Patterns of benthic oxygen uptake in a hypertrophic lagoon: spatial variability and controlling factors

Water temperature, organic matter quality and quantity and macrofauna activity generally regulate the seasonal evolution of benthic oxygen uptake in coastal areas. We hypothesize that highly productive lagoons can represent an exception in this respect, due to alternating sequences of phytoplankton bloom, dystrophy and collapse events, coupled with water anoxia and azoic sediments. In order to verify this assumption, total oxygen uptake (TOU) and diffusive oxygen uptake (DOU) were determined during the ice-free period of 2009 in the sediments of a hypertrophic basin (the Curonian Lagoon, Baltic Sea). Seasonal measurements were carried out via sediment incubation and microprofiling in littoral and pelagic areas. TOU increased from spring to summer, but it remained elevated also in autumn likely due to accumulation of labile organic matter after algal blooms. TOU and DOU closely agreed in pelagic areas, while at littoral sites TOU exceeded DOU, suggesting temporal or local importance of bioturbating organisms. Water chlorophyll a and oxygen saturation were likely the most important driving factors for benthic respiration. Very limited oxygen penetration (<1?mm) over a 6-month period possibly enhances nitrogen removal via denitrification and reactive phosphorus efflux.