Survival and energy metabolism in an oxygen deficient environment. Field and laboratory studies on the bottom fauna from the profundal zone of Lake Esrom,Denmark

The three macroinvertebrate taxa, Potamothrix hammoniensis, Chironomus anthracinus and Pisidium spp. are permanent inhabitants of the regularly microxic/anoxic profundal zone in Lake Esrom. In situ and laboratory studies (10 °C) of metabolism (aerobic and anaerobic) and anaerobic survival in P. hammoniensis and Pisidium spp. are compared with previous results from C. anthracinus. The late summer microxic conditions in the lake lasts 2–2 months, during which the three taxa display metabolic and behavioral strategies in order to survive. All three are respiratory oxy-regulators with critical oxygen levels at 1 mg O₂ l^–1 (P. hammoniensis and Pisidium spp.) or 2–3 mg O₂ l^–1 (C. anthracinus). The lethal time (LD₅₀) in experimental anoxia follows a similar trend, with 150–170 days of survival in P. hammoniensis and Pisidium spp., compared to 2–5 weeks in C. anthracinus. The glycogen stores are almost (C. anthracinus) or fully exploited (P. hammoniensis and Pisidium spp.) during anaerobis and the animals finally enter a state of quiescence or dormancy. During the late phase of anoxia, their metabolism is down at (C. anthracinus) or below (P. hammoniensis and Pisidium spp.) 1% of normoxic metabolism. The populations in the lake behave rather similar in so far that the energy gain from anaerobic degradation of glycogen maximizes 1% of normoxic conditions regardless of species. Also, in Pisidium this appears to be the only energy source during dormancy. However, as previously presented in case of C. anthracinus, P. hammoniensis maintain a partly aerobic metabolism constituting 44% of normoxia during the microxic period, compared to the 12–19% obtained by C. anthracinus. It is thus demonstrated that P. hammoniensis and Pisidium spp. possess a remarkable ability to survive in situ severe oxygen depletion. P. hammoniensis can benefit from the presence of merely traces of oxygen, whereas C. anthracinus with poorer anaerobic survival is strongly dependent on minute oxygen supplies.