What is the chemical composition of the sediments, and what does it tell us about sediments?
Knowing the chemistry of the sediment provides insight into where in the watershed the sediment might have come from, its biological availability relative to water quality, and to ultimately relate sediment fluxes to the pathways of carbon fluxes from the atmosphere through the river systems to the ocean. Comparison of the following results to similar work in the Pacific Northwest for a much more pristine basin (Ward et al. 2011. Biogeochemistry. DOI:10.1007/s10533-012-9700-9) gives insight on the dynamics of the SMV.
The concentration of particulate organic carbon (POC) and nitrogen (PON) showed a similar positive correlation to sediments with river discharge during the periods of both light and heavy rainfall during the November 2013 campaign. The percentage of organic carbon relative to total suspended sediments (%OC) slightly increased with increasing river discharge. This likely indicates that storm flow mobilizes sediments that are rich in organic material relative to base flow. The importance of hydrologic flowpaths through the uppermost OM rich soil layer in controlling soil OM concentrations has been previously demonstrated; during rapid rain events young OM has been shown to be flushed from upper soil layers, depleting soil OM concentrations and leaving behind older OM in deeper soil horizons. It has been estimated that base flow largely consists of OM from deep soil layers, whereas 50-65% and 35-50% of the OM mobilized by peak flow originates from the B and O soil horizons, respectively.
Observed (black) and simulated discharge in the Santa Maria da Vitória River near Santa Leopoldina using a 2007 land cover baseline (red), and under two scenarios—a 50% increase in eucalyptus land cover (green) and a 25% increase in agricultural land cover (blue). Figure is adapted from Tan et al. (in review).
The molar ratio of POC to PON generally increased with river discharge, but was not well correlated. The ratio of dissolved organic carbon to nitrogen, on the other hand, displayed a strong correlation with river discharge in the Pacific Northwest. The elemental composition of dissolved constituents that are mobilized into streams via storm flow are controlled primarily by solubility. That is to say, specific constituents are more readily mobilized from soils into streamwater depending on their inherent solubility even though these constituents may have been initially derived from the same source material. In the Pacific Northwest, carbon-rich compounds were relatively easier to mobilize than nitrogen or phosphorus rich compounds. The elemental composition of particulate organic material in the Rio Santa Maria da Vitoria, on the other hand, appears to be controlled primarily by variability in source materials.
The isotopic signature of both particulate organic carbon (∂13C-POC) and particulate organic nitrogen (∂15C-PON) generally became more enriched with increasing river discharge at both locations. For example, ∂15N-PON ranged from roughly 2.0‰ to 10.0‰, becoming relatively more enriched in 15N with increasing discharge during the November, 2013 storm. Organic nitrogen derived from fertilizers and wastewater is generally enriched in 15N relative to natural vegetation (McClelland et al., 1997). Storm runoff in the Mangarai and Santa Maria da Vitoria mobilizes a mixture of nitrogen sources derived from fertilizers, untreated sewage, and natural landscapes into the river network based on the observed enrichment of ∂15N-PON with storm flow. This enrichment in ∂15N-PON is significantly higher than observations made in the Pacific Northwest during a similar sampling regime In the Pacific Northwest, ∂15N-PON was not directly correlated to river discharge and only reached a maximum value of 3.8‰. In the Rio Mangarai and Santa Maria da Vitoria, however, ∂15N-PON was much more variable and reached much higher values at peak runoff. This is most likely due to stark differences in land use and wastewater treatment practices rather than latitudinal differences. For example, in the Pacific Northwest study region, N2-fixing red alder trees were the primary source of nitrogen to the system and the isotopic signal of red alder inputs dominated the isotopic composition of particulate organic matter throughout the sampled storm sequenes. Fertilizer inputs from residential neighborhoods were present, but insignificant relative to the red alder inputs. Sewage treatment in the region was primarily via septic tanks. Septic tank outfall would provide the river with enriched organic nitrogen values, however, these inputs primarily occur near the river-marine interface, and were, thus, not observed by the stream sampling regime. In stark contrast, there is very little actual wastewater treatment along many parts of the Santa Maria da Vitoria basin. Thus, the river receives direct input of raw sewage, which has a enriched ∂15N signature. Likewise, the region, especially in the lowlands, has a large abundance of agricultural activities. Whereas the Pacific Northwest streams received minimal fertilizer inputs from residential developments, the Rio Santa Maria da Vitoria receives significant fertilizer inputs from widespread agricultural activities.
