Chlorophytes) and Prochlorophytes (not phytoplankton). Chlorophyll c1/c2 (a mixture of two types of chlorophyll c which elute together on the HPLC column) indicates the presence of diatoms and dinoflagellates, with fucoxanthin generally indicating diatoms and diadinoxanthin indicating dinoflagellates. Myxoxanthophyll is found only in Cyanophytes (blue-green algae). Lutein, although specific to Chlorophytes (and Rhodophytes, not planktonic), co-elutes with zeaxanthin which is found in many types of algae as is ß-carotene. Thus zeaxanthin and ß-carotene can be used to indicate overall algal abundance, similar to chlorophyll a.

Pigment sampling and analysis are conducted as follows: Florida Bay water samples (2 liters) are collected and filtered onto 4.25 cm GF/F filters, and stored frozen at -70° C. Prior to analysis the samples are thawed and extracted according to the methods of Wright and Shearer (1984). Extracted samples are evaporated (to decrease volume) using nitrogen gas, and the final extract volume is measured to quantify results. 20 µl of a final solution are analyzed using High Performance Liquid Chromatography (HPLC) by injecting onto a Hewlett Packard 1090 HPLC with a 200 x 2.1 mm column containing 5 µm hypersil ODS to separate and identify pigments. An elution gradient similar to that of Mantoura and Llewelyn (1983) is used. Pigments are quantified and identified by an analytical software package incorporated into the system. Samples are run against purified standards of each pigment except myxoxanthophyll, which is quantified by using the standard curve for diadinoxanthin but ratioing the extinction coefficients of myxoxanthophyll and diadinoxanthin. Salinity is measured using a refractometer. [Nutrients were sampled and anlyzed by Ron Jones lab (FIU).]

II. Sampling locations and frequency. Sampling is conducted once per month. Locations vary depending on where the phytoplankton bloom is located during each sampling period. Our routine sites are basins near Rankin, Rabbit, Whipray, and Calusa keys. We also sample, depending on bloom development, at additional sites which include basins near Crocodile, Garfield, Jimmy, Johnson, Manatee, Murray, Old Dan Bank, Peterson, Pollock, Porpoise, and Twin Keys. Several of our sites correspond to sites routinely sampled by FIU's SERP monitoring program. Additional sites are sampled which exhibit the most dense phytoplankton blooms.

III. Project duration. Preliminary investigations began in 1993, and a satisfactory quantitative methodology was adopted in 1994. Quantitative data have been acquired since May, 1994 and will continue through August 1997.

Summary of results to date

At any given time, the phytoplankton bloom consists of different, patchy sub-blooms in different basins of Florida Bay which exhibit different algal communities. The dominant phytoplankton include cyanobacteria (blue-green algae), diatoms, and dinoflagellates. In addition to this spatial variability, specific sites demonstrate monthly variability. An example of the data set (in this case temporal) is shown in the following table, which presents data (pigments concentrations in µg/l) from Whipray Basin. These data show that the bloom was most pronounced in January, but was healthiest (no chlorophyllide a) in November and December. Changes in the relative amounts of cyanobacteria (myxoxanthophyll) vs. diatoms and dinoflagellates (chl c1/c2) is also evident.

