We investigated the ability of eelgrass (Zostera marina) to adjust light requirements to seasonal changes in
temperature, light and nutrient conditions through changes in metabolism, pigment and nutrient content. In
agreement with expectations we found that rates of respiration and light saturated photosynthesis of summer
acclimated plants peaked at higher temperatures (5 °C and 2 °C higher, respectively), and were lower than
of winter acclimated plants, both at sub- and supra-optimal temperatures. Moreover respiration rates were
generally more sensitive to increasing temperatures than photosynthetic rates, especially so for cold
acclimated plants in February (36% higher Q10-values). These changes were accompanied by a reduction in
chlorophyll a and nitrogen concentrations in leaves by 35% and 60% respectively from February to August. The
critical light requirement (EC) of Z. marina to maintain a positive carbon balance increased exponentially with
increasing temperature but less so for summer-acclimated than for winter-acclimated plants. However,
combining EC vs temperature models for whole-plants with data on daily light availability showed that
seasonal acclimation in metabolism increased the annual period, when light requirements were meet at the
2–5 m depth interval, by 32–66 days. Hence, acclimation is an important mechanism allowing eelgrass to
grow faster and penetrate to deeper waters. Critical depth limits estimated for different combinations of
summer temperatures and water clarity in a future climate scenario, suggested that expected increases in
temperature and nutrient run-off have synergistic negative effects, especially in clear waters, stressing the
importance of continued efforts to improve water clarity of coastal waters.