salt marshes

SALT MARSHES

Wetland Ecology and Management
NATR 5250/6250

Key Terms and Concepts

  • Neap/Spring Tide:
    Refers to the variation in tidal heights; spring tides are the highest and lowest tides occurring during a full or new moon, while neap tides are the moderate tides occurring during the first and last quarters of the moon.
  • Ebb/Flood Tide:
    Ebb tide is the period when the tide is receding (falling water levels), whereas flood tide is when the tide is rising (increasing water levels).
  • Diurnal/Semi-diurnal Tide:
    Diurnal tides result in one high and one low tide per lunar day, while semi-diurnal tides result in two high and two low tides.
  • Deltaic and Marine Sediments:
    Deltaic sediments are primarily transported and deposited by river systems, while marine sediments are classified as those deposited directly from oceanic sources.
  • High Marsh:
    The elevated section of the salt marsh that experiences less flooding and typically hosts different flora.
  • Low Marsh:
    The lower section of the salt marsh that is more regularly inundated by tidewaters.
  • Sand Barrens (Pannes):
    Areas within salt marshes characterized by low water retention, often hosting unique plant communities.
  • Mud Barrens:
    Similar to sand barrens but composed of wetter, mud-rich substrates.
  • Streamside Levees:
    Natural embankments along streams that are often colonized by specific plant species.
  • Salt Marsh Plant Adaptations:
    Special physiological and morphological traits that enable salt marsh plants to tolerate high salinity and waterlogged conditions.
  • Marsh Productivity:
    The rate at which plants produce biomass in salt marsh ecosystems, which is influenced by various environmental factors.
  • Fate of Marsh Productivity:
    Refers to what happens to the organic material produced within the marsh, including decomposition and export to adjacent habitats.
  • Outwelling Hypothesis:
    Suggests that salt marshes export organic matter and nutrients that support productivity in adjacent estuarine and coastal marine systems.
  • Marsh Consumers:
    Refers to organisms that feed on the primary producers within salt marsh ecosystems.

Salt Marsh Species

  • Smooth Cordgrass = Spartina alterniflora
  • Black Needle Rush = Juncus roemerianus
  • Saltmeadow Cordgrass = Spartina patens
  • Spikegrass = Distichlis spicata
  • Common Reed = Phragmites australis

Geographic Distribution of Salt Marshes

  • Salt marshes are primarily found in middle and high latitudes along intertidal shorelines.
  • Also included is the distribution of wetlands in coastal drainage areas of the United States, which includes freshwater tidal marshes and mangroves as well as tidal salt marshes.

Salt Marsh Characteristics

  • Salt marshes are predominantly intertidal environments, meaning they are periodically inundated by coastal tides.
  • They can be either marine or deltaic in marsh geomorphology, indicating the sources of sediments that form the marshes.
  • These ecosystems are typically sheltered from wave energy, providing a stable environment for plant growth.
  • The vegetation found in salt marshes is halophytic, meaning it has adapted to high saline conditions.

Tidal Hydrology

  • Ebb Tide: The receding phase of the tide.
  • Flood Tide: The advancing phase of the tide.
  • Spring Tide: Occurs during full and new moons; characterized by higher high tides and lower low tides.
  • Neap Tide: Occurs during first and last quarters of the moon; characterized by lower high tides and higher low tides.
  • Diurnal Tide: One cycle of high and low tides in a lunar day.
  • Semi-Diurnal Tide: Two cycles of high and low tides in a lunar day.
  • Mixed Tide: Exhibits a combination of diurnal and semi-diurnal characteristics.

Example of tidal variations: Daily tide patterns observed at Santa Barbara, CA in May 2019 illustrate the differences in spring and neap tides, showcasing the cyclical nature of tidal changes throughout the month.

Geomorphological Features

1. Marine-Dominated Marshes

  • Characterized by shelter from spits, offshore bars, and islands which guard against wave action.
  • Typically found in protected bays and estuaries.

2. River-Dominated Marshes

  • Found along coasts with shallow slopes and low wave energies. Deltas formed in these environments can build out into the continental shelf.
  • The salinity in these systems is determined by the interaction between river discharge and tidal energy, which influences freshwater inflow and saline concentrations.

Salinity Factors in Salt Marshes

Salt marsh salinity is influenced by several environmental factors:

  • Frequency of Tidal Inundation: Frequency and duration of flooding events contribute to overall salinity levels.
  • Freshwater Inflow: Surface runoff and riverine inputs can dilute saltwater, impacting salinity.
  • Rainfall: Rain can decrease salinity levels through dilution.
  • Tidal Creeks and Drainage Slopes: Structures that facilitate the flow of water and sediments during tidal events.
  • Soil Texture: Affects water retention and salinity dynamics within marsh substrates.
  • Vegetation: Impacts evapotranspiration dynamics and selective uptake of water.
  • Depth to Water Table: Influences plant interactions and resilience against salinity stress.
  • Fossil Salt Deposits: Ancient deposits may contribute to current salinity levels.

