Annual Progress Report

 

Grant number:              NA96OP0235

Amount of grant:           $56,000

Title:                             Trophic Pathways in the Pelagic Environment of Florida Bay

Grantee:                       Michael Dagg

                                    Louisiana Universities Marine Consortium

                                    Chauvin , LA 70344

Award period:              8/1/99 to 7/31/00

Report period:              8/1/99 to 4/30/00

 

Summary of progress and expenditures to date:

 

1. Work Accomplishments:

 

The objectives of this project were 1) to determine the importance of grazing by zooplankton in Florida Bay and how it varied within the bay as salinity and temperature distributions changed throughout the seasonal cycle; and 2) to determine the relative abundances of micro- and macro-zooplankton and how they vary relative to seasonal temperature and salinity cycles.

 

To meet the goals of this project, zooplankton were collected on a bimonthly basis from each of ten sites. Collections of copepod nauplii were obtained at each site by pouring 10 l of water from the surface through a 20µm mesh sieve. The organisms collected were preserved with 10% formalin (final concentration) and shipped to LUMCON for microscopic analysis. The organisms were identified to the lowest possible taxonomic level, though nauplii were not identified past the “nauplius” level. They were then measured for greatest length and width using an ocular micrometer. Net-caught zooplankton were collected by towing a 0.5m ring net with 64µm mesh for approximately 5min behind a small boat driven at roughly 2 knots. A General Oceanics flow-meter was suspended in the mouth of the net to determine the volume of water filtered. The organisms collected were preserved using the same method as for nauplii, and were sent initially to NOAA’s AOML facility for counting and identification. Beginning in the spring of 1998 newly collected samples and those samples not already processed by AOML were sent to LUMCON for identification and measurement using the same technique utilized for the nauplii. Net-caught zooplankton were only measured for the samples collected during the calendar year 1998. Sizes determined for each organism within a category (i.e. copepods, pelecypod larvae, etc…) were averaged by category and then applied to the corresponding organisms for each month of previous sample collection. Using standard formulae, the flow-meter data were used to determine the volume of water filtered, which was then combined with the numerical counts of the zooplankton present at each site to determine numerical abundances for the zooplankton. Those abundances were then used to compute biomasses for the various zooplankton categories. The average measurements of organisms within a given category were then used to calculate an average carbon content using formulae obtained from scientific journals and texts. The growth rates of microphytoplankton and grazing rates of microzooplankton were determined using the dilution technique of Landry and Hassett (1982). Dilution experiments were initially conducted at Duck Key on a bi-monthly basis beginning in July 1996. After the procedure was refined, additional sites were added in each of the remaining 3 regions. The experiments were completed in September 1998. Samples were taken for analysis of chlorophyll a using Fluorometry, as well as multiple pigment analysis using HPLC. Samples of the microplankton were also collected for microscopic analysis.  

 

Components of the phytoplankton community that "bloom" are those in which increases from growth exceed losses from grazing or other sources of mortality. In most aquatic systems, the dominant loss process is zooplankton grazing (Banse 1992). As a part of this previous award, my goal was to determine the importance of zooplankton grazing to phytoplankton dynamics in Florida Bay by comparing grazing losses to growth rates or productivity rates of the phytoplankton community.  The grazing contributions of different components of the total grazer regime in the water column have been examined. All data have been collected and analyzed, and writing is in various stages of completion, summarized below:

 

(a) mesozooplankton community (plankton > 200 um).  Grazing from the mesozooplankton community has been compared to productivity of the phytoplankton community. Bi-monthly collections of net zooplankton were made at eight stations located within each of the 4 major regions (Phlips et al. 1995) in Florida Bay, the Western region, the North-Central region, the South-Central region, and the Eastern region.  (These regions are not entirely consistent with those on the Florida Bay web page at http://www.aoml.noaa.gov/ocd/sferpm/sub.html but are similar, with the South-Central region now being referred to as the Atlantic Transitional zone). Net-caught zooplankton were moderately abundant throughout the year. In some seasons, molluscan larvae dominated zooplankton abundance and biomass but otherwise copepods dominated.  Paracalanus and Oithona species were nearly always the dominant copepods but Acartia tonsa was also significant. Other major grazers (like larvaceans) were infrequently abundant.

