Frond dynamics of the commercial seaweed Gelidium sesquipedale: effects of size and of frond history

Abstract

Ge'dium sesquipedale (Clem.) Born. et Thur. is commercially exploited along the northeast Atlantic for the production of agar. Its frond dynamics were studied by tagging individual fronds in a typlcal, dense, monospecific G. sesquipedale stand located in a commercial bed off Cape Espichel, Portugal. Storms and commercial harvesting play a key role in the regulation of the species population dynamics. Frond mortality was relatively constant throughout the year, but peaked in August-September during harvest season. High probabilities of frond breakage and low frond mortality during late fall and winter (periods coincident with severe storms) suggest that under natural disturbances, G. sesquipedale fronds break rather than detach. Frond elongation rate shows a distinct seasonal pattern: high during spring and summer and low In winter. The relat~onships among G. sesquipedalevital rates and both size and frond history (i.e. effects of frond length on mortality, growth and breakage, and effects of frond length, growth, breakage and presence of epiphytes on the next month's growth, breakage and mortality) were analysed using log-linear methods. Shorter fronds (S 10.1 cm) were less susceptible to breakage and had higher growth probability than longer fronds, while longer fronds showed higher elongation rates, particularly during the periods of faster growth (late spring and summer). During storms (late fall and winter), longer fronds had a greater chance of being detached than smaller ones. By contrast, during spring and early summer, frond mortality was greater for shorter fronds. There were no clear indications that G. sesquipedale recent biological history influenced its fate. Results indicate the potential for yield-overharvesting of the Cape Espichel G. sesquipedale bed, because the harvest season starts before peak summer production. Particularly when the previous winter and fall were stormy, as was the case during this study, could better yields be obtained by delaying the harvest season.

Fecundity, spore recruitment and size in Gelidium sesquipedale (Gelidiales, Rhodophyta)

“…In fact, reproductive potential may have been unusually low because this was a period of population decline, due to unusually strong disturbances (combined effects of harvest and storms, see Santos, 1993bSantos, , 1994Santos, , 1995 . Fronds longer than 15 cm, with more chances of being fertile, had unusually low frequencies throughout this study .…”

A model for the simulation of macroalgal population dynamics and productivity

“…The growth rates referred by Santos (1994) point to an annual maximum of 8-9 cm. However, these results were obtained by considering only the tagged algae that grew between two measurements, so they correspond to a potential maximal growth rate that is hardly ever attained.…”

Clump structure, population structure and non-destructive biomass estimation of the New Zealand carrageenophyte Sarcothalia lanceata (Gigartinaceae, Rhodophyta)

“…This stability may be attributable to S. lanceata blades eroding or tearing rather than completely detaching. A similar phenomenon has been observed in other species (Hansen and Doyle 1976, Klein 1987, Santos 1994, and may act as a dispersal mechanism in some species (Gomez and Westermeier 1991).…”