9 Seagrasses Flashcards
Characteristics of Seagrasses
- angiosperms
- herbaceous clonal habit, grow as ramets
- grows in marine and estuarine environment
- monocotyledones but polyphyletic group
- leaf bundles adapted to hydrodynamics
- colonize mobile and hard substrates
Families that includes seagrasses
- Zosteraceae
- Cymodoceaceae
- Posidoniaceae
Reproduction features of Seagrasses
- flowering plants (angiosperms) with leaves, rhizomes and roots
- flowering: underwater pollination, fruits and seeds
- both: sexual (with male and female reproductive organs) and asexual (fragments of rhizomes)
Dispersal of Seagrass
- Phase I: primary dispersal
- Phase II: Secondary dispersal
- Phase III: final resting position
Seagrasses as foundation species
- they form extensive beds or meadows
- these can be monospecific (= made up of one species) or multi specific ( >1 species co-exist)
Effect of different seagrass traits
- Seagrass traits influence the type and amount of species that grow on them
seagrasses as ecosystem engineers
seagrass regulates
- hydrodynamism
- sediment stability
- light intensity (shade)
- oxygen content of sediments (–> they bring oxygen in their roots)
- regulate disturbances
- nutrient availability
- detritus pathway
Seagrass distribution characterized by wave energy
- limited growth in areas with high hydrodynamics
fluid dynamics in seagrass beds are influenced by?
the fluid dynamics in seagrass beds are influenced by
- water flow
- seagrass density
- canopy height
- water depth
- bed size
current-driven flow in meadows
- Seagrasses reduce water velocities associated with unidirectional current-driven flows
- Within the seagrass canopy, the current velocity profile is modified from logarithmic to approximately exponential
- Seagrass presence alters the current velocity profile from a logarithmic form outside/above the meadow to an exponential form inside the meadow
- While the presence of seagrass modifies the vertical structure of the current velocity profile, the current velocity inside the seagrass meadow depends on the horizontal distance from the meadow edge.
- The near-bed current velocity decreases horizontally within the seagrass meadow
Relationship between vortex development and the ratio of canopy height to drag length scale
- The blue lines show how the shear layer, which contains the vortices, forms with increasing horizontal distance into the seagrass meadow
- The seagrass canopy is either
(a) too short to produce vortices
(b) of intermediate height and induces a shear layer which penetrates to the sediment bed
(c) is sufficiently tall to induce a shear layer which is localized only to the canopy-water interface
role of roots in sandy sediments
- Roots of plants effectively reduces later erosion rates and this effect is predominantely important in sandy sediment
seagrass - sediment -light - feedback (SSL)
- Sediment suspended in water coloumn reduces light available to seagrasses
- Seagrasses have a high light requirement
- Seagrasses reduces both near-bed currents and wave velocity
- Reduction in flow velocities allows sediment to settle, improving water clarity
- Higher water clarity favour seagrass growth
Interactions between seagrass and associated communities and trophic webs
- Canopies offer refuge from predation (e.g. crabs can’t dig bivalves out of sediment as easily)
- Seagrass canopies influence recruitment
processes through modifying hydrodynamic
regimes - Seagrass act as nursery areas
what is recruitment?
net balance between the flux of larvae into the seagrass bed and the loss of newly settled larvae due to resuspension assuming larvae are passively distributed
Seagrass act as nursery areas
Habitat for juveniles of many species survive and grow more in seagrass meados than in other habitats
communities associated with seagrass
- they are among the most important primary producers in coastal marine environment
- home to rich communities of epiphytic algae, sessile invertebrates, agile invertebrates and fish
–> higher species richness and abundance compared to non vegetated habitats
trophic food webs in seagrass meadows
- In the tropics grazing has still a significant effect on seagrasses (riesige Seekühe, die als Nahrung für Haie dienen)
- In the Mediterranean Sea there are only few grazers
- Much of the primary production enters the detritus chain locally or is exported –> eg, deep waters, where there is no primary production, or beaches
main factors that control seagrass ecology and landscape structure
- solid arrows: effects on seagrass landscape structure ( size, shape)
- dashed arrows: community and ecosystem variables affected by the structure
Nutrient fluxes in seagrasses
- main fluxes of N (Stickstoff) in a seagrass meadow
processes are:
- uptake and regeneration in the water column
- nitrification
- nitrate uptake by the sediment
- dissimilatory reduction of nitrate to ammonium
- efflux of dissolved organic nitrogen from the sediment
resuspension of sediment particulate N
bottom up control in Seagrasses
- demand for oxygen and light is high
- nutrient uptake from sediment and water column
- CO2 absorbed by leaves –> O2 production by Photosynthese
- O2 transported to rhizomes and roots
- O2 can also diffuse in sediment –> seagrass can grow in hypoxic or anoxic sediments: O2 as buffer against sulfides
- if O2 not enough –> seagrasses can switch to fermentation pathway shortly
- if period of time too long –> sulfides are formed in sediment and low O2 in roots lets sulfides in –> death!
