Salt Marsh

Introduction

Salt marshes represent a transitional zone between terrestrial and marine ecosystems resulting in one of the most biologically productive habitats on earth. This productivity plays a major role in the nutrient cycles and food webs for both terrestrial as well as marine ecosystems. Migratory birds find abundant food resources from insects, mollusks, arthropods and fish. Marine fauna utilize the nutrient rich habitat establishing critical reproductive sites for invertebrates and vertebrates groups including oysters, crabs, sharks and other fish species. This productivity is a result of major conflicting environmental factors that create harsh conditions with stressful levels of salinity, water submergence and low oxygen levels. Environmental conditions are shaped by daily tidal surges, major changes in seasonal fluctuations of freshwater, saltwater inundation during the occasional storm surges, and increased salinity during seasonal dry periods and evapotranspiration. Salinity at or greater than sea water at 34 g l-1 (3.4%) is the norm throughout the salt marsh resulting in significant Na fluxes. Regular inundation with fresh and salt water creates dramatic fluctuations in soil oxygen levels and sediment deposits. These extremes in salt and water saturations are beyond the tolerance of all but a few highly specialized organisms. Accumulated plant biomass and terrestrial runoff creates an exceptional N cycle. Essential to the productivity of the salt marsh through these various nutrient and chemical fluctuations is a diverse and unique microbial community supporting the flora and fauna of this habitat and sustaining the associated marine environment.

Physical environment

Salt marshes are influenced by three major environmental variables; salinity gradients, reoccurring water table changes and anaerobic conditions associated with saturated soils. Salt from marine water is the most dominant factor in salt marsh habitats. While sea water salinity (3.4%) is found throughout the marsh, evapotranspiration results in salinity frequently double sea water. Some soils become hypersaline (>10%) as a result of tidal flooding trapping water at higher elevations allowing evapotranspiration to concentrate salinity. Mean high water level results in substantial changes in soil salinity and subsequent plant community composition. 7% salinity prevents most salt marsh plant species from becoming established. Monotypic stands of cordgrass (Spartina spp.) develop In many temperate climate salt marshes. Spartina is one of the few plants capable of tolerating and growing in the higher saline environment. (Zedler, 2008)

     Microbes are confronted with several environmental barriers.  High salinity creates problems of osmotic regulation and disrupts enzyme activity.  A thin aerobic sediment layer with abundant organic material is accompanied by periodic submergence.  Immediately below the aerobic lay is an extended layer with frequent anaerobic conditions followed by a much deeper layer of continuous saturated soil with low organic matter and high saline conditions.  These conditions in association with a monotypic plant community provide limited resources for the microbial community specialized to cope with these conditions. 

Tidal cycles

Salt marshes are typically divided into a high, middle and low marsh zone based on elevation but responding to soil salinity established by tidal occurrence. Neap tides occur when the difference between high and low tide is the least, the lowest level of high tide occurring twice a month during the first and third quarters of the moon. Mean high water level is the average higher high-tide level during lower amplitude neap tides. Halophytic plants are usually able to become established above this mean high water level (Zedler, et al., 2008). The mean higher high water level determines the salinity maximum. The vertical width of the maximum salinity band is determined by tidal irregularities which may lead to formation of salt barrens or flats (Wang, H., 2007).

