TROPHIC PATHWAYS IN THE PELAGIC ENVIRONMENT OF FLORIDA BAY

Principal Investigator: Michael J. Dagg - Louisiana University Marine Consortium
Collaborating scientist(s):
Peter B. Ortner (AOML)
Dr. Gary S. Kleppel (USC)
Dr. Carmelo Tomas (Fla/DEP)
Objective:
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?

Rationale: 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 large 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. 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.
Method: Since October 1994 samples have been obtained in conjunction with Florida DEP at 8 sampling sites (figure 1) , (DEP sites 5,10-13,15,20,21) and since May two additional sites have been added. Zooplankton are collected with 64 um mesh 1/2m circular nets equipped with flowmeters. Tows are short (< 5 minutes duration) with the boat slowly underway at 1-2 kts. At all stations a 5 gallon bucket is used to obtain a surface sample which is filtered through 20 um mesh and the concentrate retained. Whole water filtrates better estimate the food available to fish larvae since smaller forms like nauplii are quantitatively collected. Net samples are used for both enumeration of dominant taxa (copepods and other zooplankton) and for coarse-community analysis. In net samples to be used for gut fluorescence grazing determination an aliquot of the cod-end sample is poured through a filter apparatus containing a 47mm piece of 150um mesh Nitex which is rapidly frozen in liquid nitrogen.

Since May 1997 grazing has been directly determined at four sites demarcated by crosses (figure 1) two of which are coincident with our traditional sampling sites using a modification of the dilution method (Landry 1994), one that measures the grazing impact of the entire zooplankton community, i.e. including the protozoans. At two of these sites (Captain and Rankin basins), for continuity with our previous work, we are continuing to do a limited number of experiments with the net-caught zooplankton: grazing in these is being directly measured by bottle incubations and gut content analysis while the feeding environment is being characterized by chlorophyll and HPLC analysis.

At all four experimental sites phytoplankton and protozoan communities are being taxonomically characterized via microscopy to identify key species in these trophic interactions and size-fractionated primary productivity measurements are being made for comparison with the phytoplankton growth rates from the dilution experiments.

The ingestion of phytoplankton by each copepod taxon has also been estimated determined from the product of ingestion rate per copepod and the abundance of that copepod. Grazing by other mesozooplankton as well as by nauplii is also estimated using temperature-compensated conversion factors, derived from the literature and size measurements to estimate carbon weight (mg C ind-1), respiration rate (m1 O 2 ind-1h-1), and metabolic requirements (mg C ind-1 d-1) for each animal and then some the abundances at each site to determine the metabolic requirements of the relevant planktonic community (mg C m-3 d-1).

Larval fish are sorted from 150um neuston tows to obtain individuals for gut contents analysis. Individuals are measured (notochord length) and their guts excised, measured (width) and identified. The guts of only morphologically intact individuals are included in the analysis. Identifiable gut contents will be compared to the density of food organisms enumerated in net tows, sieve samples and whole water samples.


Accomplishments to Date: During 1997 cruises continued at bimonthly intervals beginning in January and ending in November. In May the additional experimental sites were added. Preliminary analysis of some experiments is reported below.

Using the sizes, numerical densities (figure 2) , and temperatures at which the organisms were collected, metabolic requirements were calculated for the naupliar constituent of the zooplankton community. Following a trend visible since 1995, the mean naupliar metabolic rate in 1997 (figure 3) was lowest in January with a requirement of 0.49 g C l-1 day-1. The highest metabolic requirement was in September when the naupliar community of the bay required a 4.67 g C l-1 day-1. Ingestion demands can be approximated from these metabolic demands by assuming 1/3 of ingested carbon is used for metabolism.

Dilution experiment results (figure 4) are reported only for the Eastern Region Station since that has been sampled since July 96. The instantaneous grazing rate of the total zooplankton community in the Eastern Region were lowest in July/97, with a value of 0.04 day-1 and highest in September/96, 1.36 day-1. The mean grazing rate in the region was 0.62 day-1. Phytoplankton growth ranged from 0.02 day-1 in September/97 to 2.26 day-1 in January/97, with a mean growth rate of 1.22 day-1. With the exception of the September/97 experiment, phytoplankton growth exceeded total zooplankton grazing; the ratio of grazing to growth during this time averaged 0.44. This indicates that total zooplankton grazing is a significant source of mortality for the entire phytoplankton community.

