Vol. 109: 283-291, 1994 MARINE ECOLOGY PROGRESS SERIES Mar. Ecol. Prog. Ser. Published June 23 Biomass-density patterns in the temperate seagrass Zostera marina Birgit Olesen, Kaj Sand-Jensen* Department of Plant Ecology, University of Aarhus, Nordlandsvej 68, DK-8240 Risskov, Denmark ABSTRACT: Extensive studies of biomass-density patterns have led to formulation of general allo- metric 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 Mfer- ent 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, how- ever, there was continuous shoot turnover in eelgrass stands. The natural eelgrass stands did not con- form to the description of self-thinning or the -3/2 power law observed for even-aged terrestrial popula- tions undergoing density-dependent mortality. Biomass-density patterns are, nevertheless, informative descriptors of demography and disturbance of seagrass species. KEY WORDS: Zostera manna . Population structure . Maximum biomass . Thinning rule INTRODUCTION Plant biomass per unit area is the product of shoot density and mean shoot weight and, therefore, changes with the numerical size of these parameters. The interdependence of shoot weight and shoot den- sity has been examined experimentally in many even- aged terrestrial plant populations. The common find- ing has been that the populations start to self-thin (i.e. shoot density drops) as the plants grow in size and bio- mass. Logarithmic plots of mean weight versus shoot density for self-thinning populations show a -3/2 slope (Yoda et al. 1963, White & Harper 1970). Obviously, 'Present address: Freshwater Biological Laboratory, Univer- sity of Copenhagen. Helsingsrsgade 51, DK-3400 Hillered. Denmark plant biomass cannot increase indefinitely and the slope will eventually approach -1 (White & Harper 1970) such that biomass per unit area remains constant and independent of shoot density. Determinations of the thinning slope through linear regression analysis have been associated with method- ological and statistical difficulties (Weller 1987, La- Barbera 1989). One main objection is the autocorrela- tion created when mean shoot weight, calculated as biomass divided by density, is plotted as a function of density. To prevent this problem, biomass rather than mean shoot weight should be regressed to shoot density yielding a slope of -% for populations conform- ing to the -72 rule. Reanalysis of data used in support of the thinning law has revealed considerable variability among plant stands and the existence of an ideal thin- ning slope has been questioned (Zeide 1985, Weller O Inter-Research 1994 Resale o f full article not permitted