Organic matter degradation through oxygen respiration, denitrification, and manganese, iron, and sulfate reduction in marine sediments (the Kattegat and the Skagerrak)

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Abstract
A comprehensive study is reported on organic matter degradation through O2 respiration, denitrification and manganese- iron- and sulfate reduction in continental shelf sediments at 10 localities in the Kattegat and the Skagerrak (Denmark, Sweden and Norway). Total sediment community mineralization ranged from 4 to 31 mmol C m-2 d-1 with an average DIC:O2 ratio of 1.1. Most sediments exhibited an oxygen penetration depth of 0.33-0.6 cm except for a deeper-water (695 m) locality in the Skagerrak where O2 penetrated 1.9 cm into the sediment. The ratio of total O2 uptake measured by intact flux core incubation to diffusive O2 uptake rates measured with microelectrodes varied from -1 to 3.5 due to differences in benthic animal activity at the stations. Denitrification activity ranged from 0.01 to 1.03 mmol N m-2 d-1 and was primarily regulated by sediment nitrification activity. Manganese reduction was only significant at the deep-water locality in the Skagerrak. Iron reduction rates of 0 to 9.6 mmol C m-2 d-1 were found, and the activity was largely determined by the amount of poorly crystalline Fe(III) in the sediment. Sulfate reduction activity ranged from 0.1 to 9.6 mmol SO4 2- m-2 d-1at the stations investigated. A large heterogeneity was found between stations regarding the relative importance of the different carbon oxidation pathways. Within the Kattegat/Skagerrak region one may find sites where any one of the four carbon oxidation pathways, heterotrophic oxygen respiration, manganese reduction, iron reduction and sulfate reduction, is dominant. Thus, the present study demonstrates that even within short geographical distances a large variation in the relative importance of the different carbon oxidation pathways is to be expected in marine sediments.