What is the benthic production/respiration ratio?
Significance of the p/r ratio
In shallow coastal waterways in southeastern Australia, sediment p/r ratios of >0.48, 0.34 – 0.48, 0.25 – 0.34 and <0.25% are indicative of oligotrophic, mesotrophic, eutrophic and hypertrophic conditions, respectively1. In more general terms, sediment p/r ratios of > 1 and < 1 indicate net autotrophic and net heterotrophic sediment conditions2.
Sediments that are net autotrophic produce more carbon than they respire, and tend to take up ammonium from the water column2. In comparison, sediments that are net heterotrophic are decomposing excess organic matter, which consumes dissolved oxygen2. Such sediments are more at risk for anoxia, and tend to release bioavailable ammonium to the water column2.
Some indicators that are closely related to sediment p/r ratio include denitrification efficiency and benthic CO2 flux. Water column PCO2 is also a measure of net autotrophic vs. net heterotrophic conditions in aquatic systems.
What causes sediment p/r ratios to change?
Sediment p/r ratios decrease mainly in response to carbon loading, which can be estimated by benthic CO2 flux determinations. A large proportion of the carbon loading comes from algal biomass produced in the water column. The algae increases turbidity, which decreases the amount of benthic photosynthesis. The algae also load the sediment with carbon when they die, increasing respiration. The overall effect is lowered p/r ratios. Coastal discharges including stormwater and sewage outfalls also convey readily degradable organic matter from catchments to coastal waterways, which can further lower sediment p/r ratios.
Considerations for measurement and interpretation
Sediment p/r ratios in estuaries are determined using benthic chambers. Benthic chambers are open-bottom containers (usually constructed out of perspex) that enclose an area of sediment and overlying water. The chambers are deployed on the sediments/substratum, to capture solute and gas movement (including carbon dioxide and oxygen) between the sediment and the water column. Sediment p/r ratios are calculated from dissolved oxygen and carbon dioxide fluxes.References
- Eyre, B. and Ferguson, A.J.P. 2002. Sediment biogeochemical indicators for defining sustainable nutrient loads to coastal ecosystems, Proceedings of Coast to Coast 2002 – “Source to Sea”, Tweed Heads, pp. 101-104.
- Eyre, B.D. and Ferguson, A.J.P. 2002. Comparison of carbon production and decomposition, benthic nutrient fluxes and denitrification in seagrass, phytoplankton, benthic microalgae- and macroalgae-dominated warm-temperate Australian lagoons. Marine Ecology Progress Series 229, 43-59.