Biomass-density patterns in the temperate seagrass Zostera marina

Abstract

Extensive studies of biomass-density patterns have led to formulation of general allometric theories for terrestrial plant populations. Similar universal patterns have not been studied in the rhizomatous, clonal marine seagrasses despite their worldwide distribution in monospecific stands and their suitability in comparative studies. We analyzed biomass-density relationships for 29 eelgrass populations distributed between 30 and 56 N in Europe, USA and Japan. The maximum leaf biomass was independent of shoot density among populations and conformed to the law of 'constant final yield per unit area' The maxinlun~ total plant biomass of eelgrass, Including the rhizomes and roots in the sea bottom, increased with shoot density. The leaf biomass shoot density combinations wlthin Mferent eelgrass stands approximated a cyclic seasonal pattern similar to that of terrestrial clonal plants with continuous shoot formation. Most eelgrass populations predominantly allocated biomass to increased shoot size and maintained stable shoot density. However, severe disturbance that reduced leaf b~omass and opened the canopy prior to spring growth enhanced the growth and survival of new small shoots. Self-th~nn~ng, expressed as a net decline of shoot density at maximum summer biomass, was a relatively unpronounced phenomenon within natural eelgrass stands because the period of high biomass was short before optimal growth conditions vanished. Despite the constant shoot density, however, there was continuous shoot turnover in eelgrass stands. The natural eelgrass stands did not conform to the description of self-thinning or the -3/2 power law observed for even-aged terrestrial populations undergoing density-dependent mortality. Biomass-density patterns are, nevertheless, informative descriptors of demography and disturbance of seagrass species.