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No 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 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.

No abstract

Advective rates of nutrients, phytoplankton and zooplankton were estimated for the Masfjord, western Norway, in June and October 1985. The advective contribution to the phytoplankton biomass formation was dearly less than the local growth. Advection of nutrients, even below the photic zone, may have large implications for the new production of the fjord. The highest renewal rate (13.6 % d l ) due to advection was obtained for the mesozooplankton in June. While 20 % of this renewal was due to water advection alone, 80 % was due to the combined effect of the current profile and the vertical distribution of the mesozooplankton. Thus transport of mesozooplankton may exceed local growth significantly. The deep basin (494 m) of the fjord serves as a refuge for resident macrozooplankton and rnicronekton populations predating on mesozooplankton. Analysis suggests that such predators may be more sensitive to a change in the advective rate than to a similar change in the prey growth rate. Advection may be an important factor underlying the carrying capacity of mesozooplankton predators in fjords.

No abstract

The abundance of returning adult Atlantic salmon Salmo salar, in the River Orkla in mid-norway (1 sea-winter, SW, fish) and River Hals in north Norway (1-3 SW fish), was tested against the early marine feeding and the seawater temperature experienced by their corresponding year classes of post-smolts immediately after entry into the Trondheimsfjord (Orkla smolts, 22 years of data) and Altafjord (Hals smolts, 17 years of data). In both river-fjord systems, there was a significant positive correlation between the abundance of returning S. salar and the mean seawater temperature at the time of smolts descending to the sea. The number of 1SW fish reported caught in River Orkla was positively correlated to the proportion of fish larvae in the post-smolt stomachs in Trondheimsfjord. The abundance of returning S.salar was, however, neither correlated to forage ratio (R(F)) nor other prey groups in post-smolt stomachs in the two fjord systems. In the Altafjord, the post-smolts fed mainly on pelagic fish larva (70-98%) and had a stable R(F) (0.009-0.023) over the 6 years analysed. In the Trondheimsfjord, however, there was a higher variation in R(F) (0.003-0.036), and pelagic fish larvae were dominant prey in only two (50 and 91%) of the 8 years analysed. These 2 years also showed the highest return rates of S. salar in River Orkla. These results demonstrate that the thermal conditions experienced by post-smolts during their early sea migration may be crucial for the subsequent return rate of adults after 1-3 years at sea. Pelagic marine fish larvae seem to be the preferred initial prey for S. salar post-smolts. As the annual variation in abundance of fish larvae is related to seawater temperature, it is proposed that seawater temperature at sea entry and the subsequent abundance of returning adult S. salar may be indirectly linked through variation in annual availability of pelagic fish larvae or other suitable food items in the early post-smolt phase.

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