Oceanography of Papahānaumokuākea
Among the dominant natural controls over the ecosystems of Papahānaumokuākea Marine National Monument are climatic and oceanographic forces. The area lies at the northern edge of the oligotrophic tropical Pacific, a major ecological transition zone in the northern Pacific. This boundary, also known as the “Transition Zone Chlorophyll Front,” varies in position both seasonally and annually, and periodically moves across the property boundary surrounding the northern atolls of Kure and Midway. This, in turn, influences overall ocean productivity, and the resultant recruitment success of many species such as Hawaiian Monk Seals and ocean-feeding seabirds (Polovina et al. 2008; Baker, Polovina and Howell 2007). The northernmost atolls are also in a position where they are occasionally affected by an episodic eastward extension of the Western Pacific warm pool, which can lead to higher summer ocean temperatures at Kure than are found in the more “tropical” waters of the main Hawaiian Islands further to the south. This can cause greater temperature fluxes that can in turn influence the home ranges and diversity of many species. This interplay of oceanography and climate is not fully understood, but adds a level of dynamics not seen in most other tropical atoll ecosystems and is a useful natural laboratory for understanding phenomena such as periodic coral bleaching and the effects of the Pacific Decadal Oscillation, El Niño and La Niña ocean circulation patterns. Ocean currents, waves, temperature, nutrients, and other oceanographic parameters and conditions influence ecosystem composition, structure, and function in Papahānaumokuākea on both temporal and spatial scales. Spatial variability in oceanographic conditions ranges from a localized temperature regime that may affect a small portion of a reef to a temperature regime that influences Papahānaumokuākea as a whole. Temporal variability in ocean conditions may range from hourly and daily changes to seasonal, annual, or decadal cycles in nutrient inputs, sea level heights, current patterns, and other large-scale oceanographic processes (Polovina et al. 1994). Currents play an important role in the dispersal and recruitment of marine life in Papahānaumokuākea on both scales. Surface currents in the NWHI are highly variable in both speed and direction (Firing and Brainard 2006), with long-term average surface flow being from east to west in response to the prevailing northeast trade wind conditions. The direction of surface water flow also accounts for certain unusual biogeographic relationships between Papahānaumokuākea and other allochthonous areas, such as Johnston Atoll to the south (Grigg 1981), as well as patterns of endemism, population structure, and density of reef fish within the archipelago itself (DeMartini and Friedlander 2006). The highly variable nature of the surface currents is due in large part to eddies created by local island effects on large-scale circulation. The distribution of corals and other shallow-water organisms is also influenced by exposure to ocean waves. The size and strength of ocean wave events have annual, interannual, and decadal time scales. Annual extratropical storms (storms that originate outside of tropical latitudes) create high waves during the winter, greatly affecting marine and terrestrial areas, as the elevation of a large portion of terrestrial habitat is less than the height of some of the waves that pass through.