Proximate Composition and Standing Crop of Durvillaea-Antarctica (Pheophyta) in the Bay of Morbihan, Kerguelen (South Indian-Ocean)

The proximate composition (% dry weight) of Durvillaea antartica (Cham.) Hariot was similar for the frond and stipe, being 28 % ash, 3 % protein, ca 1 O/ O lipid, less than 10 % acid-soluble carbohydrate, and 60 % acid-insoluble carbohydrate. The holdfast differed by being 22 % ash and 66 % acid-insoluble carbohydrate The energetic level of all 3 components was ca 13 kJ (g dry wt).' and ca 17 kJ (g ash-free dry wt)~.'. The relative proportion of the 3 plant components varied little for plants having stipe diameters ranging from 10 to 40 mm, being ca 8, 3, and 89 % wet wt and ca 10, 4 , and 86 % kJ for the holdfast, stipe, and frond, respectively. Maximal density found in summer was 471 individual plants m-', 226 kg wet wt m-', and 457 000 kJ m-'. Juvenile plants dominated in terms of individuals, but adults were more important in gravimetric and energetic units. D. antarctjca allocates most of its biomass and energy to the frond throughout its size range. The characteristics of D. antarctica in the Bay of Morbihan are those for a sheltered shore. INTRODUCTION The purpose of this paper is to report the proximate con~position, the allocation of energy to the various The genus DurviUaea is conspicuous in the austral plant components, and the gravimetric and energetic regions because of its formation of a belt (or ceinture) density (standing crop) of Durvillaea antarctica from on high-energy rocky shores where its long, floating the Bay of Morbihan, Kerguelen. fronds can reach a length of more than 10 m (Fritsch 1945). Because it is the dominant primary producer and repository of organic material and energy in this MATERIALS AND METHODS narrow fringe, the biomass of D u r d l a e a has been measured in New Zealand (Hay & South 1979), Chile Individual plants (those with separate holdfasts) (Santelices e t al. 1980), and Marion Island (Haxen & were collected in December 1984 along the Promenade Grindley 1985). Durvillaea antarctica occurs at des Amerlocks on the north shore of the Bay of MorbiKerguelen (Delepine 1963, Arnaud 1974). han, Kerguelen (49 "S, 70 "E). The diameter of the stipes It is important to know the standing crop in terms of was measured for regression against the wet weights of energetic a s well a s gravimetric units. Because of the holdfast, stipe, and frond of the plant. differences in inorganic (water and ash) and organic The fronds, stipes, and holdfasts of the plants with composition, gravimetric values for different species stipe diameters ranging from 7 to 40 mm were sepacan vary from energetic values. In addition, energetic rated, weighed, lyophilized, and reweighed. The dried values are more useful in trophic analyses. Proximate plant material was homogenized in a Wiley mill and analysis allows one to know the relative contribution of the proximate composition measured. Ash was meavarious organic classes to the energy content of a n sured gravimetrically by ashing in a muffle-furnace at alga, e.g. the relative amounts deposited in structural 500°C for 4 h (Paine 1971); NaOH-soluble protein was and non-structural carbohydrates. This gives insight measured colorimetrically by the biuret method of into the ways in which energy and material produced GornaU et al. (1949); total lipid was measured are used and the form in which they are available to gravimetrically by the hot chloroform-methanol the next trophic level. method of Freeman et al. (1957); trichloroacetlc-acid O Inter-Research/Printed in F. R. Germany Mar Ecol. P r o g . Ser. 33: 1-5, 1986 soluble carbohydrate was measured colonmetrically by the method of Dubois et al. (1953); insoluble carbohydrate was calculated by subtraction. The number of kJ (g dry tissue)-' of the different plant components was calculated by multiplying the level of each organic class by its caloric equivalent (converted to kJ) given by Brody (1945). The total energy content was calculated by multiplying the kJ (g dry wt)-' by the g dry weight. The standing crop of Durvdaea antarctica was measured for 4 quadrats along the Promenade des Amerlocks. This was done non-destructively by ascertaining the size (the diameter of the mid-section of the stipe) frequency distribution of the plants in the quadrats. The quadrats encompassed the vertical range of the DurvlUea belt and a horizontal range to include at least 100 plants. The biomass and energetic equivalents of the stipes and fronds were estimated from the regression of the components on stipe diameter. The holdfasts were not included because they were frequently contiguous for different plants. diameter and have correlation coefficients of 0.73, 0.87, and 0.65, respectively (Fig. 1). The proximate composition for each plant component of D u d a e a antarctica showed little variation (Table 1) indicating that the composition did not vary greatly over the range of plant sizes analyzed. Levels of TCA-soluble carbohydrate, protein, and lipid were similar for the 3 plant components, but the level of ash was lower and the levels of dry weight and TCAinsoluble carbohydrate were higher in the holdfast than in the stipe and frond. Because of the lower level of ash, the energetic level in terms of g dry wt is highest in the holdfast. The differences in organic proximate composition were not sufficient to result in much difference among the plant components in energetic levels in terms of g ash-free dry wt. In both instances, TCA-insoluble carbohydrate contributed 81 to 82 % of the energy content. The relative proportion of the 3 plant components (in gravimetric and energetic units, calculated from the relations between stipe diameter and component wet weight and proximate composition) varied little with change in plant size (Table 2). The frond was always the largest component, representing 89 to 90 % of the total in terms of wet weight and 84 to 89 '10 of the total in terms of energy. The size frequency distribution of Durvillaea antarcRESULTS The wet weights of the holdfast, stipe, and fronds of Durvillaea antarctica are directly related to the stipe 0 l 10 I 0 0 STlPE DIAMETER (mm) STlPE DIAMETER ( m m ) STlPE DIAMETER ( m m ) Fig. 1. Durvrllaea antarctica. Relation between diameter of the stipe (mm) and the wet weight (g) of (a) holdfast, (b) stipe, and (c) frond Lawrence: Durvillaea antarctica at Kerguelen 3 Table 1 Durviflaea antarctica. Proximate composition and energetic levels of holdfast (H) , stipe (S), and frond (F) of samples from Kerguelen in December 1984. Values for dry weight are in terms of wet weight; other proximate values are in terms of percent dry we~ght . Values for proximate composition are i k SD, except lipid values which are for pooled samples and insoluble carbohydrate which was calculated by subtraction. Values in parentheses are percent of total. N = 10 Constituent Component