Disturbance and recovery of the macroflora of a seagrass (Halodule wrightii Ascherson) meadow in the Abrolhos Marine National Park, Brazil: an experimental evaluation of anchor damage

Anchor damage due to tourist visitation is becoming increasingly intense in the Abrolhos Marine National Park, Brazil, and is probably detrimental to the biota associated with the seagrass beds. In this study the effects of a nchor damage on an algal dominated seagrass (Halodule wrightii) bed in the national park were measured and assessed. The mean size of anchor scars was 0.16 m(2), and it was estimated that 0.5% of the seagrass beds were damage d per year by boat anchoring. The short term effect of simulated anchor dam age (over 4 d) was a reduction in seagrass density, in the standing stock o f Laurencia obtusa (Hudson) Lamouroux growing epiphytically on Udotea flabe llum (Ellis and Solander) Lamouroux, and in the total macrophyte (seagrass + algae) standing stock. The longer-term (5, 9 and 13 month) ability of the seagrass and macroalgae to recover from damage, and possible seasonal diff erences in recovery ability, were determined. H. wrightii reoccupied experi mentally cleared 0.25 m(2) areas by vegetative elongation, and in 9 mo shor t shoot, rhizome and root biomass, and short shoot and rhizome densities we re similar to controls. The ability of the seagrass to recover appeared not to be seasonal, and seasonality was only found in short shoot density. Rec overing seagrass sent up more short shoots per length of rhizome than plant s in undamaged areas. The most abundant macroalga was the rhizophytic Udote a flabellum; other algae grew on it as epiphytes or unattached. U. flabellu m could recover quickly from simulated anchor damage, though other common a lgae (Dictyota mertensii (Martius) Kuetz., D. cervicornis Kutzing, Padina s pp. and Laurencia obtusa) showed more complex patterns of recovery that wer e seasonally or species dependent. A perturbation such as an anchoring has an effect on the algal flora more than a year later, although most algae sp ecies and H. wrightii can recover more quickly. Notably large areas devoid of macrophytes within the seagrass bed may have been created by the fusion of anchor scars. (C) 1999 Elsevier Science B.V. All rights reserved.