Recycling and burial of phosphorus in sediments of an anoxic fjord-the By Fjord, western Sweden

Abstract

Recycling and burial of sediment phosphorus were studied in the By Fjord, western Sweden, during the years 2009 to 2010 using autonomous benthic landers and sediment sampling. The By Fjord is a small fjord with a shallow sill at its narrow mouth, which limits water exchange of the fjord's basin water. The water in the basin is exchanged only every 3 to 5 years and the water below sill level is anoxic or sulfidic between water renewals. Five sites were examined in the By Fjord; three shallow sites above the sill level with oxic bottom waters and two deeper sites with anoxic bottom waters. Contents of sediment organic carbon, total nitrogen and organic phosphorus were higher at deep stations when compared to shallow ones, whereas the contents of sediment inorganic phosphorus was higher at shallow than at deep stations both in surficial and buried sediment. One shallow oxic site and one deep anoxic site were also examined in the adjacent Koljo Fjord having similar characteristics as the By Fjord. In situ measurements of benthic fluxes of dissolved inorganic phosphorus (DIP) showed that the fluxes from sediments with oxic overlying water (0.05-0.23 mmol m(-2) d(-1)) were much lower than fluxes from sediments with anoxic overlying water (1.25-2.26 mmol m(-2) d(-1)). The DIP flux increased with increasing flux of dissolved inorganic carbon (DIC) not only at anoxic but also at oxic bottoms, which is different from observations in brackish water environments. The average ratio between the DIC and DIP fluxes at oxic bottoms was almost 10 times higher than the Redfield C: P ratio indicating partial immobilization of P in oxic sediments. In contrast, the C: P ratio in fluxes was on average 1.5 times lower than Redfield at the anoxic bottoms. The benthic fluxes from anoxic bottoms were P rich not only in relation to C, but also to N. The low C: P flux ratio at anoxic sites coincided with an about 2.5 times higher than Redfield C: P ratio of organic matter in the sediment solid phase clearly suggesting preferential regeneration of P at anoxic bottoms. Burial of inorganic P was higher than organic P burial at both anoxic and oxic sites; the former made up 59 to 60% of the total P burial at the deep anoxic stations, and 80% at the main shallow oxic station. The burial efficiency for organic P at anoxic bottoms was estimated to be only 1 to 3%, which indicates extremely efficient recycling of deposited organic P under anoxic conditions in this fjord environment.