SEASONAL-VARIATION IN THE INTERTIDAL SEAGRASS ZOSTERA-NOLTII HORNEM -COUPLING DEMOGRAPHIC AND PHYSIOLOGICAL PATTERNS

The considerable seasonal variation in biomass (2-130 g ash-free dry w eight (AFDW) m(-2)) and cover of intertidal Zostera noltii Hornem. in the Zandkreek estuary (SW Netherlands) was mainly caused by changes in shoot density (1000-23000 m(-2)) and not in shoot size (shoot weight 1-3.6 mg AFDW, shoot leaf area 0.3-1.5 cm(2)). The spring increase in shoot density was realised through continuous monopodial branching of the rhizome, which commenced when light available during low tide incr eased above 15 E m(-2) day(-1), in mid April. Branching stopped by the end of July, about 6 weeks before the onset of the annual biomass dec line, due to a combination of: (a) self-shading during low tide; (b) h igh respiratory demand by the expanded rhizome network; (c) the season al decline in light availability during late summer. Nutrients were pr obably not limiting since concentrations in shoots remained high: 3.6% N and 0.6% P of dry weight are seasonal means. During the period of m aximal biomass the primary rhizome axes decayed, leaving single shoots on short pieces of rhizome, the former secondary axes. The rapid decl ine in biomass from mid September onwards could be attributed to grazi ng by herbivorous migratory waterfowl. It was estimated that brent gee se, Branta bernicla (L.), and wigeon, Anas penelope L. (about 200 and 300 birds, respectively, on the bed of 30 ha) together removed 45 g AF DW m(-2) month(-1), whilst autumn storms were insignificant. As establ ished experimentally, winter survival was by single shoots enclosing a n active meristem, and not by rhizome fragments without leaves. Sucros e was the main storage carbohydrate. The rhizome was the main storage organ, with maximal carbohydrate content observed in mid July (190 mg g(-1) dry weight, sucrose + starch in glucose weight equivalents), and a gradual decline during autumn and winter. We estimated that the sto rage carbohydrates could cover 28% of the respiratory needs during win ter, which would necessitate a substantial photosynthesis to meet the remaining 72%. From iteratively fitted photosynthesis-light curves we conclude that this intertidal Z. noltii population is high-light adapt ed compared with permanently submerged seagrasses and freshwater angio sperms: estimates for the light compensation point (LCP) and half-satu ration constant (K-m) were comparatively high (July LCP and K-m: 98 mu E m(-2) s(-1) and 236 mu E m(-2) s(-1), respectively), maximum photos ynthetic rate was high (P-max: 236 mu g O-2 g(-1) AFDW min(-1)) and th e initial slope of the curve was low (alpha: 0.63 mu g O-2 g(-1) AFDW min(-1)/mu E m(-2) s(-1)). Estimated daily oxygen balances confirmed t hat positive net photosynthesis was largely limited to low tide daylig ht in this turbid estuary (mean high tide light attenuation coefficien t: 2.1 m(-1)).