The Influence of River Discharge on Nutrient Export and Phytoplankton Biomass Off the Central Chile Coast (33°–37°S): Seasonal Cycle and Interannual Variability

Italo Masotti,Pilar Aparicio-Rizzo,Lucy Belmar,Mariela A. Yevenes,René Garreaud,Laura Farías

doi:10.3389/fmars.2018.00423

Abstract

Using in situ hydro-chemical data and MODIS-SeaWiFS ocean color images as a proxy of river plumes and phytoplankton biomass from 2000 to 2014, this study documents the temporal co-variability of river discharge, plume area, nitrate and phosphate export and phytoplankton biomass in the coastal waters off Central Chile (33o-37oS). Five major rivers (Maipo, Mataquito, Maule, Itata and Biobio) drain into this region with annual mean discharge ranging from 120 to 1000 m3 s-1. River discharge and coastal plume area present a marked seasonal cycle, reaching maximum values during the winter rainy season (June-September). Export of riverine nutrients also peaks in winter, leading to an increase in phytoplankton biomass within the plumes that can be twice larger than the background values in coastal areas away from the river mouths. Wintertime river discharge, plume area and nutrient export are also correlated at interannual time scales. During a recent extended dry period (2010-2014), river discharges, plume areas and nutrient export clearly decreased by about 50% compared to historical values, reducing significantly the size of the chlorophyll pool within plumes off Central Chile during winter. The potential impacts of droughts are discussed in terms of coastal ecology and primary production, a highly relevant issue considering the projections of a dry climate over Central Chile in the future. Systematic evidence of mega-drought effects upon coastal productivity still does not exist, but it remains a priority to further investigate and quantify these impacts.

Highlights

Phytoplankton biomass (PB) and primary production (PP) in coastal areas are highly variable over temporal and spatial scales, primarily regulated by nutrient supply, light availability and physical processes driving stratification/mixing and advection (e.g., Hickey and Banas, 2003)
Influence of River Discharge Off Central Chile through rivers, which can support both PB and PP, as well as regulate coastal biogeochemical cycles of Nitrate (NO3−), Phosphate (PO4−3), and Silicate [Si(OH)4] (Hopkinson and Vallino, 2005; Cotrim et al, 2007)
Several studies have reported a positive relationship between increased river discharge, nutrient input, and high PB in adjacent coastal areas to river mouths

Summary

Introduction

Phytoplankton biomass (PB) and primary production (PP) in coastal areas are highly variable over temporal and spatial scales, primarily regulated by nutrient supply, light availability and physical processes driving stratification/mixing and advection (e.g., Hickey and Banas, 2003). Influence of River Discharge Off Central Chile through rivers, which can support both PB and PP, as well as regulate coastal biogeochemical cycles of Nitrate (NO3−), Phosphate (PO4−3), and Silicate [Si(OH)4] (Hopkinson and Vallino, 2005; Cotrim et al, 2007). Through their impact on precipitation, both natural variability (e.g., El Niño Southern Oscillation, ENSO) and anthropogenic climate change may alter river discharge into coastal areas over interannual and longer time scales, leading to considerable impacts on coastal PB and PP. Ecological studies have indicated a deterioration of the planktonic food web during a period of drought in the coastal waters of the North Pacific, with negative consequences on fish populations due to cascading effects on higher trophic levels (Cloern et al, 1983; Nichols, 1985; Wetz et al, 2011)

Methods

Results

Conclusion