Nutrient and oxygen sediment-water fluxes in Burgas Bay. 2022 IBER-BAS BRIDGE-BS WP5 survey

Sediment samples for studying nutrient fluxes at the sediment/water interface were collected in 2022 from Burgas Bay, southwestern Black Sea. A total of 23 samples were collected from 6 locations by diver sampling. The seabed depth varied between 3 and 11 meters. Three of the locations were vegetated with Zostera sp. and three were non-vegetated. The sediment differed between locations and ranged between fine sand, medium sand and shelly gravelly mud.

The sediment surface water layer (within 2 mm above the sediment surface) of each core was sampled with a syringe (5 mL water sample). Up to 1 mL was used to immediately measure dissolved oxygen concentration with a SI130 Microcathode Oxygen Electrode equipped with MC100 Microcell thermostatic chamber (Strathkelvin Instruments, Scotland). Porewaters (up to 5 mL) were collected by inserting a graduated stainless-steel needle (G-17, ~1.5 mm OD) into the sediments at 1 cm intervals, and gently sucking using a syringe. To prevent surface and/or upper porewater layer intrusion and mixing, the needle was equipped with a sampling flange as suggested in EPA (2013). In addition, each depth layer was sampled at a different position on the core surface. After the particulate matter has settled down, the remaining amount was transferred to precleaned plastic tubes, sealed and kept frozen until further analysis of the N-NO3, N-NH4 and P-PO4 concentrations (Grasshoff, 1976). Results lts are presented in mmol/m2/day.