ABSTRACT. The feeding dynamics and predation impact of the hyperiid amphipod Themisto qaudichaudi on mesozooplankton were studied in the vicinity of South Georgia, Southern Ocean, during austral summer 1994. Data show that 7, gaudichaudi is a visual opportunistic predator, consuming primarily the most abundant species of copepods, euphausilds and pteropods. In situ estimated daily rations were equivalent to 6.3 % of body dry weight, and similar to the value of 7.1 % of body dry weight derived using an energy budget approach which takes into account the total daily energy requirements. In vitro estimates produced daily rations higher than these, ranglng from 8.5 to 21.8% of body dry weight, increasing with prey density. The predation impact of T. gaudichaudi adults, averaged over a 0 to 200 or 0 to 100 m layer, never exceeded 2.1 of mesozooplankton standing stock per day but accounted for up to 70% of the daily secondary production This suggests that in the vicinity of South Georgia T. qaudichaudi adults are able to control the local mesozooplankton community and may contribute significantly to the downward flux of biogenic carbon.
Predation rates of the 9 most abundant Antarctic meso-(4 copepods) and macrozooplankton (3 euphausiids, 1 hyperiid and 1 salp) species on microplankton (20 to 200 pm) were estimated using in vitro incubations during the fourth cruise of the South African Antarctic Marine Ecosystem Study (SAAMES IV) to the ice-edge region of the Lazarev Sea during austral summer (Dec/Jan) 1994/1995. Chlorophyll a concentrations during the incubations ranged between 0 187 and 1.410 pg 1.' and were dominated by ice-associated chain-formlng microphytoplankton ( > 2 0 pm) of the genera Nitzschia and Chaetoceros. The microplankton assemblages were entirely dominated by protozoans comprised of ciliates and dinoflagellates. Densities of protozoans ranged from 1375 to 2690 cells I-' Based on previously published results, meso-and macrozooplankton species generally consumed >120% of their minimum daily ration, i.e. minimum carbon uptake (MCU), when offered microplankton. Exceptions were Euphausia crystallorophias and Vibilia antarctica for which microplankton carbon contributed 68 and 30% of MCU, respectively. Microplankton carbon contributed between 17 and 24 % of the total carbon requirements for the 4 copepod species examined and between 21 and 73% for the macrozooplankton. The daily rations of juveniles were, however, twice those of the adults, suggesting that the relative importance of microzooplankton to the daily ration of macrozooplankton shifts with life stage. Carnivory by metazoan grazers may, therefore, potentially reduce the high grazing impact of microzooplankton on the local phytoplankton stock KEY WORDS: Antarctica . Carnivory . Zooplankton . Microzooplankton INTRODUCTIONRecent observations have shown that the role of microzooplankton (20 to 200 pm) in aquatic food webs is more important than previously thought. Microzooplankton consume a significant proportion of daily primary production (Paranjape 1990, Froneman & Perissinotto 1996, in press, for review see Pierce & Turner 1992) and are important agents in nutrient regeneration (Probyn 1987, Goeyens et al. 1991. In addition to these roles, microzooplankton are considered to be an important source of carbon for larger zooplankton (Stoecker & Capuzzo 1990). Since microzooplankton consume bactivorous flagellates, they may be regarded as important trophic intermediaries between bacterioplankton and larger meso-and macrozooplankton (Gifford & Dagg 1988. On the basis of these observations, the classical paradigm of pelagic food webs simply composed of diatoms, copepods and fish has been revised (Sherr & Sherr 1984).Microzooplankton are an ubiquitous component of the plankton assemblages in the Southern Ocean (Garrison 1991, Garrison et al. 1993 and are now recognised as major consumers of phytoplankton production (Bjornsen & Kuparinen 1991, Garrison et al. 1993, Burkill et al. 1995, Froneman & Perissinotto 1996. Phytoplankton consumed by microzooplankton contribute less to particulate organic flux due to the O Inter-Research 1996 Resale of full article not permitted
Size-fractioned primary production and zooplankton grazing impact were estimated along a repeat grid dunng seasonal ice melt in the Lazarev Sea aboard the MV 'SA Agulhas' (voyage 77) in austral summer (December/January) 1994-1995. During the survey, the phytoplankton size composition shifted from a community dominated by nano-and picophytoplankton ( c 2 0 pm) during the first grid to one dominated by microphytoplankton (>20 pm) during the second grid. Total areal production during the first grid was generally dominated by nanophytoplankton and ranged between 133 and 356 mg C m-2 d-'. During the second grid survey, total areal production was higher, ranging between 263 and 400 mg C m-2 d ' Protozoan grazing removed between 0.5 and 31 'Yo of the initial phytoplankton stock or between 33 and 94':c1 of the potential phytoplankton product~on per day during the first grid, and between 0.5 and 8 % of the initial phytoplankton stock or between 9 and 25% of the potential phytoplankton production per day during the second grid The grazing impact of meso-and macrozooplankton during the flrst grid corresponded to < O 5% of chlorophyll stock or < 2 3 % of the daily primary production Dunng the second grid, the grazing impact of meso-and macrozooplankton was higher, removlng on average 1.12"/0 of integrated chlorophyll or 28.7% of daily production These results suggest that the partitioning of carbon between the various size classes of zooplankton dunng seasonal ice retreat is largely determined by the size structure of the phytoplankton.
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