Soil and Geochemical Profile

Plants will slow water flow thus trapping sediment with roots and rhizomes. This trapped sediment is easily lost at the next water surge unless immobilized. Biofilms (algae, fungi and bacteria) produce mucilage which cements sediment particles to form initial soils. Biofilm of cyanobacteria will also coat plant stems. Floating mats of green macro algae collect sediment and help build additional soils when trapped on higher elevations. Marsh soils have high levels of sulfur from the marine water easily observed as the blacken soil which smells like rotten eggs. Coloration and odor are a result of the microbial formation of sulfide. This higher sulfide is another factor reducing the number of plants capable of growing in this habitat. Soils are often anoxic immediately below the surface due to high organic matter and abundant moisture for microorganisms. Roots and burrowing invertebrate bioturbation create micro pores within the soil which aid in aeration. Bioturbation also causes re-suspended and erosion during tidal activity. Biofilms help counter bioturbation thus inhibiting erosion. (Zedler, et al., 2008) Wilms, R. et al., (2006) produced a geochemical profile of tidal-flat sediments close to the island of Spiekeroog, Germany. Sulfate concentrations ranged from 30 mM at the surface. Beneath 50 cm sulfate decreased to concentrations below 1 mM. At 250 cm, a second maximum of pore water sulfate was detected, with concentrations ranging from 5.3 to 11.5 mM. Sulfate was depleted below 400 cm with concentrations below .2 mM. Methane was depleted along the anoxic part of the sediment with concentrations below detection limit at the surface. Highest methane value (125 uM) was measured in the sulfate minimum zone (100 and 200 cm). At the deepest layer (450 cm) 100 uM methane was detected. Maximum ammonium concentrations (4.6 to 8.8 mM) were found within the lower part of the sand-dominated interval and the upper shell layers (120 to 200 cm). Beneath this peak, concentrations did not decrease below 1 mM. TOC in the sand-dominated sediments was generally below 0.5%. Higher values (up to 1.3%) were found only for thin intercalations of black mud. These values indicate several different chemical gradients are occurring simultaneously.

Carbon

Organic matter is provided by a very limited number of resources. Roots and stems from Spartina provide a thin layer of carbon on the sediment surface. Autotrophs, cyanobacteria and green algae, provide additional useable carbon. Characteristic of the salt marsh is the very limited organic matter restricted to the upper few centimeters of sediment. Anoxic conditions slow the decomposition with carbon accumulating as peat (Zedler, 2008). Microbial community immobilizes and mineralizes most of the carbon produced (Kuehn, 2000). While wetland plants contribute to the majority of the primary production, only a small fraction of the produced biomass accumulates in the marsh with the majority relocated to other areas through tidal activity (Sousa, 2010)

Nitrogen

Salt marshes are often the recipient of large concentrations of nitrogen because of their small size in relations to the size of the watershed that provides freshwater runoff. Most of the water sourced nitrogen simply passes through the salt marsh and becomes part of the aquatic portion of the nitrogen cycle in the ocean. Nitrogen fixation and atmospheric deposition are major factors in salt marsh soils. Approximately 55% of the nitrogen from land and fixation are exported to the ocean. The remainder is stored or denitrified making salt marshes both sinks and sources of nitrogen. (Hopkinson, 2008)

Phosphorus

Phosphorus is not directly limiting to salt marshes but it provides an indirect limitation. Sundareshwar, ( 2003 ) has shown that while plants are limited by nitrogen, bacteria that support the plants in marsh habitat are limited by phosphorus. Phosphorus limits inhibit the growth of nitrogen-transforming bacteria which also affect carbon fixation, storage and release by plants. Phosphorus availability has major implications for salt marsh communities.

Biological interactions

Plants

Most plants are unable to cope with the near surface salt water conditions. Some adopt unique means for dealing with salt. Turtle weed (Batis maritime) continually drops its older leaves with high salt concentration as a means of salt removal. This provides organic matter for microbes immediately below the plant and enriches the carbon cycle. In temperate climates cordgrass (Spartina spp) often becomes the lone botanical representative. Near surface water limits the root system depth and subsequently the organic resources needed by microbes at lower depths. At higher elevation where brackish conditions prevail, cattails (Typha spp.) become the dominate species. The more competitive cattail prevails in the less harsh conditions while the more stress tolerant species of cordgrass will dominate the salt marsh. In contrast, the microbial community including green algae, cyanobacteria, diatoms and flagellates are diverse in species and functional groups (Zedler, 2008).

Animals

Salt marshes are limited in the number of animals that utilize this habitat. Fish find a rich nutrient source in the water associated with the marshes but have little impact on the habitat. Predominate fauna inhabitats are birds and invertebrates particularly fiddler crabs. While most birds are migratory using the fish population as a nutrient source they provide a nitrogen source for the microbial community in the form of urea. Fiddler crabs provide a unique function by extending aerobic conditions deeper within the sediment. Burrowing activity increases oxidation within the soil sometimes as deep as 50 cm below the sediment surface. Labile organic matter from plant material is largely restricted to the surface creating a sharp redox gradient with the highest density of aerobic microbes in the upper 10 cm. Roots and crab burrows enhance the limited mixing that occurs in the vertical separation between the aerobic and anaerobic conditions (Koretsky, 2005).