Grazing rates and behaviors of Neocalanus plumchrus: implications for phytoplankton control in the subarctic Pacific

Abstract

Grazing rates and behaviors of the copepod Neocalanus plumchrus were investigated in shipboard experiments during the first SUPER Program cruise (May, 1984). N. plumchrus can exploit cells in the 2 to 30 /m size range with equal clearance efficiency but displays considerable flexibility in responding to changes in concentration and size composition. Its functional response helps to stabilize phytoplankton at low densities. In 60-liter microcosms, a density of one copepod liter-' was sufficient to maintain the ambient abundance and structure of the phytoplankton community for a week. In the absence of the copepod, phytoplankton bloomed to unnaturally high levels, and the community composition was dramatically altered. Despite its grazing potential, N. plumchrus was not present in sufficient density to control phytoplankton blooms in the subarctic Pacific. However, the copepod may have an important role in regulating the abundance of smaller grazers and the size structure of the phytoplankton community.

Omnivory by copepods in the New Zealand Subtropical Frontal Zone

“…The 1998 results indicated that in the 1997 experiments, which were sampled only at 24 h, it was likely that the animals had reduced ciliate food concentrations to low levels in <12 h. This may have caused food limitation, and reduced estimates of ciliate clearance, determined over 24 h. The biomasses of copepods used in these experiments (0.2, 0.7, 2.4 mg DW l -1 for small, medium and large animals, respectively) were sufficient to clear the 2 litre flasks about once in 12 h, based on the August 1998 0-12 h clearance rates. The fact that some ciliates were left in the 2 litre bottles after 24 h suggests that feeding rate had indeed slowed as prey stocks became very dilute, consistent with a sub-threshold feeding functional response (Frost, 1975;Landry and Lehner-Fournier, 1988). In summary, clearance rates determined for phytoplankton and ciliates over 24 h incubations October 1997 and between 12 and 24 h in August 1988 appeared to be large under-estimates, due to prey depletion and trophic interactions within bottles.…”

“…We also assumed that copepods consumed phytoplankton as small as 2 µm with 100% efficiency. Although this size is a commonly used minimum for feeding studies of both large (Landry and Lehner-Fournier, 1988) and small (Lampitt and Gamble, 1982;Støttrupp and Jensen, 1990) copepods, it is possible that clearance efficiency declines for larger copepod species at cell sizes less than 3 to 6 µm (Støttrupp and Jensen, 1990;Nakamura and Turner, 1997). If the 2-5 µm phytoplankton fraction was unavailable to the large copepods, their estimated ingestions were reduced by about 20%.…”

Mesozooplankton grazing in the coastal Gulf of Alaska: Neocalanus spp. vs. other mesozooplankton

“…We know that Neocalanus spp. and other copepods do not feed on small cells (Frost et al, 1983;Landry and Lehner-Fournier, 1988;Liu et al, 2005). One objective of this study was to determine if mesozooplankton dominated by Neocalanus spp.…”

“…are suspension-feeders that rely on the establishment of a feeding current to collect food particles. They are efficient at taking in particles .5 mm (Frost et al, 1983) or .2 mm (Landry and Lehner-Fournier, 1988) and have been reported feeding on phytoplankton, microzooplankton, and detrital particles (Greene and Landry, 1988;Dagg, 1993b;Gifford, 1993;Liu et al, 2005). Because microheterotrophs are the major grazers of phytoplankton in the Subarctic Pacific (Landry et al, 1993b;Rivkin et al, 1999;Liu et al, 2002;Strom et al, 2007), mesozooplankton may exert an indirect effect on phytoplankton production as predators of microzooplankton (Landry et al, 1993a;Liu et al, 2005).…”

“…It was later demonstrated that mesozooplankton grazing at ineffective at controlling total phytoplankton production (Dagg, 1993a;Tsuda and Sugisaki, 1994;Boyd et al, 1999). However, mesozooplankton may still play an important role in regulating the abundance of micrograzers (Gifford, 1993) and therefore alter the size structure of the phytoplankton community (Landry and Lehner-Fournier, 1988;Landry et al, 1993a;Shiomoto and Asami, 1999;Liu et al, 2005).…”