200 μm mesh nets demonstrate low amplitude (about two-fold) seasonal cycles
around an annual mean abundance of approximately 0.7 g C m−2 in daytime, 1.0 g C
m−2 at night (Sheridan & Landry 2004). In addition, they showed a convincing
increase of the overall totals between 1994 and 2002 (Fig. 11.32). Their analysis
shows that the change was primarily in the non-migrants that remain near the surface
during the day, since the night minus day difference in biomass was nearly
unchanging. Animals migrating below 160 m are important in carrying carbon and
nutrients down out of the euphotic zone by respiring and excreting at depth in
daytime. Hannides et al. (2009), based on estimates of biomass and oxygen
consumption, calculated the amount of migrant-delivered carbon flux at about 15% of
the estimates from traps at 150 m, the total being about 2.6 mmol C m−2 d−1. The
seasonal cycling of both trap flux and zooplankton abundance shows seasonal cycles
similar to those of primary production (Fig. 11.24), possibly peaking somewhat earlier
in the spring–summer season.
Fig. 11.32 Mesozooplankton dry-weight biomass measured monthly during the HOT
program at Station ALOHA from 1994 to 2002. Mean biomass is shown for plankton
collected during the (a) night, (b) day, and (c) the biomass of migrating zooplankton
(night biomass – day biomass). Evidently, it was primarily the non-migrant biomass
that increased.
(^) (After Sheridan & Landry 2004.)