Recent oxygen depletion and benthic faunal change in shallow areas of Sannäs Fjord, Swedish west coast

Abstract

Sannäs Fjord is a shallow fjord (<32m w.d.) with a sill depth of 8m, located at the Swedish west coast of the Skagerrak (North Sea). The anthropogenic impact on the fjord represents combination of sewage from the local village of Sannäs and land run-off from agricultural areas. Sewage impact has been reduced since 1991 and today the fjord is included into several nature conservation programs administrated by the European Union. Yet, observations during the summers of 2008–2011 show that the shallow inner fjord inlet experiences severe oxygen depletion at 5–12m water depth. To explore if the oxygen depletion is only a recent phenomenon and to evaluate the potential of fjord sediments to archive such environmental changes, in 2008 and 2009 seven sediment cores were taken along a transect oriented lengthwise in the fjord. The cores were analysed for organic carbon, C/N, benthic foraminifera and lead pollution records (as relative age marker). Carbon content increases in most of the cores since the ~1970–80s, while C/N ratio decreases from the core base upward since ~1995. Foraminiferal assemblages in most core stratigraphies are dominated by agglutinated species. Calcareous species (mainly elphidiids) have become dominant in the upper part of the records since the ~late 1990s or 2000 (the inner fjord and the deepest basin) and since the ~1950–70s (the outer fjord). In the inner Sannäs Fjord, an increase of agglutinated foraminiferal species (e.g. Eggerelloides scaber) and organic inner linings occurred since the ~1970s, suggesting an intensification of taphonomic processes affecting postmortem calcareous shell preservation. A study of living vs. dead foraminiferal assemblages undertaken during June–August 2013 demonstrates that in the shallow inner fjord, strong carbonate dissolution occurs within 1–3months following the foraminiferal growth. The dissolution is linked to corrosive conditions present within the sediment - bottom water interface, and is likely caused by the organic matter decay, resulting in severe hypoxia to anoxia. Oxygen depletion at <10m w.d. develops fast due to the small water volume and limited bottom water exchange caused by a close proximity of pycnocline to the fjord bottom. Sediment cores from the deep fjord basin and the outer fjord are, on the contrary, characterized by good to excellent preservation of foraminiferal shells, higher sediment accumulation rates, and the greatest potential for high-resolution paleoenvironmental studies. Increased frequencies of low-oxygen tolerant species (e.g. Stainforthia fusiformis) in the outer fjord after ~ the 1970s suggests that increased primary productivity and seasonal oxygen deficiency have existed in the area over the last century.Recent milder winters, absent sediment reworking by freezing and grounding of sea-ice, increased nutrient load due to higher precipitation and land run-off, and the luxuriant growth of filamentous green algae followed by the organic matter decay are discussed among the mechanisms driving formation of recent oxygen deficiency in the shallow fjord inlets.