Controls on Phytoplankton Populations in Florida Bay: Clams and Eggs

Gabriel A. Vargo, University of South Florida, Department of Marine Science, St. Petersburg, FL; Robert Erdman, Eckerd College, St. Petersburg, FL; and Gary Kleppel and Carol Burkhart, University of South Carolina, Columbia, SC.

Water column microalgal blooms were first noted in Florida Bay in 1987 following the die off of seagrasses (primarily Thalassia testudinum). Blooms of a small cyanobacterium, Synechococcus sp. in the central portions of the bay, and diatom blooms in the western regions have occurred with increased frequency and duration. One hypothesis that has been suggested to account for these blooms is a reduction in loss rates due to reduced grazing by water column phytoplankton populations by zooplankton and benthic filter and suspension feeders.

The impact of the cyanobacterial bloom on zooplankton populations and their grazing and reproduction rates was unknown. Also, a major loss of benthic organisms particularly in the central and southern portions of the Bay occurred. Such information led us to suggest the following hypotheses:

1. Reduced grazing activity by benthic filter and suspension feeders has contributed to the maintenance and prolongation of water column phytoplankton blooms.

2. Reproduction rates and subsequently grazing rates by zooplankton are reduced within the area of the cyanobacterial bloom.

The first hypothesis was tested in a pilot study at Keys Marine Laboratory during June, 1996. The purpose of the study was to determine the magnitude of filtering and ingestion rates of representative benthic suspension and filter feeding invertebrates common to Florida Bay; to test experimental protocols for obtaining these rates and recommend modified procedures; to determine the general groups of phytoplankton that were ingested in field collected samples and in laboratory grazing studies; and ultimately to estimate the impact of filter and suspension feeders on water column phytoplankton biomass in Florida Bay.

Seven species of benthic invertebrates were used in our experiments: The bivalves Argopecten irradians, and Chione cancellata; the mangrove tunicate, Ecteinascidia turbanata; and the sponges, Chondrilla nucula, Cinachyra sp., Halichondria sp. and Haliclona sp. Representative specimens of each species were exposed to a range of chlorophyll concentrations derived from water collected from 3 locations; Sprigger Bank, Captain Keys and Rankin Basin with suitable controls. Three approaches were used to estimate grazing rates: 1. a static protocol with triplicate specimens of each species exposed to a range of 5 chlorophyll concentrations; this approach was discarded for logistical reasons and specimen requirements; 2. a static protocol with triplicate specimens incubated in water with a range of chlorophyll concentrations sequentially from lowest to highest concentration; and 3. A closed system protocol with a specimen enclosed in a chamber through which phytoplankton enriched filtered sea water is pumped through a fluorometer which records changes in fluorescence over time. In all the approaches incubation time ranged from 20 to 40 minutes at ambient light in the laboratory and calculations of filtering and ingestion rates are based on control corrected initial and final extracted chlorophyll determinations and all values are normalized to dry weight. Samples were also taken for cell counts but have not been completed. Qualitative determinations of the types of phytoplankton ingested will be based on HPLC analysis of selected carotenoid pigments. These analyses are also awaiting completion.

The second hypothesis was tested using a combination of feeding and egg production experiments using animals and water collected from several locations in Florida Bay. Egg production was determined by incubating representative species for 24 hours in water from four locations. An additional 24 hour incubation was used to determine hatching success. Microzooplankton grazing rates were determined monthly by the Landry dilution method with water from two locations, Rankin Basin and Captain Keys while macrozooplankton grazing was determined by standard incubation methods every other month.

