Variability in the inorganic carbon system over the southeastern Bering Sea shelf during spring 1980 and spring—summer 1981

Abstract

Inorganic carbon system measurements made over the southeast Bering Sea shelf during March to June 1980 and April to July 1981 reveal extreme temporal variability that is governed primarily by biological processes. Inorganic carbon uptake by organisms reduced pCO 2 values from near or above atmospheric levels to minimums of 125 ppm by volume during both experiments, andΣCO 2 (CO 2 + H 2CO 3 + HCO 3 − + CO 3 2−) concentrations were reduced from∼2mM to minimums of∼1.75mM. The accumulation of organic carbon due to net community production, as estimated from changes in the inorganic carbon system, attained maximum values of∼100gC m −2 during each experiment. Although the dominant signals were of biological origin, effects of increasing insolation during spring and early summer, vertical mixing during storms, gas exchange with the atmosphere, and oscillatory currents were also evident. Although basic similarities existed between the 1980 and 1981 data, there were some interannual differences. For example, within the∼50to 150 m isobaths, the spring blooms of 1980 and 1981 both began in or near a front that is found near the 100 m isobath, but in 1981, the bloom began further seaward and about one week later (sometime between 11 and 22 April). The accumulation of organic carbon observed during our experiments tended to be lower in 1981, in the middle shelf where the 1980 and 1981 spring bloom values were∼100and 60 gC m −2, respectively, and average net community production rates were2.4gC m −2 d −1 during 12 April to 24 May 1980 and1.2gC m −2 d −1 during 12 April to 3 June 1981. The lower mid-shelf values in 1981 are in line with data on the interannual differences in wind mixing, grazing pressure, and nitrate levels. Our last ΣCO 2 and pCO 2 observations from 1980 (in late May) suggest that much of the organic carbon produced remained unmineralized. By the end of the 1981 experiment (mid-July), considerable remineralization had occurred near the middle of the shelf, but net inorganic carbon uptake continued at other locations. Ratios of change of inorganic carbon to inorganic nitrogen appeared to be higher than the ‘standard’ ratios. Atomic ratios of change for C:N may have been9 (vs Redfield, Ketchum and Richards' ratio of 6.6) even after correcting for the possible effect (∼30%) of carbonate precipitation, a factor that causes the ΔΣCO 2:ΔN ratio to be even higher.