Responses of the Neocalanus spp. - microplankton community to physical forcing in the coastal Gulf of Alaska

Principal Investigators:

M. Dagg (LUMCON)

S. Strom (Shannon Point Marine Center)

Neocalanus cristatus in the Northern Gulf of Alaska - April 2003

Project Summary:

Three species of large calanoid copepods of the genus Neocalanus dominate mesozooplankton biomass throughout the subarctic Pacific and its marginal seas in the spring and early summer. All three species of Neocalanus are particle-grazing copepods that consume both phytoplankton and microzooplankton.

As a part of the GLOBEC CGOA Process Study, we conducted grazing experiments during cruises in April, May and July 2001, and in April and July 2003. On each cruise, 4 locations in the coastal water of the Gulf of Alaska were occupied to study the effects of Neocalanus spp. grazing on the structure of the pelagic web. The ingestion of three phytoplankton size classes (<5, 5-20 and >20 mm) was measured. Additional samples were preserved for enumerating and identifying phytoplankton and microzooplankton.

Based on the chlorophyll analyses, all three species of Neocalanus fed primarily on phytoplankton cells larger than 20 mm. In April 2001, CIV and CV of N. cristatus and N. flemingeri were abundant in the surface waters. Mean clearance rates are 186 and 432 ml copepod-1 d-1 for CIV and CV N. cristatus and 63 and 205 ml copepod-1 d-1 for CIV and CV of N. flemingeri. In May 2001, all three species were abundant and the mean clearance rates were 492, 148 and 146 ml copepod-1 d-1 for CV of N. cristatus, N. flemingeri, N. plumchrus, respectively. The abundance of all three Neocalanus species was low in the surface water in July and most of them were not feeding. Direct effects of Neocalanus spp. grazing on microzooplankton are currently being determined.

Indirect effects of Neocalanus spp. grazing were also apparent. In many experiments, especially ones with low total concentrations of phytoplankton, there was an increase in cells of < 5 mm in size. We attribute this to a reduction in their mortality from larger microzooplankton associated with Neocalanus predation on larger microzooplankton. In some cases, this cascade effect can be seen in the picoplankton and bacteria populations also. As additional samples are analyzed, more detailed understanding of the direct and indirect effects of Neocalanus spp. grazing on pelagic food webs will become more apparent.

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