Mr. Landry et al., MESOZOOPLANKTON GRAZING IN THE SOUTHERN CALIFORNIA BIGHT .2. GRAZING IMPACT AND PARTICULATE FLUX, Marine ecology. Progress series, 115(1-2), 1994, pp. 73-85
Mesozooplankton grazing on phytoplankton, as inferred from gut pigment
contents and gut evacuation rates, was studied in relation to primary
production and particulate export flux on 6 cruises in the Santa Moni
ca Basin, California, USA. Gut evacuation rates did not vary significa
ntly among different taxa or size classes examined and were consistent
with extrapolations of published temperature relationships. Shipboard
incubations with cultured phytoplankton and net-collected zooplankton
indicated a seasonal difference in the extent to which gut passage co
nverts chlorophyll to non-fluorescent by-products. In autumn experimen
ts, only about 5% of ingested chlorophyll could not be recovered as ph
aeopigment. In winter-spring experiments, approximately 70% of ingeste
d chlorophyll (chl) was destroyed. In contrast, other indices of pigme
nt destruction, the ingestion rates of a dominant copepod species and
the ratio of water-column phaeopigment:silica fluxes, did not reveal a
significant gut passage effect during winter-spring cruises. Mesozoop
lankton community grazing impact varied from 1.7 to 7.3 mg chi m(-2) d
(-1), with higher grazing during the winter-spring period (mean = 5.8
mg chi m(-2) d(-1)) as compared to the autumn (mean = 2.3 mg chi m(-2)
d(-1)). On average, mesozooplankton grazing accounted for a loss of 1
1.7% of chlorophyll standing stock d(-1) with a 6 cruise range of 6 to
18% d(-1). Mesozooplankton grazing on phytoplankton accounted for 29
to 44% (mean = 39%) of measured primary production for the winter-spri
ng cruises, but only 16 to 24% (mean = 19%) of production in the autum
n. From measured phaeopigment fluxes into sediment traps below the eup
hotic zone, only 27.5% (range 23 to 32%) of this grazing on phytoplank
ton could be accounted for as export flux. Thus, in terms of contribut
ion to particulate flux or remineralization, most mesozooplankton graz
ing in the Santa Monica Basin was functionally equivalent to that of m
icrozooplankton. Direct grazing on phytoplankton contributed 15 to 38%
of carbon flux into sediment traps during winter-spring and 8 to 13%
during autumn. Nonetheless, if feeding on nonpigmented prey is conside
red from the available information on carbon:phaeopigment ratios of fr
esh fecal pellets, over 70% of the carbon flux to traps could have a m
esozooplankton grazing origin.