Date Chl a Chlorophyllide a Chl c1/c2 myxoxanthophyll

5/4/94 3.74 0.21 0.16 0.34

6/8/94 3.37 0.13 0.00 0.30

7/7/94 4.41 0.37 0.40 0.38

8/3/94 3.24 0.14 0.19 0.28

10/5/94 5.32 0.00 0.26 0.21

11/2/94 5.07 0.00 0.49 0.48

12/1/94 5.39 0.00 0.60 0.38

1/5/95 10.11 0.85 1.73 0.93

3/22/95 1.07 0.12 0.14 0.13

Each pigment within the data set was also linearly regressed against water quality data acquired at the same time. We found no strong correlation between pigment distribution and variations in salinity, nutrients, or dissolved O2. Salinity, believed by some to be the cause of the Florida Bay bloom, had a correlation coefficient (R2) of 0.003 when regressed against chlorophyll a, and 0.053 against chlorophyllide a. Similarly, pigment regressions against nutrients and dissolved oxygen were non-significant. The only (mildly) significant correlations were found between salinity and temperature (R2 = 0.317). Chlorophyll a and temperature were not significant (R2 = 0.185). The distribution of pigments which occur within the same taxa were, as expected, significantly correlated (for example R2 of fucoxanthin and chlorophyll c = 0.950).

Outlook for remaining work

This work will continue through August 1997, and is supported by the Ocean Biology program withing NASA's Mission to Planet Earth program. In addition to the data gathering efforts detailed above, the NASA-supported effort consists of determining the feasability of using hyperspectral imaging remote sensing data to study phytoplankton bloom dynamics in Florida Bay. Monthly sampling for determination of accessory pigments, salinity, pH, water temperature and optical measurements will be used to generate a background for analysis of AVIRIS (Advanced Visible-Infrared Imaging Spectrometer) data, acquired by flying this prototype sensor on NASA's ER-2 high altitude aircraft. As the data base increases, perhaps a correlation between algal distribution and water quality will become apparent.

References

Mantoura, R.F.C. and C.A. Llewellyn, "The rapid determination of algal chlorophyll and carotenoid pigments and their breakdown products in natural waters by reverse-phase high performance liquid chromatography", Anal. Chim. Acta, Vol. 151, pp. 297-314, 1983.

Wright, S.W. and J.D. Shearer, "Rapid extraction and high performance liquid chromatography of chlorophylls and carotenoids from marine phytoplankton", J. Chromatog., Vol. 294, pp. 281-295, 1984.

Karen Steidinger, William Richardson, Earnest Truby, Rachel Bray, Nancy Diersing, and Dave Eaken, Florida Department of Environmental Protection, Florida Marine Research Institute, 100 Eighth Ave. SE, St. Petersburg, FL 33701.

Persistent microalgal blooms have occurred in the western and central portions of Florida Bay since 1991. Blooms of mixed microalgae populations are often dominated by pico- and ultraplankton consisting of blue-greens (cyanobacteria), diatoms, flagellates, and an unidentified sphere. The abundance of these pigmented algae contributes to extensive surface water discoloration, which ranges from yellow-green to brown. Resuspended carbonate sediments and bottom organic material can add to the discoloration and turbidity. Several of the objectives of the Florida Marine Research Institute's Florida Bay Microalgae Study are to 1) determine the planktonic and benthic microalgal species composition, abundance, biomass, and dominant assemblages (biological turbidity), 2) determine the non-biological components of turbidity associated with resuspended inorganic sediments and organic detritus on the same spatial scale, and 3) isolate dominant species into culture for a) laboratory growth limitation experiments and b) exposure studies with filter feeders to determine acceptability of species as food items or the toxicity of species to selected filter feeders. These three objectives address questions B.3 (what limits growth of phytoplankton), B.5 (what is the biological and non-biological cause of turbidity), C.3 (what is the cause of sponge mortality), and D.1 (have altered environmental conditions and habitat affected growth/survival of animals through altered trophic structure and dominance of different microalgae) in the 1994 Science Plan for Florida Bay.

