PNNL

Abstract

We used field and laboratory measurements to develop light criteria to support short- and long-term growth and survival requirements for eelgrass in Pacific Northwest, USA, estuaries. Photosynthesis-irradiance experiments and field depth distribution studies indicated that eelgrass photosynthesis was saturated at a photosynthetic photon flux density (PPFD) of about 350 µmol quanta m-2 s-1, and the compensation irradiance was on the order of 50 µmol quanta m-2 s-1. Winter plants had approximately three-fold greater net photosynthesis rate at the same irradiance as compared with that of 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 d-1 on average during spring and summer, and that growth was limited below 7 mol quanta m-2 d-1. We propose criteria that state that non-light limited growth and survival requires an average of at least 7 mol quanta m-2 d-1 during spring and summer. These criteria may be used to evaluate the effects of short- and long-term reductions in irradiance reaching eelgrass, especially that related to turbidity and overwater structures.