Abstract Over the last decades, regional climate changes, erosion, and heightened agricultural runoffs have increased nutrient and particle input in rivers, unbalancing the biogeochemical cycle of this suspended material along estuaries. In this context, particle fluxes in the euphotic zone, a very productive layer and important to maintain the food chain and estuarine preservation, require better understanding. This study aims to evaluate particle sedimentation fluxes in the photic zone of the Cananéia-Iguape Estuarine-Lagoon Complex (CIELC), considering sectors under different salinities and trophic statuses. A cylindrical sediment trap was installed at the base of the photic layer to measure particle fluxes and photopigments. Meanwhile, water samples were taken to measure temperature, salinity, pH, dissolved oxygen, and nutrients. Valo Grande (a freshwater domain) showed high nutrient concentrations, in which high fluxes of phaeopigments and chlorophyll-b highlighted the contribution of vegetable detritus to particle sinking. In the Batatais mangrove creek, the high fluxes of organic particles (675.32 mg m-2 d-1) and chlorophyll-a (98.40 mg m-2 d-1) evinced a high contribution of microplankton to carbon sinking. During the winter of 2018, flocculation processes were evinced in the flood tide of the Cananéia Bay as an important driver of sedimentation rates, with considerable fluxes of inorganic particles (1161.20 mg m-2 d-1) and chlorophyll-a (83.27 mg m-2 d-1). In the summer of 2019, we observed a lower flux in total particles (451.24 mg m-2 d-1) in a period of haline stratification. In the Ararapira Channel, the lowest organic particle fluxes (<100 mg m-2 d-1) were associated with ultra-oligotrophic conditions, indicating a low influence of microplankton on sinking particles. These findings suggest that freshwater input, tidal variation, and trophic status are relevant controls of sinking particles in different CIELC sectors. decades changes erosion rivers estuaries context preservation understanding CananéiaIguape Iguape EstuarineLagoon Estuarine Lagoon CIELC, , (CIELC) statuses photopigments Meanwhile temperature salinity pH oxygen nutrients domain concentrations chlorophyllb chlorophyll b creek 675.32 67532 675 32 (675.3 m2 m 2 m- d1 d 1 d-1 chlorophylla 98.40 9840 98 40 (98.4 2018 rates 1161.20 116120 1161 20 (1161.2 83.27 8327 83 27 (83.2 d1. . 2019 451.24 45124 451 24 (451.2 stratification Channel <100 100 (<10 ultraoligotrophic ultra oligotrophic conditions variation (CIELC 675.3 6753 67 3 (675. d- 98.4 984 9 4 (98. 201 1161.2 11612 116 (1161. 83.2 832 8 (83. 451.2 4512 45 (451. <10 10 (<1 675. 6 (675 98. (98 1161. 11 (1161 83. (83 451. (451 <1 (< (67 (9 (116 (8 (45 < ( (6 (11 (4 (1

Abstract This study presents the implementation of a portable module designed for autonomous analysis of sea-surface inorganic nutrients onboard vessels of opportunity, as an additional tool for assessing the levels of eutrophication risk. The study was carried out during August-mid-September 2019 in the Ria de Arousa and outer shelf area (NW-Iberian coastal upwelling system). During this period, the distributions of the measured sea-surface concentrations of nitrate and phosphate were compared according to three Oceanographic Environments (OEs). The OEs were defined according to the interplay between upwelling/downwelling events and river discharge on the coastal system. The nutrient measurements agreed well with the OEs, showing that the portable module is a useful tool for opportune measurements of sea-surface nutrients and can serve as a complement for the available monitoring networks. An initial evaluation of the eutrophication risk in this area indicated low risk levels (following the Environmental European Agency criteria) for most of the measured points in summer, except for some vulnerable areas under certain OEs. Nutrient concentrations are sensitive to periods of Sustained Upwelling events, reaching medium risk levels (7.14 - 9.05 µmol L-1 for nitrate and 0.39 - 0.64 µmol L-1 for phosphate) in inner parts of the Ria de Arousa. These areas are characterized by abrupt bathymetric changes that channel and intensify the upwelling processes, increasing sea-surface nutrient concentrations. High eutrophication risk levels of phosphate (1.53 µmol L-1) were detected close to the coastline during Upwelling Relaxation periods. Under these conditions, continental flows, previously retained by the upwelling, are able to expand. The location of these samples and the difference in concentration between phosphate and nitrate indicate a most likely source in wastewater outflows. Our results highlight the need for deeper studies on the synergy between upwelling/downwelling processes and the continental water discharges and its modulation of sea-surface nutrients.