Download Hydrosphere (water on or near the earth) Shellfish

Hydrosphere (water on or near the earth) Shellfish
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Hydrosphere (water on or near the earth)
Salinity: The Pacific Oyster can live under a wide range of salinity
concentrations from 10 to 35 ppt. Oyster reefs in the intertidal are best created
at intermediate salinities around 20-25 ppt (Troost, 2010).
Nutrients and Pollution: Oyster reefs are not directly sensitive to high nutrient
levels, but eutrophication has some indirect effects on oyster survival.
Eutrophication can indirectly kill oysters by inducing local anoxic conditions at
the bottom (Lenihan, 1999) or increasing the probability of harmful algal
blooms. Oysters feeding on the algae in the water can get killed by so called d
inoflagellates. High nutrient concentrations and the presence of harmful algal
blooms or bacteria can also make shellfish unsuitable for human consumption
(Rippey, 1994; Etheridge, 2010). Polluting substances can kill oysters directly,
or accumulate in the oysters meat, making them unsuitable for human
consumption (Baudrimont et al., 2005).
Inundation time: The inundation time depends on the position in the intertidal
zone and the tidal regime. Inundation time is crucial for the occurrence and
survival of oysters as it determines factors such as food availability, larval
Fig. 7. Habitat requirements hydrosphere
settlement, predation and heat stress. Short inundation times can be tolerated,
but means the oysters have less time to feed and more time for overheating and dehydration. Experiments in the Netherlands revealed that oyster
larvae settle most and grow best in the intertidal zone 60 cm below the mean sea level (NAP, Normaal Amsterdams Peil)(Walles, unpublished).
This value will be variable across sites, as it depends on local tidal regimes and other environmental conditions such as predation and ambient
temperatures. The tidal range in which oysters can survive is therefore best tested at specific locations. Roughly speaking, self-sustaining oyster
reefs can best be created at places that are inundated 40-50% of the time or more.
Waves & currents: Wave action (and strong water currents) can dislodge oysters, re-suspend sediment and cause erosion. Whether oyster reefs
can establish depends on local hydrodynamics and the presence of suitable (natural or artificial) substrate. In sheltered areas, living reefs may be
created by supplying loose hard substrates for oyster larvae settlement, whereas at more exposed sites (in particular wave-dominated) substrates
need to be fixed in cages. Such cages will keep the substrate from being spread around and getting smothered, thus increasing the survival rate
of settled oyster larvae. Especially when aimed at coastal protection, the use of a certain type of cage will be necessary in most cases (see
lithosphere). In the Eastern Scheldt, experiments with loose oyster shells at an exposed site resulted in the complete loss of the shells during
stormy conditions. Therefore, the use of gabions filled with oyster shells was introduced in the Case Shellfish Reefs.
Turbidity and SSC: Increased suspended sediment concentrations can affect filterfeeding organisms like oysters through clogging and damaging
their feeding and breathing facilities. When the Suspended Sediment Concentration (SSC) is too high, gills of filtrating oysters can get clogged
(Troost, 2010). On the other hand, oysters can also be used as a biofilter to reduce phytoplankton concentrations and turbidity in shallow waters
(Grabowski et al., 2007).