Results

Average filtration and ingestion rates for all benthic species, irrespective of the water type and chlorophyll concentration ranged from 100 to 850 ml/g dw/hr and 0.08 to 2.11 ugChl/g dw/hr, respectively. Maximum filtration and ingestion rates exceeded 3000 ml/g dw/hr and 20 ugChl/g dw/hr, respectively. Higher filtration and ingestion rates were obtained in the closed system compared to the static systems for the purple sponge, Haliclona sp. (1000 ml/g dw/hr vs. 335 ml/g dw/hr, respectively). Both filtration and ingestion rates were dependent upon the source of water. Water from the KML raceway produced consistently lower rates than any other source water. Average rates for Chione and Ecteinascida were higher in water from Captain Key whereas Argopecten had little preference. All species except Chondrilla and Cinachyra had relatively high rates in water from Rankin Basin. There were no obvious relationships between filtration and ingestion rate and chlorophyll concentration for any species tested.

Microzooplankton grazing rates were highest in samples taken from the cyanobacteria bloom in Rankin Basin. Macrozooplankton (copepod) gazing rates were also high in this region however the species investigated were mainly feeding on microzooplankton and flagellates rather than on the cyanobacterium. Egg production at all locations was low relative to other areas or estuaries however, the hatching rate was extremely high; usually 80%. Egg production in Rankin Basin water was higher than other locations during summer but the overall seasonal average was essentially the same for all locations.

Estimates of the impact of grazing by benthic filter and suspension feeding on the water column cannot be calculated until quantitative areal estimates of biomass distribution are available. A preliminary assessment can, however, be made based on some assumptions and a limited amount of data. Turney and Perkins (1972) report areal distribution values for Chione cancellata in several areas of Florida Bay. Values range from 3 to 24 animals/ft2 for the interior region of Florida Bay which translates to 32-258 animals/m2. With a maximum filtering rate of approximately 200 ml/animal/hr, Chione could filter 6400 - 51,600 ml/m2/hr or 67% to 386% of a 1m3 water column (10,000 liters). A similar calculation can be made using the filtration rates for Halichondria and Haliclona sp.; 300-500 ml/animal/hr. If we assume a population density of 2 animals/m2, such values translate into particle removal from 6% to 10% of a 1 m3 water column per hour; or essentially the entire water column in one day. Our rates may also be underestimates relative to rates recalculated from Reiswig (1974). His rates for several different species of sponges, normalized to gram dry weight, range from 5400 to 18,000 ml/g dw/hr; approximately 5 to 10 times higher than our measured maximum rates. Thus, the impact of filtration of water column phytoplankton by benthic suspension feeders can be significant on a daily basis.

In addition to the benthic filter feeders, microzooplankton grazing rates were also elevated in the bloom areas. Macrozooplankton appeared to take advantage of the presence of these micro-grazers by using them as a food resource. With added egg production and high hatching success rates, zooplankton populations were being maintained within the bloom area of Florida Bay. Therefore, at least on first analysis, water column grazing and benthic grazing should be operating as a control mechanism in the region of the cyanobacterial bloom. However, we were not able to find any viable benthic suspension and/or filter feeders with the bloom area on our sampling trip. Therefore, losses by grazing from this component of the ecosystem potentially did not occur. A final analysis will be based on the results of the FDEP benthic study currently underway.

Florida Bay Microalgal Blooms: Composition, Abundance, and Distribution

Karen A. Steidinger, Florida Department of Environmental Protection, Florida Marine Research Institute, St. Petersburg, FL; and Edward J. Phlips, University of Florida, Department of Fisheries and Aquatic Sciences, Gainesville, FL.

Persistent blooms of microalgae have occurred in Florida Bay for the last several years. The blooms consist of mixed microalgal populations often dominated by pico- and ultraplankton consisting of blue-green algae (cyanobacteria), diatoms, flagellates, and a eukaryotic picosphere. The composition (size classes and species composition) and distribution of blooms varies by region,e.g., western, central, and eastern portions of Florida Bay as well as seasonally. One of the regions, the central, can be seasonally divided into north and south components when the blue-green or blue-green-diatom-picosphere blooms spread from the apparent northern seed area around Rankin Basin to the southern portion of the Bay in late fall/winter. The abundance of these pigmented algae contributes to extensive surface water discoloration. Resuspended carbonate sediments and organic material can add to the discoloration and turbidity. Any persistent autochthonous bloom of planktonic blue-greens or other picoplankter in an estuary isabnormal and a symptom of system dysfunction or alteration.

