Conolly & Drew, 1985a; Sjotun, 1993). *Present address: Department of Fisheries and Marine Biology, University of Bergen, Bergen High Technology Centre, ...
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European Journal of Phycology
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Seasonal growth pattern of an Icelandic Laminaria population (section Simplices, Laminariaceae, Phaeophyta) containing solid- and hollow-stiped plants Kjersti Sjøtunab; Karl Gunnarssona a Marine Research Institute, Reykjavík, Iceland b Department of Fisheries and Marine Biology, University of Bergen, Bergen High Technology Centre, Bergen, Norway First published on: 01 November 1995
To cite this Article Sjøtun, Kjersti and Gunnarsson, Karl(1995) 'Seasonal growth pattern of an Icelandic Laminaria
population (section Simplices, Laminariaceae, Phaeophyta) containing solid- and hollow-stiped plants', European Journal of Phycology, 30: 4, 281 — 287, First published on: 01 November 1995 (iFirst) To link to this Article: DOI: 10.1080/09670269500651061 URL: http://dx.doi.org/10.1080/09670269500651061
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Eur. J. Phyco]. (1995), 30: 281-287.
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Seasonal growth pattern of an Icelandic Laminaria population (section Simplices, Laminariaceae, Phaeophyta) containing solidand hollow-stiped plants
KJERSTI SJOTUN* A N D KARL G U N N A R S S O N Marine Research Institute, PO Box 1390, 121 Reykjavt'k, Iceland
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(Received 31 August 1994; accepted 1 March 1995) Lamina elongation and content of mannitol, Iaminaran and nitrate were measured during one year in Laminaria saccharina sensu &to from Iceland. The population contained both solid- and hollow-stiped plants. Growth rate was at its minimum from October to December, and started to increase in mid-winter, slightly earlier at 3 m than at 5 m. The increase in growth rate coincided with a strong reduction in stored carbohydrates and an increase in nitrate content of the laminae, indicating that stored mannitol and laminaran provided extra energy for increased lamina growth and/or for nitrate uptake. The results showed that stored mannitol was utilised before laminaran. The growth rate was at its maximum from April to June, and was reduced from June to July. The ambient nitrate concentration at the locality was low from May to August. The nitrate content of the lamina tissue in relation to dry weight was high during spring but was reduced to low values by July, indicating that nitrate levels limited growth during summer. However, high nitrate concentration of the sea-water and high levels of storage carbohydrates in the plants during autumn indicate that the low growth rate at this time cannot be attributed to lack of nitrate or energy in the form of stored carbon. The Laminaria population in Iceland that was examined showed morphological similarity with L. longicruris populations in Canada (hollow stipe), while the growth pattern corresponds with European L. saccharina populations.
Key words: growth, kelp, Laminaria saccharina, nitrate Iimitation, Phaeophyta, storage carbohydrates.
Introduction
Studies of Laminaria ]ongicruris de la Pylaie from Canada have shown that its annual cycle of growth and storage of carbohydrates and nitrate in the lamina is dependent on the seasonal pattern of nitrate content in the sea-water. At localities with low concentrations of nitrate in the seawater during summer and autumn the plants show decreasing and minimum growth during ~his period, and increasing and high growth from mid-water to early summer (Chapman & Craigie, 1977; Gagn6 eta]., i982). In nitrate-rich localities maximum growth during summer and minimum growth during winter is found (Anderson et a]., 198I; Gagn6 et al., 1982). At a locality with intermediate nitrate availability Gagn6 et a]. (1982) found a third growth pattern, with maximum growth during early summer and a continued high growth rate during the autumn. Different seasonal patterns of growth have not, as yet, been found in European L. saccharina (L.) Lamouroux, which consistently exhibits minimum growth during late summer and autumn irrespective of the nitrate availability during this time (Parke, 1948; L/ining, 1979; Conolly & Drew, 1985a; Sjotun, 1993). *Present address: Department of Fisheries and Marine Biology, University of Bergen, Bergen High Technology Centre, 5020 Bergen, Norway.
