Ch 5: Intertidal Zonation, Estuarine Dynamics, and Coastal Ecosystems

Vertical Zonation in the Rocky Intertidal

This ecosystem is characterized by strong physical and biological gradients. It is a universal phenomenon where distinct horizontal bands of species occupy specific heights on the shoreline. Major divisions include the Supralittoral (spray) zone , the Littoral zone , and the Infralittoral zone.

Determining Upper and Lower Species Limits

The upper limit of a species is generally determined by physical factors related to physiological tolerance, such as desiccation, tide levels, and high temperatures. Conversely, the lower limit of a species is typically dictated by biological factors , specifically competition for space and predation.

Critical Tide Levels

These are specific elevations on the rocky shore where the duration of continuous exposure to air or submergence in water changes abruptly. A species' survival can depend on its position relative to these levels due to the drastic shifts in desiccation stress they represent.

Barnacle Zonation (Chthamalus vs. Semibalanus)

In New England, the upper limit for both species is set by physical factors. However, the lower limit for Chthamalus is set by competition , as Semibalanus typically moves up the shore and displaces it. Chthamalus survives only in the highest zone because it has a higher tolerance for desiccation than Semibalanus.

Keystone Species Predation (Pisaster)

The predatory sea star Pisaster sets the lower limit for mussels. By consuming mussels, Pisaster prevents them from monopolizing all available space. This predation allows other species to establish themselves, thereby maintaining higher community diversity.

Three Models of Succession

1. Facilitation Model: Opportunistic species modify the environment to make it appropriate for climax species. 2. Inhibition Model: The first species to colonize space holds it, and progression only occurs when those species die. 3. Tolerance Model: Any species can start the progression, but it moves along in an orderly fashion based on which species are most tolerant of the conditions.

Alternate Stable States in Tide Pools

Tide pools can exist in two distinct ecological states. High-diversity pools with snails contain a mixture of palatable green seaweeds and well-defended species like Irish moss. Low-diversity pools without snails are dominated by fast-growing, competitively dominant green seaweeds like Ulva because green crabs limit snail recruitment.

Sandy Beach Structure

The physical structure of a sandy beach is divided into three primary zones: the Dunes (vegetated hills), the Berm (flat platform), and the Beach Face (sloping area washed by waves).

Dissipative vs. Reflective Beaches

Dissipative beaches have a flat, concave beach face with spilling breakers where wave energy is spread over a wide area. Reflective beaches feature a steep beach face and surging breakers where energy is reflected back off the shore.

Feeding Mechanisms on Sandy Beaches

Inhabitants utilize two main strategies: 1. Suspension Feeders, which filter particles from the water column (e.g., mollusks, beach crabs, polychaetes). 2. Deposit Feeders, which eat the substrate they live in to extract organic matter (e.g., mud snails, fiddler crabs, polychaetes).

Sediment Grain Size and Biodiversity

Coarse-grained beaches have large interstitial spaces and strong wave action but support few organisms. Fine-grained beaches have small spaces, retain water at low tide, and host many organisms. On artificial reefs, the absolute number of organisms often decreases when approaching the reef due to reduced grain size.

Redox Potential Discontinuity (RPD) Layer

In muddy shores, oxygen is available only in the upper surface layer where oxidized molecules abound. Within centimeters of the surface, the RPD layer marks the transition between this oxidized zone and the deeper anoxic zone.

Chemical Environment of the Anoxic Zone

Below the RPD layer, the sediment is black and characterized by anaerobic decomposition. This zone contains several reduced compounds, including methane ($CH_{4}$), hydrogen sulfide ($H_{2}S$), ammonia ($NH_{3}$), and ferrous iron ($Fe^{2+}$).

Primary Production in Muddy Shores

Production occurs in two distinct layers: the surface, which is dominated by diatoms, algae, and marine grasses , and the RPD layer, where organic production is driven by chemolithoautotrophic bacteria.

Salt Marsh Progression

Marshes form through a sequence involving Spartina alterniflora. The grass reduces wave action and increases sedimentation. As the plants die and decay, they produce peat, which allows the marsh surface to build up over time.

Aerenchymal Tissue

This is specialized tissue characterized by a series of circular passageways found in Spartina. It is vital because it allows the plant to exchange gases even when its root and rhizome system is surrounded by anoxic soil.

Salt Marsh Plant Zonation

The low marsh is dominated by tall Spartina alterniflora. The high marsh consists of the Spartina patens zone and the Juncus gerardi zone. The marsh border often features the marsh elder, Iva frutescens.

Salt Pans and Disturbance

Salt pans are disturbances within the marsh where standing water evaporates, leading to high salt buildup that prevents plant growth. These hypersaline depressions often form in intermediate elevations or large open patches.

Salt Marsh Dieback Theories

Dieback can be caused by "Top-Down" or "Bottom-Up" factors. A prominent top-down example involves the crab Sesarma reticulatum, which overgrazes Spartina. These diebacks can shift the marsh into alternate stable states.

Outwelling Hypothesis

This theory suggests that salt marshes produce an excess of organic material and nutrients that are "outwelled" into the coastal ocean. This transport supports offshore food webs and serves as a nutrient source for coastal waters.

Four Geological Types of Estuaries

1. Coastal Plain: Rising sea levels invade rivers (e.g., Hudson, Chesapeake). 2. Tectonic: Ground sinks to create the basin (e.g., San Francisco Bay). 3. Lagoon: Sandbars build up and enclose water (e.g., Pamlico Sound). 4. Fjord: Glaciers scour a deep valley with a shallow sill.

Positive, Neutral, and Negative Estuaries

A Positive Estuary is river-dominated and stratified with a salt wedge where salinity decreases upriver. A Neutral Estuary is marine-dominated and homogenous. A Negative Estuary is evaporative; salinity increases as you move up the estuary.

Factors Affecting Estuarine Salinity

The salinity gradient is influenced by river flow, tidal strength, the Coriolis Effect (which deflects water), and the nature of the substrate.

Euryhaline vs. Stenohaline Species

Euryhaline species are marine organisms that can tolerate a wide range of salinities and are common in estuaries. Stenohaline marine species can only tolerate a narrow range of salinity and are usually restricted to the mouth of the estuary.

Osmoregulators and Osmoconformers

These are physiological adaptations for estuarine life. Osmoregulators actively maintain their internal salt balance regardless of external salinity, while osmoconformers allow their internal salinity to change with the environment.

Estuaries as Nurseries

Many transitory marine organisms use estuaries as nurseries. They provide abundant food and protection for young crabs and fish before they migrate back to the open ocean as adults.

Estuarine Flushing Time

This is a physical factor referring to the time required for a given volume of water to be replaced or circulated out of the estuary. It is critical for determining how long nutrients or pollutants remain in the system.

Turbidity in Estuaries

Estuaries often have high turbidity, which refers to the amount of suspended solids in the water. High turbidity can limit light penetration, affecting primary production by phytoplankton and submerged plants.

Anthropogenic Changes to Near-Shore Habitats

Coastal environments are threatened by filling/embankment, dredging of channels, and eutrophication (excessive nutrient loading). Sea level rise is also a significant long-term concern for these habitats.

Fetch

In the context of coastal physical factors, fetch refers to the distance of open water over which the wind blows. It directly influences the size and strength of waves that impact the shoreline or estuary.

Physical factors: Different Rock Texture, Crevices, exposure to waves, exposure to the sun, slope

Biological: predation, territoriality