On the calibration and use of in situ ocean colour measurements for monitoring algal blooms

Simultaneous in situ measurements of chlorophyll concentration and water co lour are reported at three diverse sites: a sea loch, the west Scottish she lf and the north Atlantic. A good (R-2 = 0.97) log-log relation exists betw een the ratio, gamma, of the irradiance reflection coefficients at 490 and 570 nm and the chlorophyll concentration, C, over a range of chlorophyll co ncentrations from 0.01 to nearly 50 mg m(-3). This relation can be expresse d as C = 1. 87 gamma (-1. 81). The equation is very similar to that used in remote sensing algorithms for the conversion of ocean colour data into chl orophyll concentrations. The root mean square ( RMS) variation of observed chlorophyll about the values predicted by this equation is 33%. The robustn ess of this algorithm implies that this colour ratio can be used to monitor chlorophyll concentrations in case 1 waters with a minimum of additional c alibration information. As an illustration, a time series is presented of c olour-ratio-derived chlorophyll concentration during the spring bloom in a Scottish sea loch in 1994. The data show reasonable agreement with the chlo rophyll time series measured by a recording fluorometer, and a comparison o f the two time series serves to highlight the advantages and disadvantages of each instrument.