Changes in reproductive effort, lamina-area index, and standing crop with depth in the deep-water alga Phyllariopsis purpurascens (Laminariales, Phaeophyta)
1997
Flores-Moya, Antonio
Density and aggregation, reproductive effort, lamina-area index, and standing crop of the deep-water alga Phyllariopsis purpurascens (C. Agardh) Henry et South (Laminariales, Phaeophyta) were examined as a function of depth (20, 25, 30, 35, 40, and 48 m) for two consecutive years (1994 and 1995) at a locality in the Strait of Gibraltar. No significant temporal differences of the variables examined were found for a given depth, but a clear influence of depth was observed. Peak average density (4.2 ± 0.9 plants m⁻²) occurred at 30–35 m. The variance: mean ratio of plant density was significantly < 1 at all depths, indicating a regular distribution of sporophytes on the sea bottom. Photon flux density beneath a single blade layer of P. purpurascens growing at 30 m depth was 3 ± 2 μmol m⁻² s⁻¹, lower than the photosynthetic light compensation point (6.5 μmol m⁻² s⁻¹). Thus, light competition among sporophytes may be responsible for the uniform plant distribution. Reproductive effort dropped from 27 ± 6% at 20–35 m depth to 13 ± 4% at depths ≥40 m. Lamina-area index and standing crop showed similar patterns, with peaks at 30–35 m depth (0.7 ± 0.2 m² m⁻² and 6.2 ± 1.4 g dry weight m⁻², respectively) and lower values at other depths. The lack of an appropriate substratum and interspecific competition with other large kelps could be responsible for the low density of P. purpurascens at 20–25 m depth, and the reduction of irradiance to the photosynthetic compensation level at ≈35 m may account for the decline in vegetative growth and reproduction at depths >35 m. Similar values of water temperature, plant density, reproductive effort and standing crop of P. purpurascens at 30 m depth in this and a previous study at this site indicate a stable habitat, which may account for the regular seasonal growth pattern of this alga.
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