2. Neocalanus spp. and the Structure of the Pelagic Ecosystem in the Subarctic Pacific Ocean M. Dagg, Louisiana Universities Marine Consortium H. Liu, Hong Kong University of science and Technology S. Strom, Western Washington University

3. Surface Mixed layer Thermocline Permanent halocline N. plumchrus/flemingeri zone N. cristatus and E. bungii zone data from Mackas, D.L., H. Sefton, C. B. Miller and A. Raich. 1993. Vertical habitat partitioning by large calanoid copepods in the oceanic subarctic Pacific during spring. Prog. Oceanogr. 32: 259- 294. 0 m 35-45 m 100-120 m

4. May 1984 - 10 stations: mean abundance = 152 CVs / m3 (from Mackas et al. 1993)

5. June 1987 – mean abundance = 204 ind / m3 (from Mackas et al. 1993)

6. (from Levitus World Ocean Atlas 1994)

7. Chlorophyll concentration: < 5  m = 0.577  g l -1 5-20  m = 0.143  g l -1 > 20  m = 0.051  g l -1

8. Chlorophyll concentration:Clearance rate: < 5  m = 0.577  g l -1 -0.068 L cop -1 d -1 5-20  m = 0.143  g l -1 0.088 L cop -1 d -1 > 20  m = 0.051  g l -1 0.208 L cop -1 d -1

10. are there enough copepods in situ to structure the nano- and pico-food web in the eastern subarctic Pacific?

11. For 200 copepods m -3 and 1 L cop -1 d -1 = 200 liters d -1 m -3 (0.2 d -1 ) Concentration of N. plumchrus/flemingeri CV = 25 - 500 m -3 Clearance rate on large particles (> 20  m) = 0.2 – 2.0 L cop -1 d -1

12. growth rates of large diatoms in this Fe limited system are 0.2 - 0.5 d -1 growth rates of large ciliates and dinoflagellates?? other copepodid stages must contribute also Other considerations: top down effect by N. flemingeri/plumchrus

13. Conclusions Neocalanus flemingeri and N. plumchrus exert a strong structuring effect on the food web of the N. Pacific Ocean, driving the food web towards a small-cell dominated system by: removal of large cells (diatoms and microzooplankton) cascade effect that stimulates small cells

14. Conclusions Neocalanus flemingeri and N. plumchrus exert a strong structuring effect on the food web of the N. Pacific Ocean, driving the food web towards a small-cell dominated system by: removal of large cells (diatoms and microzooplankton) cascade effect that stimulates small cells These effects are strongest: at low food concentrations when copepod clearance rates are highest, and under nutrient or food limiting conditions when growth rates of prey are reduced

15. The END