The use of gut fluorescence to estimate grazing by oceanic salps

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Abstract
The fluorometric measurement of chlorophyll and its degradation products in the guts of salps was used as a means of calculating in situ grazing rates. Salps are particularly suitable for this method because they are non-selective filter-feeders large enough to be individually collected by divers. Analysis of the pigment in the gut showed that chlorophyll degradation products can be used as tracers to estimate in situ ingestion. Filtering rates are calculated from the amount of pigment, the gut clearance time and the concentration of pigment in the water. For Pegea confoederata and Salpa maxima, gut-pigment rates are compared with rates measured in laboratory particle-clearance experiments. S. cylindrica proved impossible to maintain in the laboratory and the gut-pigment results are the 1st feeding data for this species. Filtering rates for all species examined are high, on the order of liters per hour. Variability in the amount of pigment found in S. cylindrica suggests this species, unlike the others, may feed sporadically. The gut-pigment method permits measurements of filtration and ingestion by herbivores on an almost instantaneous basis and can be a useful tool for assessing grazing impact in field situations.

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