Zooplankton Densities/Abundance

Principal Investigator: Lewis S. Incze (lincze@usm.maine.edu)
Research Technicians: Ford Dye, Beth Novak, Nicholas Wolff

Methods:

Using a pumping system appended to the CTD unit, measured water volumes (typically 40 liters) were sampled at descrete depths and filtered through 40 micron mesh prior to sorting. A subsample of the filtered sample was used for the zooplankton counts and identification. The size of the subsample varied and depended on the amount of zooplankton present - subsamples were larger when there were fewer animals.

From the subsamples, all zooplankton were counted and identifed. Counts were then converted into number per cubic meter as follows: Number counted / (Subsample size X Amount filtered).

It is worth noting that the frequency of repeated densities is an artifact of the small subsample volumes screened in order to process a large number of samples. The small sample volumes did not give us the ability to resolve small differences in abundance at individual species and stages (e.g., Cal fin Nauplius I). At this level of sorting many counts were reported as zeros, ones, twos or threes. These counts are at the threshold for detection, and the results give the false impression of many identical densities (concentrations). This also shows up in gaps in the life histories as well. For example, a single depth/station might have NI and NIII and NIV stages, but not II or V. As a result, samples were often recombined to give the population characteristics of an integrated water column, or a group of samples. The approach worked well for our research objectives and we are well aware of its limitations. A single station and depth in our data can be deceiving and does not tell the whole story.

Note: Sampling for cruise EL9904, events EL10999.24 and EL111299.6 are intentional replication sites. All sampling was made at same depth level. These stations were used to test the repeatability of methods.

Cruises

SJ9508 (Seward Johnson Cruise 9508 to Southwest Georges Bank, 6 - 16 June 1995)
OC303 (Oceanus Cruise 303 to Georges Bank, 6 - 23 May 1997)
EL9904 (Edwin Link Cruise 9904 to Georges Bank, 14 - 28 April 1999)
EL9905 (Edwin Link Cruise 9905 to Georges Bank, 10 - 29 May 1999)

 Any questions, contact:

Lewis S. Incze
University of Southern Maine
P.O. Box 9300
96 Falmouth Street
Portland, ME 04104-9300

Voice:  207 228 8070
Fax:    207 228 8057
Email:  (lincze@usm.maine.edu)

 updated:  23 November 2004; gfh