Effects of Oil Spills on Fringing Red Mangroves (rhizophora Mangle): Losses of Mobile Species Associated with Submerged Prop Roots

Extrapolation of results from empirical studies of natural or anthropogenic perturbations are a useful tool in estimating potential effects of future impacts, For 5 years we studied the direct effects of a major oil spill on fringing red mangroves (Rhizophora mangle) and the organisms that live attached to their submerged prop roots, Based on these data, we here estimate the effects of this oil spill on associated mobile species using (I) direct measures of the abundance of wood-boring isopods and (2) losses of epibiota used as shelter, settlement and juvenile habitat by non-boring species associated with submerged prop roots. Using a descending hierarchical analysis. we examined losses (a) in the density of submerged prop roots per unit shoreline fringed with red mangroves, (b) in the suitability of submerged prop roots, and (c) in the amount of settlement substrate available on roots. For boring isopods in sheltered channel:; and lagoons, there was no significant difference in isopod abundance on suitable (live) roots at oiled and unoiled sites. However, suitable (live) submerged prop roots were less abundant at oiled sites, leading to a reduction of over 60% in the number of submerged prop roots containing isopods at oiled as compared to unoiled sites 5 years after the oil spill. We estimated effects on non-boring species using two groups of epibiota (foliose red algae and arborescent hydroids and bryozoans) utilized as juvenile habitat by spiny lobsters (Panulirus argus). Changes in two mangrove habitats, the exposed open coast and sheltered channels and lagoons, reduced the estimated amount of juvenile habitat by -4050% for at least 5 years after oiling. In channels and lagoons, losses were directly proportional to reductions in the number of submerged prop roots-unicl of shoreline, because average root size did not change after oiling and approximately the same area on root surfaces was covered with foliose red algae and arborescent hydroids and bryozoans. On the open coast, there were additional losses in juvenile habitat, because roots were significantly shorter at oiled than unoiled sites in years 2-4 post-spill. For arborescent hydroids and bryozoans (but not foliose red algae), reductions in cover were greater than reductions in the amount of potential habitat, indicating further negative effects of oiling. These additional reductions may have been caused by toxic hydrocarbons or further physical changes related to oiling. The effects of natural or anthropogenic perturbations depend on the type of habitat affected. Habitats created at least in part by their constituent plants or animals are especially vulnerable to negative impacts; such biogenically-structured habitats include salt marshes, seagrass beds, coral reefs and mangrove forests. Damage to the structuring organism(s) can cause loss of the habitat (i.e., erosion of the salt marsh), with cascading effects on the suite of associated species (loss of salt marsh crab, snail, fish and bird populations). If a perturbation is also accompanied by persistent toxic compounds, then negative effects may be prolonged due to the action of the toxins. Interactions between physical damage, toxic compounds and the process of recovery are complex and largely unknown (Teal et aI., 1992). Studies of natural or anthropogenic perturbations are most important when results can be extrapolated to other studies or used as part of planning for future impacts. For 5 years, we studied the effects of an oil spill on fringing red mangroves (Rhizophora mangle) and the plants and animals that live attached to submerged mangrove roots (Bums et al. (1993) and included references). Thickets of submerged red mangrove prop roots provide hard substrata in areas of soft