Phosphorus storage and mobilization in coastal Phragmites wetlands: Influence of local-scale hydrodynamics

Abstract

Coastal Phragmites wetlands are at the interface between terrestrial and aquatic ecosystems and are important for nutrient retention and regulation. They can act both as nutrient sinks and sources for phosphorus, depending on environmental conditions, sediment properties as well as antecedent nutrient loadings and sorption capacities of the sediments. The Darss-Zingst Bodden Chain is a shallow lagoon system at the German Baltic coast with a long eutrophication history which is lined almost at its entire length by Phragmites wetlands. In order to elucidate under which conditions these wetlands act as sources or sinks for phosphorus, in-situ data of chemo-physical characteristics of water and sediment samples were combined with hydrodynamic measurements and laboratory experiments. A basin zone within the wetland serves as accumulation sink for fine-grained particles rich in phosphorus, iron, manganese and organic matter. The Phragmites stems reduce the water flow and allow the accumulation of organic matter and minerals. Without turbulent mixing and downward flow the bottom water and the sediment surface lack replenishment of oxygen. The relationship between oxygen saturation in the water and soluble reactive phosphorus (SRP) concentrations showed a threshold-type behavior. Under oxygen deficiency soluble reactive phosphorus concentrations in the water were significantly higher than under aerated conditions (oxygen saturation <10%: mean SRP = 0.22 mg/L vs. oxygen saturation >10%: mean SRP = 0.02 mg/L). During stagnant periods with low water level, low-turbulence and thus low-oxygen conditions phosphorus from the sediments is released. But the sediments are capable of becoming sinks again once oxygen is resupplied. When vertical downward flows are dominant in the water and turbulent kinetic energy rises, oxygen saturation increases. A thin oxic sediment surface layer rich in iron and manganese is able to readsorb phosphorus quickly as kinetic sorption experiments show, but due to its redox sensitivity the immobilization is – at least in part – only temporary. This study demonstrated that sediments in coastal Phragmites wetlands can switch their function from sink to source of soluble reactive phosphorus on a very short time-scale, depending on local-scale hydrodynamic conditions and the state of the oxic–anoxic sediment interface.