According to Wilce (1965) the Simplices section of Laminaria in the North Atlantic consists of L. ]ongicruris, L. saccharina and L. so]idungu[a J. Agardh. A fourth species, L. faeroensis Borgesen, was originally described from the Faeroes (Borgesen, 1903), and later recorded from Iceland (J6nsson, 1903) and the Shetland Isles (Kain, 1976). It has been considered to be an ecotype of L. ]ongicruris (Wilce, 1965; Kain, 1976; Egan & Yarish, 1988) although the two entities have not been compared genetically. During the last two decades the taxonomic status of L. saccharina and L. ]ongicruris has also been questioned. The two species readily hybridise (Bolton et al., 1983) and analysis of ribosomal DNA sequence variation shows no difference between North American L. saccharina and L. longicruris (Bhattacharya et al., 1991). On the other hand, Egan et aI. (I990) recommended that the two entities should remain separate. This paper focuses on the seasonal pattern of growth of an Icelandic Laminaria population containing both plants with hollow stipes, described as L. faeroensis by J6nsson (1903), and plants with solid stipes. Because of the confused taxonomy in the L. saccharina-longicruris group we will use the name L. saccharina sensu lato for the Icelandic plants examined. The purpose of the study was to determine whether the Laminaria population in Iceland responded to the seasonal pattern of nitrate
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K, Sjotun and K. Gunnarsson availability in the same way as the Canadian L. ]ongicruris. The Icelandic and the Canadian plants with hollow stipes may differ genetically, and this possible ecotypic differentiation could result in a different response to environmental factors which influences the seasonal growth pattern. Ecotypic differentiation resulting in different ecological responses has been described in the Laminaria saccharina]ongicruris group. For instance, genetic differences in nitrogen assimilation were demonstrated in L. Iongicruris (Espinoza & Chapman, 1983). Gagn6 eta]. (1982) suggested that L. [ongicruris may show genetic variation in assimilation of storage carbon. In the present study of the seasonal growth pattern, lamina growth of tagged plants was measured, and seasonal variation of storage carbohydrates and nitrate in laminae was analysed. The observed growth pattern is discussed in relation to environmental factors which may influence growth, and the results are compared with earlier studies of Laminaria ]ongicruris and L. saccharina. Materials and methods
The study site was a sheltered locality in Berufj6rdur, southeast Iceland (Fig. i). J6nsson (I903) found both Laminaria faeroensis and L. saccharina at this locality. He separated the two species from this site by the occurrence of hollow or solid stipes, respectively. We also found that the plants of this population were morphologically similar except for this difference in the stipes. Plants growing at 3 and 5 m below chart datum were tagged with lock-tight plastic strips. Lamina elongation was measured by punching a hole in the lamina 10 cm above the junction between stipe and lamina and measuring the length of lamina added between successive dates
Fig. 1. The study site at Foss~rvikin Berufj6rdur,with its location in southeast Iceland (inset).
282 (Parke, 1948). The increase in lamina length was measured at time intervals of between I and 2 months from October 1985 to August 1986. The presence or absence of sort on the laminae was noted at each visit. The plants were growing attached to small stones and shells on a sandy bottom, and we experienced a considerable loss of tagged plants. Initially 30 plants were tagged, and of these only 1 persisted to the end of the investigation. New plants were therefore tagged regularly. Occasionally, we also recovered lost plants. The number of plants examined at each interval ranged from 17 to 32, except in April 1986 when only 4 plants were found. During the last visit to the locality, the tagged plants (n = 28) were harvested and their stipes examined. During each visit to the locality 10 plants growing in the vicinity of the tagged plants were collected, 5 with solid stipes and 5 with hollow stipes. The plants were transported in a cooling bag to the laboratory, frozen, and transported to Reykjavlk. In Reykjavlk, tissue samples for chemical analyses were taken from each plant, two each from the distal and the proximal part of the lamina. The distal third of the lamina was defined as distal lamina, and the basal third as proximal lamina. Samples of proximal lamina were always taken more than i0 cm above the junction between stipe and lamina in order to avoid meristematic tissue; otherwise, the tissue samples were taken haphazardly within the proximal and distal lamina. Tissue samples were taken without regard to horizontal position on lamina. Earlier studies have either shown no gradient of chemical content across the lamina (Black, 1954), or that the seasonal pattern of variation is more pronounced than a gradient across the lamina (Chapman & Craigie, 1978). Extraction of inorganic nitrate in tissue samples was carried out as described by Chapman & Craigie (1977). Samples of lamina were extracted with ethanol and ether and the combined extracts evaporated to dryness. After redissolving the salts with distilled water the amount of NO 3 was determined using a Technicon Autoanalyser (Rosenberg & Ramus, 1982). The extracted dry tissue was treated as described by Chapman & Craigie (1977) and laminaran content analysed according to Yemm & Willis (1954) using an anthrone reagent and D-glucose as reference sugar. The content of mannitol was analysed in dried samples according to a titration method described by Larsen (1978). The method is based upon the rapid oxidation of sugar alcohols by periodic acid. The plants collected in June and August 1986 were not analysed. Because the plants were frozen before they were analysed, a test was carried out to see if this would affect the measured amount of nitrate in the plants. Ten pairs of adjacent samples were taken from a plant, one of each pair being from the flesh plant and the other after freezing. Nitrate was thereafter extracted from the samples and analysed as described above. At each site visit, the surface temperature was measured and water samples from 3 and 5 m depth were collected and transported in a cooling bag to the
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Growth pattern of solid and hollow-stiped Laminaria from Iceland laboratory where they were frozen. The concentration of nitrate in the sea-water was measured later using a Technicon Autoanalyser. The salinity of the samples was also measured. Plants with hollow and solid stipes were compared with respect to nitrate, mannitol and laminaran content of laminae during each month. The number of plants in each group was too low (n = 5) to allow ANOVA or nonparametric analysis, and in order to compare the two groups the observations were ranked from the smallest to the largest values. The ranking sums (used in ranking tests: Sokal & Rohlf, 1981) were thereafter compared for each month. Cumulative growth of hollow and solidstiped plants during the summer was compared in a t-test. On the basis of the results from the ranking sums, the results for hollow and solid-stiped plants were combined in the subsequent statistical analysis. The precision of the measurements of nitrate content was found to be uncertain (see below), and these results were therefore not analysed. Tests of homoscedasticity (Fmax-test) showed that the variances of laminaran content in distal and proximal parts of the laminae and growth at 3 and 5 m depth were heterogeneous, and these results were not analysed further. A one-way ANOVA analysis and an unplanned comparison among means (Tukey-test) were carried out on the results of mannitol content in distal and proximal parts of laminae. Percentage values were arcsinetransformed before the tests were carried out. Growth rates and frequency of plants with sori were compared in a correlation test. Results
Elongation rate and fertility The growth in length of lamina was at its lowest during the autumn. From October to December a growth rate of about 0"1 cm day -~ was found (Fig. 2). Growth started to increase in mid-winter, slightly earlier at 3 m than at 5 m depth (Fig. 2). A tendency for higher growth at 3 m than at 5 m depth was observed until May. Lamina growth reached a peak of about 0-8 cm day -~ in April and May.
283 During summer the growth rate decreased, with the largest reduction taking place from May to June (Fig. 2). Plants with sori were found during all seasons except spring (Fig. 2). The highest frequency of reproductive plants was found from November to February, when 5 0 75% of the plants examined (3 and 5 m depth combined) had sori. A correlation test showed a significant negative relationship between lamina elongation and the frequency of plants with sori (Pearson's product-moment, r ~ - - 0 . 8 6 , p < 0.05). The inspection of the tagged plants at the end of the investigation in August showed that 5 of 28 plants examined had hollow stipes. Most of the plants with hollow stipes in August had been tagged in April. Since each plant was marked by a number which was registered each time growth was measured, the growth rates of plants in each group could be calculated from April to July. Both groups showed maximum growth in May and a marked decline in growth rate by June (Fig. 3). No significant difference in cumulative growth of hollow and solid-stiped plants was found in July (t-test, p < 0"05). Comparisons of the ranking sums of mannitol, laminaran and nitrate for each month also showed that the differences in ranking sums between hollow and solidstiped plants were generally small, and no consistent differences were found. It is therefore concluded that the pooling of hollow and solid-stiped plants is justified.
Nitrate content of plants and sea-water From October to December, nitrate did not exceed 2 0 # m o l g -I dry weight in the plants (Fig. 4). A slight rise in nitrate content was seen in both the proximal and distal parts of the lamina from December to January. In February and April, the nitrate content of laminae peaked, reaching values of between 100 and 190 #tool g-1 dry
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