Challenges of the Salt Marsh Environment

          Organisms of the salt marsh face a number of challenges and difficulties imposed by both physical and biotic factors. The tidal environment means that organisms are alternately covered by salt water at high tide and exposed to the air during low tide. Wave action can impose difficulties of physical stress. Physical and chemical conditions can change relatively rapidly. Temperature, light intensity, salinity, pH, oxygen levels, water availability, and other factors can all fluctuate considerably during a tide cycle. They can also fluctuate seasonally. What follows are some examples of these challenges and how organisms have adapted to them. These are only a few of the many examples of adaptations or behaviors that salt marsh species use to cope with a difficult environment.

Water loss/Desiccation

           At high tide organisms of the low marsh are fully or at least partially covered by water.  At low tide they are exposed to the air and they start to dry out.  This is accelerated by exposure to the sun and wind.  Mobile organisms can move to more protected areas such as crevices, under rocks, or under seaweed to reduce desiccation.  They can also enter burrows in the sediment.  Snails, barnacles and mussels can either seal off or close their shells when desiccation begins.  Some algae have a mucilaginous covering that reduces water loss.  However, they can lose up to 75-90 percent of their water content during low tide and still rehydrate and function normally after the tide returns.  These are three examples of ways organisms can survive desiccation: use microhabitats; seal themselves up in shells, or simply dry out and rehydrate.  

Submergence/Immersion

            Plants and animals in the low marsh are covered by tidal water twice a day.  This flooding reduces the availability of oxygen to some organisms and increases it for others that are able to acquire oxygen from the water.  Among plants, like smooth cordgrass (Spartina alterniflora), being submerged reduces the oxygen level available to the roots of the plants.  This inhibits aerobic respiration.  One means to overcome this is the development of a spongy tissue of enlarged and gas-filled spaces that runs from the leaves to the roots of the plants.  This tissue, called aerenchyma, increases the availability of oxygen to the roots by allowing it to diffuse from the leaves down to the roots.  Aerenchyma is produced in many types of plants in submerged or waterlogged situations.  This is a response to increased levels of the plant hormone ethylene, which is elevated in many cases given submergence of the plant.

            Some organisms of the salt marsh, especially those found in the high marsh, are more terrestrial than they are aquatic.  They are better adapted to living in the air and must adjust to being covered by salt water during the high spring tides.

Wave action

            Considerable forces are transmitted to the shore and shore organisms due to the breaking of waves.  The grinding action of stones and sand suspended in the water is also an important factor.  Having a hard shell or living in a burrow in the sediment provides protection from wave action. 

Smooth cordgrass is a major factor in dissipating wave velocity and the impact of waves in the low marsh.  As waves enter the dense stems of the grass the energy of the wave is dissipated.  This greatly reduces shore and beach erosion and provides a much more stable environment for many organisms in the low marsh. 

Temperature fluctuations

Many poikilothermic (ectothermic) organisms have a relatively narrow temperature range over which they can function effectively. In intertidal areas of the salt marsh daily 10-30 deg C temperature swings are common. During low tide the surface temperatures in the summer often rise to 30-40 deg C.  When the tide returns the temperature drops suddenly. Organisms often use microhabitats to escape these extremes. 

During the winter freezing temperatures as low as –20 deg C can occur.  The grinding action of ice is an especially destructive force.  It can scrape off vegetation and attached animals.  It prevents the growth of extensive algal communities in salt marshes in northern New England.

Salinity fluctuations

            Organisms can be classified into two broad types in relation to their ability to survive in variable salinities.  Stenohaline organisms can survive over a relatively narrow range of salinity.  The open ocean has a very stable salinity of about 35 ppt and therefore most organisms found in the open ocean are stenohaline types.  Many of the invertebrates in the ocean have body fluids that have the same osmotic pressure as seawater.  They are said to be isosmotic.  They do not osmoregulate and are known as osmoconformers.  Mytilus edulis, the common mussel, is a osmoconformer.  It is found in the low marsh partly buried in the sediment.  During low tide it closes its two shells and retains a supply of seawater.  This allows the mussel to keep a fairly stable osmotic pressure and salinity for a number of hours.  It opens up again when the tide returns. 

