What is Seagrass

Seagrass, such as Zostera capricorni and Halophila ovalis, are flowering plants and as such reproduce by pollination, just like land flowering plants. This is more than a quirk of nature but a case of convergent evolution; seagrass have evolved similar strategies that promote pollination. Interesting to note, terrestrial pollen bursts on contact with water.

Halophila ovalis is common in intertidal zones, tolerates salinity changes and various substrates. Zostera capricorni is long thin and a transluscent light green in colour, brown leaf blades are common when exposed at low tide. The leaf blades are rounded at the tips and have five longitudinal veins with each leaf arising directly from horizontal stems.

Halophila spinulosa is a widespread species but only occurs below low tide. The rhizome of H. spinulosa has erect lateral shoots, up to 15 cm long, bearing compound leaves. Each compound leaf is made up of 10-20 pairs of opposite leaflets, lying in a single plane and obliquely arranged around the common stalk. The leaflets are oblong-linear, 1.5-2.5 cm long and 3-5 mm wide, with a serrated edge distinctly visible to the naked eye. H. spinulosa is 'fern-like' in appearance, as the pinnate nature of the leaves bears some resemblance to fern fronds. The rhizomes are thin, sometimes 'woody', and have prominent shoot scars.

Halophila ovalis	Halodule uninervis

Cymodocea serrulata has a smooth, herbaceous robust rhizome system, which produces short, erect shoots often with fibrous rootlets at each node, each shoot bearing 2-5 leaves. The leaf sheath of C. serrulata is broadly triangular and narrowed at the base. It is often a purple colour in the living plant. When shed, the sheaths leave open circular scars on the shoot. A ligule is present. The leaf blade is linear to somewhat curved, 6-15 cm long and 4-9 mm wide, with 13-17 longitudal veins. The leaf is narrowed at the base and the leaf tip is bluntly rounded and distinctly serrated. Tannin cells are present in circular-shaped groups. Phenotypic variation is generally minimal.

Seagrass productivity

Seagrass beds are highly productive. Primary production is a major process driving the carbon and nutrient dynamics within the seagrass system. The seagrass itself makes up the majority of this primary production but epiphyte productivity also contributes and is typically 20 to 60% of the aboveground seagrass productivity (Hemminga and Duarte 2000).

On average, across Moreton Bay, Zostera capricorni produces approximately 0.5 to 2g dry weight per square metre per day (Dennison and Abal 1999). This translates into approximately each seagrass shoot producing a new leaf each week while shedding the oldest leaf (J. Udy personal communication). Each Zostera shoot along a rhizome has three to four leaves contained within its sheath. Seagrass production is slightly higher in spring and summer, and it is more noticeable when easterly winds cause the old shed leaves to pile up at the high tide mark along our shoreline.