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. 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. 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.
Resultant rates are similar to rates calculated by the energy demand method.
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.