            Euryhaline species can tolerate broader ranges of salinity.  Most salt marsh invertebrates are euryhaline types.  They are generally osmoregulators.  As is the case with all trends in biology, there are many exceptions.  For example, the green crab (Carcinus maenas) is an osmoconformer at higher salinities (30+ ppt) but an osmoregulator in brackish water with salinities around 15 ppt.  Carcinus increases its rate of urine production in less saline water up to 30 percent of its body weight per day when the salinity is reduced from 35 to 14 ppt.  Another response of Carcinus to lower salinity is an increase in sodium ion uptake by the gills.  As is often the case, it regulates both its water balance and its ionic balance.  This crab’s ability to live in a range of salinities has made it very successful.  Even though it is an alien species, it is now a dominant organism along most of the Atlantic shoreline.   

            Yet another adaptation is to avoid the fluctuations of salinity during the tide cycle by escaping to microhabitats.  Burying into the sediments is a major way to escape fluctuations in salinity.   For example, during a tide cycle, at a depth of 5 cm the salinity was 21 ppt while at the surface it ranged from 2 to 29 ppt.  Many organisms in the intertidal mudflats use burrows at low tide.  They serve a number of functions. They reduce salinity fluctuations, reduce desiccation, and reduce predation.

            The influence of tidal flooding in controlling salinity concentrations in the sediments in the salt marsh is an important factor.  Salt concentrations can increase in the high or irregularly flooded marsh as evaporation concentrates the salt that is introduced by spring tides.  Since tidal input to the high marsh occurs only during these spring tides, the leaching (washing out) and dilution of this salinity occurs infrequently.  As a result it is possible for extremely salty (hypersaline) conditions to occur in areas of the high marsh called salt pannes.  Only a limited number of plants and few animals can survive in these extremely saline conditions.

            Salt can slow the growth of plants because the sodium acts as a competitive inhibitor of ammonia uptake by the plant’s roots. For most plants, ammonia is a major source of the critical element nitrogen.  As a result, many salt marsh plants are somewhat stunted and will grow larger in less saline situations. 

Oxygen fluctuations

            Organisms in the tidal mud flats live in a situation where oxygen is relatively available near the surface of the sediments but decreases rapidly with sediment depth.  These low oxygen sediments limit the occurrence of many organisms.   Burrowing crabs and worms are important in reworking the sediments and bringing oxygen to lower levels in their burrows.  By removing organic material from the sediments as they eat it, deposit feeding organisms also reduce the amount of microbial respiration in the sediment.  This also helps reduce the oxygen depletion.

            Ribbed mussels and barnacles are both sedentary organisms in the intertidal zone.  They open their shells when submerged by water and filter feed.  They extract oxygen from the water using gills.  When the tide goes out the organisms close their shells to prevent water loss and desiccation.  However, as time goes along they begin to deplete the oxygen reserves inside their shells.  They have adaptations that allow them to secure oxygen.  The ribbed mussels open their shells and “airgape”.  The barnacle has a small opening where the shell does not close completely.  Oxygen enters there but the space is to small to allow much water to escape.