Six fixed stations, (Sprigger Bank, Sandy Key, Johnson Key Basin, Rankin Basin, Captain Key, and Twin Key) were chosen for monthly water column sampling based on the availability of a historical database or because of their location in relation to past microalgal bloom events. Four of these stations are also sampled for primary productivity and nutrient enrichment bioassays by another group within this Study. Two bloom stations are chosen monthly based on aerial observation of discolored water. Up to 35 additional stations are sampled by The Nature Conservancy's Bay Watch volunteers. Locations for these incidental stations are selected the day before based on aerial overflight observations of discolored water in the western, central, and eastern portions of the Bay. The following variables are measured at the fixed and bloom stations with a Hydrolab Surveyor II or III: salinity, temperature, D.O., pH, and water depth. In addition secchi depth is measured and cloud cover, wind direction, and wind speed are estimated. Four to 20 liters of surface water are collected for laboratory processing and testing for Total Particulate Matter (inorganic and organic seston fractions) following a modification of Strickland and Parsons (1972) and EPA (1990); blue-green algal (cyanobacterial) abundance following a modification of the epiflourescence techniques of Booth (1993) and MacIssac and Stockner (1993); diatom, dinoflagellate, flagellate and other algal group species composition, abundance, and cell volume following the Utermohl method of Lugol's fixed and sedimented samples counted on an inverted microscope and scanning electron microscopy of Lugol's fixed concentrated samples, and single cell isolation of dominant species for establishing clonal cultures in "f/10" or "K/10" media following Guillard and Keller (1984). The incidental stations are treated the same for Total Particulate Matter, blue-green algal abundance but other microalgae are counted by group and not species and abundance is reported numerically rather than by cell volume. In addition, field measurements are salinity by hydrometer, temperature by thermometer, and secchi depth. Monthly sampling started in March 1994 and continues today for the above variables. Data are current for physical-chemical measurements, total particulate matter, and blue-green algal abundance. Results for species composition and abundance at the fixed stations are complete for July 1994 through April 1995 (10 months) and are being calculated for cell volume. Remaining samples are being worked up. Physiological studies (e.g., salinity, temperature, and light) using recently established clonal cultures of the dominant microalgae Synechococcus elongatus, Cyclotella choctawhatcheeana, Chaetoceros wighamii, Rhizosolenia imbricata, and the 2mm picoplankter are being formulated, as are exposure studies with Synechococcus and Cyclotella.

Over 120 diatom, 70 dinoflagellate, and 30 other algal taxa have been identified from Florida Bay during this study; 160 have been identified to species. Scanning electron microscopy and workup of the remaining samples will no doubt increase the identified species to over 250. Of the fixed stations, the two most western stations, Sprigger Bank and Sandy Key, appear to be subject to continual resuspension events as reflected by the presence of organic detritus, benthic diatoms, sponge spicules and inorganic seston (median values of 8.33 mg L-1 and 8.85 mg L-1 respectively). Turbidity is also influenced by blue-greens (103 to 106 per ml), summer blooms of Chaetoceros spp. (6+) and winter blooms of species in the Rhizosoleniaceae (Rhizosolenia imbracata, R. setigera, R. styliformis, Proboscia alata, Pseudosolenia calcar-avis, Dactyliosolen fragilissima, and Guinardia striata). Various combinations of sediment and microalgae can cause the surface waters to appear tan to murky brown. At Sprigger in December 1994, blue-greens dominated numerically and by cell volume. In February 1995, blue-greens dominated numercially but diatoms dominated in cell volume. Many of the Rhizosolenia cells were senescent and apparently the product of rapidly dividing populations. In April, blue-greens dominated numerically but diatoms dominated in cell volume. The two north central stations, Johnson and Rankin basins, have some of the highest chlorophyll a values recorded (up to 40 µg L-1). Johnson is more influenced by the Gulf as are the most western stations, e.g., resuspension, summer Chaetoceros and winter Rhizosolenia blooms. Also, this is the station where the picoplankter and small flagellates start to appear in substantial numbers. Rankin literally is the hot spot for the small-sized blue-greens, picoplankters, flagellates, and Cyclotella, and it also has resuspension events as evidence by benthic species and detritus. In December 1994, blue-greens dominated numerically but diatoms and flagellates dominated by cell volume. In April, blue-greens and picoplankters dominated numerically, but diatoms dominated by cell volume. For the most eastern fixed stations, Captain and Twin, dominance can shift between blue-greens, flagellates and picoplankters to diatoms and dinoflagellates. Typically lower chlorophylls were recorded and resuspension events were not common, e.g., median inorganic seston values were 3.13 mg L-1 and 2.16 mg L-1 respectively.

