OXYGEN-CONSUMING PROCESSES AT THE PROFUNDAL AND LITTORAL SEDIMENT WATER INTERFACE OF A SMALL MESO-EUTROPHIC LAKE (LAKE VECHTEN, THE NETHERLANDS)

Oxygen consumption in the profundal gyttja and littoral sands of Lake Vechten was determined with pore-water analyses and core incubations. Methane (58-64%), ammonium (10-13%), iron (2-6%), and sulfide (< 12%) oxidation accounted for approximately 75% of the oxygen consumed in the profundal sediments. Almost the complete diffusive flux of CH4 out of the anoxic layer into the oxic surface layer (7.9-9.4 mmol m-2 d-1) was oxidized, while only 27-36% of the NH4+ flux (5.1-5.2 mmol m-2 d-1) was oxidized in the oxic surface layer and 64-73% diffused into the overlying water. In contrast to the profundal sediments, oxidation of reduced end products of anoxic respiration was of minor importance (< 15%) in littoral sandy sediments. Also, sediment oxygen consumption rate was higher in the profundal gyttja (29 mmol m-2 d-1 at 7-degrees-C) during overturn than in the littoral sandy sediments in both winter (8.7 mmol m-2 d-1 at 7-degrees-C) and summer (18.6 mmol m-2 d-1 at 22-degrees-C). Poisoning the sediment cores with formaldehyde stopped bacterial activity, but oxygen profile measurements showed that this treatment does not reveal the bacterial or chemical nature of the oxidative processes.