The marsh is chasing the ocean... And losing the race
James Morris, Ph.D.*
What does this mean and how do I know this?
Starting in 1984, permanent plots established in the North Inlet salt marshes (Fig. 1) have been monitored monthly for primary production, elevation, and sediment. The objective of this long-term research is to quantify and explain the temporal variability in these parameters.
The resulting data on demography (e.g. stem survival, density-dependent growth and mortality, etc.), productivity, and sediment chemistry have led to a new paradigm about the regulation of marsh productivity and are providing insights about the responses of coastal wetlands to sea level and climate change. The data demonstrate there is significant interannual variation in net aboveground production controlled proximally by soil salinity, hypoxia, and other variables.
Data have also revealed there are complex interactions involving relative sea level, flooding frequency, and solute balance that extend to the entire estuary. Sea level is lower in the winter months and higher during summer due to seasonal changes in sea temperature and thermal expansion of water. Monthly mean sea level varies seasonally, with a range of about 30 cm. Changes of this magnitude have a great effect on the flooding frequency of salt marshes. Mean sea level for the summer growing season (April-August, red dots) is highly variable, and encompass the peak months (July and August) for the growth of marsh plants. Furthermore, flooding frequency during the summer in conjunction with the higher evaporation of water can cause soil salinity can rise to levels that reduce the growth of the marsh plants. (Morris 1995).
The variability observed in annual production of Spartina alterniflora was the spark for an entire line of research (Morris et al. 1990, Morris and Haskin 1990). The interannual variation in production of Spartina alterniflora correlates positively with mean summer sea level (Morris 2000, Morris et al. 2013). Observing a trend of rising productivity in established plots between 1984 and 1996 led me to hypothesize in 1996 that the marsh was not keeping pace with rising sea level. Reasoning that if productivity was high in years when summer sea-level and flood duration were high, then a trend of increasing plant growth could imply that sea level was rising faster than the marshes could keep pace.
I set out to test this hypothesis by measuring the change in marsh elevation (Morris et al. 2002) using Sedimentation-Erosion Tables (SETs) (Cahoon et al. 2002) installed at Goat Island near our permanent plant census plots. One half of each SET plot was fertilized with nitrogen and phosphorus to test the hypothesis that nutrient additions increase the decay of organic matter and lead to a loss of elevation. The current results demonstrate 1) the marsh is not keeping pace with sea level and 2) contrary to the original hypothesis, nutrient additions greatly increased marsh elevation, that is, until we stopped fertilizing in 2005. The rate of sea-level rise recently has been about 4 mm/yr, and our control marsh has gained only about 1 mm/yr since the experiment began. The fertilized sites gained an average of almost 5 mm/yr during the time they were treated.
The result that fertilized sites gained elevation at a greater rate than controls led to the Marsh Equilibrium Model (MEM) (Morris et al. 2002). When the growth of vegetation in a marsh is stimulated, whether by nutrients or rising water level, the plants trap more of the suspended sediment in tidal flood water (due to the increased density and biomass), and they make more biovolume in the soil (i.e., they grow more roots). This is the idea behind the model. See the paper by Wigand et al. (2015) for further details about the root response to nutrients.
Increasing the inundation time of the marsh with rising water levels cannot increase production indefinitely. There is a limit. If the flooding is too great, plant growth will decrease and, consequently, sediment trapping and biovolume growth will also decrease, leading to marsh collapse.
In North Inlet and elsewhere, the plant response to relative elevation is being measured using devices known popularly as marsh organs. A marsh organ is a bioassay that holds marsh turf at different elevations in the field. When the plants are harvested we typically see a parabolic response. We have found that there is an optimum relative elevation for growth where the plants are exposed to the perfect amount of flooding. At one extreme the marsh is too dry, and at the other it is too wet. If the elevation of the marsh is in the range that is super-optimal for growth, then rising water level will stimulate growth, increase sedimentation, and raise the elevation of the marsh. If marsh elevation is sub-optimal, then rising water will have the opposite effect. Our data on the growth-elevation response is the basis for my marsh equilibrium theory (Morris et al. 2002, Morris in press).