Light intensity fluctuations

            Plants in the salt marsh are using light to photosynthesize to make their own energy rich molecules.  Specific wavelengths of light are captured by specific pigments in different kinds of algae.  Light intensity fluctuates in the intertidal zone as the tides come and go.  During high tide the water depth is generally more shallow near the shore and deeper away from the shore.  The intensity and wavelength of light are changed by water depth.  Different groups of algae are adapted to live in these different intensities and wavelenghts of light.  Near the shore the shallow water allows higher light intensities and  a more complete spectrum (range of wavelenths) of light.  These conditions favor green algae such as Ulva and Enteromorpha.   These species rely on chlorophyll pigments to photosynthesize.  In somewhat deeper water the  light intensity falls and the spectrum is shifted toward longer wavelengths.  This favors the brown algae such as Fucus.  These algae utilize xanthophyll pigments such as fucoxanthin in addition to chlorophyll.  In the deepest parts of the intertidal zone the light intensity is the least and the wavelengths are shifted toward the red parts of the spectrum.  In this zone, red algae such as Chondrus crispus are most abundant.  They use phycobilin and carotenoid pigments along with chlorophyll to photosynthesize.  Therefore, different groups of algae use different pigment combinations to capture the different wavelengths of light found at different water depths.

            When the tide goes out the algae are left lying on the sediment.  It is often possible to see three zones of algal plants lying on the shore at extreme low tide.  The greens are nearest to the shore, the browns are next and the reds are the furthermost out.  While the tide is out most algae stop photosynthesis and lose a substantial portion of the water in their tissues.  When the tide returns they rehydrate and photosynthesis begins again. 

Hydrogen sulfide accumulations

           In salt marsh ecosystems H2S accumulates in waterlogged soils. It is one of the contributors to the distinctive smell of a salt marsh at low tide. It is phytotoxic (poisonous to plants) and probably limits the availability of ammonia, an important plant nutrient. Because of these two factors, plant growth is more limited in waterlogged soils compared to soils which are more well drained which tend to be more oxygen rich.

Seasonality

     Being in a temperate climate means having seasons during the year.  The changing daylength and temperature conditions in the salt marsh during the seasons have a large impact on the organisms.  During the summer the salt marsh is most active from a biological standpoint.  Most organisms are at their most active metabolically.  As the fall season approaches, temperatures and daylength decrease and the activity of both plants and animals declines.  Below about 10 deg C the activity of many of the salt marsh’s ectothermal animals slows dramatically.  As the daylength drops below 12-14 hours per day hormonal changes in the plants promote the onset of dormancy. 

     The most challenging season in the salt marsh is during the winter.  At this time most organisms are dormant.  Birds are among the few active species.  It is the time when the most damage can occur to the marsh in the form of ice sheets which scrape and scour where they develop and are pushed and pulled by the tides.  They cut off the cordgrass and scrape areas of the sediment.  This can destroy areas of the marsh.  At the same time, it begins the process of rendering standing cordgrass into detritus which will eventually be the major energy supply of the marsh as it is broken down and decomposed.

Predation

            Predators feed on their prey.  In the salt marsh predators can come and go with the tide. For example, some crabs and fish enter parts of the marsh during both high and spring tides.  They prey on snails, insects, worms, and other organisms.  Prey species have adapted by escaping predators.  Snails climb into the stems of plants during high tide to escape.  Insects fly to other areas.  Amphipods and isopods hide under detritus and rocks.  Fiddler crabs retreat to their burrows and wait for low tide.

Competition

            Competition occurs when two organisms need the same resource(s) and there isn’t enough of the resource for both of them.  Competition can occur between members of different species or among members of the same species.  Competition is an important factor that determines a great deal about where organisms can live and how successful they can be.

            An example is the distribution of the two principle grasses found in the salt marsh. Both grasses are halophytes and are in the same genus, Spartina.  In the low marsh, smooth cordgrass (S. alterniflora) predominates.  In the high marsh, the smaller salt marsh cordgrass (S. patens) is the dominant species.  Smooth cordgrass makes loose, open spaced, long runners in its root system.  Salt marsh cordgrass makes dense turf-forming roots.  Even though smooth cordgrass can tolerate the conditions of the high marsh, the dense roots of salt marsh cordgrass prevent it from forming extensive stands.  Competition for root space keeps smooth cordgrass from invading and dominating in the high marsh.