Abstract
River estuaries are connectors of terrestrial and marine ecosystems. Riverine particulate organic carbon (POC) is discharged into oceans after a series of biogeochemical reactions in estuaries. Satellite monitoring of POC will improve our understanding of the carbon dynamics of these water bodies. Based on in situ data from four seasonal survey cruises, we developed an algorithm for estimating POC concentrations in the Pearl River Estuary (PRE). Reflectance ratios, Rrs(678)/Rrs(488) and Rrs(748)/Rrs(412), were set as inputs to calculate POC concentration in the PRE. The algorithm was then applied to MODIS/AQUA data to inverse POC concentrations in the PRE from 2002 to 2014. Additionally, sources, impact factors, and seasonal distributions of POC were also investigated. Phytoplankton contributed more to POC in off-shore waters than that in in-shore waters in autumn and spring, but showed the opposite pattern in winter. Under the influence of freshwater from the Pearl River, underwater topography, tides, winds, etc., the seasonal POC concentrations along a specific section, vertical to water depth gradient, decreased in different seasons. These decreases could be described by exponential functions (y = aebx, b < 0). The distribution of POC concentrations in the PRE resulted from complex physical and biogeochemical processes, which can change spatially and seasonally.
Highlights
Carbon is found widely in aquatic ecosystems in the form of dissolved organic carbon (DOC), particulate organic carbon (POC), dissolved inorganic carbon (DIC), and particulate inorganic carbon (PIC)
In accordance with Ni et al [15], we found a positive correlation between total suspended matter (TSM) and POC concentration in the Pearl River Estuary (PRE) (Figure 9b)
In the PRE, phytoplankton contribution to POC differed in different waters and changed with the seasons
Summary
Carbon is found widely in aquatic ecosystems in the form of dissolved organic carbon (DOC), particulate organic carbon (POC), dissolved inorganic carbon (DIC), and particulate inorganic carbon (PIC). Correlations between POC concentration and the diffuse attenuation coefficient at 490 nm, K490, or the beam attenuation coefficient due to particles, Cp, were used to estimate ocean POC concentration from satellite data [8] All these algorithms were designed for retrieval of POC concentrations in open sea surface water or over the euphotic depth [10], where POC is mainly derived from phytoplankton and can even be estimated from Chl-a concentrations [5,11]. The variable particle assemblages (many mineral particles for example) and high concentrations of optically important dissolved organic matter (DOM) in coastal waters mean that open ocean POC algorithms do not perform well [3]. Based on spectrum and POC data from the cruises, we first developed an estimation algorithm for surface POC concentrations in the highly turbid waters of the PRE. We applied the algorithm to the MODIS/AQUA data to calculate seasonal POC in the PRE; sources, impact factors, and temporal and spatial variations of POC in the PRE were examined
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