Title The effect of sea ice on the oceanographic structure and phytoplankton biomass in the Nemuro Strait, a nursery area of salmon
Authers Ikutaro SHIMIZU
Keywords sea ice, phytoplankton, salmon, Nemuro Strait, Okhotsk Sea
Citation Bull. Fish. Res. Agen. No.27, 43-94, 2009
Coastal waters of the Okhotsk Sea and the Nemuro Strait of east Hokkaido are important nursery grounds for salmon fry released from hatcheries to the rivers of these districts. As an annual event in these coastal regions, sea ice drifting from the north of the Okhotsk Sea covers the area in winter and phytoplankton blooms occur following the retreat of sea ice in the spring. Spring phytoplankton blooms are thought to be important precedents to zooplankton production which supports the growth of salmon fry during their first oceanic stage. With establishment of a better management scheme of salmon resources as the ultimate goal, the objective of this study is to fill the gaps in our knowledge about mechanisms through which sea ice affects hydrography and phytoplankton production in the Nemuro Strait.
 Through analyzing water temperature, salinity, nutrients (nitrate, phosphate and silicate) and chlorophyll a collected during January and July in 1998-2002, the water under the ice cover was revealed as being nutrient-rich. As judged by nutrient ratios (N:P=11.2 and Si:P=23), the origin of the water under sea ice was considered to be Okhotsk Intermediate Cold Water which entered the Nemuro Strait before sea ice encroached. Nutrient-poor Soya Warm Water was found in the Nemuro Strait during summer only.
 Intensive study in 1998 revealed that massive phytoplankton blooms occurred after sea ice retreat in late March. Size fractionated chlorophyll a data showed the majority of phytoplankton was composed of >10 μm size, but small size fractions (2-10 μm and <2 μm) increased as the season progressed. Zooplankton abundance peaked following the spring phytoplankton bloom in 2001, and the most dominant component was Copepoda, followed by Sagittoida or jellyfishes.
 Massive phytoplankton blooms occurred after sea ice retreat in most years with the exception of 1999 when sea ice retreat was delayed to late April. The major mechanism of the onset of the phytoplankton blooms is the water column stratification due to ice melt combined with the development of a deep mixing layer within the critical depth. A moderate phytoplankton bloom in 1999 was attributed to a shallow mixing depth which limited nutrient availability for phytoplankton.
 The present results regarding the effect of the timing of sea ice retreat on the magnitude of phytoplankton blooms are opposite to those predicted from Hunt et al.’s hypothesis for the Bering Sea. This difference in the consequences of sea ice-phytoplankton bloom relationships between the Nemuro Strait and the Bering Sea may be reconciled if one takes into account of dissimilar daily solar radiation at equivalent seasons which would have a dissimilar effect on sea ice melting. The Bering Sea is located at higher latitudes than the Nemuro Strait, hence daily solar radiation in the former are much less than in the latter, and in the former early retreat of sea ice is not associated with meltwater leading to water column stratification.
URI http://www.fra.affrc.go.jp/bulletin/bull/bull27/simizu.pdf