The annual cycle of phytoplankton primary production and hydrography in the Disko Bugt area, West Greenland

Abstract

The distribution and size of phytoplankton production and biomass in relation to physical and chemical parameters in the upper 50 m at Godhavn and in Kangikerdlak in the inner part of Disko Fjord was investigated through 2½ years (1973-75). Some data from other parts of Disko Bugt are presented.

In both locations the hydrography alternates between an unstable winter situation with isothermal (-1.75°C) and isohaline (33.5-34.0‰) conditions throughout, and a highly stable summer situation when dilution and heating, especially of the upper 20- 30 m, raise the temperature at the surface to 9.9°C and at 50 m to 3.8°C at Godhavn, and to 12°C and 3.5°C respectively in Kangikerdlak. Salinities drop correspondingly to 30.6‰ in Kangikerdlak.

The 1% depth for green light is greatly reduced beneath ice and snow. During the ice free period at Godhavn it varies from 12 m during the spring phytoplankton bloom to more than 60 m from Oct. through the winter. In Kangikerdlak the 1% depth reaches only 40 m in winter, and outflowing turbid fresh water creates I% depths of as little as 4-5 m in June-Aug.

At Godhavn NO₃-N reaches highs of 10.05 μgat /liter and 10.15 μgat /liter at 0 and 50 m respectively in winter, whereas during the summer, depletion to less than 0.01 μgat /liter occurs in the upper 40 m and to 1.0 μgat / liter at 50 m. PO₄-P is similarly reduced from 0.8 μgat/liter and 1.1 μgat/liter to less than 0.01 μgat / liter in the upper 20 m and to 0.21 μgat /liter at 50 m. The N:P ratio drops from 13 to less than 0.01 in the upper 30 m and to 1.0 at 50 m. In Kangikerdlak depletion of NO3-N is similar to conditions at Godhavn, whereas PO4-P reaches a low of 0.1 μgat/liter only, while in mid summer it reaches 1.88 μgat /liter at the surface, giving an N:P ratio which is below 0.1 in the upper 5 m only.

At Godhavn primary production is about 90 gC • m^-2 • yr^-1  (75-104 g) with a maximum of about 5 .5 gC • m^-3 • yr^-1   at 5- 10 m, whereas in Kangikerdlak production was concentrated near the surface with about 6.0 gC • m^-3 • yr^-1and a total of 35 gC • m^-2 • yr^-1 at most. Production at Jacobshavn off the glacier fjord is probably greater than at Godhavn, whereas at Christianshilb and Egedesminde it is definitely lower. Phytoplankters larger than 56 μ contribute about 50% of annual and up to 90% of daily production.

Due to the great stability, production usually extends no deeper than compensation depth, and most of the chlorophyll is usually in the nutrient rich water below this depth, where it sinks, is consumed, or degrades into phaeopigment. P/B is highest where there is least chlorophyll. Light reduces production in the upper 5-10 m, and inhibition may extend to 30 m. Correlations between production, P/B, or P/8 / light and nutrients reveal possible saturation values of 0.08-0. 78 μgal NO₃-N /liter and 0.17-0.22 μgat PO4-P/liter. PO4-P seems to be the limiting nutrient in some cases, although NO₃-N is most quickly and thoroughly depleted.

Dark fixation at Godhavn is about 24 gC • m^-2 • yr^-1, and at Kangikerdlak about 15 gC • m^-2 • yr^-1. 55-60% of dark fixation is presumed to be biotic and 16-64% is associated with particulate matter larger than 56 μ.

Although oxygen is never at a minimum in Disko Bugt, saturation as well as absolute 0 2 values and pH show profiles in the bay that clearly reflect the high degree of stratification compared to waters south of the bay.