Abstract
We report in situ ingestion rates of individual oikopleurid appendicularians from the Northeast water Polynya (NEW) determined by a modification of the gut pigment technique. Appendicularians were most abundant at ice edge stations and were rare at open water stations in the centre of the polynya. Gut passage time (GPT), which was determined in experiments onboard ship, was not related to body size (mean ± SD = 74 ± 23 min). Gut chlorophyll content (GCC) excluding phaeopigments (i.e. GCC = chlorophyll a + b + c), averaged 4.06 ng ind.-1, with quartiles of 1.20 to 4.73 ng ind.-1. There was no evidence of a diel feeding rhythm. GCC was higher and more variable at non-bloom (i.e. at chlorophyll concentrations 5 μm in size. This suggests inhibition of ingestion rates at ice edge stations with diatom blooms. The ingestion rate (IR, ng chlorophyll ind.-1 d-1) of individual appendicularians in the NEW can be predicted by IR = 2.5 BL:BT-0.41 TL2.14 (r2 = 0.43) where BL:BT is the ratio of chlorophyll a in particles >5 μm in diameter to the total chlorophyll biomass, and TL (mm) is the trunk length.
Highlights
Polynyas are mesoscale ice-free areas in the midst of ice-covered seas that serve as important foraging areas for large animals, including birds, seals, walruses, whales and man (Stirling 1980).Polynyas are created and maintained by physical factors such as winds, tides, upwelling or regional currents (Smith et al 1990, Schneider & Budeus 1995).Dunng spnng and summer 1993, a n international, multidisciplinary expedition to the Northeast Water Polynya (NEW)tested the hypothesis that opening of the polynya leads to the formation of a short food chain based upon diatom production while the under-ice food w e b is a complex microbial loop based upon very small producers
Since Gut Passage time (GPT) in Oikopleura vanhoeffeniis essentialiy constant over a wide range of food concentrations (Bochdansky et al 1998) and the slope of log (TL) (Fig. 7a), ingestion rate can be directly calculated from Gut chlorophyll content (GCC) by, Acuna et al : In situ Ingestion rates of Olkopleura where IR = ingestion rate, GCC = gut chlorophyll content,GPT = gut passage time (d)and k = a correction factor to account for that portion of ingested chlorophyll which is converted either to phaeopigments or to non-fluorescent products, both of which are lost to our analytical technique (See 'Methods'). kcould not be determined during the present study
The gut pigment technique is especially suited for the study of arctic oikopleurid appendicularians, because of the absence of die1 feeding rhythn~s(Fig. 3; See Redden 1994),the wide size range of ingested particles (Urban et al 1992) and the lack of particle selection by the mucous filters within this range (Deibel & Lee 1992, Bochdansky et al 1998).Oikopleura vanhoeffeni is a relatively large appendicularian, allowing for the pigment analysis of individual guts
Summary
1954, Grainger 1965, Buchanan & Browne 1981),and because appendicularian populations frequently have a particle removal capacity equivalent to that of all of the copepod species in the cornmunity combined (Knoechel & Steel-Flynn 1989, Buck & Newton 1995), we were interested in quantifying the individual and population ingestion rates of these animals in the NEW. Cetta 1984, Madin & Purcell 1992, Madin & Kremer 1995) and copepods (Head 1986).To date, only 2 studies have considered in situ feeding by appendicularians determined using gut fluorescence information (Jansa 1977, Landry et al 1994a,b). Neither of these studies included the determination of gut Passage time, so the derivation of ingestion rates was not possible. We aimed at describing the vanability of ingestion rates of appendiculanans in the NEW over large space and time scales (10s to several hundred km over 6 wk), and at determining the most likely physical and biological factors accounting for this variability
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