Benthic nutrient fluxes in the Eastern Gotland Basin (Baltic Sea) with particular focus on microbial mat ecosystems

• High seabed release of PO43 −and inorganic N in the Eastern Gotland Basin
• Total seabed P and N release was 8 and 2-fold higher than the external nutrient load
• A microbial mat ecosystem apparently affected benthic P and N fluxes

Benthic fluxes and water column distributions of dissolved inorganic nitrogen (DIN) and total dissolved phosphate (PO43 −) were measured in situ at 7 sites across a redox gradient from oxic to anoxic bottom waters in the Eastern Gotland Basin (Baltic Sea). The study area was divided into the oxic zone (60 to ca. 80 m water depth, O2 > 30 μM), the hypoxic transition zone (HTZ, ca. 80 to 120 m, O2 < 30 μM) and the deep anoxic and sulfidic basin (> ca. 120 m). Sediments in the HTZ were covered by mats of vacuolated sulfur bacteria. Ammonium (NH4+) fluxes in the deep basin and the HTZ were elevated at 0.6 mmol m− 2 d− 1 and 1 mmol m− 2 d− 1, respectively. Nitrate (NO3−) fluxes were directed into the sediment at all stations in the HTZ and were zero in the deep basin. PO43 − release was highest in the HTZ at 0.23 mmol m− 2 d− 1, with a further release of 0.2 mmol m− 2 d− 1 in the deep basin. Up-scaling the benthic fluxes to the Baltic Proper equals 109 kt yr− 1 of PO43 − and 266 kt yr− 1 of DIN. This is eight- and two-fold higher than the total external load of P (14 kt yr− 1) and DIN (140 kt yr− 1) in 2006 (HELCOM 2009b). The HTZ makes an important contribution to the internal nutrient loading in the Baltic Proper, releasing 70% of P (76 kt yr− 1) and 75% of DIN (200 kt yr− 1) despite covering only 51% of area.