Distribution, seasonal abundance and shoot size of the seagrass Halodule wrightii near its southern limit at Rio de Janeiro state, Brazil

A search for Halodule wrightii. Aschers. carried out along the most coast of Rio de Janeiro state revealed 21 populations. Populations were usually small, sparsely distributed and essentially monospecific. To quantify the intensity of seasonal variation within these southerly populations, samples were collected every 45 days at the site with most seagrass—Ilha do Japonês, Cabo Frio. Variation in biomass and density, and size distributions of short shoot lengths were measured. Short shoots made up between 21 and 44% of H. wrightii biomass. Seasonal changes in seagrass parameters (e.g. biomass from 12.1–37.3 g dry weight m -2, short shoot density from 2080–8482 m −2 and short shoot size from 28–106 mm), were strongly pronounced in the population between 1995 and 1997, although inter-annual variation in the measured parameters was also evident. The variation (coefficient of variation) of shoot size distributions also showed seasonal differences but differences in size skewness (skewness coefficient) were not found. No evidence of sexual reproduction was found. A comparison of the same variables in ten populations distributed along the coast of Rio de Janeiro state revealed substantial between site variation in shoot density, biomass distribution and short shoot size distributions. Biomass values for H. wrightii were relatively low. In the populations at Rio de Janeiro, rhizome biomass was positively correlated with root biomass, while shoot biomass was positively correlated with mean shoot length rather than shoot density.