 

The body size of each species and stage of mesozooplankton was determined for each sample. From this and the ambient temperature, the metabolic demand of each individual can be calculated using relationships derived from vast numbers of measurements of zooplankton respiration rate throughout the world (Ikeda 1985). Summation of these metabolic demands for all individuals at each station provides a measure of the carbon requirement for respiration of the entire mesozooplankton community.  The amount of carbon ingested by the mesozooplankton community can be approximated as 3 x the metabolic requirements.  Using this method we determined that, during 1998, the mesozooplankton community ingested between 12.5 – 38.3 mg C m-3 d-1 in the South-Central region, 2.9 – 93.9  mg C m-3 d-1  in the North-Central region, 14.1 – 46.2  mg C m-3 d-1   in the Western region and 16.6 – 25.6  mg C m-3 d-1  in the Eastern region. Ingestion was highest during the summer and early autumn in each region. These ingestion rates are equivalent to a significant portion of the daily phytoplankton production and frequently more than the phytoplankton stock.  The diet of these copepods is, of course, not only phytoplankton but also includes microzooplankton and probably some forms of detritus, but it is likely the mesozooplankton community consumes a significant fraction of the daily phytoplankton production.  A manuscript summarizing this work is in preparation:

 

 (b) crustacean microzooplankton community. Copepod nauplii were sampled bimonthly at ten sites in Florida Bay between September 1994 and November 1998. Samples were collected from each of the 4 regions identified by Phlips et al. (1995),  the Western region, the North-Central region, the South-Central region, and the Eastern region.

 

Concentrations of copepod nauplii varied widely during the study. In the Western, North-Central, and South-Central regions, there was a clear seasonal cycle with maxima in the fall during each year. Seasonality was not apparent in the Eastern region. Regional differences in nauplius concentration were apparent, although only 3 regions were identifiable statistically (p = 0.0001)--the Western and South-Central regions were not significantly different.  The North-Central region had the highest mean concentration (236.0 l-1) while the Eastern region had the lowest (71.0 l-1).  Also, the mean body size of individual nauplii in the Western region was larger than in the other three regions, which did not show any significant differences between them. This suggests either different species or better nutritional conditions in the Western region.

           

Biomass of the nauplius community was computed as the product of the abundance and size composition of the community. Three regions were clearly defined. The North-Central region had the highest mean biomass (6.37 mg C m-3), followed by the Western region (4.42 mg C m-3) and the not significantly different South-Central (2.97 mg C m-3) and Eastern regions (1.87 mg C m-3). Seasonally, the North-Central, South-Central and Western regions were marked by annual biomass maxima in the summer/fall and minima in the winter or spring. There was no clear seasonal pattern in community biomass within the Eastern region.

 

Community metabolic requirements were calculated from nauplius abundance, community size composition and water temperature, using equations for zooplankton respiration (Ikeda 1985).   In all 4 regions, demands were lowest during the winter and highest during the summer/fall. Regional differences were strong (p = 0.0001). Highest metabolic demands were in the North-Central region. There was no significant difference between the Western and South-Central regions or the South-Central and Eastern regions. Carbon required to meet metabolic demands of the nauplius community was generally between 0.5 and 4.0 mg C m-3 day-1, with occasionally higher values in the North-Central region, up to maximum of 12.3 mg C m-3 day-1 in September 1997.

 

The amount of carbon ingested by the nauplius community can be approximated as 3 x the metabolic requirements and is  < 20 mg C m-3 day-1, with exceptions in the North-Central region during September 1995 and 1997.  Comparison of this community ingestion with the phytoplankton stock indicates the nauplius community of Florida Bay consumes an amount of carbon equivalent to 1 to 80 % of the phytoplankton stock daily, exceeding 20% nearly half the time. A manuscript summarizing this component of our grazing work has been submitted:

 