Florida Bay microalgal blooms have been studied by several groups including a study group from the Florida Department of Environmental Protection's Florida Marine Research Institute (DEP) and a study group from the University of Florida's Department of Fisheries and Aquatic Sciences (UF). The studies were designed to address several questions, one of which is "What regulates the onset, persistence, and fate of planktonic algal blooms in Florida Bay". To address these questions you need to know the microalgal composition, biomass, and distribution of blooms in time-space and how the blooms are influenced by physical, chemical, geological, and biological variables. What initially fuels and drives the microalgal blooms can ultimately influence total community structure and ecosystem function. Although sampling stations, time of collections, and methods differ between the two groups, their data bases are being evaluated for their unique contributions as well as their commonality. It would appear that both data bses support the division of Florida Bay into four zones or regions: the western region influenced by the Gulf of Mexico; the north and central regions influenced by resident populations of blue-greens-diatoms-picospheres and runoff, and the eastern region which is less variable and only occasionally influenced by micoalgal blooms.In addition, there are seasonal differences in physical, chemical, and biological variables within these regions that need to be defined by hydrological seasons. Biologically, the background community components appear to be pico- and ultraplankters such as Synechococcus elongatus and S.spp., Cyclotella choctawatcheeana,and other very small centrics <10 um, and the picosphere which are influenced by salinity, light and other environmental variables. Superimposed on this oscillating background there are peaks of larger diatoms and dinoflagellates. For example, the winter blooms of Rhizosoleniaceae (e.g., Rhizosolenia imbricata) diatoms in the western region, the summer blooms of Chatocerotaceae (e.g., Chaetoceros cf. wighamii) diatoms in the north Central region, or the blooms of large dinoflagellates such as Pyrodinium bahamense, Prorocentrum mexicanum, Ceratium hircus, Gyrodinium instriatum, Gymnodinium sanguineum, and others seasonally in different regions of the Bay, particularly from June through September and occasionally in winter. Even the Rankin station can be dominated by abundant large desmokont and dinokont dinoflagellates that have high cell volume.

The DEP database for February 1994 to September 1996 represents monthly sampling at six permanent stations and up to 34 nonfixed stations. In total, there have been 230 station samples in the western region, 102 samples in the north central region, 125 samples in the south central region, and 167 samples in the eastern region. Currently, Principal Component Analysis and other tests are being applied to the database (salinity, total particulate load, organic load, inorganic load, blue-green algal numerical abundance, and chlorophyll a) to assess variability by region and season. Eventually the intent is to assess community composition (over 120 diatom, 75 dinoflagellate, and 30 other algal taxa have been identified from Florida Bay),biomass, seasonality, and distribution in relation to environmental variables. The north central region has the highest chlorophyll a (mean:8.76 ug L-1; range: 0.29 to 40.58 ug L-1 while the western region is second, the south central region is third, and the eastern region is fouth in chlorophyll a mean values. Organic particulate load values follow the same trend as for chlorophyll a and ranged from a mean of 1.55 to 6.42 mg L-1. The mean numerical abundance of blue-greens was highest in the north central region and second in the south central region followed by the western region and then the eastern region.The values for inorganic particulate load differed in that the western region had the highest values; the means for the regions varied from 3.88 to 10.31 mg L-1. It would appear that the western and north central regions are most influenced by resuspension events. The UF database shows similar regional differences in physical, chemical, and biological variables. The DEP and UF databases can help further refine the designation of bloom regions.

