Effects of mesoscale phytoplankton variability on the copepods Neocalanus flemingeri and N. plumchrus in the coastal Gulf of Alaska

M.J. Dagg1,*, H. Liu1,a and A.C. Thomas2

 

1Louisiana Universities Marine Consortium, 8124 Highway 56, Chauvin, Louisiana 70344, USA

2School of Marine Sciences, University of Maine, 5741 Libby Hall, Orono, Maine 04469-5741, USA

 

*Corresponding author: mdagg@lumcon.edu

 

Abstract: The copepods Neocalanus flemingeri and N. plumchrus are major components of the mesozooplankton on the shelf of the Gulf of Alaska where they feed, grow and develop during April-June, the period encompassing the spring phytoplankton bloom. Satellite imagery indicates high mesoscale variability in phytoplankton concentration during this time. Because copepod ingestion is related to food concentration, we hypothesized that phytoplankton ingestion by N. flemingeri and N. plumchrus would vary in response to mesoscale variability of phytoplankton. We proposed that copepods on the inner shelf, where the phytoplankton bloom is most pronounced, would be larger and have more lipid stores than animals collected from the outer shelf, where phytoplankton concentration are typically low. Shipboard feeding experiments with both copepods were done in spring of 2001 and 2003 using natural water as food medium. Chlorophyll concentration ranged widely, between 0.32 and 11.44 mg l-1 and ingestion rates varied accordingly, between 6.0 and 627.0 ng chl cop-1 d-1. At chlorophyll concentrations < 0.50 mg l-1, ingestion was always low, < 50 ng cop-1 d-1. Intermediate ingestion rates were observed at chlorophyll concentrations between 0.5 and 1.5 mg l-1, and maximum rates at chlorophyll concentrations > 1.5 mg l-1. Application of these feeding rates to the phytoplankton distribution on the shelf allowed locations and time periods of low, intermediate and high daily feeding to be calculated for 2001 and 2003. A detailed cross-shelf survey of body size and lipid store in these copepods however, indicated they were indistinguishable regardless of collection site. Although the daily ingestion of phytoplankton by N. flemingeri and N. plumchrus varied widely because of mesoscale variability in phytoplankton, these daily differences did not result in differences in final body size or lipid storage of these copepods. These copepods efficiently dealt with small and mesoscale variations in their food environment such that mesoscale structure in phytoplankton did not affect their final body size.

 

REFERENCE: Dagg, M.J., H. Liu and A.C. Thomas. 2006. Effects of mesoscale phytoplankton variability on the copepods Neocalanus flemingeri and N. plumchrus in the coastal Gulf of Alaska. Deep-Sea Res I 53: 321-332.

 

a Current address: Department of Biology; Atmospheric, Marine, and Coastal Environment (ACME) Program, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong