In the beginning of July oxygen depletion in the Kiel fjord
Longer lasting high temperatures and intense solar irradtiation have not only led to an increase in phytoplankton but also enhanced an explosion like increase in filamentous algae. Right now, a new problem is imminent in the sheltered areas of Kiel fjord: Oxygen depletion due to microbial degradation of organic substances.
Sulphuric bacteria (Beggiatoa) on the sediment are a distinct indicator of oxygen depletion near the seafloor. (Picture: H. auf dem Venne)
We were able to see for ourselves what engaged participants informed us via mail about: At several places in the Kiel fjord (Strande, Schilksee, Falckenstein Beach) oxygen depleted zones are obvious. One just has to look for dark spots on the seafloor - there you find a carpet like growth of white sulphuric bacteria. Although these bacteria do need oxygen for metabolism, they also require high concentrations of hydrogen sulfide (H2S).
During scuba diving near Friedrichsort Lighthouse and in Strande/Schilksee we noticed a high number of dead common starfish (Asterias rubens). Those that were still alive protruded their arms away from the seafloor and looked really bad. Another clear sign of severe oxygen depletion is the fact that many of the infauna organisms living beneath the seafloor now can be seen on the surface of the sediment - if at least partially. This was observed in Razor clams (Ensis directus) and soft-shell clams (Mya arenaria). Additionally, some impressive polychaetes with more than 20 cm in length (e.g. Nereis) are driven from the shelter of their sediment tubes and dwell on the seafloor during daytime.
Oxygen depletion situations are quite common in the Baltic Sea and occur every once in a while, especially in summer. Indications of oxygen depletion this early in the year however are exceptional and definitely related to quite high water temperatures of 17°C down to 14m depth.
