Dynamics of shallow-water assemblages in the Saipan Lagoon

Abstract

The Saipan Lagoon (Northern Mariana Islands) was first examined for benthic compo- sition and habitat distribution in the late 1940s. Here, we employ a 4-stage approach to evaluate and explain change in the distribution of the 9 habitats previously demarcated. We show that there have been considerable reductions in seagrass, staghorn Acropora, and Acropora palifera dominated habi- tats (-3.72, -1.26, and -1.37 km 2 , respectively) that were replaced by 6.16 km 2 of sand (~20% of the lagoon area). Multiple regressions showed that the spatial extent of Enhalus seagrass was positively related with, and best predicted by, the adjacent watershed area and lagoon width. Although the interaction was not as strong, Enhalus also increased in accordance with human development. The size of the Halodule seagrass and macroalgal habitat was negatively related to water-flow velocities and positively related to human development, while its integrity (i.e. the density of seagrass within a given habitat) decreased with human development. Taken together, the results suggest Enhalus and Halodule respond differently to proxies of watershed pollution, and contradict contemporary doctrine linking pollution with reduced seagrass density. This study found no relationship between offshore habitats and watershed characteristics, but suggests their expansion and contraction on relatively short time scales is a result of large-scale disturbances such as typhoons. We posit that, while habitat integrity can be altered by human disturbances, habitat identity is only altered under extreme condi- tions. In summary, this study advances habitat mapping by increasing resolution and accuracy, which, in turn, improves the texture at which reef ecology is used by management.