The planktonic and benthic chlorophyll-bearing microalgae of Florida Bay are diverse and range from temperate to tropical species, many are euryhaline, some are stenohaline. Many have been recorded from south Florida waters previously, some are new to the region, and some are just undescribed species. The dinoflagellates Gambierdiscus toxicus, Prorocentrum belizeanum, Pyrodinium bahamense var. bahamense (bioluminescent species) and others identify this Bay as tropical/subtropical. From a microalgal perspective, the Bay is unique in several combined features: it is rich in species, many of which are mucus producers; it has persistent mixed microalgal blooms, often dominated by the smaller size fractions; it has at least 15 known toxic dinoflagellate, diatom, and flagellate species; and it lacks the dominance of two common and abundant estuarine/coastal diatoms, Skeletonema costatum and Asterionellopsis glacialis. As in many Florida bays and other coastal areas, microalgal biomass as chlorophyll a and primary production can exceed the water column biomass and production (Zimba, unpublished).

C. R. Tomas, and B. Bendis, Florida Department Environment Protection, Florida Marine Research Institute, St. Petersburg.

The development of extensive phytoplankton blooms in Florida Bay marked a major departure from a previous status where primary production was dominated by benthic macrophytes resulting in a limpid, nutrient poor water column having a low phytoplankton biomass. These pelagic blooms, ushering major changes in biomass distribution also imposed altered trophic dynamics and structure. Among the major questions regarding these blooms are what major species compose the blooms, how are they distributed and how do they change with space and time? Equally important are questions related to the rates of change, growth, and factors stimulating and limiting the blooms. Finally, how do these blooms influence the trophic structure and how is this translated into the presence and loss of species from Florida Bay? The first questions relate to the description and quantification of biomass while the latter two relate to dynamic processes regulating the biomass. Studies addressing both types of major questions are included in the FDEP/FMRI phytoplankton program for Florida Bay.

Ongoing studies were begun nearly 2 years ago to define the spatial and temporal changes in phytoplankton species composition, abundance and biomass measured as chlorophyll a in Florida Bay. Additional studies of phytoplankton processes were also initiated 20 months ago. Since March 1994, nutrient bioassay studies have been conducted monthly on natural populations from four fixed stations in the Bay. These studies were prompted from the previously published results of Fourqurean, Jones and Zieman (1993) describing nutrient analyses from the Bay indicating the general paucity of inorganic nitrogen and phosphorus and the conclusion that phosphorus was limiting the blooms in most of the Bay regions. While nutrient ratios are informative, our objectives were to verify independently the type and degree of limitation as demonstrated directly by the phytoplankton components of the Florida Bay ecosystem. Growth stimulated by the nutrient addition, preference for specific nutrients, and inhibition of lacking major nutrients could be used to verify limitation as well as to compliment the biomass data. These observations would be useful in defining the intrinsic growth rates of phytoplankton in the Bay and what specific nutrients were limiting it.

In addition to the nutrient bioassays, estimates of primary production were begun in September 1994 on samples from the same 4 fixed stations where bioassays were being conducted. Using the 14C labelled bicarbonate, measurements at 10 natural light levels for duplicate bottles of samples from each station were made to define daily primary production rates. The production values would be helpful in defining the variation in productivity within Florida Bay, to give data from which comparisons can be made to other coastal regions and to allow a comparison of pelagic and benthic production.

A third component, begun in June 1995, is examining secondary production as measured by zooplankton grazing, copepod egg laying and microzooplankton feeding. This portion of the trophic dynamics studies are presently being conducted under contract to the FMRI phytoplankton group. Primary and secondary production will, at a future date, be coupled with species abundance and composition in defining the influence of blooms on trophic flow. This component will not be discussed here.