For microalgal species to bloom and persist, there must be a seed population or inoculum present, a suitable physical environment, available and sustained nutrient input, and a competitive advantage over other microalgal species. Species occurrence, distribution, and biomass have been attributed to species specific life cycle strategies and reproduction and loss rates in response to a number of environmental and biotic factors. A portion of the DEP's 1996-1997 study is to determine through laboratory experiments, why the dominant microalgal species (e.g., Synechococcus elongatus, Cyclotella choctawatcheeana, Chaetoceros cf. wighamii, Rhizosolenia imbricata, and a eukaryotic picosphere have an advantage in the different portions of Florida Bay and outcompete the other microalgae. If salinity, turbidity, light, and/or nutrient availability regulate the dominant microalgae and consequently bloom development, then knowing the environmental regulators or modulators will help evaluate restoration options and theirpotenital success. Laboratory studies involve a series of light growth rate experiments, salinity growth rate experiments, growth kinetic (Monod-type) experiments, and nutrient-limited competition experiments involving static and fluctuating salinities. Preliminary data for salinity confirms that the salinity tolerance curves for the study species differ in their optimal ranges and upper and lower tolerance limits. Both the DEP and UF results for Synechococcus suggest that this blue-green algal group tolerates a wide salinity range, high turbidity and low water clarity. The preliminary results for experiments with four dominant microalgae in Florida Bay are given in the poster presentation by Bill Richardson, Florida Department of Environmental Protection's Florida Marine Research Institute, entitled "Salinity, light and nutrient requirements of several dominant microalgal taxa of Florida Bay".

Larry E. Brand and Gary L. Hitchcock, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL.

One of our research goals is to determine the source of the nutrients that promote microalgal blooms in Florida Bay. The two primary potential sources our research is focussing on are benthic sediments and human activities on land. Another part of our research is focussing on the extent to which Florida Bay water with high nutrients, suspended sediments, and/or phytoplankton is transported as far as the coral reefs. If such transport occurs often enough, the ecological structure of the reef community could be altered.

Shallow tropical marine ecosystems are generally characterized by clear waters (low nutrients and microalgal biomass in the water column) and by much of the productivity being associated with the benthos (seagrasses, macroalgae, and microalgal turfs and films). Most of the nutrients in these ecosystems are sequestered in either the biota or in the sediments. In the case of Florida Bay, the system may have become destabilized by the loss of a major portion of the benthic producers (Thalassia testudinum, with a release to the water column of excessive nutrients from plant decay). Loss of seagrass cover can also lead to more sediment resuspension. The blooms observed today may be supported by nutrients formerly in the benthic sediments and biota.

Another potential source of nutrients is human activities that generate nutrients that enter the marine ecosystem by surface runoff or groundwater. It has been estimated that there are 25,000 cesspools and septic tanks, 281 injection wells, 4 active and 10 inactive landfills, 182 marinas with 2707 wet slips, and 1410 live-aboard boats in the Florida Keys. These sources, along with several sewage outfalls, are thought to be injecting nutrients into local waters, but in most cases are not yet proven to be significant. We are investigating the extent to which these nutrients may be reaching various coastal ecosystems.

To address these issues, the following questions are being asked.

How large is the pool of nutrients in the sediments of Florida Bay relative to that in the water column above? Depending on the size of the sediment pool, it could potentially sustain a phytoplankton bloom long after the initial nutrients in the water column are removed.

Over a wide range of habitats, is there a general correlation between benthic and planktonic nutrient concentrations, between benthic nutrient concentrations and benthic microalgal biomass, and between benthic nutrient concentrations and phytoplankton biomass? Such correlations would provide some evidence of the strength of coupling between sediment-water column nutrient cycling and microalgal population dynamics.

Is it possible that episodic resuspension of sediments is a major mechanism for transporting benthic nutrients into the water column (where they can support microalgal blooms) and out to the reefs?

Instead of nutrient rich groundwater flow into coastal waters being uniform along the coastline, does it occur in local "hotspots" because of preferential flow along structural faults and solution holes?

Are nutrient inputs and the resulting chlorophyll concentrations higher after large amounts of rainfall, particularly after a long period of no rain because of nutrient concentration build up on land during the dry period that is then flushed out by the rain?

