In the shore laboratory, frozen samples were thawed and sorted copepods were analyzed for the amounts of phytoplankton pigments retained in their guts. Gut contents were converted to ingestion rates by application of the gut residence time, determined from the generalized equation for copepods relating temperature to gut residence time. A correction was also made for the average 34 % destruction of pigment that occurs during ingestion/digestion. Subsamples from the formalin preserved sample were taken with a stempel pipette to determine the concentration of each numerically abundant organism. The ingestion of phytoplankton by each copepod taxon is determined from the product of ingestion rate per copepod and the abundance of that copepod. The sum of the contributions from all copepod taxa is termed copepod community ingestion in this paper but in reality this is a slight underestimate because rare copepods are not included.
Community grazing rates determined at specific sites can be scaled up to larger areas if information on water properties and zooplankton abundance and distribution is available. During a cruise in April 1993, zooplankton abundance was mapped on a series of transects along and across the shelf. A V-fin tow vehicle was towed continuously in the near surface. It simultaneously measured with temperature, salinity, chlorophyll fluorescence, visible (400- 700nm) light transmittence and acoustic backscatter at six frequencies ranging from 256KHz to 3.0MHz. The latter data will eventually be used to estimate particle size distribution. At the same time an optical particle counter/video system was used to sample water continuously pumped aboard through the ship's MIDAS system. The data generated by the latter device facilitates identication of animal targets and direct comparision with acoustics.
In general, gut pigment levels were greater at the plume site than at the shelf site during each cruise. However, gut pigment levels within each site were higher during 9/91 and did not reflect the generally higher chlorophyll concentrations observed in the water during 5/92.
The contribution from each species is derived by summing the contributions from the late developmental stages. Analysis of variance indicated community ingestion rates were significantly (p < 0.05) different at the 9/91 plume site than at the other 3 sites. Temora turbinata and Eucalanus pileatus are important contributors to copepod grazing at all sites, accounting for 54 % and 43 % of the grazing by the copepod community at the plume and shelf sites respectively during 9/91, and 38 % and 86 % during 5/92. Hourly rates are integrated over 24h to obtain daily ingestion rates of phytoplankton by the copepod community.
Comparison of the ingestion of phytoplankton with phytoplankton production can be made by applying carbon:chlorophyll ratios, determined using a modification of the chlorophyll labeling method originated by Redalje and Laws. Conversions of ingested chlorophyll to carbon are made accordingly and compared to phytoplankton production rates measured on these same cruises. Except during 5/92 at the plume site, the copepod community ingested a significant fraction of the daily phytoplankton production.
Grazing by the copepod community was only 4 - 5 % of daily phytoplankton production in plume waters during 5/92. This contrasts with higher rates, 14 - 62 %, in the plume during 9/91 and at the shelf site during both cruises. It is not possible to develop a clear understanding of patterns from only these two cruises but this observation appears consistent with the suggestion that copepods will not be an important component of the grazing community in the river plume until they have some time to numerically respond to the presence of phytoplankton food. In the plume region, copepod populations should increase throughout the spring in response to the spring maximum of riverine-stimulated phytoplankton production. Numerical response rates should also increase in the spring as water temperatures increase. Patterns observed in this study are consistent with these generalizations but more information is required to properly develop these arguments.
Other components of the zooplankton community may also ingest significant amounts of phytoplankton. Organisms less than 200um, including protozoans and small metazoans, may be important grazers but are not included in this analysis. High grazing rates by this microzooplankton community have been observed at both sites during other times of the year.
Another potentially important grazer is the larvacean Oikopleura dioica. Larvacans are typically abundant during spring, summer, and fall. For example, a detailed cross-shelf description of their abundance and the associated physical and biological system parameters was provided during April/93. The transect was run from slope waters directly north towards the inner shelf just south of the Atchafalya. Salinity, temperature and light transmission increased dramatically towards the inner shelf. Fluorescence was very low offshore and increased during the transect. From about the 20km mark onwards, fluorescence was maximal for the instrument setting and the apparent plateau is not real. Since it appears to be closely correlated, albeit inversely, with transmittence it is likely fluorescence likely continued to increase shoreward. Larvaceans were abundant over most of the transect but especially so from km 25-35, attaining concentrations of several/liter at intermediate salinities and temperatures (ca. 25ppt and 19 deg.C). Lowest concentrations were at the innermost part of the shelf. Note that larvaceans were more abundant than small copepods except well offshore over the entire transect and that their changes in abundance were not well correlated. Moreover the larvacean distribution appears to be more patchy (at least on large spatial scales). These differences are entirely consistent with larvacean biology since their rapid growth rate permits them to bloom and increase dramatically in abundance over periods as short as a few days. Interestingly the dominant peak in larvacean (and small copepod) abundance are seen in acoustic returns at high frequencies but not at lower frequency.
The grazing impact of the larvacean population has not yet been not estimated for this transect but during the 9/91 and 5/92 cruises is estimated by calculating the carbon requirements for growth of the observed populations. These O. dioica were small, ranging between 200 and 600um trunk length (n=200 for each cruise). Calculated carbon requirements, which need not be met by phytoplankton only, are greater than requirements for the entire copepod community during 9/91 and only slightly less than requirements for the copepod community during 5/92.
A more direct estimate of phytoplankton removal by larvaceans can be obtained by applying the filtering rate of the larvacean community to the chlorophyll stock. Filtration rates are computed for each size of O. dioica from a relationship between filtration rate and trunk length. Alldredge's measurements were made at 23.5oC and her relationship is applied directly to O. dioica for the 5/92 cruise. Resultant rates are similar to rates calculated by the energy demand method. Estimates are not made with this method for the 9/91 cruise because of uncertainty in scaling up the calculation to the higher temperature. These high abundances and the calculated high ingestion rates support the argument that larvaceans are important grazers on phytoplankton in this region. There importance in other contexts is discussed in a recently accepted manuscript.
The input of new nutrients onto the continental shelf of the northern Gulf of Mexico greatly stimulates phytoplankton production, and large phytoplankton stocks are typically observed at intermediate salinities in plumes of the Mississippi River. Grazing by the copepod community is an important source of mortality for this phytoplankton. Grazing from other components of the zooplankton community is as yet unquantified but indications are that protozoans and larvaceans can contribute significantly. If so, the primary fate of phytoplankton production in this plume system is to be grazed by zooplankton.
Ortner, P.B., and M.J. Dagg (1995), Nutrient Enhanced Coastal Ocean Productivity Explored in the Gulf of Mexico, EOS 76: 97,109.
Ortner, P.B., L.C. Hill, and S.R. Cummings (1989). Zooplankton community structure and copepod species composition in the Northern Gulf of Mexico. Cont. Shelf Res. 9: 387-402.
Dagg, M.J., P.B. Ortner, and F. Al-Yamani (1988), Winter-time distribution and abundance of copepod nauplii in the Northern Gulf of Mexico. Fisheries Bulletin, 86(2): 219-230.
Dagg, M.J. and P.B. Ortner (1994), Zooplankton grazing and the fate of phytoplankton in the Northern Gulf of Mexico. Nutrient Enhanced Coastal Ocean Productivity, Proceedings of the 1994 Synthesis Workshop, Baton Rouge, LA.