Salinity and Tidal Range Relationships

An analysis of average salinities of various salt marshes presents a connection between tidal range (saltwater inflow) and freshwater supply (river inflow). Salinity levels are measured in parts per thousand (ppt). Examples include:

  1. Wash, England: Salinity 60 ppt
  2. Laguna Madra, TX: Salinity 59 ppt
  3. Camargue, France: Salinity 40 ppt
  4. Atachalalaya, LA: Salinity 15 ppt
  5. Barataria, LA: Salinity 10 ppt

Vegetation Zonation in Salt Marshes

  • The idealized zonation of vegetation communities in typical North Atlantic salt marshes is heavily influenced by elevation above mean high water levels.
  • Common vegetation types include:
    • Smooth Cordgrass - Spartina alterniflora
    • Saltmeadow Cordgrass - Spartina patens
    • Spikegrass - Distichlis spicata
    • Black Needle Rush - Juncus roemerianus
    • Common Reed - Phragmites australis
  • Vegetation zonation varies by coast:
    • Southeast Atlantic coast features a mix of meadows and marshes with Spartina alterniflora dominant.
    • Eastern Gulf of Mexico exhibits different types of vegetation zones due to climatic variances.

Hydrology of Salt Marshes

The hydrological distinction between low marsh and high marsh in salt marshes is characterized by

  • Maximum periods of submergence, varying per day and year, which defines plant communities:
  • Low Marsh: Submergence for ≥ 360 days per year (maximum continuous exposure for ≤ 9 days).
  • High Marsh: Submergence for < 1.2 days per year (max exposure ≥ 10 days).

Other Features of Salt Marshes

  • Sand Barrens (Pannes): Specific areas within marshes that exhibit low water retention conditions.
  • Mud Barrens: Similar features that exhibit wetter soil conditions.
  • Streamside Levees: Natural levees created along streams or creeks that are integral to marsh architecture.

Plant Adaptations in Salt Marshes

Salt Stress Management

Salt marsh vegetation exhibits adaptations similar to desert species:

  • High salt concentrations challenge water retention capabilities.

Photosynthesis Adaptations

  • C4 Photosynthesis:
    A biochemical pathway allowing marsh grasses to utilize CO2 more efficiently, enabling them to thrive in saline environments while conserving water.

Sulfide Avoidance Mechanisms

  • Sulfide Toxicity:
    Sulfide represents a reduced form of sulfur, toxic to plant tissues in marsh habitats.
  • Adaptations include:
    • Creation of an oxygenated rhizosphere, which can oxidize sulfide to sulfate.
    • Some plants can sequester sulfur in vacuoles, while others demonstrate metabolic tolerance to sulfide.

Primary Production in Salt Marshes

Salt marshes are among the most productive ecosystems on earth, rivaling agricultural productivity. The three main autotrophs are:

  1. Marsh Grasses.
  2. Mud Algae.
  3. Phytoplankton.
  • Productivity is highest in areas along creek channels and low marsh due to greater tidal exposure and flushing; however, various factors cause variability, such as:
    • Soil salinity affects plant growth and nutrient uptake.
    • Anaerobic conditions can lead to toxin buildup (e.g., sulfides) and low oxygen levels.
    • Temperature variations influence production; warmer climates generally yield higher productivity.

Relationships in Salt Marsh Ecosystems

  • Relation of Salinity to Vegetation: The relationship between interstitial salinity and vegetation types occurs based on active flood frequency.
    • For instance, a critical gradient occurs as follows:
    • Tall Vegetation Zones (e.g., Spartina alterniflora) thrive at salinities of 40-80 ppt, while Short Vegetation Zones (e.g., Salicornia spp.) may occur at higher salinities exceeding 100 ppt.
  • Variation of macrophyte primary production across different geographic locations in North America shows productivity differences influenced by latitude, with aboveground productivity measured in grams of carbon per square meter per year (g C m^{-2} yr^{-1}). Examples from various regions include:
    • Canadian Arctic: Low productivity.
    • Mid-Atlantic: Moderate productivity.
    • Southern California: Higher productivity due to favorable climate conditions.

Fate of Primary Production in Salt Marshes

The fate of primary production includes three major processes:

  1. Decomposition:
    • Initial decomposers, primarily fungi, increase the nitrogen content of the decomposed organic matter.
  2. Consumption:
    • Algae serve as critical components of the food web; nitrogen requirements for marsh consumers may surpass carbon demands.
  3. Organic Export:
    • The Outwelling Hypothesis suggests that organic material and nutrients produced in marshes are exported to adjacent estuarine and coastal marine environments, akin to the upwelling of deep ocean waters.

Salt Marsh Consumers

Food Web Dynamics

  • The salt marsh food web displays interactions among various consumer groups in differing habitats:
    • Aquatic Habitat: Detritivores (bacteria, fungi), herbivorous invertebrates (crabs, shrimp), and predatory fish.
    • Aerial Habitat: Includes insects (herbivores), spiders, and bird species (sparrows, egrets).
  • Energy transfer shows a greater loss in respiration through consumption, thus connecting nutrient dynamics between these compartments.

Invertebrate Consumers

Above Ground:
  • Organisms include herbivores, such as arthropods and snails, that graze on marsh vegetation.
Benthic Invertebrates:
  • Comprising of micro- and meio-fauna living within the sediment, such as fiddler crabs and mussels.
Aquatic Invertebrates:
  • Represented by blue crabs and various shrimp species, which play integral roles in this habitat.

Salt Marsh Fish

Resident Fish:

  • Fish that depend on the marsh for all life stages, utilizing this habitat for reproduction and protection while exhibiting high tolerance to salinity and oxygen variability.
    Transient Fish:
  • Species that utilize marshes periodically, often during high tide, certain seasons, or specific life stages; these generalist species are less adapted to extreme environmental variations compared to residents.