(c) microzooplankton grazing  We also wanted to determine the contribution of the microzooplankton community to total grazing on phytoplankton. In many other regions, this component of the zooplankton community is the most significant grazing component. We began measuring the consumption of phytoplankton by microzooplankton at one site in July and September 1996 and thereafter at 4 sites on a bi-monthly basis until September 1998. As before, our stations were selected to represent each of the regions identified by Phlips et al. (1995) and further verified by our analysis of nauplius community structure and grazing (Brenner and Dagg, submitted).  Using the dilution method (Landry and Hassett 1982; Landry 1994) we measured  phytoplankton growth rates (d-1) and microzooplankton grazing rates (d-1).  Rates reported here are based on disappearance of  chlorophyll a from experimental bottles and therefore represent ingestion of phytoplankton only. Additional samples were collected for HPLC analysis to determine pigment-specific growth and grazing rates. Converting grazing rates measured in these experiments into the same units as presented in sections (a) and (b) indicates the microzooplankton community ingests between 17.2 and 260.7 mg C m-3 d-1 in the South-Central region, between 10.3 and 255.7 mg C m-3 d-1   in the North-Central region, between 43.6- 422.0  mg C m-3 d-1  in the Western region and between 4.7 – 83.0 mg C m-3 d-1  in the Eastern region. In contrast to the seasonal patterns indicated for grazing by the mesozooplankton and nauplius communities, a clear seasonality was not apparent for microzooplankton community grazing. In more than 50 % of the experiments however, grazing on phytoplankton exceeded phytoplankton growth rate, measured in these same experiments at 50 % light. Further analysis of these data is ongoing but it is apparent that microzooplankton grazing is the major fate of phytoplankton production at almost all stations and times. A manuscript summarizing this component of our grazing work is in preparation:

 

(d) synthesis. 

                                                     Ingestion (mg C m-3 d-1)

                                    S-Central         N-Central         Western           Eastern

mesozooplankton*        13-38              3-94                14-46              17-26

nauplii*                        < 12                 1-36                < 15                < 12

microzooplankton**     17-261            10-256            44-422              5-83

 

            *    all carbon sources

            **  phytoplankton carbon only

 

The carbon demands for the mesozooplankton community and the nauplius community are equivalent to a significant % of the phytoplankton stock daily. Although non-phytoplankton foods are important diet items for these grazers, it is probable that these components of the grazer community can affect the phytoplankton community structure (i.e. what blooms) but do not exert sufficient grazing pressure to prevent a bloom or to control a bloom. In contrast, grazing by the microzooplankton community is sufficiently high that it often exceeds growth of the entire phytoplankton community measured in the same experiment. At the present time, we can unequivocally state that:

 

·        microzooplankton grazing has significant control over phytoplankton community structure; and

·        microzooplankton grazing can prevent or control blooms under some conditions.

 

The sum of these grazing estimates does not include benthic grazers, many of which consume particles suspended in the water column and thereby add to the “grazing pressure” on pelagic phytoplankton.

 

All field work related to this project was completed within the period covered by this progress report. The remaining portion of the funded year will be devoted to sample analysis, data analysis, and writing.

 

2. Applications:

 

Brenner, Robert, J. The contribution of net-caught and microzooplankton to the phytoplankton dynamics of Florida Bay. PhD thesis (expected completion – April 2001)

 

Brenner, R.J., M.J. Dagg and P.B. Ortner. 2001. The distribution, abundance and metabolic demands of net-caught zooplankton in Florida Bay, USA. Estuaries (accepted with revisions).

 

Brenner R.J. and M.J. Dagg. Microzooplankton grazing and phytoplankton growth in Florida Bay. Estuaries (to be submitted in Feb 2001).

 

“Regional Microzooplankton-Phytoplankton Interactions in Florida Bay”, R. J. Brenner and Michael J. Dagg. Oral Presentation. Florida Bay and Adjacent Marine Systems Annual Meeting. 1999

 

"Microplankton Dynamics in Florida Bay. USA”, R. J. Brenner and Michael J. Dagg. Oral Presentation. American Society of Limnology and Oceanography Winter Meeting. 1999.

 

“Microplankton Growth and Grazing Dynamics in Florida Bay”, R. J. Brenner and Michael J. Dagg, Poster Presentation, Florida Bay Science Conference. 1997

 

3. Expenditures:

 

Total expenditures on this award during the period of this progress report were $31,289.96. Remaining funds were for salary  and for small amounts of supplies to be used in sample analyses.  These expenditure rates are approximately as planned and as projected in the original proposal.

 

 

 

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