To date, over 2000 individual bioassay tests have been conducted on natural phytoplankton populations from the four fixed stations. These stations were chosen to reflect the variability within the bay and monitor major regions. The Captain Key basin station reflects features generally found in the oligotrophic eastern sector of the Bay while samples from Rankin Lake, where maximum phytoplankton biomass persisted, represents the most heavily impacted bloom station. Samples from Sprigger Bank and Sandy Key in the western Bay are areas where waters from the South West Florida shelf and Bay intermix. Eight treatments run in triplicate were tested for each station each month. Two treatments included low and high phosphorus (5, 10 µM/L P), another two low and high levels of nitrogen (15 and 30 µM/L) equally as nitrate and ammonia, two additional treatments with low and high combination additions of nitrogen, phosphorus and constant level of silica. A total addition consisted of N, P, Si, vitamins, and trace metals including chelated iron. A non enriched control completed the eighth treatment. Growth was monitored by in vivo fluorescence using a Turner Design fluorometer and run in a constant temperature waterbath having continuous illumination of approximately 50-90 µEm-2s-1 cool white fluorescent light. The assays were run for 3 days then terminated and harvested for final extracted chlorophyll analysis. By the third day, clear responses were detectable and further incubation resulted in declines of high biomass samples. An alternate exclusion assay was also done to confirm the limitation. For these assays, all nutrients were added to sample tubes except one giving -N, -P and -Si treatments. These assays were run in duplicate along with the addition assays.

Captain Key basin is the one station that continuously showed phosphorus limitation. Stimulated by phosphorus or limited by its absence, populations from this station indicated the limited availability of this nutrient to the eastern sector. The biomass of this station was consistently low compared to the other stations. With the exception of a late winter expansion of the blooms this station responded consistently in this manner. Rankin Lake area which often had the highest phytoplankton biomass responded modestly to the nutrient bioassays enrichments indicating a varying limitation between nitrogen and phosphorus. The response at Rankin Lake was as much a function of the heavy biomass nearing the carrying capacity of the system as it was to the exact limiting nutrient(s). During periods when biomass was low at Rankin, bioassays responded in a similar manner to those done at other stations. Both Sprigger and Sandy Key stations gave bioassay results indicating varying limitation of nitrogen, rarely phosphorus but definite stimulation by the addition of silica. This presumably is due to a major diatom component of the phytoplankton population which can dominate most of the year in this area. Other inner stations, particularly Rankin Lake, have a shared dominance particularly between bluegreen algae and diatoms. The emergent nutrient picture for Florida Bay is that the nutrient limitation of phytoplankton reflects the complicated morphometry of the whole basin offering a mosaic of limitation segmenting the isolated eastern basin, heavily impacted mid central basin and shelf influenced western area.

To date, 1,400 individual estimates of primary production were made for the 4 fixed stations in Florida Bay. Primary production measurements from the bay stations varied directly with the biomass and consistently showed high light limitation throughout the year. Light saturated production varied from 0.8 to > 1 g C/m-3/day-1 with highest values occurring in the western bay stations. Primary production at the station having the highest biomass (Rankin Lake) was often exceeded by the western stations particularly when diatoms were abundant at the near shelf stations. Presumably the phytoplankton composition, being dominated by a bluegreen algal component (Synechococcus) at Rankin Lake, had different photosynthetic efficiencies than stations having lower biomass but different species composition.

The lowest production was consistently measured at the Captain Key station which accompanied the low biomass found there. A complete cycle of these studies is yet to be completed and the data is presently being analyzed. Estimates of photosynthetic efficiencies and carbon turnover rates are expected to further define the full range of variability in phytoplankton primary production for Florida Bay.

Last updated: 04/22/98
by: Monika Gurnée
gurnee@aoml.noaa.gov