Seasonal variation in the intertidal seagrass Zostera noltii Hornem.: coupling demographic and physiological patterns

Abstract

The considerable seasonal variation in biomass (2–130 g ash-free dry weight (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 increased 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 decline, due to a combination of: (a) self-shading during low tide; (b) high respiratory demand by the expanded rhizome network; (c) the seasonal decline in light availability during late summer. Nutrients were probably 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 maximal biomass the primary rhizome axes decayed, leaving single shoots on short pieces of rhizome, the former secondary axes. The rapid decline in biomass from mid September onwards could be attributed to grazing by herbivorous migratory waterfowl. It was estimated that brent geese, Branta bernicla (L.), and wigeon, Anas penelope L. (about 200 and 300 birds, respectively, on the bed of 30 ha) together removed 45 g AFDW m −2 month −1, whilst autumn storms were insignificant. As established experimentally, winter survival was by single shoots enclosing an active meristem, and not by rhizome fragments without leaves. Sucrose 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 storage carbohydrates could cover 28% of the respiratory needs during winter, 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 adapted compared with permanently submerged seagrasses and freshwater angiosperms: estimates for the light compensation point (LCP) and half-saturation constant ( K m) were comparatively high (July LCP and K m: 98 μE m −2 s −1 and 236 μE m −2 s −1, respectively), maximum photosynthetic rate was high ( P max: 236 μg O 2 g −1 AFDW min −1) and the initial slope of the curve was low (α: 0.63 μg O 2 g −1 AFDW min −1/μE m −2 s −1). Estimated daily oxygen balances confirmed that positive net photosynthesis was largely limited to low tide daylight in this turbid estuary (mean high tide light attenuation coefficient: 2.1 m −1).