The Salt Marsh Science Project is an example of science in progress. As we learn new things, we alter the study and adjust our hypotheses. The following is a list of what we've learned so far about salt marshes and what we still hope to discover:
Previous Studies | What We Think We Know | Questions | Hypotheses | Methods | Educational Extensions | Additional Questions | Conclusions
What Previous Studies Have Shown
- Phragmites australisis an invasive plant species that often grows in monotypic (one type of plant) stands. Therefore, it is a plant that spreads rapidly over an area growing tall and so densely that other plants are killed off.
- Spartina alterniflora (saltmarsh cordgrass) grows close to ditches and other waterways. It grows tall near ditches. It grows in a shorter form away from the ditches if water is impounded (kept in one place for a while). It grows in a short form in areas that water does not drain from immediately.
- Typha (cattails) prefer fresh water and grow in a stunted (shorter) form in brackish water.
- Phragmites prefers to grow in fresh or brackish (mixed fresh with salt) water and cannot tolerate full seawater.
What We Think We Know
- In a salt marsh affected by tidal restrictions, such as a culvert sized too small to allow the normal amount of seawater, ground water salinity (salt content) is lowered. Less of the marsh surface is flooded at high tide, and there is decreased sedimentation which may lead to a relative drop in the marsh surface elevation. This favors the growth of Phragmites, and decreases the amount of Spartina (typical salt marsh grasses) growing.
- With the reintroduction of normal tidal flow, restoration of the system can happen and Phragmites should decline.
- The height of Phragmites decreases with increased salinity.
- Salt marshes help cushion the impact of rising sea level, migrating inward as sea level rises. As the tide level increases, it creates more area where salt marsh plants can grow. Tidal restriction prevents this from happening.
Questions We're Currently Exploring
- How fast is Phragmites growing? How far will it be able to grow into the salt marsh (what is its range in the salt marsh)?
- How quickly will a stand of Phragmites grow into an area ?
- Is the Phragmites range and height limited by salinity?
- What other factors control the growth and spread of Phragmites on salt marshes?
- How do different bird species use Phragmites and other types of vegetation on the salt marsh?
- Are fish species and size of individuals affected by tidal restrictions?
- Phragmites will decrease in height and extent of cover after tidal flow is restored.
- Phragmites stands are still growing and will cover more of the salt marsh in the future if no measures to restore natural tidal circulation are taken.
- The depth of the water level has an impact on the growth of Phragmites.
- Phragmites stands harbor less biodiversity than the Spartina patens and cattail stands.
- Phragmites prefer to grow where there is lower salinity levels (between 5-20 parts per thousand /00). Spartina grasses grow where salinity is higher (over 20 /00.). Salinity is higher where tide is not restricted and lower within an area impacted by a tidal restriction.
- Other salt marsh plants, such as Spartina grasses, will reestablish themselves after a tidal flow is restored.
- Birds and other animal species prefer Spartina grasses and other salt marsh plants to Phragmites.
- Fish are less abundant and there are fewer species within a tidal restriction than outside a tidal restriction.
Our Research Methods
Items with an * indicate that students are also doing this at this time (since 1998).
- Measuring and comparing the length, volume*, and amount* of fish, upstream and downstream of a tidal restriction (two traps upstream, two traps downstream, measure fish caught per hour).
- Measuring salinity* (salt content) within Phragmites and outside of Phragmites in the transition, and also at a similar distance from the upland with no Phragmites.
- Monitoring vegetation* along transects over time to observe and record the height and presence of different species.
Educational Extensions of the Project
- Do an additional bird/vegetation study or an insect/vegetation or macroinvertebrate/vegetation study.
- Make a herbarium (pressed samples) of all the plant species found on this salt marsh.
- Study the growth of Phragmites roots.
- Update the salt marsh vegetation key to include additional species found unique to each school site.
- Observe how plants are interacting with animals and other plants.
- Observe how birds are interacting with plants, animals, etc.
- Monitoring elevation* changes caused by addition of sediments the marsh surface.
Additional Questions To Investigate
What is the impact of school groups studying the salt marshes? How can we minimize this impact?
The weight of students will compact the marsh, decreasing the ability of plants to get oxygen to their roots and, therefore, the plants will die. Trampling will directly kill vegetation. Salicornia will grow back where trampling killed vegetation. Other plants may grow in a progression as a result of students impacting the site. Large amounts of students standing in one place may depress the area enough to create a salt panne (depression filled with water.)
Minimizing the amounts of students standing in one area at one time will help minimize the compaction of the salt marsh and prevent some killing of vegetation.
Possible methods of investigation:
Use a soil compression meter to measure how compressed soil is before and after students have stood in an area. Compare this to soil compressed by ice. Compare vegetation over time.
Why is that plant species growing in a certain place in the marsh? What abiotic and biotic features affect the growth of plants on the salt marsh? What is a typical successional pattern on the salt marsh? (Is there a typical pattern?)
There are predictable patterns of succession on the salt marsh.
Are more insects species and individuals found amongst the Spartina grasses and cattails than amongst the Phragmites stands?
Are snails and other macro-invertebrates more abundant in the salt marsh compared to Phragmites stands?
How is the area responding to sea level rise?
Tidal restricted areas do not rise with the sea level. Areas that are not tidally restricted keep up with the sea level rise.
Salinity (salt content)
Salt water is heavier (more dense) than fresh water. When a river flows to meet the sea, it flows on top of the heavier salt water. Wave action eventually mixes the two. Deer can drink fresh water from rainfall off salt pannes because the fresh water floats on top. Students may have personal experience at floating more easily in salt water, than fresh water.
Why did students at Pine Grove measure salinity higher in the shallow well and lowest in the deeper well?
Soil does not filter out salt.
Massachusetts Audubon data collected in 1996 revealed that salinity changes throughout the year. The shallow well shows the greatest variety of salinities and seems to be responding to outside factors. Dave Burdick, scientist at UNH, noticed that salinity in the shallow well is lowest in the springtime and suggests that it is responding to an inflow of freshwater due to snowmelt etc. During this time, the medium and deep wells have higher salinity. At the end of the summer (where drought has meant fewer fresh water inputs and sunlight has meant increased evaporation) the salinity of the shallow well is greatest and the medium and deeper wells have less salinity.
Phragmites is growing in areas with salinities as high as 27 ppt.
Fresh groundwater from the upland could be flowing underneath salt water. This would account for lower salinity at greater depths.