Zooplankton response to a phytoplankton bloom near South Georgia, Antarctica

Abstract

A shelf site near the island of South Georgia was sampled during a spring bloom in January 1994. Chlorophyll a (chl a ) values declined from 19 to 6 mg mv3 during the 8 d of sampling. The bloom comprised mainly long pennate diatoms and large colonies of centric diatoms; a 200 pm sieve retained over two-thirds of the chl a. Mesozooplankton biomass was high, 12.3 g dry mass me2 within the top 200 m, and comprised mainly copepods. A series of Longhurst Hardy Plankton Recorder profiles showed that the numerical dominants (Oithona similis, Drepanopus forcipatus and pteropods) resided mainly within the top 20 m, whereas the large, blomass-dominant copepods had secondary maxima rather deeper within the thermocline. D~e l vertical migration was not a feature of this community, being limited to inetridlid and euchaet~id copepods. Gut fluorescence measurements on 7 large copepod species showed that all fed during both day and nlght, although guts tended to be fullest during afternoon and night. About 20% of chl a grazed by these copepods occurred below the mixed layer, thus representing a potentially direct export of carbon from the system via sinking faecal pellets. Algal carbon rations (% body carbon ingested per day) of mixed layer copepods ranged from 3 % (Rhincalanus gigas) to 20% (small copepods). With the exception of R. gigas, these values from gut fluorescence agreed with independent estimates from the site following the d e c l~n e of chl a during incubations in ambient seawater Despite low clearance rates, ingestion rates (per copepodid) were at the upper end of recorded Antarctic values, suggesting food saturation. Calanoides acutus and R gjgas cleared diatoms (including the hlghly elongated 0.5 to 1 mm forms) at maximal rates. Metridla spp., Calanuspropinquus and small copepods, by contrast, cleared dinoflagellates and ciliates faster than diatoms of similar size The total mixed layer zooplankton probably removed < 5 % of daily primary production and < 5 % of protozoan standing stocks per day.

Regional differences in the life cycle of Calanoides acutus (Copepoda:Calanoida) within the Atlantic sector of the Southern Ocean

“…Calanoides acutus is considered to be m a n l y herbivorous (Voronina & Sukhanova 1976, Hopkins & Torres 1989, Atkinson et al 1996 and it is also agreed that individuals at depth do not feed (Andrews 1966, Schnack-Schiel et al 1991, Bathmann et al 1993. Therefore their presence in the upper mixed layer (Figs.…”

Mesozooplankton structure and functioning during the onset of the Kerguelen phytoplankton bloom during the KEOPS2 survey

“…before the full bloom conditions. Moreover, smaller copepod species and copepodids of large copepods may better exploit these low food concentrations (Atkinson et al, 1996), allowing individuals to develop and grow, whereas large copepods are food limited.…”

N&lt;sub&gt;2&lt;/sub&gt; fixation as a dominant new N source in the western tropical South Pacific Ocean (OUTPACE cruise)

“…However, zooplankton might play a significant role, although it is hard to quantify. Zooplankton living below the photic layer migrate to the surface at night, and when going down, can increase the export of dissolved and particulate organic and inorganic N through defecation, excretion or mortality (Atkinson et al, 1996;Le Borgne and Rodier, 1997;Dam et al, 1993Dam et al, , 1995Longhurst et al, 1989Longhurst et al, , 1990Longhurst and Williams, 1992;Zhang and Dam, 1998) if it occurs below a barrier to vertical mixing (i.e. nitracline or pycnocline; Longhurst et al, 1989Longhurst et al, , 1990.…”