red tides

red tides are caused by blooms of planktonic marine plants (phytoplankton) that discolour the water. They occur especially in regions where upwelling stimulates rapid growth in the algae, but their geographical range has been expanding and the frequency of their occurrence has been increasing, probably because of eutrophication, an important environmental issue. Some red tides are merely nuisances, producing unsightly foams or bad smells. In some the algal species are very abrasive and mechanically damage the gills of fish. Others are seriously toxic. Blooms of one type of phytoplankton, dinoflagellates, not only kill fish but also make shellfish exceedingly poisonous. One species that has appeared off the eastern seaboard of the USA aggressively attacks fish and has caused the death of a researcher.

Ballast waters discharged from bulk carriers have spread many toxic species from Japanese waters to Australia and the USA. The exotic species are introduced as dormant cysts. Since they arrive without their normal suite of predators and diseases, if they become established they tend to out-compete the native species. Around Australia shell fisheries have been destroyed, and local people have experienced health problems. So if you see dense blooms of algae discolouring the sea, be careful about swimming and refrain from eating the local seafood.

M. V. Angel