To address questions of water column-sediment interactions, 10 stations along the southeastern part of Florida Bay are being sampled every 3 months. To address questions concerning ephemeral inputs of nutrients and sporadic transport offshore to the reefs, as many water samples as possible are being taken from both bayside and oceanside waters from Elliott Key down to Key West, particularly after storms. Water samples are also being taken in the western area of Florida Bay to examine its interaction with the Gulf of Mexico.

To date we have collected over 600 samples throughout the Florida Keys and analyzed them for temperature, salinity, chlorophyll and turbidity. Salinity has ranged from 8 to 38 ppt. Chlorophyll concentrations have ranged from 0.12 ug/l to 9.55 ug/l. The overall trend in the observations in Hawk Channel out to the reefs so far is for lower chlorophyll concentrations in the upper Keys and higher concentrations in the lower Keys. Chlorophyll concentrations are also considerably higher in Florida Bay than on the ocean side of the Keys. Another clear trend is for higher chlorophyll concentrations in deadend canals than in the water basins to which they are connected. It also appears that chlorophyll is consistently lower in areas with extensive benthic plant communities than in nearby areas of similar depth that have no significant benthic plants. We have not observed higher chlorophyll concentrations immediately next to the coastline compared to a kilometer away. Further offshore, chlorophyll concentrations decline to "oceanic" levels, but how far offshore the decline occurs is highly variable, depending on wind and current patterns. High concentrations of chlorophyll and turbidity have been observed on occasion over the reefs and several kilometers beyond.

Turbidity ranges from as high as 99 NTUs in Florida Bay to as low as 0.01 NTUs offshore. Turbidity is much more variable, due to the influence of winds. It is persistently high in many parts of Florida Bay but quite different in different parts of the bay. In Hawk Channel and out to the reefs, it is fairly sporadic, with generally lower turbidity in the upper Keys compared to the lower Keys, and as one moves offshore.

Dissolved phosphate concentrations in the water column range from 0.01 to 0.16 uM, nitrate from 0.02 to 4.68 uM, ammonia from undetectable to 28.85 uM, and silicate from 1.83 to 127.91 uM. Nutrients are considerably higher in Florida Bay than at the offshore stations. Silicate decreases linearly from Shark River to the Keys with increasing salinity, indicating a freshwater source. Porewater ammonia concentrations range from 15.86 to 1531.15 uM, porewater nitrate from 0.09 to 2.25 uM, and porewater phosphate from undetectable to 10.75 uM. The highest porewater nutrient concentrations are at stations in Florida Bay closest to Upper Matecumbe Key.

To date, our work has focused on developing and testing methods, setting up sampling programs, surveying Florida Bay for selection of sampling stations, and collecting and analyzing samples. Now that we have about six months worth of data, we are beginning to analyze them for correlations. We plan on using those correlations to develop more specific hypotheses and then design both field and laboratory experiments to test them more rigorously.

The Role of Nutrients in Initiating and Supporting Florida Bay Microalgal Blooms and Primary Production

Carmelo R. Tomas, Florida Marine Research Institute (FDEP), St. Petersburg, FL.

Since March 1994, monthly phytoplankton studies conducted by FDEP/FMRI were made to survey the bloom areas consisting of joint observations by aerial surveillance to map bloom locations and at the same time sampling from 6 fixed locations and as many as 30+ additional stations for bloom composition, distribution, chlorophyll a and phaeopigments, total particulate solids and dissolved and particulate nutrients. In addition to this routine monitoring activity, process measurements were made from four selected locations to define the nutrient dynamics and limitations of the natural populations, their potential growth rates and primary production. In addition to these process measurements, secondary trophic studies were conducted at the same time to define the linkage between the primary and secondary trophic compartments.

