Sea power

From Brian Wood I was delighted to see your report on generating power from ocean heat exchanges (1 March, p 48).  However, I was disappointed to see the warmed, nutrient-rich water from below the thermocline described in terms of unwanted algal blooms. This water should  be compared with the natural upwelling caused by deep ocean currents which results in important fisheries, such as  those off western South America. The balanced nutrient input from deep ocean water can create phytoplankton growth and support a full food web. Since it is a by-product of the power station, it must also be cheaper and less controversial than enriching the open ocean with iron. Lenzie, East Dunbartonshire, UK From Martin J. Greenwood As a cynical retired engineer, I note the lack of any discussion of the major issue that bedevils all active structures in the ocean. Growth of marine organisms on and in the pipes, pumps and heat exchangers will rapidly reduce heat transfer efficiencies and require regular costly maintenance. At an onshore gas processing plant of my acquaintance, emergency pumps draw in seawater to fight fires (fortunately seldom needed). On one routine test, the pumps were found to be inoperable, blocked solid by mussels that had grown in the inlet pipe. Stirling, Western Australia