Linking benthic algae to sediment oxidation-reduction dynamics: implications for sediment-water interface nutrient cycling

Abstract

Invasive macroalgae threaten the diversity and function of ecosystems and have the potential to alter environmental biogeochemistry. In Hawaiʻi two red macroalgae, Acanthophora spicifera and Gracilaria salicornia, are successful invasive species. Morphologically these species are different, A. spicifera is more flexible and forms less dense canopies than the dense mats created by G. salicornia. This dissertation examined the influence of A. spicifera on diel variations in water column nutrient concentrations and sediment oxidation-reduction (redox) chemistry and the effect of G. salicornia on water flow and redox chemistry. Diel changes were evident through changes in the concentration of water column oxygen and sediment Mn2+ and ammonium concentrations, especially in sites colonized by A. spicifera. Additionally, sites with A. spicifera had higher sediment ammonium concentrations and ammonium flux to the water column. In situ electrochemical profiles revealed that hydrogen sulfide was present in the water column within A. spicifera canopies. The detection of Mn2+ and dissolved iron, both in surface waters and at the sediment water interface, provide evidence that colloidal Fe3+ and manganese oxides may be sorbing phosphate, contributing to the low dissolved phosphate concentrations observed throughout the study. Despite the difference in sediment redox chemistry and sediment nutrient concentrations between sites with and without the invasive macroalga, no difference in water column nutrients was found. Similar nutrient concentrations between areas with A. spicifera and devoid of the alga are likely due to interception and uptake of nutrients at the SWI by the macroalga. Gracilaria salicornia was found to reduce water velocity (xRMS), turbulent kinetic energy and turbulent xRMS compared to adjacent macroalgae free regions. Thin, (less than 1 cm) vertically isolated suboxic zones occurred within the otherwise oxic water column of algal mats, which corresponded to regions within the mat where turbulent xRMS of the dominant flow direction dropped to speeds near 0.001 m/s. These suboxic microniches provide environmental conditions suitable for redox sensitive nutrient transformations, such as reductive solubilization of phosphate bound to iron oxides, to take place. Invasive algae are altering sediment and water column redox conditions which may provide an ecological advantage to the algae.