Characteristics of conservative and non-conservative CDOM of a tropical monsoonal estuary in relation to changing biogeochemistry

Highlights

• CDOM captures spatiotemporal processes of a medium tropical estuary.

• CDOM was conservative during non-monsoon; during monsoon it was related to pH.

• Silicate covaried with conservative CDOM being co-leached from silts.

• 90% of annual river flux to sea has 2.75 m${}^{-1}$ a₃₅₀ and 0.016 nm${}^{-1}$ ${\mathit{S}}_{300-500}$.

• The FDOM B:T ratio was a new proxy of OM diagenesis during no-discharge.

Abstract

The absorbance and fluorescence characteristics of colored dissolved organic matter (CDOM) were investigated in the tropical monsoonal Godavari estuary over a year during 18 field surveys attempting to capture different river discharge scenarios ranging from nil to >5000 ms ⁻¹ and the accompanying environmental factors. Key biogeochemical constituents were also analyzed to explain the abundance and structure of CDOM and fluorescent DOM (FDOM) in terms of spatiotemporal controls. The high flow water of the monsoon season was rich in CDOM of spectral slope (${\mathit{S}}_{300-500}$) typical for river water but of lower fluorescence. The first flood water was most rich in CDOM of lowest ${\mathit{S}}_{300-500}$, attributed to land washings during summer. The ratio between the UV–visible​ humic-like (A) and marine humic-like (M) fluorophores was linearly related to that of tryptophan protein-like (T) and Tyrosine protein-like (B) fluorophores and indicated linkage between terrestrial inputs and marine processes. The low and nil flow water was composed of seasonally and spatially variable CDOM but which was conservative against salinity (S) fitting to the model, a${}_{\mathrm{CDOM}}$(350) (m⁻¹) = 2.196-0.036*S. During the discharge season, pH held an inverse linear relationship to $a_{\mathrm{CDOM}}$(350): a₃₅₀ (m⁻¹) = 9.346-0.535*pH. The salinity co-varying (conservative) CDOM was linear versus silicate. A laboratory experiment showed that CDOM, FDOM and silicate were co-leached from the estuarine silts. During the non-discharge season unique, transient factors affected the CDOM e.g., (i) hot summer conditions in the upper estuary, (ii) benthic flux by bacterial metabolism from the consolidated silts after the flood season, and (iii) occasional anthropogenic input of petroleum hydrocarbons. The B:T fluorophore ratio was higher and linear versus NH${}_4^{+}$ during no-discharge and is a potential index of the bacterial metabolism i.e., protein un-folding of the silt-held organic matter accumulated during the erstwhile flood season.

Introduction

Colored dissolved organic matter (CDOM) is the fraction of dissolved organic matter (DOM) having exponentially decreasing absorption of the UV–visible light in short to long wavelength continuum (Coble, 2007). Fluorescent DOM (FDOM) is a further fraction, of variable magnitude, of the CDOM. Humic and protein components of DOM, the more often studied structural classes, exhibit both absorption (CDOM) and fluorescence (FDOM) (Carter et al., 2012, Yakimenko et al., 2018). Estuarine CDOM, when dominated by the riverine humic matter behaves conservatively as to why its absorption as well as fluorescence have been used as tracers in estuarine and coastal mixing studies (Dorsch and Bidleman, 1982, Baker and Spencer, 2004, Callahan et al., 2004, Kowalczuk et al., 2005, Xie et al., 2012), and in tracking sources from large rivers in oceanic regions far away from river mouths (Hu et al., 2004).

The river water derives its CDOM from the organic phases of the river basin e.g., Organic matter adsorbed on aluminosilicate minerals (Kalbitz et al., 2000), by the chemical weathering reaction when silicate goes in solution (Gaillardet et al., 1999). As the silicate and CDOM rich water pass through the estuary, under conditions of insignificant cycling within the estuary compared to the size of the inputs, both should emerge from the estuary, retaining their common source signature. To our knowledge, as to how CDOM relates to silicate in an estuary has not been explored so far.

In estuaries, compared to the coastal and oceanic regions, not only the abundance of DOM is greater due mainly to the influx from land sources e.g., rivers, but also because the fraction of CDOM in the bulk DOM is significantly higher (Traving et al., 2017, Bowers and Brett, 2008). Being the more labile fraction of DOM, the CDOM undergoes mineralization, metabolism or photo-degradation more readily than the bulk DOM, contributing to the ecosystem fertility (Shank et al., 2009). The summer floods introduce large amounts of CDOM (DOM) and (organic) particles of land (plant) origin (Bouillon et al., 2003) from which new CDOM is formed (Brym et al., 2014). In the case of a tropical monsoonal estuary e.g., the Godavari estuary in which discharge rates oscillate between extreme limits, seasonal forcings control the CDOM concentration and its slope parameter (Sarma et al., 2018) as well as molecular weight (Chari et al., 2019). During the no-discharge season, the benthic flux is an important contributor to the in situ inputs (Maher and Eyre, 2011). .

We have taken up a study of CDOM and FDOM over a full annual cycle in the Godavari estuary with the objectives, (i) what is their concentration and structural variability in response to the ecosystem changes, and (ii) can the estuarine silicate be a proxy of the benthic CDOM flux?