On Long Island, eelgrass thrives in the coolest and clearest waters of our estuaries, including the easternmost reaches of the Peconic Estuary and Long Island Sound, and near inlets in the South Shore Estuary. Eelgrass puts on the majority of its growth in the fall and spring months, or when water temperatures are at an optimal range of 50-68 °F (10-20 °C). This is when our eelgrass restoration program conducts the majority of its planting efforts. As water temperature rises above its optimal range, leaf respiration increases more rapidly than photosyntheses resulting in a decrease in the photosynthesis-to-respiration ratio. In other words, the plant begins to use stored energy in order to meet energy requirements, and ceases to grow. Exposure to temperatures above 77°F (25°C ) have resulted in plant mortality (Greve et al. 2003, Reusch et al. 2005).

Here on Long Island, warmer temperatures peak in the late summer, and eelgrass growing within our bays and creeks can become stressed and even die back. In some rare cases here at our latitude, eelgrass has been known to completely die back during the summer and return via belkow ground biomass or seed once extreme temperatures subside. This is known as summer suppression, and is more common down south in the Chesapeake and in North Carolina where eelgrass is near its limit of thermal tolerance. If the population can survive by returning from the temperature stressor via seed recruitment year after year, it becomes known as an annual population.

Temperature also affects the timing of flowering and seed germination in Zostera marina. In our region, when water temparatures reach about 37°F / 3°C in early spring, inflourescence primordia present themselves. At approximately 59°F / 15°C, anthesis occurs, and by 70-73 °F (21-23°C), seeds have reached maturity. Seed germination occurs once water temperatures have dropped below 59°F/15°C in the fall or following spring (Sept-March), although low salinity (5-10ppt) can also initiate germination. To learn more about eelgrass sexual reproduction, check out the life history section.


Churchill, A.C. 1983. Field Studies on Seed Germination and Seedling Development in Zostera marina L. Aquatic Botany 16: 21-29.

Greve TM, Borum J, Pedersen O. 2003. Meristematic oxygen variability in eelgrass (Zostera marina). Limnol. Oceanogr. 48: 210-216.

Larkum, A.W.D., Orth, R.J., Duarte. C.M. (Editors), 2006. Seagrasses: Biology, Ecology and Conservation. Springer.

Orth, R.J. Moore, K.A., Roberts, M.H. and Silberhorn, G.M. 1981. The biology and propegation of eelgrass, Zostera marina, in the Cheasapeake Bay, Virginia. Final Report (Contract No R805953), US EPA, Washington D.C. 227 pp.

Reusch TBH, Ehlers A, Hämmerli A, Worm B. 2005. Ecosystem recovery after climatic extremes enhanced by genotypic diversity. Proc. Natl. Acad. Sci USA. 102: 2826-2831.


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