Internal loading of phosphorus from sediments of Lake Pontchartrain (Louisiana,USA) with implications for eutrophication |
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Authors: | Eric D Roy Nhan T Nguyen Sibel Bargu John R White |
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Institution: | (1) Wetland & Aquatic Biogeochemistry Laboratory, Department of Oceanography and Coastal Sciences, School of the Coast and Environment, Louisiana State University, Baton Rouge, LA 70803, USA;(2) Phytoplankton Ecology & HAB Laboratory, Department of Oceanography and Coastal Sciences, School of the Coast and Environment, Louisiana State University, Baton Rouge, LA 70803, USA; |
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Abstract: | Diffusive flux of bioavailable soluble reactive phosphorus (SRP) across the sediment–water interface is one mechanism by which
sediments can be a source of phosphorus to the water column in aquatic systems and contribute to primary productivity. This
process is dependent on sediment biogeochemistry and SRP concentration gradients at the sediment–water interface. In systems
subjected to episodic external pulses of nutrient-rich water, SRP concentration gradients can have potential implications
for diffusive flux. In this study, we sought to investigate two hypotheses: (1) diffusive flux of SRP from sediments is a
significant source of SRP in the annual budget for the oligohaline Lake Pontchartrain estuary and (2) under SRP-depleted water
column conditions following large episodic, external pulses of nitrogen-rich Mississippi River water to the estuary, internal
SRP loading by diffusive flux can regenerate SRP in the water column to previously observed levels rapidly. Our specific objectives
were to: (i) determine sediment, water column, and phytoplankton characteristics at multiple locations in the estuary, (ii)
measure rates of SRP diffusive flux from sediments using intact cores under aerobic and anaerobic incubations, (iii) estimate
the potential for water column SRP regeneration by diffusive flux under SRP-depleted conditions using a simple model, and
(iv) estimate the annual load of SRP from the sediments by diffusive flux. Results indicate that diffusive flux of SRP from
Lake Pontchartrain sediments likely contributes ~30–44% of the annual SRP load to the estuary. Further, internal SRP loading
by diffusion has the potential to regenerate SRP in SRP-depleted waters to previously observed concentrations in <60 days.
Our findings suggest that a sequence of events is feasible where external pulses of nitrogen-rich water produce phosphorus-limited
conditions, followed by an internal pulse of SRP from sediments to restore nitrogen-limited conditions. This internal SRP
load may be an important contributor in promoting blooms of nitrogen-fixing harmful algae under summertime low-nutrient conditions. |
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