Critical summer irradiance requirements for biomass accrual of the seagrass Zostera muelleri

Seagrass meadows are valuable coastal habitats that require sufficient light to flourish. A laboratory experiment was performed to measure the response of the seagrass Zostera muelleri to different levels of irradiance. Plants from a New Zealand estuary were grown submersed in artificial seawater for six weeks at five different daily irradiance levels ranging from 0.3 to 7.3 mol m(-)(2) d(-1). Plants were extinguished within three weeks at <= 1.4 mol m(-2) d(-1), showed some growth but then declined at 2.9 mol m(-2) d(-1) while biomass, quantified as total leaf area, doubled at 7.3 mol m(-)(2) d(-1). The results indicate that Z. muelleri is likely to require at least 7.3 mol m(-)(2 )d(-1) of daily irradiance under average summer water temperatures of 20 degrees C to maintain growth and above-ground biomass accrual. The irradiance requirements for this and other seagrass species that grow in the subtidal zone are anticipated to increase as ocean water temperatures rise, highlighting the critical importance of actions to maintain and improve coastal water clarity.