DEMOGRAPHY OF SHALLOW EELGRASS (ZOSTERA-MARINA) POPULATIONS - SHOOT DYNAMICS AND BIOMASS DEVELOPMENT

1 Although Zostera marina (eelgrass) is a widespread and well examined seagrass we provide here a first study of shoot demography and growth , conducted by following cohorts of eelgrass shoots during an annual c ycle in six perennial populations in Limfjorden, Denmark. 2 The format ion of leaf shoots on side-branches occurred throughout the year but p eaked in May-June and declined at reduced light availability. Maximum mortality of over-wintering leaf shoots coincided with the flowering s eason, whereas new leaf shoots had the highest mortality in late summe r and most were lost at an early age (mean half-life of 50 days) presu mably due to shading within the dense stands. The cumulative productio n of new leaf shoots was between 525 and 1296 m(-2) year(-1) and total shoot mortality ranged from 541 to 1483 m(-2) year(-1), so that net s hoot density changed only little (16-491 shoots m(-2) year(-1)). Becau se no seedlings were established within the vegetation, population mai ntenance was achieved by rhizome branching. 3 Average leaf production per shoot was high (17 leaves shoot(-1) year(-1)) compared to formatio n of new shoots (0.97 shoots shoot-1 year-1). Biomass losses due to th e continuous leaf turnover constituted 74% of total biomass losses, an d growth of the over-wintering shoot population was the major contribu tor to seasonal biomass increases. Thus, during an annual cycle with l ow frequency of disturbance, biomass in these stable eelgrass populati ons was predominantly allocated to increased shoot size, despite the h igh shoot recruitment. 4 Eelgrass is not easily assigned to a particul ar growth strategy but resembles clonal terrestrial plants with phalan x growth by forming dense meadows of restricted horizontal spread and maintaining control of shoot recruitment. Cost effective reconstructio n techniques, not applicable to eelgrass with short lived rhizomes, ho ld great promise far future comparative studies of demography and grow th strategy of long-lived seagrasses and terrestrial rhizomatous plant s.