At the confluence of western boundary currents: A 3D Biophysical Nutrient-Zooplankton-Phytoplankton-Detritus (NPZD) Model for the Benham Rise

The Benham Rise, which is a 135,506 km² underwater feature rising 2 km from the 5 km deep sea floor located 250 km east of Luzon (119°30’E - 132°E, 12°10’N - 20°30’N), is an extended continental shelf territory recently claimed by the Philippines through the approval of the United Nations Convention on the Law of the Sea. An exploratory oceanographic cruise was conducted for the first time on May 2014 to obtain baseline biogeochemical and hydrodynamic data in the area. The Benham Rise is influenced by major North Pacific western boundary currents - the Kuroshio Current and the bifurcating North Equatorial Current (NEC) that can produce seasonally-propagating eddies in the vicinity of the rise. The eddy field is hypothesized to enhance biological productivity. To understand the lower trophic ecosystem dynamics in the Benham Rise, a size-structured (micro- and meso-zooplankton, diatom and small phytoplankton <5μm) and multi-nutrient (NO₃, PO₄ and SiO₃) NPZD model is coupled to the three-dimensional Hybrid Coordinate Ocean Model (HYCOM). Growth rates are light and temperature regulated and uptakes are generally represented by a Michaelis-Menten formulation. Preliminary model results show that nutrient vertical profiles reproduce the standard nutrient depletion trend near the water surface. Plankton accumulation is observed at eddy-like structures produced by the HYCOM velocity field. Spatial variations for the different plankton groups in relation to circulation patterns are explored.