Abstract
Estuaries are biologically productive transition zones between land and sea that play a vital role in transforming, recycling, and sequestering nutrients and organic matter, thus influencing nutrient loading to coastal systems. Yet, the processes involved in phosphorus (P) transformation and cycling among inorganic and organic P forms are poorly known in estuaries. To better understand the potential for P transformation and sequestration, we identified P forms and estimated their contributions to total P in intertidal wetland sediments of a river-dominated estuary (Columbia River, Oregon, USA) using solution 31P nuclear magnetic resonance spectroscopy (P-NMR). Inorganic P forms dominated sediment P extracts throughout the estuary, with orthophosphate accounting for 71–84% of total extracted P. However, biologically-derived inorganic and organic P forms were also detected. Polyphosphates were found in sediment extracts throughout the estuary, contributing as much as 10% of extracted P. Similar to other wetlands, orthophosphate monoesters and diesters made approximately equal contributions (~ 20%) to total extracted P. However, monoesters (e.g., phytate) were more abundant in sedimentary environments characterized by low organic matter content, while diesters (e.g., DNA) were more abundant in sedimentary environments with high organic matter, regardless of salinity. Collectively, the data show strong evidence for P transformation in sediments of a large, river-dominated estuary, which influences its transport to the coastal Pacific Ocean via the expansive Columbia River plume.
Highlights
Phosphorus (P) is an essential but often limiting nutrient in freshwater aquatic and terrestrial ecosystems (Daniel et al, 1998); downstream systems are increasingly under threat from nutrient-stimulated eutrophication (Conley et al, 2009; Tiessen et al, 2011)
Watershed losses result from inefficient recycling, or poor sequestration, and lead to P transport from soils to surface waters; excess P ends up in estuaries and coastal systems where it often results in nutrient-driven eutrophication
Combined with downstream transport in rivers, estuaries are vulnerable to nutrient pollution because dissolved inorganic P desorption from sediment surfaces occurs when anions in seawater compete with phosphate anions for binding sites, leading to P efflux (Fox et al, 1986; Froelich, 1988; House and Warwick, 1999; Monbet et al, 2010)
Summary
Phosphorus (P) is an essential but often limiting nutrient in freshwater aquatic and terrestrial ecosystems (Daniel et al, 1998); downstream systems are increasingly under threat from nutrient-stimulated eutrophication (Conley et al, 2009; Tiessen et al, 2011). Estuaries are biologically productive transition zones comprising a variety of habitats including wetlands, mudflats, and seagrass beds between freshwater and marine systems They play a vital role in transforming, recycling, and sequestering nutrients and organic matter (OM) prior to entering the coastal ocean (Barbier et al, 2011). The reducing conditions associated with intense OM remineralization increase P desorption, setting up a positive feedback loop whereby eutrophic conditions contribute to P mobilization and limit nutrient sequestration (Howarth et al, 2011; Li et al, 2013) This impairs the critical function of estuaries and wetlands, which is to reduce nutrient loads before they reach coastal waters (Barbier et al, 2011)
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