Vol. 67: 97-103, 1990 MARINE ECOLOGY PROGRESS SERIES Mar. Ecol. Prog. Ser. Published September 20 Seagrass colonization: biomass development and shoot demography in Cymodocea nodosa patches Carlos M. Duartel, Kaj Sand-~ensen~ ' Centro de Estudios Avanzados, Camino de Santa Barbara, E-17300 Blanes, Gerona, Spain Freshwater Biological Laboratory, University of Copenhagen, Helsingersgade 51, DK-3400 Hillered, Denmark ABSTRACT: Patches of Cymodocea nodosa colonizing highly mobile sandy sediments were studied to determine the temporal and spatial course of biomass and demographic development. Results show (1) a close coupling between rhizome growth and shoot density and biomass; (2) an average b~on~ass accumulation rate of ca 250 g fresh wt m-2 yr-l for rhizomes and 480 g m-2 yr-' for shoots; and (3) a close relationship between plant biomass and distance from patch edge Short shoots are produced in annual cohorts, subject to an average mortality of ca 1.0 exponential unit yr-' regardless of cohort age, but shoot mortality decreases with distance from patch edge. Patch maintenance is achieved by shoot recruitment rates closely matching shoot mortality rates. Mean shoot age, variability in shoot age structure, and associated shoot characteristics (i.e. length and number of standing leaves), increase with time after patch establishment. These results demonstrate the importance of shoot recruitment and mortality for the maintenance and growth of C. nodosa patches and, hence, for seagrass recovery. INTRODUCTION Seagrass recovery from physical disturbance and catastrophic decline involves the colonization by prop- agules and the overgrowth of bare substratum through rhizome expansion (Duarte & Sand-Jensen 1990). Rhizome growth is supported by reallocation of photo- synthetic products, and must, therefore, be closely coupled to the development of leaf-bearing shoots. Furthermore, biotic communities associated with sea- grasses often use their leaves as substratum or refuge. and their recovery relies, therefore, on the develop- ment of above-ground material. Thus, knowledge of the coupling between rhizome growth and shoot development is important for forecasting the recovery of seagrasses and their associated biota. As seagrass colonization proceeds, the age structure of growing seagrass patches becomes increasingly complex, with a gradient from young colonizing shoots in the periphery to older shoots at the patch origin. Maintenance of the seagrass community over the col- onized substratum requires that shoot senescence and subsequent mortality be balanced by shoot recruit- ment. This should lead to an increasingly skewed shoot age distribution with time since the plant became established. Because new shoots are often smaller and, therefore, less productive than mature shoots, shoot O Inter-Research/Printed in F. R. Germany mortality and recruitment should generate consider- able small-scale variability in shoot demography and growth within expanding patches. Here we examine the relationship between rhizome and shoot development in patches of the seagrass Cymodocea nodosa colonizing highly dynamic sandy sediments in a shallow NW Mediterranean bay (Al- faques Bay, Ebro Delta, NE Spain; cf. Duarte & Sand- Jensen 1990). Patterns of rhizome and shoot biomass development during colonization are described by examining the biomass build-up from the patch edge (newly colonized substratum) to the patch origin. We then describe short-shoot demography by examining age structure along the patches, and in relation to the time elapsed since the stand was established. Because age structure is the outcome of shoot recruitment and mortality, we quantify these rates and search for pat- terns to explain the variability in shoot recruitment and mortality within the patches. Finally, we assess the age-dependence of shoot characteristics (i.e. number of leaves per shoot and leaf length). METHODS Seagrass biomass, short shoot characteristics, and short shoot age structure were studied in 10