From the early years of this decade, phytoplankton blooms have become a permanent feature of the Florida Bay ecosystem. The questions regarding the appearance of these blooms center around two major hypotheses, namely 1) that the blooms are a function of a few opportunistic species which have been able to exploit open nitches afforded by the presence of excess nutrients, and/or 2) that a major trophic dysfunction has occurred whereby grazing pressure has been reduced or eliminated, thus allowing the accumulation of phytoplankton biomass. The latter hypothesis is being addressed in another presentation at this meeting and will not be discussed here. The first hypothesis was examined via the study of nutrient bioassays and primary production measurements. These studies were oriented at defining the limiting nutrient(s) and an estimate of potential growth by the natural populations exposed to these nutrients. The primary production was conducted to give an independent estimate of carbon fixation and therefre growth and as a measure of rates from the first trophic level for subsequent trophic studies.

Natural populations from four stations were sampled monthly from March 94 through July 96. The stations included, Sprigger Bank and Sandy Key areas located in the western regions of the Bay which are heavily influenced by marine waters from the West Florida Shelf where blooms were noted previously. Another bloom area studied was the Rankin Lake region in the north central portion of the Bay. Again, this regions was found to be heavily impacted by the phytoplankton blooms dominated by cyanobacteria. The Rankin area is also one where sea grasses have disappeared and where extremes in salinity in excess of 40-50% were found. The fourth area routinely sampled was the basin adjacent to Captain Key. This basin had consistently low phytoplankton biomass values, sparse sea grass and physical-chemical characteristics more similar to the eastern region of the Bay. The full suite of monitoring parameters were conducted at these four stations as well as the nutrient bioassay and productivity studies.

Nutrient bioassays were conducted using the natural phytoplankton populations for the four stations by first prefiltering through 163 mm mesh netting to remove larger animals and detritus and dispensing as 40 ml aliquots into triplicate 60 ml screw cap culture tubes. Two general types of treatments were made. One set had eight treatments of nutrients additions either as single additions or in combination. A second set had nutrient enrichments including all but one of the major nutrients and thus was tested for the deletion of either nitrogen, phosphorus or silica. A total nutrient enrichment included additions of nitrogen (as nitrate and ammonia), phosphate, silicate, trace metals, and vitamins while a control had no nutrients added. Phosphorus was added at two levels, low (5 mM/L) and high (10 mM/L) singly and combined with nitrogen and silica at the low and high treatments. Nitrogen, as an equal combination of nitrate and ammonia, was added at two levels, low (15 mM/L) and high (30 mM/L), singaly or in combination as low and high treatments with phosphorus and silica. Silica was added at one level (30 mM/L) to the combined low and high nitrogen and phosphorus addition as well as in the complete additions. In summary, the nutrient addition bioassays included 3 low phosphorus, 3 high phosphorus, 3 low nitrogen, 3 high nitrogen, 3 low phosphorus + low nitrogen and silica, 3 high phosphorus + nitrogen and silica, three full enrichments and 3 unenriched controls. In addition, a nutrient exclusion series consisted of three replicates containing all nutrients except, nitrogen, phosphorus or silica. The minus N, P and Si treatments were compared with the enrichments series to confirm the potential limitation. All tubes were incubated in a constant temperature water bath at a temperater similar to that of the stations sampled. All tubes were under constant illumination from cool white fluorescent light at a fluence rate of approximately 100 mE/m2 sec. Growth was measured as in vivo fluorescence measured with a Turner Designs 10 AU fluorometer twice daily at the same time each day for 4 days. At the end of the incubation period, the triplicate tubes were pooled from which duplicate chlorophyll determinations were made and from which a small sample for species composition was preserved for examination at a later date.

On separate aliquots from each station, primary production was measured using the standard C14 bicarbonate incubation at 10 light intensities during incubations generally lasting 4 hours. The productivity measurements were run in duplicate, corrected for dark absorption of C14 and maintained at ambient temperatures in a tube incubator having circulating sea water and natural light. The gradient was produced by the use of neutral density screening. All samples were harvested on GFF filters, placed into glass scintillation vials with 10 ml of Optima Gold fluor and read on a Packard TR900 liquid scintillation counter. Net activity was converted to weight carbon per meter square per day. Monthly values were used to obtain an integrated estimate of yearly production from each of the regions tested.

