Comparative-Study on Populations of Zostera-Marina L (Eelgrass) - in-Situ Nitrogen Enrichment and Light Manipulation

The combined effects of light and nitrogen availability on population dynamical characteristics (e.g. growth rate, biomass, density, flowering) of a perennial and semi-annual population of Zostera marina L. were experimentally studied in two non-tidal lagoons (Grevelingen and Veerse Meer). Light reduction (approximate to 70%) significantly reduced all population dynamical features measured both populations. Under reduced light uptake of nitrogen occurred by plants of the semi-annual population, but the nutrient could not be used for growth. After enrichment with nitrogen plants of neither population stored nitrogen; the nutrient was immediately used. Light limitation of growth remained undetermined for the semi- annual population, although mean values for biomass and density characteristics doubled at light enhancement (studied in the Veerse Meer only). Nitrogen enrichment had a significantly positive effect on the perennial population; leading to enhanced above-ground relative growth rate, flowering and above-ground biomass. A lower probability of recruitment through generative reproduction (flowering) of the perennial population caused by nitrogen limitation apparently does not become compensated for by a possible higher vegetative production of shoots. Nitrogen limitation is thought to be a concomitant factor in the sharp decline of Z. marina populations since 1985 in the Grevelingen. [KEYWORDS: Flowering; growth; light manipulation; nitrogen enrichment; population dynamics; zostera marina Nutrient enrichment; depth distribution; sediment ammonium; pore water; growth; seagrass; availability; lagoon; beds; bay