The stable isotopic composition of particulate organic carbon also became relatively more enriched with increasing storm runoff. For example, ∂13C-POC varied from -27.1‰ to -24.0‰ throughout the week-long November sampling with the most enriched values occurring at peak discharge. The range of values measured indicates a primary input of C3 terrestrial organic matter and an enhanced input of C4 organic matter during peak discharge. This likely indicates that POC export from pasture and some agricultural land uses is enhanced relative to forest inputs during heavy rainfall reference to another work that also shows this?., the trend in ∂13C-POC values observed here was the opposite of what was previously observed in the Pacific Northwest during a similar sampling regime. In the Pacific Northwest ∂13C-POC became more depleted in 13C with increased discharge. This observed decrease in ∂13C-POC likely indicated that the particulate material mobilized by rapid surface runoff in the Pacific Northwest was relatively “more degraded” than that which was mobilized by base flow. The majority of particulate organic matter mobilized by early autumn storms in the Pacific Northwest was the product of litterfall accumulation during a long dry summer period. The region was primarily characterized by coniferous and deciduous forest land cover, whereas the Rio Santa Maria da Vitoria study contains a much higher abundance of agricultural and pasture land uses. In the Pacific Northwest, prior to mobilization by surface runoff, the accumulated litterfall experienced enhanced degradation relative to the particulate organic matter buried deep in soil layers. The particulate fractions present in deep soil layers also experience an exchange of matter between the dissolved phase via physical sorption, which can greatly alter its composition. It is likely that both biodegradation and sorption play an important role in the composition of both dissolved and particulate organic carbon compounds that are mobilized by base flow and rapid storm flow.
The trend of relatively more degraded particulate organic matter being mobilized by surface runoff compared to baseflow that was observed in the Pacific Northwest was not clearly evident in the tropical/sub-tropical Rio Santa Maria da Vitoria basin. Instead, the systematic enrichment of ∂13C-POC revealed the mobilization of a unique source of organic material. Interestingly, the base flow values of ∂13C-POC are very similar in both Espirito Santo and the Pacific Northwest, but opposite trends occur during rapid runoff. It is likely that old forest-derived organic carbon is the primary contributor to deep soil carbon that is mobilized by baseflow. Since forested land cover dominated the Pacific Northwest study region, the carbon that accumulates in shallow surface layers is, in a sense, of the same origin as the carbon that is buried in deep soil layers. However, these two pools experience different degradation and sorption regimes based on their lifespan (e.g. deep soil carbon was at one point present in the shallow surface pool and was subsequently altered by a series of physical and biological processes). The Rio Santa Maria da Vitoria basin, on the other hand, is much less uniform and contains significant landscapes that are characterized by agricultural or open pasture land uses. The observed variability in ∂13C-POC is most likely related to the mobilization of carbon from unique landscapes that become functionalized during periods of rapid rainfall. Depending on the exact vegetation type, agricultural and pasture land use regimes typically express a more enriched 13C signature. Thus, the observed increase in ∂13C-POC during storm flow in the Santa Maria da Vitoria basin indicates enhanced carbon inputs from pasture and agricultural land use regimes during periods of rapid rainfall. Whereas deep baseflow was primarily comprised of inputs from forested lands, rapid surface runoff mobilized relatively more particulate organic matter from pasture and agricultural lands.
In summary, the results show clearly that water transport during storm events is enhanced in fertilizers and wastewater and export from agricultural lands.