SEASONAL CHANGES IN SIZE STRUCTURE OFSARGASSUMANDTURBINARIAPOPULATIONS (PHAEOPHYCEAE) ON TROPICAL REEF FLATS IN THE SOUTHERN RED SEA¹

Abstract

Seasonal variation in density, thallus length and biomass, population size structure, and allometric length-biomass relationships was investigated in populations of Sargassum ilicifolium (Turner) C. Agardh, Sargassum subrepandum (Forssk.) C. Agardh, and Turbinaria triquetra (J. Agardh) Kütz. (Phaeophyceae) on shallow reef flats in the southern Red Sea. Thallus length and biomass varied strongly with season, with the highest values occurring in the cooler months. Thallus densities showed no significant temporal variation. Log-total biomass versus log-density relationships were positive throughout the growth season without any decrease in the slope of the relationship. In two populations, biomass-density combinations approached the interspecific biomass-density line, but the massive annual shedding of modules occurred before self-thinning would set in. Allometric length-biomass relationships varied with season in all populations and were associated with seasonal module initiation, growth, and shedding. Evidence of a strong asymmetric competition was found in two high-density populations. These populations showed a predominance of small thalli during peak development, asymmetrical Lorenz curves, increasing Gini coefficients, and increasing thallus length relative to biomass during the main growth phase. In two other less crowded populations, small thalli were absent during peak development, Lorenz curves were symmetrical, and Gini coefficients decreased during the main growth phase. In these populations, size equalization appears to be due to responses at the modular level rather than sizedependent mortality. We conclude that changes in size structure in this highly seasonal environment are determined by module dynamics, modified by asymmetric competition in some populations, with a minor role of recruitment and no regulatory effect of self-thinning.Key index words: allometry; asymmetric competition; biomass; density; self-thinning; size hierarchies; size inequalityAbbreviations: ANCOVA, analysis of covariance; ANOVA, analysis of variance; dwt, dry weight; G, unbiased Gini coefficient; IBDR, interspecific biomass-density relationship; IRF, inner reef flat; MRF, middle reef flat; ORF, outer reef flat; PCA, principal component analysis; SSI, Sheikh Said Island study site; TW, Tewalet study site As in terrestrial plants Thomas 1986, Weiner et al. 2001), seasonal changes in population structure of large canopy algae appear to be associated with seasonal patterns of growth and strong changes in density due to seasonal recruitment and density dependent mortality (Ang and De

Seasonal variation ofSargassum ilicifolium(Phaeophyceae) growth on equatorial coral reefs

“…Similar density patterns have been observed for S. ilicofolium in the southern Red Sea (Ateweberhan et al. ). Survival of Sargassum holdfasts is considered vital in maintaining population densities by securing suitable substratum for potential regrowth (Fletcher and Fletcher , De Wreede ).…”

“…), or even the same location (e.g., Eritrea: Ateweberhan et al. , ) have been reported. Further work examining recruitment and mortality rates of S. ilicifolium is required to better understand the population dynamics of S. ilicifolium in Singapore.…”

“…Seasonal variation in Sargassum populations therefore typically takes place at the level of lateral branches instead of whole individuals due to persistence of established holdfasts (Ateweberhan et al. ). However, Sargasssum thallus densities can also vary by several orders of magnitude over a year due to strong seasonal fluctuations in recruitment and adult mortality (Kendrick and Walker , Vuki and Price , Ateweberhan et al.…”

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“…The temporal variation in density observed here for the three species is consistent with the available descriptions of seasonality in biological characteristics in other temperate regions (McCourt, 1984;Plouguerné et al, 2006;Thomsen et al, 2006). The density of Sargassaceae populations is known to be affected by seasonal changes in water movements and quality (Stiger and Payri, 1999b;Ateweberhan et al, 2006Ateweberhan et al, , 2009Plouguerné et al, 2006). High recruitment events in autumn and the fast growth of plantlets from spring to summer explain the significant variation in S. muticum density over short periods (see below and Thomsen et al, 2006).…”

Simulations of Long-Term Community Dynamics in Coral Reefs - How Perturbations Shape Trajectories

“…Macroalgae which are mainly parametrized with data from Sargassum ilicifolium [57], [58] grow 30 cm per month, and can reach a maximum height of 60 cm and a maximum diameter of 45 cm. To account for the process of fragmenting we assumed that macroalgae fragment from a threshold height of 30 cm (a size at which surge can impact the alga) and for each produced fragment, their height is reduced by 5 cm.…”