The early stages of marine biofouling and its effect on two types of optical sensors

This paper records the results of an investigation into the effects of biofouling on optical marine sensors and the organisms responsible for the deterioration in sensor accuracy. Two kinds of commercial sub-surface optical marine sensor, commonly used to measure water quality, were operated in a natural marine environment and allowed to foul while measurements of the actual conditions were made daily using clean instruments. A number of glass and acrylic coupons were placed in the same environment and were removed at intervals throughout the trial. These coupons were examined and the biological population quantified. Deterioration in the optical properties of the coupons was measured using image analysis and UV-visible spectroscopy. The results from the coupons were compared with the results from the commercial instruments. It was observed that the major deterioration in instrumental accuracy occurred when a bacterial population exceeding 10 5/mm 2 was found on the coupons. The algal population had little effect on the instruments over this time period. The acrylic coupons supported a lower fouling population, apparently due to the increased solubility of acrylic in seawater. The two optical techniques returned similar patterns of results for the surface area fouled, although the numerical values returned by each technique were different. Neither of these two techniques returned values directly comparable with the deterioration in accuracy of the commercial instruments. The trial took place on the Isle of Cumbrae in the Firth of Clyde on the west coast of Scotland, U.K.