Impacts of hydroclimatic variability on surface water and porewater dissolved organic matter in a semi-arid estuary

Highlights

• Changing hydroclimatic conditions impact seasonal DOM composition.

• Increased submarine groundwater discharge during flood recession reduced C:N ratios.

• Increased N and S heteroatom content and DOM diversity under high flow conditions.

• PCA and Volcano Plots concur that flood recession DOM is significantly different.

• Greatest influence from benthic flux on DOM composition during flood recession.

Abstract

Hydroclimatic disturbances, like flooding, may greatly impact dissolved organic matter (DOM) composition in coastal waters by altering hydraulic gradients, groundwater residence times, and seafloor recirculation. This study provides new evidence regarding seasonal molecular DOM changes, in a submarine groundwater discharge influenced semi-arid estuary, across different hydroclimatic conditions: from flood peak (summer) to flood recession (fall), to typical dry (winter) to semi-wet (spring) conditions. DOM molecular characterization of seasonal surface- and pore- water samples was conducted using PPL solid phase extraction and UPLC-Orbitrap Fusion mass spectrometry, in positive mode. The proportion of CHO compounds present was lowest during flooding (34%), increased during fall flood recession (39%), and was highest under semi-wet baseflow (spring) conditions (42%). While CHON compounds increased from the flood and flood recession events (27% and 22%, respectively) to dry and semi-wet conditions (35% and 34%, respectively), sulfur and phosphorous containing compounds decreased (flood ~34% and flood recession ~6%; dry <25% and semi-wet <3%, respectively). During the flood recession, DOM was considerably different from all other events, with over 1000 compounds significantly specific to this period. These significant changes were likely driven by submarine groundwater discharge, particularly as the compounds identified in surface- and pore- water were very similar, and surface water DOC and DON showed porewater influence. Baseflow characteristics were highly similar, with low saturation and high likelihood of rings or double bonds. It was evident that the influence of riverine inputs decreased within six months from the flood peak, which also showed the limited fresh riverine inputs outside extreme wet precipitation events when benthic fluxes became one of the major drivers of DOM characteristics. Thus, surface water DOM molecular characteristics showed significant responses to hydrologic changes, due to surface runoff inputs and benthic fluxes, which reflect changes in source and processing of OM within the estuary.

Introduction

Estuarine dissolved organic matter (DOM) consists of a diverse array of allochthonous (i.e., terrestrial runoff, river discharge, and benthic fluxes) and autochthonous (i.e., phytoplankton metabolism and excretion, viral lysis, and sloppy feeding) sources (Carlson and Hansell, 2015; Repeta, 2015; Sipler and Bronk, 2015). Annually, an estimated 0.25 × 10¹⁵ g C are transported from land to the sea in the form of dissolved organic carbon (DOC) (Hedges et al., 1997) while the DOM sediment benthic fluxes are equivalent to riverine fluxes in coastal regions (Burdige et al., 1999). However, these estimated fluxes may underestimate allochthonous DOC fluxes to regions where groundwater discharge, diffuse sediment fluxes, and effects of extreme wet events (e.g., flooding and drought) are still not well constrained (Webb et al., 2019).

Generally, bulk DOC concentrations decrease along salinity gradients from the river to the open ocean while the DOM composition is simultaneously changing. With increasing salinity, it is expected the molecular weight, carbohydrate content (Abdulla et al., 2010a), heteroelement content (Sleighter and Hatcher, 2008), photoammonification (Aarnos et al., 2012), and lability (D'Andrilli et al., 2015) will increase while the aromaticity (Abdulla et al., 2010a), photoproduction of dissolved inorganic carbon (DIC) (Aarnos et al., 2012), and carbon:nitrogen (C:N) ratio (Sipler and Bronk, 2015) of the DOM decreases. In tropical and temperate marine basins, the riverine DOM pool is rapidly transformed through a multitude of biotic (microbial) and abiotic (photochemical) processes (Spencer et al., 2009), such that terrigenous DOC constitutes approximately 1–2% of total DOC in the oceans (Hernes and Benner, 2006). Thus, the molecular composition of terrestrially derived DOM appears to have reduced concentrations and less richness than marine DOM (Abdulla et al., 2010b; Medeiros et al., 2015; Powers et al., 2018).

The quantity and composition of riverine DOM was shown to affect biogeochemical cycling in estuaries with magnitudes that vary with changes in hydroclimatic conditions (Letourneau and Medeiros, 2019; Liu and Lu, 2019; Majidzadeh et al., 2017; Raymond and Saiers, 2010; Spencer et al., 2009). Additionally, changes in riverine DOM composition have been shown to lag behind changes in DOM fluxes during storm events (Yang et al., 2013). Extreme flooding events result in large “pulse” releases of terrestrial DOM (tDOM) into fluvial systems, thus, increasing DOM concentrations that are “shunted” rapidly downstream due to increased discharge and flushing (Raymond et al., 2016). A survey of 31 forested watersheds in the eastern United States, demonstrated that 86% of annual DOC export occurs following major precipitation events and that 60% of this export occurs during large flow events that account for 5% of the year (Raymond and Saiers, 2010). For example, during a storm event three small watersheds in Oregon were shown to experience approximately 3-fold increases in DOC concentrations with an increase in the proportion of humic DOM (Hood et al., 2006). Thus, low-frequency large flow events, anticipated to increase with climate change (Stocker, 2014), are responsible for a significant portion of annual tDOM input to estuaries and coastal oceans. Furthermore, the export of DOM during extreme wet events approaches the mean annual DOM export during non-extreme flow event years (Asmala et al., 2021; Caverly et al., 2013; Jung et al., 2014; Raymond and Saiers, 2010; Sánchez-Murillo et al., 2019; Yoon and Raymond, 2012). Therefore, in the face of a rapidly changing climate, there is an immediate need to understand the mechanisms that control the fate and transport pathways of DOM for successful management of aquatic food webs and sustainable ecosystem health.

To date, studies that focused on the DOM composition that accompany temporal hydroclimatic changes have largely focused on riverine surface water (Liu and Lu, 2019; Raymond and Saiers, 2010; Yang et al., 2013), temperate systems with perennial stream flow (Dixon et al., 2014; Osterholz et al., 2016; Powers et al., 2018), and the arctic (Rossel et al., 2020). Most estuarine and coastal DOM characterization studies have not considered inter- and intra-seasonal changes in both surface- and pore- water DOM molecular composition (Mori et al., 2019; Rossel et al., 2020) in response to extreme hydroclimatic events, such as flooding. To our knowledge, no such studies exist for semi-arid estuaries where precipitation is infrequent and riverine discharge is flashy. Further high-resolution studies of the DOM molecular composition along terrestrial to marine gradients under different hydroclimatic conditions are still necessary, especially in semi-arid estuaries.

To fill this knowledge gap, this study investigated how the DOM molecular composition varies across four seasons with different hydroclimatic conditions through the application of ultra-high performance liquid chromatography coupled to Orbitrap Fusion Tribrid mass spectrometry (UPLC-OT-FTMS). The study focused on Nueces Bay, a semi-arid region, where previous studies have shown that significant temporal and spatial variability of submarine groundwater discharge (Douglas et al., 2020; Murgulet et al., 2018) represent a source of inorganic and organic nutrients in excess of riverine inputs, which influence nutrient budgets and cycling (Douglas et al., 2021). This study sought to better constrain the influence of benthic-derived DOM inputs in relation to hydroclimatic changes through the molecular characterization and comparison of seasonal surface water and bottom sediment porewater within the estuary.