BIOMASS-DENSITY PATTERNS IN THE TEMPERATE SEAGRASS ZOSTERA-MARINA

Extensive studies of biomass-density patterns have led to formulation of general allometric theories for terrestrial plant populations. Simi lar universal patterns have not been studied in the rhizomatous, clona l marine seagrasses despite their worldwide distribution in monospecif ic stands and their suitability in comparative studies. We analyzed bi omass-density relationships for 29 eelgrass populations distributed be tween 30-degrees and 56-degrees-N in Europe, USA and Japan. The maximu m leaf biomass was independent of shoot density among populations and conformed to the law of 'constant final yield per unit area'. The maxi mum total plant biomass of eelgrass, including the rhizomes and roots in the sea bottom, increased with shoot density. The leaf biomass - sh oot density combinations within different eelgrass stands approximated a cyclic seasonal pattern similar to that of terrestrial clonal plant s with continuous shoot formation. Most eelgrass populations predomina ntly allocated biomass to increased shoot size and maintained stable s hoot density. However, severe disturbance that reduced leaf biomass an d opened the canopy prior to spring growth enhanced the growth and sur vival of new small shoots. Self-thinning, expressed as a net decline o f shoot density at maximum summer biomass, was a relatively unpronounc ed phenomenon within natural eelgrass stands because the period of hig h biomass was short before optimal growth conditions vanished. Despite the constant shoot density, however, there was continuous shoot turno ver in eelgrass stands. The natural eelgrass stands did not conform to the description of self-thinning or the -3.2 power law observed for e ven-aged terrestrial populations undergoing density-dependent mortalit y. Biomass-density patterns are, nevertheless, informative descriptors of demography and disturbance of seagrass species.