Oceanographic and chemical conditions in the water column overlying to the Nazca Ridge

Abstract

The Nazca Ridge is a deep-sea ecosystem spanning 1,000 km in length and 300 km in width as well as forming part of National Reserve of the same name. This area, located off the coast of the Ica Region (Peru), is renowned for its remarkable biodiversity. To enhance our understanding of the biological and oceanographic characteristics, a comprehensive study was conducted during the summer of 2020 in collaboration with the Peruvian Directorate of Hydrography and Navigation (DHN) and the crew of BAP Carrasco. We aimed to characterize the oceanographic, chemical, and geological conditions in the region. The investigation focused on analyzing nutrient levels, dissolved oxygen, productivity, and their correlation with water masses and oceanographic variables. A total of nine hydrographic stations were established along the Nazca Ridge, reaching depths of up to 2,800 m. The study revealed the influence of various water masses on the system, including Subtropical Surface Waters (SSW), Equatorial Subsurface Waters (ESSW), Antarctic Intermediate Waters (AAIW), and Pacific Deep Waters (PDW). Each water mass exhibited distinct temperature and salinity characteristics consistent with previous research. Notably, SSW displayed high dissolved oxygen levels, low nutrient concentrations, and elevated productivity. Dissolved oxygen was found to be highest within the uppermost 30 m, with the base of the oxycline occurring around 30 m. Between 50 m and 750 m, the oxygen minimum zone (OMZ) (<0.5 mL/L) corresponded to the distribution of ESSW, while concentrations increased below 1,500 m, nearing 2 mL/L in association with deep waters. Nutrient concentrations, including phosphates, silicates, and nitrates, were relatively low in surface waters due to phytoplankton consumption. However, concentrations increased with depth, particularly in relation to AAIW and PDW.