Spatial variation of biogeochemical properties of landfast sea ice in the Gulf of Bothnia, Baltic Sea

摘要

Horizontal variation of landfast sea-ice properties was studied in the Gulf of Bothnia, Baltic Sea, during March 2004. In order to estimate their variability among and within different spatial levels, 72 ice cores were sampled on five spatial scales (with spacings of 10 cm, 2.5 m, 25 m, 250m and 2.5 km) using a hierarchical sampling design. Entire cores were melted, and bulk-ice salinity, concentrations of chlorophyll a (Chl a), phaeophytin (Phaeo), dissolved nitrate plus nitrite (DIN) as well as dissolved organic carbon (DOC) and nitrogen (DON) were determined. All sampling sites were covered by a 5.5–23 cm thick layer of snow. Ice thicknesses of cores varied from 26 to 58 cm, with bulkice salinities ranging between 0.2 and 0.7 as is typical for Baltic Sea ice. Observed values for Chl a (range: 0.8–6.0 mg Chl a L–1; median: 2.9 mg Chl a L–1) and DOC (range: 37–397 mM; median: 95 mM) were comparable to values reported by previous sea-ice studies from the Baltic Sea. Analysis of variance among different spatial levels revealed significant differences on the 2.5km scale for ice thickness, DOC and Phaeo (with the latter two being positively correlated with ice thickness). For salinity and Chl a, the 250m scale was found to be the largest scale where significant differences could be detected, while snow depth only varied significantly on the 25m scale. Variability on the 2.5m scale contributed significantly to the total variation for ice thickness, salinity, Chl a and DIN. In the case of DON, none of the investigated levels exhibited variation that was significantly different from the considerable amount of variation found between replicate cores. Results from a principal component analysis suggest that ice thickness is one of the main elements structuring the investigated ice habitat on a large scale, while snow depth, nutrients and salinity seem to be of secondary importance.