Light is key!
Seagrasses are most commonly found in shallow depths where light levels are high. This is because although most seagrasses complete their entire life cycle, under water they still need sunlight to produce energy and oxygen (photosynthesis). Sediment, or soil and dirt particles in the water can make the water murky or ‘turbid’. Sunlight cannot easily ﬁlter through turbid water. Phytoplankton (microscopic ﬂoating plants), epiphytes (plants growing on plants) or algae that cling to seagrass leaves can also restrict the amount of sunlight that actually reaches the leaves of the seagrasses.
The depth range of seagrasses is usually controlled by the availability of light for photosynthesis. Many seagrass species live in depths of 1 to 3 meters but the deepest growing seagrass (Halophila decipiens) has been found as deep as 58m in clear waters of the Indo-Pacific (Lee Long et al. 1996).
The water temperature determines which species of seagrass can grow, with tropical areas having higher species diversity. Seagrasses are adapted to survive in the intertidal zone, especially in sites sheltered from wave action or where there is entrapment of water at low tide, (e.g., reef platforms and tide pools) to protect the plants from drying out. Some species are well adapted to grow on intertidal mud flats, surviving exposure to the sun for several hours at low tides.
Seagrass have adapted to living in saltwater environments. The most common way to record salinity is to measure the amount of salt in 1,000 g of water, referred to as ‘parts per thousand’ or ppt. The ocean has an average salinity of between 34 ppt and 36 ppt. Some seagrass species can tolerate salinity of 4 to 65 ppt, however most grow best at 35ppt.
Nutrients & Sediment
Like terrestrial plants seagrass need nutrients for growth, these are nitrogen and phosphorous. Typically nitrogen is limited in coastal environments. Excessive nutrients may be toxic to the seagrass and allow algae to take over. Seagrass need sand or mud sediment to anchor themselves to the floor.