Light requirements for growth and survival of eelgrass (Zostera marina L.) in Pacific Northwest (USA) estuaries

We developed light requirements for eelgrass in the Pacific Northwest, USA, to evaluate the effects of short- and long-term reductions in irradiance reaching eelgrass, especially related to turbidity and overwater structures. Photosynthesis-irradiance experiments and depth distribution field studies indicated that eelgrass productivity was maximum at a photosynthetic photon flux density (PPFD) of about 350-550 mu mol quanta m(-2) s(-1). Winter plants had approximately threefold greater net apparent primary productivity rate at the same irradiance as summer plants. Growth studies using artificial shading as well as field monitoring of light and eelgrass growth indicated that long-term survival required at least 3 mol quanta m(-2) day(-1) on average during spring and summer (i.e., May-September), and that growth was saturated above about 7 mol quanta m(-2) day(-1). We conclude that non-light-limited growth of eelgrass in the Pacific Northwest requires an average of at least 7 mol quanta m(-2) day(-1) during spring and summer and that long-term survival requires a minimum average of 3 mol quanta m(-2) day(-1).