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Marine Ecology Progress Series

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MEPS 387:39-49 (2009)  -  DOI:

Coastal water darkening and implications for mesopelagic regime shifts in Norwegian fjords

Dag L. Aksnes1,*, Nicolas Dupont1, Arved Staby1, Øyvind Fiksen1, Stein Kaartvedt2, Jan Aure3

1Department of Biology, University of Bergen, 5020 Bergen, Norway
2Department of Biology, University of Oslo, 0316 Oslo, Norway
3Institute of Marine Research, 5817 Bergen, Norway

ABSTRACT: The light regime of the water column has a strong structuring effect on aquatic food webs and it has been previously hypothesized that coastal water darkening has increased the success of tactile predators relative to visual predators such as fish. Due to a general lack of time-series of optical parameters, we applied a proxy for light attenuation that depends on salinity and dissolved oxygen at a time of the year when chlorophyll concentrations were low. We present evidence that coastal waters of Norway have darkened as a result of freshening over the period 1935 to 2007. Our results suggest that locations where coastal water penetrates into deep basins have been prone to water column darkening, particularly if dissolved oxygen has also declined. We have estimated that salinity and oxygen variation on the ranges 33 to 35 PSU and 1 to 6 ml O2 l–1 are associated with up to 8 orders of magnitude difference in photon flux at 200 m depth in a water column devoid of pigments. Our results suggest such darkening needs to be considered in analyses of mesopelagic regime shifts involving mass occurrence of the jellyfish Periphylla periphylla.

KEY WORDS: Light attenuation · Regime shift · Freshening · Darkening · Coastal ecosystems

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Cite this article as: Aksnes DL, Dupont N, Staby A, Fiksen Ø, Kaartvedt S, Aure J (2009) Coastal water darkening and implications for mesopelagic regime shifts in Norwegian fjords. Mar Ecol Prog Ser 387:39-49.

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