-> happens when light is low because that limits O2 -production
- if light limitation occurs simultaneously with e.g. increased organic loading –> sudden dramatic die-offs with positive feedback on organic-matter-degradation-O2-demand
symbiosis reducing sulfide stress for seagrasses
- three-stage symbiosis between seagrass,lucinid bivalves and sulfide-oxidizing gill bacteria
–> bivalve-sulfide-oxidizer-symbiosis
-symbiosis between bivalves and sulfide-oxidizer reduce sulfide levels and that enhances seagrass production
- bivalves and their endosymbionts profit from organic matter accumulation and oxygen release from seagrass roots
top-down control in Seagrass meadows
(conceptual models)
scenarios of seagrass dominant vs. unvegetated substrate
top-down control in Seagrass meadows (impact on juveniles)
Fundamental role of herbivores both directly (pasture) and indirectly (epiphyte abundance control)
Mutualism between mesograzers and seagrasses (–> what happens when the amount of mesograzers is reduced?)
- no mutualism via mesograzers reduction caused a bloom of epiphytic algae and a steep decline in seagrass biomass
Mutualism of mesograzers and seagrass
- mutualism has sensitive outcome to environmental conditions and species composition and food web structures
- relationship can turn antagonistic (one benefits at the other one’s expense)
value of seagrass
extremely valuable
natural products provided by seagrasses
ecosystem services provided by seagrass
– Direct and indirect food source
– Oxygen and organic matter production
– Starting point of complex trophic webs
– Nursery sites
– Nutrient cycles
– Water quality control
– Sediment compaction and reduction of resuspension
– Hydrodynamic control
– Coastal protection
– Carbon sink
Ecosystem services provided by
seagrasses (Coastal protection)
- P. oceanica effectively reduces wave energy by 30% to 60%, even if it occupied only 20% of the water column
- Impact on the beach morphology, modifying the berm – bar exchange of sediment and seagrass litter;
- The sediment removed during the removal is permanently lost from the beach system
- Depletion of nutrient and biomass for the coastal ecosystem
Ecosystem services provided by
seagrasses (Carbon sequestration and nutrient cycling)
- also very high value (money-wise)
extension, production ans losses of vegetated coastal ecosystems
- seagrasses are one of the most productive ecosystems
- highly threatened system
global losses of seagrasses
- large scale declines as a result of natural processes and human activities
- overall historical losses 65%
losses of seagrass in the Adriatic sea
(main drivers? when?)
increased load of sediments and nutrients as main driver of loss in Po river delta (between 1840-1870)
losses of seagrasses in Mediterranean Sea
estimated 40-80% of Posidonia
alterations in top-down control due to humans
- mesograzer control (facilitation)
alterations in bottom-up control due to humans
- light
- nutrients
- temperature
Eutrophication
anthropogenic increases in nutrient loading are inversely related to various indices of seagrass health
Eutrophication
alteration of trophic cascades
- direct toxicant effects at elevated levels of water column nitrates (nitrates in sediment seem to have less toxic effects). These effects so far little studied (see effects of excess nutrients before)
- indirect effects, due to growth of phytoplankton and epiphytic macroalgae that ultimately block light penetration.
effect of anchoring on seagrasses
effect of alien species on seagrasses
Seagrass Restoration challenge
- Challenge is to re-create an entire ecosystem by shifting unproductive lanscapes (e.g. Sand) into productive and structurally-complex habitats (e.g. Seagrass)
- Ecosystems are complex, so there are some successes, but many failures from which we learn
estimated costs for seagrass restoration
- Estimated cost for restoration of tropical seagrasses in the USA range from 100,000 to a million dollars per acre (0.4 hectares)
- very expensive
three common methods of seagrass restoration
- transplanting individuals
- artificially supplementing recruitment
- enhancing natural recruitment
Pro’s and Con’s of Method: transplanting individuals
Pro’s
- immediately increase habitat abundance
- robust adults likely to survive
Con’s
- fragment and weaken source population
- labor-intensive
Pro’s and Con’s of Method: Artificially supplementing recruitment
Pros:
- maximise number of recruits by providing ideal conditions (e.g. laboratory)
- can grow to predetermined size ideal for outplanting
Con’s:
- remove potential recruits from nature
- recruits need time to grow into adult habitat
- ‘new’ technology
Pro’s and Con’s of Method: Enhancing natural recruitment
Pro’s:
- minimal impact on natural populations
- can be cost-effective
Con’s
- recruits need time to grow into adult habitat
- ‘new’ technology
What are Artificial Seagrass Shields?
