Most bio-optical work and remote sensing for the estimation of biomass and productivity has concentrated on clear ocean waters (optical Case-1 waters). Coastal waters (optical Case-2 waters), however, tend to be the most productive areas of the world oceans, and it is therefore important to estimate biomass here. However, because of the presence of suspended particulate matter (SPM), and coloured dissolved organic matter (CDOM) coastal waters are optically very complicated, and remote sensing imagery from coastal waters is difficult to interpret.
A study of bio-optical properties of Case-2 waters is reported using the Menai Strait and the open Baltic Sea as examples. Both in situ and laboratory methods were applied. Laboratory methods focused on spectrophotometric measurements of optically-active in-water constituents (OICs) and a four-channel colour sensor (CS) was used for in situ measurements of upwelling irradiance.
The Menai Strait was optically dominated by suspended particulate matter (SPM), whereas in the open Baltic Sea coloured dissolved organic matter (CDOM) was the prime optical determinant. Multiple regression analysis (MRA) and a semi-empirical model (Harker, 1997) were applied to interpret the CS data, and to investigate the possibility of deriving OICs from CS measurements. The MRA algorithms were regional.
The semi-empirical model was tested using data from a 9-month deployment of the CS in the Menai Strait. Only the red:green ratio had a good fit when plotting predicted against observed values. This may be due to the lack of information about backscattering. Instead, MRA was used to derive a time series of OICs. When inverted, the resulting algorithm provided good estimation of SPM. For chlorophyll and SPM, a time series was produced that agreed with historical data. For CDOM no satisfactory algorithm could be derived, because CDOM absorption was relatively small in the Menai Strait. Chlorophyll a had a seasonal signal, and SPM showed a seasonal as well as a tidal signal, whereas CDOM was highest in summer.
The Harker model was adapted to the Baltic Sea. Only the red:green ratio had a highly predictive power, although it was underestimated substantially. The MRA yielded good predictive algorithms for chlorophyll and CDOM in the open Baltic Sea. A high correlation was found between chlorophyll and SPM. CDOM absorption was higher in summer than in spring.
Complementary chromatic adaptation of Baltic Sea cyanobacteria was investigated, and suggestions for algorithm development were made.