THE TROPHODYNAMIC ROLES OF ZOOPLANKTON IN FLORIDA BAY

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Principal Investigator: Peter B. Ortner (AOML)
Collaborating scientist(s):
Michael J. Dagg - Louisiana University Marine Consortium
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: Samples are obtained in conjunction with Florida DEP at 8 sampling sites. Zooplankton are collected with 64 um mesh 1/4m 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 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.

Grazing rate is measured by the gut fluorescence method. The ingestion of phytoplankton by each copepod taxon will be determined from the product of ingestion rate per copepod and the abundance of that copepod. Grazing by other numerically abundant mesozooplankton is estimated by applying either literature values for grazing at similar temperatures or size specific rates estimated from the copepods to the abundances derived from our net tows.

Community grazing rates determined at specific sites will be scaled up to larger areas based on more extensive measurements of zooplankton abundance and concomitant water properties and the degree to which taxonomic distribution and abundance can be determined by hydrographic variation determined using multivariate statistics like correspondence analysis.

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 will be included in the analysis. Identifiable gut contents are be compared to the density of food organisms enumerated in net tows, sieve samples and whole water samples. The same procedures are being used with small-mouthed juvenile and adult fishes captured in Fla. DEP trawl and seine net samples.

Using Acartia tonsa collected in Biscayne Bay, we amplified a partial region of the large subunit of ribosomal DNA using the PCR (polymerase chain reaction) method. We 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. This probe is used to identify unrecognizable larval fish gut content fragments with a competitive PCR detection technique. The approach is not only qualitative but quantifiable.

Accomplishment: To date samples have been collected over one annual cycle at a bimonthly interval commencing in September 1994. All ninety-six samples have been enumerated but only the first three fully analyzed. In addition one Night-Day comparison series of twelve 150um net tows was made at Twin Key basin in the South Central region and has been analyzed. Forty eight macro zooplankton samples from September and November have been enumerated and analyzed as have twelve January Night-Day intercomparison samples and sixteen microzooplankton bucket samples. Initial results are discussed below:

The most abundant net caught copepods were Acartia tonsa, Oithona nana and Paracalanus crassirostris. In September when salinity ranged from 33-43ppt Acartia abundances ranged from 16 to 8216/m3, Oithona from 307 to 40460/m3, Paracalanus from 10 to 785/m3. In November when salinity ranged from 22-32 ppt. Acartia abundances ranged from 79 to 5948/m3, Oithona from 427 to 29952/m3, Paracalanus from 204 to 15384/m3. Only Paracalanus was significantly more abundant in November. In the one January station already analyzed salinities were slightly lower than in November and Acartia abundances were considerably higher (fourfold or more in comparable 150um net tows). In September, bucket sampled copepod nauplii ranged from 21 to 284 liter-1 while in November from 36 to 235 liter-1. The average length of a copepod nauplius was 116 um and the average width was 55 um. Such small nauplii are expected since the dominant copepods are comparatively small but from morphological examination alone it is difficult to determine the species of the nauplii enumerated. 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. In September they ranged from 553 to 69,732/m3 and in November from 42 to 187,436/m3. Other meroplankton included zoea, decapod larvae, echinopluteus and heteropod larvae.

Comparison of 150um mesh tows taken during the day and at night indicated that large Acartia (0.5-1.0mm body length) and Calanopia americana were significantly more abundant. In fact Calanopia was rarely sampled during the day. The abundances of Oithona, Paracalanus, Longipedia and Euterpina and chaetognaths were insignificantly different night and day. In other shallow estuarine systems older Acartia are reported to have a semi-demersal behavior, staying close to or on the bottom during the day and rising up into the water column at night. Because Acartia is the largest of the dominant species and is well known to have a high tolerance for varying salinity, additional night-day comparisons are planned.

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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