Natural recovery of Zostera noltii seagrass beds and benthic nematode assemblage responses to physical disturbance caused by traditional harvesting activities

摘要

In the intertidal seagrass beds of Zostera noltii of Mira estuary (SW, Portugal) the harvesting practices are frequent.The traditional bivalve harvesting not only affects the target species as the remaining biological assemblages.The main aim of this study was to assess the disturbance caused by sediment digging in the recovery ofthe seagrass beds habitat, through an experimental fieldwork. The responses of the seagrass plant condition,the sediment microbial activity and the nematode assemblages were investigated after the digging activity inseagrass beds. A total of four experimental plots were randomly demarcated in situ, two plots were subjectedto the disturbance - “Digging” - while other two were “Control”; the sampling occurred in five occasions, fromMay to October: T0–before digging; T1–14 days after digging; T2–45 days; T3–75 days; and T4–175 days. The environmentalvariables measured in the sediment and the photosynthetic efficiency (α) of the Z. noltii plants ineach plot and sampling occasion registered similar values, throughout the experiment. The extracellular enzymaticactivity (EEA) clearly presented a temporal pattern, although no significant differences were obtained betweendigging and control plots. Nematode assemblages registered high densities, revealing the absence of thedigging effect: control plots maintained similar density and diversity throughout the experiment, while the densityand diversity between digging plotswas significantly different at T0 and T4; the trophic compositionwas similarfor both control and digging plots, characterizedmainly by non-selective deposit feeders (1B) and epigrowthfeeders (2A).Organicmatter, nitrate and mean grain size explain a significant amount of the variation in the nematodegenera composition. This study demonstrated the capacity of the seagrass habitat to recover under low intensityphysical disturbance associated to harvesting.