Seagrasses Support abundant life Provide complex habitat –Trophic support –Refuge –Recruitment –Nursery

Characteristics and Distribution Approximately 48 species Family Hydrocharitaceae – Thalassia testudinum Family Cymodoceaceae – Halodule, Cymodocea filiforme Distribution is limited to temperate and tropical waters worldwide Requirements for survival: salt tolerance submergence anchoring system in turbulent environment hydrophilous pollination

Cymodocea filiforme Manatee Grass

Thalassia testudinum Turtle-grass

Halodule sp. Shoalgrass

Morphological Adaptations Flattened, strap-like leaves Extensive root/rhizome system Halophytic Light sensitive Nutrient uptake through roots and leaves Reproduction: sexual (pollination) and asexual through prolific vegetative propagation Hydrophilic pollination pollen transported on water surface (Halodule, Ruppia) pollen transported beneath water surface (Thalassia)

Seagrasses-Anatomical Adaptations Leaves Lack stomata; thin cuticle to allows gas and nutrient exchange Large thin-walled aerenchyma facilitate gas diffusion within the leaf & provides buoyancy to the leaves Roots and Rhizomes Oxygen transport to the roots creates an oxic environment around the roots, facilitating nutrient uptake All produce root hairs

Ecophysiology and Productivity Wide temperature tolerance 0-36 degrees C (tolerance), 0-30 degrees C (growth) Wide salinity tolerance 0-90 ppt (tolerance) 0-56 ppt (growth) Minimum 18% surface irradiance PAR requirements Blade productivity: g C/m 2 /day Biomass up to 8,000 g drywt/m 2

Factors affecting species composition & zonation Substrate composition –Mud or muddy sand Wave energy –low Water depth –11-25% SI –Intertidal to m Salinity tolerance –Optimal vs tolerance Successional stage Latitude

Succession ECOSYSTEM DEVELOPMENT SANDY SUBSTRATE MUDDY SUBSTRATE Halodule/Halophila Cymodocea Thalassia STABLE ENVIRONMENTAL CONDITION DISTURBANCE

Trophic Support Large amounts of primary productivity –Supports detrital food webs Large amount exported to adjacent habitats & ecosystems –Supports some grazing food webs Direct grazing on roots & rhizomes Epiphytes grazed

Trophic Interactions Herbivore/predator control –Distribution & abundance –Population biology –Sexual reproduction Subtle interactions between producers & consumers –Epiphyte removal & control by grazers –Overfishing of top predators & release of epiphyte grazing pressure

Increased nutrient loading Epiphyte & phytoplankton increase Seagrass loss Seagrass decomposition Remineralized seagrass nutrients Overfishing Large predator decline Decreased recruitment Small predators increase Grazers decrease Epiphytes increase Seagrass loss

Nursery & Refuge Support more animals than adjacent unvegetated areas –Many animals present as juveniles –Provide protection: increased vegetation = increased protection Not much evidence for greater growth rates Role as nursery probably greatest in tropical & subtropical areas Some species may compete for habitat & refuge

Influence of Habitat Structure Complex, at several levels of spatial scale –Above & belowground complexity –Macroalgae & epiphytes –Gaps

Spatial Scales Individual leaf shoots (mm) –Gradient of epiphytic cover (biomass) Centimeters –Changes in seagrass & algal species Meters –Patchy distribution & gaps Kilometers –Patch configuration

Animal Response Amphipods preferred high SA/volume ratios Pipefish preferred long leaves & dense meadows Heterogeneity in meadows increases numbers & species Mobile predators found along edges

Linkages Animal movements –Migration to and from beds Export of detritus

Human Influences Algal overgrowth due to eutrophication or top predator removal Introduction of non-native plants & animals Dredging & filling Propeller scarring