Abstract
AbstractNutrient and carbon dynamics in river ecosystems are shifting, and climate change is likely a driving factor; however, some previous studies indicate anthropogenic modification of natural resources may supersede the effects of climate. To understand temporal changes in river ecosystems, consideration of how these agents act independently and collectively to affect watershed biogeochemistry is necessary. Through the Georgia Coastal Ecosystems Long‐Term Ecological Research Project, we assessed nutrient (phosphorus, nitrogen, silicate) and carbon dynamics, with specific regard to import and export, in the Altamaha River Basin from 2000 to 2012. This is the first study in the region to document the biogeochemical patterns in the Altamaha's four main tributaries, the Little Ocmulgee, Ocmulgee, Oconee, and Ohoopee rivers, and the relationships between biogeochemistry and historical precipitation and discharge patterns as well as agricultural and population census data. As discharge patterns are a primary driver of nutrient loads, we determined that water use was a dominant factor in the shifting ecosystem dynamics. Dissolved inorganic nitrogen loads were primarily driven by population density and dissolved inorganic phosphorus loads were strongly influenced by livestock biomass. Taken together, we conclude that both the transportation and biogeochemical cycling of nutrients within the Altamaha River Watershed were highly impacted by anthropogenic influences, which were then further exacerbated by continued climate change. Furthermore, the N‐ and P‐loads in the Altamaha River and tributaries were dominated by dissolved organic nitrogen and dissolved organic phosphorus, emphasizing a need to further study the bioavailability of these species and the mechanisms driving their potential ecological impacts.
Highlights
The interplay between climate- and human- related impacts on water resources has historically been a challenge to quantify
An analysis of the data points lying outside the 95% confidence interval of the log–log regression showed that across the Altamaha Watershed, the instances in which Total dissolved nitrogen (TDN) concentrations were lower than expected generally occurred during the summer or early fall (June, July, August, or September)
The predominance of both organic nitrogen and phosphorus, correlations between population, livestock density, and the proportion of agricultural land with P- and N-d ynamics, and the decrease in overall v www.esajournals.org discharge as a result of increased populations found in this study underscore the need to (1) assess both the inorganic and organic nutrients expor ted into and out of watersheds and (2) integrate the effects of anthropogenic influences and climate change when assessing the health of watershed ecosystems
Summary
The interplay between climate- and human- related impacts on water resources has historically been a challenge to quantify. Drawdown of groundwater resources along the coast, exacerbated by climate-d riven sea-level rise (SLR), generates saltwater intrusion into local groundwater (aquifer salinization) and into watershed surface waters (Whitehead et al 2009, Rotzoll and Fletcher 2013). This decreases the availability of potable groundwater and stimulates the mineralization of organic carbon currently stored in freshwater marshes (Weston et al 2006, Rotzoll and Fletcher 2013)
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