UQx TROPIC101x 3.2.3 Seagrass Ecology


Well in the first two parts of the
lecture we talked about the distribution of seagrass and a brief introduction to what it is, next
we covered biology so a little bit about how plants work
and how they can persist underwater in the coastal zone. But now we’re going to talk a little bit
about their ecology. Now seagrass form the base of some extremely important food webs, they support a diverse community of
invertebrates and vertebrates and these include
commercially important fish species. And these tend to either feed or organisms tend to feed
directly on the seagrass, they can feed on some of the dead
matter that’s associated with seagrass, which we call detritus, and they’re also feeding on algae or
seaweeds and microalgae that occur within the seagrass beds. So if we look at this food web what we can see here is that these three parts of the system seagrass, epiphytes/algae and
detritus/microalgae are supporting consumers so for example seagrass directly are grazed on by dugongs and turtles and some echinoderms which include sea
urchins, for example. The epiphytes are grazed on by small fish and crustaceans, while the microalgae and
detritus, the dead stuff in the system, feed fish. They feed other organisms like sea
cucumbers or bêche-de-mer. Now these animals that
graze on the the lower parts of the food web are then in turn grazed on by other organisms and
these include the silver fish that we like to eat like
whiting, They include prawns and other carnivorous fish, large white fish
that prey upon smaller things that humans like to eat. So finally you are
moving up the food chain to organisms like sharks, birds of
prey. And there’s one thing that’s really
missing from this picture and that’s humans. So in later lectures with Professor Mumby, Peter Mumby, you’re going to be talking about the influence, or you are going to learn about the influence of humans on marine food webs. Now one of the
things that I’ve referred to again and again through this lecture is
mega-grazers, the large grazers of seagrass communities. This is a picture of a dugong grazing
seagrass. Now these are important animals and they are endangered in parts of their range, the manatees of Florida, for example, are an endangered species while dugongs are threatened. So obviously if we wish to conserve these species then conserving seagrass beds is a very
important part of that solution. Now animals like dugongs and manatees
disturb the community and they actually alter how the plant community, how the
seagrass bed is and how it looks. Their repeated grazing means that fast-growing smaller species are favoured and so this way the dugongs are actually cultivating a habitat that’s
more suitable for them and this is sometimes referred to as
cultivation grazing. But in addition to grazing, floods intense storms, they can also remove
seagrass beds and they also favor the regrowth of small
species. Small fast-growing species. And over
time usually the diversity of the meadows
increases. So we have thought about how seagrasses respond to disturbance, now this here I want to talk about what are some
of the ecosystem functions that seagrasses perform, that make them valuable assets to
communities that have seagrasses in their coastal zone. The first thing is
that they’re really important for coastal protection. If you look at this slide you can see that what seagrasses do is attenuate
wave energy and flow rates of water. By doing so
they’re actually reducing the chance of erosion on the
coast. This process of slowing water down and slowing wave energy or dampening wave
energy results in particles that are suspended
in the water column dropping out to the seabed and that’s a
very important process in increasing the water quality, so
increasing the clarity of the water. And it also supports what in this slide
we have called vertical accretion, which is the capacity
of the seabed to rise up or to keep pace with
sea-level to add volume over time. Seagrass also stabilize sediments and
stop the sediment from being kicked up every time that there’s a
storm. Now in addition to these sort of very
physical processes, they also support fisheries production and we looked at that in the food web diagram and in some places there is extraction
of products. For example you do see seagrass matting
on sale in many furniture shops throughout the
world. The final thing that we will talk about is
carbon sequestration. Now below there is a link to an abstract
from a recent paper that asks you to consider the role of
seagrass in climate mitigation and the importance of soft engineering solutions to adapting to climate change. So seagrass are important for commercial fisheries
and I think this deserves special attention in this lecture. They do this because seagrass meadows
are nurseries for some fish species, so they are vital for
some stages in the life cycle of commercial fish. Another thing that the science around
coastal zone management and coastal productivity is giving us to think about, is that
seagrasses are more productive or there is more fish
when sea grasses are adjacent to mangrove forests
or coral reefs. So if you have these habitats
collectively in good shape in the landscape you will
maintain higher productivity of fish species, so
higher abundances. Now using an example from my own country you can see that in this example 235,000 dollars worth of lost fisheries production occurred when 12,700 ha of seagrasses in South Australia were lost. Now if you’re interested in the
valuation of ecosystem services of marine habitats then I point you to the paper by Barbier et al., or Barbier and others in the journal
Ecological Monographs. Now one of the services that seagrasses
perform for communities is that they sequester
carbon dioxide or carbon that was carbon dioxide and they
sequester it within their sediments, so within the below-ground parts of the
seagrass meadows. Now science has recently shown us that
seagrass sediments are a globally significant stock of carbon and this mainly occurs because of the low oxygen concentrations
in seagrass sediments that basically limit decomposition of that biomass that was deposited there from
the roots. They also trap carbon from elsewhere and
we talked about that in the slide where we talked about that seagrass can slow water and particles
drop. So, for example, particles that come from
the land, carbon that’s in the soil of a forested systems that when it arrives
in the coastal zone the slowing of the water velocities by seagrass lead to
the trapping of that carbon within the seagrass meadow sediments. Additionally, with adding of sediments
and root growth, the volume of seagrass sediments
increases over time so carbon is continually sequestered. Now because of these huge stocks that
are within the seagrass meadows it’s very prudent of us to try and
conserve them to prevent emissions of carbon
dioxide to the atmosphere. Now if you want to read a paper on this topic then please see
the link below to a very recent paper on the carbon
stocks within seagrass beds of the coastal zone.

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