To date samples have been collected at eight sites since 1994. These encompass the Western Bay, the Central Bay and the Eastern Bay as well as the Atlantic Transition Zone both near an inlet and relatively far from one. We can summarize the data for the first two annual cycles. The most abundant net caught copepods were Acartia tonsa, Oithona nana and Paracalanus crassirostris. Other copepods that were common but much less abundant included Tortanus setulosis, Euterpina acutifrons, Longipedia helgolandicus and Calanopia americana. The most abundant meroplankton in net samples were gastropod larvae and pelecypod larvae. While more highly variable than the copepods these were on occasion the most abundant organisms in the 64um net tows. Other meroplankton included zoea, decapod larvae, echinopluteus and heteropod larvae. Total net zooplankton biomass (figure 5) was higher during the Fall of 1984 except in the Atlantic Transition Zone samples and not very different than the naupliar biomass sampled over the same period. Higher values are generally seen toward the west and the lowest in the Eastern Bay. Acartia were relatively constant across various regions while demersal plankton and Paracalanus were more abundant in the Bay interior and Oithona at the western perimeter (figure 6) . The difference between regions and years may well be related to a systematic difference in salinity across the Bay. Shortly after our sampling began freshwater inflow markedly increased and only at the end of the two year period did it drop back to more typical levels. Some of the zooplankton types enumerated appeared to have distinct salinity preferences although Acartia abundance seemed unrelated to salinity which is consistent with its well known physiological tolerance perimeter (figure 7).

Assuming a metabolism to daily food ration ratio of 3 and a mean depth of 2 meters at our sampling locations we can calculate the potential grazing impact of the naupliar population. This ranged from 2 - 68 mgC/m2/day. Application of the same procedures to the net-caught zooplankton data yields estimates over a similar range of 2-85 mgC/m2/day. Over the past few years FIU has conducted monthly surveys of Bay water quality including chlorophyll concentration. Using their data (and corrected for a recently reported systematic calibration offset), a carbon to chlorophyll ratio of 30 and a growth rate of one doubling per day, estimated phytoplankton production is 50 to 1790 mgC/m2/day. This is within the range of the limited phytoplankton production data available for the same regions in Florida Bay over that period which ranged from 50-4500 mg C m-2 d-1(unpublished data of C. Tomas, DEP). Dilution experiments reported above confirmed doubling rates consistent with these primary production estimates. Although this approach involves numerous assumptions the patterns that emerge seem reasonable. The excess of primary production over grazing is most intense in the western Bay where blooms are most frequently reported while the balance is closest in the eastern Bay and along the Atlantic Transition zone where blooms are rare perimeter (figure 8) . Moreover, the greatest imbalance occurs during those months when blooms are reported to occur. Estimates in the eastern Bay are also roughly consistent with the dilution experiments reported above.

The gut contents of a few juvenile fish have been examined to date. These confirm utilization of holoplanktonic and meroplankton fauna by both pelagic anchovies and canopy dwelling killifish albeit on an opportunistic basis perimeter (figure 9) .

Another aspect of our present research focuses on the relationships between food concentration, food quality, the diets and egg production of an important calanoid copepod in the bay ecosystem, Acartia tonsa. A conceptual model was developed from data collected at four stations in the bay. The conceptual model compares bay-wide average copepod dynamics (figure 10) with those observed at Rankin Lake, a perturbed site (figure 11) characterized by extensive cyanobacteria blooms.

The model suggests that the perturbed site is the location of a much enhanced planktonic biomass but, perhaps more importantly, that a shift in energy flow has occurred relative to the bay-wide average. Bay-wide, micro-phytoplankton accounts for ca. 72% of the copepod diet; microzooplankton (e.g., ciliates and heterotrophic dinoflagellates) for 16% at the perturbed site when temperature did not affect egg production. We have found no correlation between food availability (micro-phytoplankton accounts for 26% of the diet, microzooplankton for 69%. The egg production of Acartia at the perturbed site was significantly higher (14.2 +/- 7.7 eggs/female/d) than the baywide average (5.8 +/- 0.81 eggs/female/d), though both rates are relatively low compared to egg production rates observed in other warm-temperate and subtropical estuaries. We have begun to suspect that qualitative aspects of the food environment are important in driving egg production and perhaps other aspects of secondary production in Florida Bay.


Key reference:
Science Plan for Florida Bay, A science planning document provided to the Interagency Working Group on Florida Bay, April 1994.
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