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 O 2 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.

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We evaluate the hypothesis that the vertical migration of Periphylla periphylla is governed by its sensitivity to light intensity. By applying an individual-based model where random walk is combined with assumed individual responses to light, we compare the predicted vertical distributions with acoustical observations. Important features of the observed P. periphylla distributions can be explained by a simple proximate light response where individual P. periphylla avoids light above a certain threshold but also has a preference for very low light intensities. In addition to accounting for the observed synchronous diel vertical migration phenomenon in P. periphylla, this simple mechanism also accounts for simultaneous asynchronous vertical migrations observed for part of the population.

a b s t r a c tIn marine ecosystem models, the underwater light intensity is commonly characterized by the shading of phytoplankton in addition to a background light attenuation coefficient. Colour dissolved organic matter (CDOM) is an important component of the background light attenuation, and we investigate how variation in CDOM attenuation affects euphotic zone properties in a general marine ecosystem model. Our results suggest that euphotic zone properties are highly sensitive to CDOM variations occurring in nature. While the nutrient input to the euphotic zone scales the magnitude of the primary production, the vertical structure of nutrients and phytoplankton is largely determined by the variation in CDOM attenuation in our simulations. This suggests that knowledge of CDOM variation is useful to constrain uncertainties in predictions of water column structure in marine ecosystem modelling, but also in analyses utilizing the oceanic nutricline depth as proxy for primary production. Finally, according to our sensitivity analysis, many coastal areas experiencing high loads of terrestrial CDOM are expected to show eutrophication symptoms induced by altered optics.

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