Species Adaptations to Estuarine Conditions
Dealing with Changes in Salinity, Oxygen, Light and Temperature
Mar 23, 2009
Variances in salinity, temperature, light and oxygen levels can occur frequently due to seasonal changes, tidal range and the structural characteristics of the estuary itself: depth, width and type of estuary. It is clear that the organisms that survive and adapt to these varying conditions are quite remarkable.
Dealing With Changes in Salinity
The marine species generally reside near the mouth of the estuary, the freshwater species in the low salinity areas and the estuarine species somewhere in the middle. Many estuarine species are osmoregulators, meaning they can maintain a constant salt balance, no matter what the salinity of the water is.
Organisms that are capable of dealing with varying salinities are euryhaline, and organisms that can only deal with small changes in salinity are stenohaline. Stenohaline animals rely on coping methods such as moving out of the area, burrows in the sand, excreting excess salts or closing their shells, and worms, molluscs and fish can produce mucus or slime to cover sensitive body parts.
The mussel, Xenostrobus securis, deals with salinity of 2g/L by closing up their valves for up to many months. When an organism is unable to move or reduce contact with the unfavorable salinity, it then relies on physiological responses such as osmosis (become iso-osmotic with estuarine water) or changes its activities or its metabolic rate.
Generally mature organisms are better able to handle the stress of salinity changes than reproducing adults or newly hatched eggs. Many sessile organisms die if conditions become too harsh and have to re-colonize when conditions are once again suitable.
Euryhaline crustaceans respond to changes in salinity by molecular processes. Vertebrates respond by regulated blood osmotic concentrations and controlling ion fluxes and organic osmolytes. Invertebrates can regulate blood osmotic concentration, cell volume or both.
Sodium chloride is very important in regulating blood osmolarity in euryhaline crabs, and regulating fluxes and permeability’s of these ions is how the crabs deal with the salinity. Green shore crabs respond quickly to salinity variations and within six (6) hours, their blood Na+ level reaches a steady state.
It is thought that the crabs possess a sensory organ on their legs known as “hair peg” which is what responds to the salinity variations and sets the wheels in motion for response. Gill structure is also helpful in dealing with changes in salinity as the gills contain two (2) different sets of epithelial cells, which are believed to be crucial to ion and gas exchange.
Dealing With Changes in Oxygen Levels
In water containing low amounts of oxygen, organisms have physiological and behavioral mechanisms to survive. In sessile animals that cannot move to a more oxygen rich environment, they must lower their energy demand, maintain their metabolism or use a method of creating energy without oxygen.
Many animals do this by increasing their heart rate and the flow of water past respiratory surfaces. Some organisms have respiratory pigments, which allow them to maximize the oxygen they do get. Others are very good regulators of oxygen uptake and maintain constant rates of uptake until the oxygen falls to a ‘critical pressure’.
They then switch to anaerobic metabolism. This can only be maintained for a short time. Some organisms, such as oyster larvae, have been shown to lower there aerobic metabolism in response to hypoxia.
Dealing With Changes in Light Intensity
Water-column mixing is estuaries results in changes in light that reaches phytoplankton, and vertical mixing can affect phytoplankton productivity. Photoacclimation (shade adaptation) has been shown to increase chlorophyll in phytoplankton that are exposed to low light intensity.
Photoacclimation can take anywhere from three (3) hours to three (3) days. Chlorophyll will respond to the light intensity it is exposed to. If in a well-shaded or light-limited depths productivity will decrease but there will be increased cellular chlorophyll content.
Dealing With Changes in Temperature
Organisms that are able to withstand varying temperatures are eurythermal. Those that can’t must use other responses such as moving, burying themselves, or to transform to their resting stage. Temperature can fluctuate over seasonal cycles in temperate estuaries. Bacteria have adapted to this by reorganizing their biochemical pathways and adjusting protein and DNA synthesis rates.
Further information on Salinity, Oxygen, Light and Temperature is available.
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