The annual cycle of primary production in the Sargasso Sea off Bermuda

Abstract

Gross and net primary production have been measured at bi-weekly intervals for 18 months in the North-western Sargasso Sea 15 miles SE. of Bermuda in 1500 fm of water. Ancillary data include temperature, salinity, phosphate, nitrite, nitrate, disolved oxygen, plant pigments, light penetration and incident radiation.

A seasonal cycle of production was observed with high levels in the winter and early spring, low levels in the later spring, summer and early fall. Gross production ranged from 0·13 to 2·00, averaging 0·44 g carbon assimilated/m²/day or 160 g C/m²/year. Net production ranged from 0·05 to 0·83 and averaged 0·20 g C/m²/day or 72 g C/m²/year.

Production was closely dependent upon vertical mixing, high levels occuring when the water was isothermal and mixed to or near the depth of the permanent thermocline (400 m), low levels being associated with the presence of a seasonal thermocline in the upper 100 m. Nutrient concentrations were extremely low (maxima of 1·8 μgAN/L and 0·16 μg AP/L as inorganic compounds in the upper 100 m) and showed little seasonal variability. Mixing and enrichment from the permanent thermocline is negligible, production being largely dependent upon the rapid re-cycling of nutrients within the upper 400 m.

The environmental factors controlling production which make this region differ from temperate or boreal waters, and which permit high production throughout the winter are: (1) low nutrient concentrations, (2) clear water, (3) relatively high incident radiation in winter, (4) a shallow winter mixed layer and (5) a rapid re-cycling of nutrients, possibly due to higher temperatures.