Seasonal dynamics and disturbance of phytoplankton biomass in the wake of Tahiti as observed by Biogeochemical-Argo floats

Abstract

Abstract. The South Pacific Subtropical Gyre (SPSG) is a vast and remote area where large uncertainties on variability in phytoplankton biomass and production remain due to the lack of biogeochemical in situ observations. In such oligotrophic environments, ecosystems are predominantly controlled by nutrients depletion in surface waters. However, this oligotrophic 20 character can be disturbed in the vicinity of islands where enhancement of biological activity is known to occur (i.e. the island mass effect, IME). This study mainly focuses on in situ observations showing that an IME can be evidenced leeward of Tahiti the second one remained in the Tahitian wake (around 45 km from the island coasts). In the oligotrophic central SPSG, the wintertime increase in upper layer chlorophyll a concentration is likely due to photoacclimation process. Vertical observations show a light-driven deepening of the deep chlorophyll maximum (DCM) from winter to summer, consistently with previous descriptions. At the opposite, within the Tahitian wake, the DCM temporary widens during late spring in association with a biological enhancement in the upper layer. Combining in situ measurements with meteorological data along the Tahiti coasts, 30Hybrid Coordinate Ocean Model outputs and satellite-derived products (i.e., horizontal currents and associated fronts), the physical mechanisms involved in the disturbance of phytoplankton seasonal cycle in the Tahitian wake have been investigated. This disturbance results from the concomitant occurrence of strong precipitations and a zone of weak currents leeward Tahiti.We conjecture that the land drainage induces a significant supply of nitrate in the ocean upper layer (down to ~100 m) while a zone of weak currents in the southwestern zone behind Tahiti forms an accumulation zone, hence allowing phytoplankton 35 growth up to 20 km away from the coastlines. Moreover, bio-optical measurements suggest that the composition of

Role of Iron in the Marquesas Island Mass Effect

“…This float was equipped with a seabird standard conductivity, temperature, and depth, completed with additional optical sensor packages including the WET Labs ECO Puck Triplet that noticeably measures the fluorescence of chlorophyll‐ a . The fluorescence observations were converted in Chl following the procedure described in Sauzède et al ().…”

Long-Term Presence of the Island Mass Effect at Rangiroa Atoll, French Polynesia

“…Important sources of nutrients and organic material are inputs from island terrestrial sources such as rivers or submarine groundwater discharge (Dandonneau and Charpy, 1985). In the water surrounding the islands of Tahiti, increased nutrients were found to occur after rain events caused runoff of nutrient-rich land sediment (Sauzède et al, 2018). Further, populated islands and atolls may cause anthropogenic inputs such as human waste or agricultural fertilizer enhancing nutrient and organic matter inputs to nearshore waters (Dandonneau and Charpy, 1985;Gove et al, 2016).…”

“…Internal waves interacting with bathymetry can lead to instabilities in the otherwise stably stratified water column and deliver nutrient rich water from below to the euphotic zone (Carter et al, 2006). Many studies have presented significant spatial and temporal variations in IME, as well as diversity in key forcing mechanisms and their importance in driving IME (Dandonneau and Charpy, 1985;Charpy, 1996;Charpy et al, 1997;Martinez and Maamaatuaiahutapu, 2004;Gove et al, 2016;Sauzède et al, 2018). Gove et al (2016) analyzed chlorophyll-a measurements recorded over 10 years via satellite for 24 islands and 11 atolls in the tropical western Pacific and found the primary drivers of IME magnitude to be reef area, bathymetric slope, geomorphic type, and population status.…”