Another fascinating product of this project is the observation that the growth of marsh vegetation is tightly connected to sediment biogeochemical cycles. A synthesis of 20-yr of data from harmonic analyses of monthly measurements of plant growth and soil chemical parameters (concentrations of ammonium, phosphate, and sulfide) shows that these cycles are periodic and synchronous. The cycles of N, P, and S appear to be controlled by the plants. Plant growth rate is represented by the color of the line and ranges from red (fast growth) to blue (very slow). One revolution (up and down) of the line is a single annual cycle, moving counterclockwise, i.e. the nutrient concentrations rise as plant growth rises and they fall as growth is declining. The entire figure, which looks like a basket, traces a larger cycle of 18.6-yr duration, moving clockwise, driven by the lunar nodal cycle. The interaction of nutrients and primary producers is usually viewed like a predator-prey cycle – nutrients rise, stimulating growth, plants (algae) exhaust the supply and then crash. Marsh nutrient cycles seem to be timed to coincide exactly with growth. Based on the data from my long-term research I think that the Spartina plants are orchestrating these cycles, and that will be the subject of future research.
What does all this mean? It means that North Inlet salt marshes in particular are unlikely to survive to the end of this century. There are areas in North Inlet where vegetated marsh is already succumbing to rising seas. The future North Inlet is likely to lose its inlet and its barrier island, and the beach will retreat to the upland edge west of present day marshes. It’s not just a local effect. Marshes up and down the East and Gulf Coasts are also likely to retreat in coming decades, sooner or later, depending on the turbidity of the water and tide range (Morris et al. 2014, Morris et al. 2016). Furthermore, the water chemistry of a marsh like North Inlet, where the influence from surface water inputs from surrounding watersheds is minor, is greatly influenced today with exchanges of water from the marsh soils. As the marshes recede, the chemistry and fertility of North Inlet will change. It will look like the chemistry of the coastal ocean. The productivity of North Inlet will decline. On the bright side--there will be some compensation for this lost productivity as marshes migrate inland. They will migrate to the extent that topography and people allow. Whether the migration and colonization of new marsh habitat can fully compensate for the loss of existing marsh lands is an open question.
Cahoon, D. R., J. C. Lynch, P. Hensel , R. Boumans, B. C. Perez, B. Segura, and J.
W. Day, Jr. 2002. A device for high precision measurement of wetland sediment elevation:
I. Recent improvements to the sedimentation-erosion table. Journal of Sedimentary Research 72(5):730-733.
Morris, J.T. 1995. The salt and water balance of intertidal sediments: results from North Inlet, South Carolina. Estuaries 18:556-567.
Morris, J.T. 2000. Effects of sea level anomalies on estuarine processes. p. 107-127. In: J. Hobbie (ed.), Estuarine Science: A Synthetic Approach to Research and Practice. Island Press. 539 pp.
Morris, J.T. 2007. Estimating net primary production of salt-marsh macrophytes, pp. 106-119. In Fahey, T.J. and Knapp, A.K (eds), Principles and Standards for Measuring Primary Production. Oxford University Press.
Morris, J.T. In press. Marsh equilibrium theory. In: ICI 2014 Spartina Conference Proceedings. University of Rennes Press, France.
Morris, J.T. ,D.C. Barber, J.C. Callaway, R.Chambers, S.C. Hagen, C.S. Hopkinson, B.J. Johnson, P. Megonigal, S.C. Neubauer, T.Troxler, and C.Wigand. 2016. Contributions of organic and inorganic matter to sediment volume and accretion in tidal wetlands at steady state, Earth’s Future. 4, doi:10.1002/2015EF000334.
Morris, J.T. and B. Haskin. 1990. A 5-yr record of aerial primary production and stand characteristics of Spartina alterniflora. Ecology 71:2209-2217.
Morris, J.T., B. Kjerfve, and J.M. Dean. 1990. Dependence of estuarine productivity on anomalies in mean sea level. Limnology and Oceanography 35:926-930.
Morris, J.T., K. Sundberg, and C.S. Hopkinson. 2013. Salt marsh primary production and its responses to relative sea level and nutrients in estuaries at Plum Island, Massachusetts, and North Inlet, South Carolina, USA. Oceanography 26:78-84.
Morris, J.T., P.V. Sundareshwar, C.T. Nietch, B. Kjerfve, and D.R. Cahoon. 2002. Responses of coastal wetlands to rising sea level. Ecology 83:2869-2877.
Wigand, C., E. Davey, R. Johnson, K. Sundberg, J. Morris, P. Kenny, E. Smith, and M. Holt. 2015. Nutrient effects on belowground organic matter in a minerogenic salt marsh, North Inlet, SC. Estuaries and Coasts 38(6):1838-1853.