The nutrient bioassay studies identified three major types of limitation for phytoplankton in Florida Bay. The stations in the western Bay (Sprigger and Sandy) showed limitation by either nitrogen or silica. This was consistent with the blooms of diatoms common in that region. The exclusion assays confirmed the nitrogen and silica limitation. In addition, occasionally very high growth and biomass was developed in the total enrichment indicating that other factors besides nitrogen and phosphorus were involved. Here trace metals, perhaps iron, may be also limiting. Growth at these stations could be elevated and did exceed 1 division per day particularly during bloom periods suggesting a rapid total community growth consistent with the biomass in Chl a measured there. The central region of the Bay at Rankin Lake indicated an alternation between nitrogen and phosphorus limitation for most of the year with a rare occurrance of silica limitation. The extensive blooms there, dominated by cyanobacteria and ditoms are apparently receiving enough nutrients to maintain the extensive blooms there. While growth rates here were much lower (< 0.6 div./day) than at the western stations, the biomass present during the bloom times indicated that these modest growth rates represented a sizable growth overall. The exclusion assays confirmed that both nitrogen and phosphorus were limiting with with little difference between the presence of absence of that nutrient. This suggest that slight nutrient imputs can alternate the limitation from nitrogen to phosphorus and back again. The nutrient delivery and cycling is very important in maintaining the bloom populations observed there. The easternmost station, Captain Key, was one where phosphorus was consistently shown to be limiting. Again, both additions of low and high levels of phosphorus as well as exclusion of phosphorus showed the degree of limitation by this nutrient. This station also consistently had low phytoplankton biomass and modest to low sea grass abundance Comparing this station with other regions, it behaved consistently with the eastern Bay areas and was less like the central and western areas. The modest phytoplankton biomass was consistent with the low growth rates (<0.4 div/day) and pronounced phosphorus limitation found there.

The variations in primary production closely followed biomass fluctuations. The highest daily rates measured were observed at the Rankin Lake station during bloom periods. Daily fixation rates in excess of 3 gC/m2 day were common during blooms. Values less than 1 were found during the summer months when chlorophyll biomass was less than 1 mg/L. Annual production at this station exceeded 400 gC/m2 for each of two years studied. The western area was the next most productive area. Here again, daily rates between 1 and 3 gC/m2 day were found when phytoplankton biomass levels were elevated. Although these production rates were not as high as those from the north central bloom area, they do represent a sizable carbon production equivalent to 200-300 gC/m2 year. The least productive area was the Captain Key site where daily values rarely exceeded 300 mgC/m2 day and annual production was below 75 gC/m2 year. These low production values were consistent with the low biomass and growth rates measured there under chronic phosphorus limitation.

The results of these studies indicate that the nature of the nutrient limitation of the phytoplankton blooms varies with region of the Bay and may be related to the delivery and/or cycling of nutrients locally. Nitrogen plays a more important role in the central and western regions whereas, phosphorus which can be important at times in the central region limits the eastern portion of the Bay. Growth from natural populations from the four stations indicated a potential for rapid growth in excess of 1 div./day for the central and western regions. These chlorophyll based growth rates were in general agreement with the primary productivity rates when examined as carbon doublings. The agreement of nutrient based doubling times and carbon based doubling times suggest that the phytoplankton populations in Florida Bay are extremely active in maintaining population densities and that their growth far exceeds losses in producing blooms. The eastern Bay station, consistently phosphorus limited, having the lowest pytoplankton biomass for most of the time and with low or modest growth, showed features which contrasted the other Bay regions. This region may reflect the availability and delivery of nutrients as well as an uncoupling or weak cycling of nutrients needed to maintain the populations at the other regions. The quantity and supply of nutrients to the Bay regions is a major regulator in the inception and maintenance of these blooms. The fate of these blooms may be more closely tied to the cyclic nature of nutrient delivery and assimilation of the nutrients but also importantly tied to the control exerted by secondary trophic levels.

