Seasonal dynamics and leaf growth of Zostera noltii hornem., A perennial intertidal seagrass

From May to October 1983, seasonal dynamics and leaf growth of Zostera noltii Hornem, were studied in three permanent quadrats (PQ) in the Zandkreek (SW Netherlands). Dense Z. noltii beds covered 100 ha of tidal flats together with occasional patches of Zostera marina L. Leaf growth was estimated weekly using a modified leaf-marking method. Seasonal curves for biomass and shoot-density development were identically shaped for all plots but had different timing, with a strong incline in biomass increase in PQ 1 in the middle of June, in PQ 2 at the end of July and in PQ 3 at the end of August. Vigorous growth occurred above a shoot density of 1000–2000 m −2. Mean shoot density of PQ 2 and 3 in May did not differ much (300–400 m −2) but distribution in PQ 2 was patchy while PQ 3 was homogeneous. It is hypothesized that mutuality is the cause of this difference in timing. Above a threshold density, patches may suffer less from uprooting by currents. Total leaf growth of Z. noltii shows maxima in spring and late summer (0.06g g −1 ash-free dry weight day −1), a more or less constant summer level (0.04g g −1 day −1) and declining values in autumn (≤0.03 g g −1 day −1). A comparison of aboveground biomass increase in PQ 1 and net leaf growth rates revealed no significant difference, thus validating the leaf-marking method applied. Total leaf production of Z. noltii was 135 g m −2 during the study period, including leaf loss (51% of the production). Estimated annual aboveground biomass production of Z. noltii was 230 g m −2.