Molecular Traits of Dissolved Organic Matter in the Subterranean Estuary of a High-Energy Beach: Indications of Sources and Sinks

Abstract

Advective flows of seawater and fresh groundwater through coastal aquifers form a unique ecohydrological interface, the subterranean estuary (STE). Here, freshly produced marine organic matter and oxygen mix with groundwater, which is low in oxygen and contains aged organic carbon (OC) from terrestrial sources. Along the groundwater flow paths, dissolved organic matter (DOM) is degraded and inorganic electron acceptors are successively used up. Because of the different DOM sources and ages, exact degradation pathways are often difficult to disentangle, especially in high-energy environments with dynamic changes in beach morphology, source composition, and hydraulic gradients. From a case study site on a barrier island in the German North Sea, we present detailed biogeochemical data from freshwater lens groundwater, seawater, and beach porewater samples collected over different seasons. The samples were analyzed for physico-chemistry (e.g., salinity, temperature, dissolved silicate), (reduced) electron acceptors (e.g., oxygen, nitrate, and iron), and dissolved organic carbon (DOC). DOM was isolated and molecularly characterized via soft-ionization ultra-high-resolution mass spectrometry, and molecular formulae were identified in each sample. We found that the islands’ freshwater lens harbors a surprisingly high DOM molecular diversity and heterogeneity, possibly due to patchy distributions of buried peat lenses. Furthermore, a comparison of DOM composition of the endmembers indicated that the Spiekeroog high-energy beach STE conveys chemically modified, terrestrial DOM from the inland freshwater lens to the coastal ocean. In the beach intertidal zone, porewater DOC concentrations, lability of DOM and oxygen concentrations, decreased while dissolved (reduced) iron and dissolved silicate concentrations increased. This observation is consistent with the assumption of a continuous degradation of labile DOM along a cross-shore gradient, even in this dynamic environment. Accordingly, molecular properties of DOM indicated enhanced degradation, and “humic-like” fluorescent DOM fraction increased along the flow paths, likely through accumulation of compounds less susceptible to microbial consumption. Our data indicate that the high-energy beach STE is likely a net sink of OC from the terrestrial and marine realm, and that barrier islands such as Spiekeroog may act as efficient “digestors” of organic matter.

The Beach Aquifer Microbiome: Research Gaps and Data Needs

“…One of the emergent issues in beach aquifers is the behavior and lability of terrestrially derived dissolved organic matter, as well as the source-sink dynamics of carbon (Waska et al, 2021). In a thorough review on groundwater, Griebler and Lueders (2009) highlighted the important association between microbial communities and dissolved organic carbon as a driver of freeliving versus attached communities in addition to other variables such as the availability of nutrients, sediment grain size and minerology.…”

Cross-Shore and Depth Zonations in Bacterial Diversity Are Linked to Age and Source of Dissolved Organic Matter across the Intertidal Area of a Sandy Beach

“…Furthermore, vascular plant-derived DOM strongly interacts with reactive iron oxides, resulting in adsorption under oxic, and release under reducing conditions [ 73 , 81 , 82 ]. Significant positive correlations of humic-like FDOM concentrations with those of dissolved iron have been reported as an indication that a substantial fraction of FDOM interacts with iron cycling [ 47 , 82 ]. Such a geochemical link would explain the co-occurrence of high FDOM and iron concentrations related to cluster III.…”

“…PO 4 ³− was determined photometrically, following the methods of Itaya and Ui [ 54 ] for samples with concentrations below 2.1 µM and Laskov et al [ 55 ] for samples exceeding 2.1 µM PO 4 ³− . Porewater samples used for DOC analysis were treated and processed according to Waska et al [ 53 ], and DOM samples were extracted and analyzed with ultra-high resolution Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) according to Waska et al [ 47 ]. In addition to the chemical characteristics (e.g., elemental ratios and compound classes) and molecular indices (e.g., Ideg and ITerr), which were calculated in the same way as in Waska et al [ 47 ], we included here the homologous series network number, which is described in Merder et al [ 56 ] as part of the ICBM-OCEAN data processing output.…”

“…* The molecular indices Ideg, MLB_L, and ITerr were calculated using the whole DOM datasets from the sites which were associated with each of the clusters (all seasons combined). Information on compound classes can be found in the text as well as in Waska et al [ 47 ].…”

“…As a model study site, we chose a high-energy beach in the German North Sea. Previous studies found indications of rapid and efficient organic matter degradation, resulting in the accumulation of inorganic nutrients and recalcitrant DOM along the advective porewater flow paths [ 19 , 46 , 47 ]. Studies on microbial diversity revealed a large core community as well as cross-shore gradients across the intertidal area [ 19 , 48 , 49 ].…”