Michael J. Dagg,Louisiana Universities Marine Consortium,Chauvin, LA; and Peter B. Ortner, NOAA, Atlantic Oceanographic and Meteorological Laboratory, Miami, FL.

Components of the phytoplankton community that "bloom" are those in which increases from growth exceed decreases from loss processes. In most aquatic systems, the dominant loss process is zooplankton grazing. One of the objectives of our studies has been to determine the importance of zooplankton grazing in Florida Bay, and to compare grazing losses to algal growth rates or productivity rates. We hypothesized that growth in small phytoplankton is closely balanced by losses to microzooplankton grazing, but populations of larger zooplankton are not sufficient to control larger phytoplankton, which consequently can form blooms. In addition, although not the subject of this presentation, we are interested in the trophodynamic significance of the various zooplankton species; we will identify which taxa are important food sources for planktivorous larval and juvenile fishes.

Bi-monthly collections of zooplankton have been made since September 1994 at eight stations: 1/2 mile N. of Murray Key, Whipray Basin, Eagle Key Basin, Cross Bank Basin, Twin Key Basin, Johnson Key Basin, 1/4 mile SE of Duck Key and the south end of Shell Key Channel. For gut content analyses, fish larvae are being collected with 150 um neuston nets, and planktivorus fish juveniles by parallel FNMRI/DEP trawl net deployments at the same stations. After some initial comparative experiments (earlier reported) our procedure has been to quantitatively collected larger zooplankton by towing nets of 64 um mesh, and smaller zooplankton with bucket samples passed through a 20 um screen. All samples have been sorted and all zooplankton identified and enumerated. The sizes of representative individuals from each enumerated category have been measured and corrected for preservation effects.

Zooplankton are moderately abundant throughout the year. In some seasons, molluscan larvae dominate zooplankton abundance and biomass but otherwise copepods dominate. Paracalanus and Oithona species are nearly always the dominant copepods but Acartia tonsa is also significant throughout the year. Other major grazers (like larvacea) are infrequently abundant.

The potential grazing impact of copepod nauplii on the phytoplankton community was estimated. The metabolic requirements of each nauplius was determined from its size using literature-derived conversion factors corrected for temperature conditions in Florida Bay. Summation of the demands from all nauplii provided an estimate of the total metabolic demand by the nauplius community. Other microzooplankton 20 um, and all microzooplankton <20 um, are excluded from this calculation. The potential grazing impact of the larger copepods and meroplankton on the phytoplankton was separately estimated by a similar calculation. Measured body dimensions and temperatures where combined with literature-derived conversion factors to determine the metabolic demand of each taxa, and then multiplied by the abundance of that taxa to obtain the total community demand. Summation of the food demands of the nauplius community and the community of larger zooplankton yielded an estimate of the total demand of the metazoan zooplankton community.

These feeding estimates indicate protozoan grazers are the most important component of total grazing community.

Direct measurements of whole community zooplankton grazing (i.e. including protozoa) and whole phytoplankton community growth have been made at a single station during the past three sampling intervals. These twenty-four hour in situ experiments have shown that grazing by the whole zooplankton community markedly exceeds estimates described above. This indicates the organisms <20 um, the protozoans, are the most important grazers in this system. Total community grazing is sufficient to nearly balance the daily growth of the whole phytoplankton community. Enumeration of the phytoplankton species composition at the outset and termination of these experiments samples will identify which components of the phytoplankton community are increasing and which are being controlled by grazers.

Although protozoans are more significant grazers than metazoans, the latter are more significant food sources for planktivorous fish. Close coordination of our studies with studies of phytoplankton growth, larval and juvenile fish abundance and food requirements, and physical circulation, as well as proper scaling through integrated modeling studies, is required to quantitatively assess the factors controlling bloom dynamics in Florida Bay.

Last updated: 2/26/98
by: Monika Gurnée
gurnee@aoml.noaa.gov