Estuaries (17)

Estuaries

• Definition: Inlet of the ocean extending into a river valley to the upper tide limit.

• Function: Acts as a transition zone between freshwater and seawater ecosystems.

• Productivity: Highly productive regions characterized by constant environmental changes.

• Borders: Often surrounded by salt marshes or mangroves.

Types of Estuaries (Geomorphology)

• Influence of Geology: The geomorphology and geological history of a region affect the types of estuaries present.

• Geologically Ephemeral: Some estuaries can change over geological timescales.

Types of Estuaries

Coastal Plain Estuary

• Formed by glacial melt, which raises sea levels and floods low-lying rivers and plains.

• Example: Chesapeake Bay.

Tectonic Estuary

• Created by earthquakes that cause land to sink, allowing seawater to inundate it.

• Example: San Francisco Bay (referenced as Levinton Fig. 2.26).

Bar-built Estuary

• Sediments deposit at the river mouth, forming a barrier between the river and the ocean.

• Example: Pamlico Sound.

Fjord

• Created by glaciers carving deep valleys into coastlines; valleys filled with water upon glacier retreat.

• Examples: Saanich Inlet, Bonne Bay.

Salinity Gradients in Estuaries

• Salinity Range: Varies from 35 (ocean) to near 0 (river).

• Mixing Factors: Influences include river input, tidal action, and basin shape.

• Basic Estuarine Flow: Low-density river water flows downstream while high-density saline water flows upstream.

Types of Estuaries (Salinity Gradients)

• Highly Stratified: Distinct layers due to high river flow.

• Moderately Stratified: Mixing occurs due to wind and tides, resulting in inclined isohalines.

• Vertically Homogeneous: Strong mixing in shallow estuaries (referenced as Levinton Fig. 2.27).

Salinity Structure

• Influenced by topography, river size, estuary channel size, and tidal flow.

• River Discharge: Seasonal variations; more freshwater in spring, lower flows in summer contribute to nutrient addition (leading to O2 depletion). (Refer to Levinton Fig. 16.48).

Other Physical Features

• Substrates: Soft mud.

• Temperature Variability: Higher than coastal waters.

• Wave Action: Reduced.

• Current Concentration: Focused in channels leading to erosion.

• Turbidity: High due to suspended sediments.

• Oxygen Levels: Abundant in the water column but limited in sediments due to high Biological Oxygen Demand (BOD).

Estuarine Biology

• Productivity: Among the most productive habitats; high production at higher trophic levels (e.g., benthic invertebrates, fish, birds).

• Diversity: Generally low with many generalist feeders.

• Fisheries: Estuaries serve as major fisheries zones impacted by human activities.

Estuarine Productivity

• Nutrient Sources: Freshwater runoff carries nutrients (nitrogen, phosphorus, silica) and detritus.

• Ocean Contributions: Ocean water is rich in phosphorus and other nutrients.

• Mixing Effect: Shallow photic waters support primary productivity including phytoplankton, diatom mats, seaweeds, and seagrasses.

• Food Web Base: Most estuarine food webs rely on detritus, resulting in a high detritivore biomass.

Estuarine Fauna

• Components: Consist of marine, freshwater, and brackish species.

• Salinity Tolerance: Varies across species:

  • Marine: >10-15 psu

  • Freshwater: <5 psu

  • Brackish: 5 to 18 psu

• Critical Salinity Range: Most sustainable at 3-8 psu (referenced as Levinton Fig. 16.50).

Benthic Fauna

Brackish-water Benthic Fauna

• Salinity Range: Species found in salinity of 5 - 18 psu.

• Habitat Limitations: Often excluded from marine environments due to biological interactions.

Infauna and Epifauna

• Infaunal Species: Experience less salinity variation, residing within sediment.

• Epifaunal Species: Predominantly crustaceans, are good osmotic regulators, enabling colonization of more upstream areas than planktonic species (referenced as Levinton Fig. 16.49).

Estuarine Fish

• Osmoregulation: Many are adept osmoregulators (e.g., salmon, striped bass), migrating between fresh and saltwater quickly.

• Role: Often serve as spawning grounds or nurseries for fish due to nutrient abundance and predator scarcity.

Estuaries & Shelf: Two-phase System

• Life Cycle: Numerous fish and invertebrate species alternate between estuaries and oceanic shelf habitats for breeding, feeding, and larval development (referenced as Levinton Fig. 7.26).

Estuarine Suspension-Feeders

• Large populations, particularly bivalves (e.g., oyster reefs), significantly control phytoplankton density in shallow estuaries.

• Ecosystem Health: Restoration of oyster reefs can enhance overall ecosystem health (referenced as Levinton Fig. 16.53).

Estuarine Food Webs

• Base of Food Web: Detritus, colonized by microbes, forms the foundation.

• Feeding Process: Detritus is consumed directly by suspension feeders or settles and is consumed by deposit feeders.

• Predator Dynamics: Fish and invertebrates occupy higher trophic levels, consuming bivalves and annelids in abundance.

Human Impacts on Estuaries

• Nutrient Inputs: High nutrient levels cause negative effects on eelgrass beds, increasing epiphyte loads and turbidity.

• Oxygen Dynamics: Seasonal hypoxia or anoxia in sediments leads to adverse effects.

• Overfishing: Targets molluscs and top predators, resulting in trophic cascades.

• Invasive Species: Presence of species like Corbula and Dreissena disrupts native ecosystems.

Estuaries

Definition: An estuary is defined as an inlet of the ocean that extends into a river valley, reaching the upper limit of tidal influence. This unique landform acts as a crucial interface within ecosystems by facilitating interactions between freshwater from rivers and saline water from the ocean.

Function: Estuaries serve as transition zones between freshwater and seawater ecosystems, which are vital for the biodiversity they support. They act as nurseries for many marine species, providing a safe environment for juvenile fish and invertebrates to grow and develop. Additionally, they play a role in filtering pollutants and sediments, thereby improving the water quality before it enters the open ocean.

Productivity: Estuaries are among the most productive regions globally, characterized by their dynamic environmental changes, including frequent fluctuations in salinity, temperature, and nutrient levels. These changes stimulate diverse biological habitats, leading to high levels of primary productivity and food webs that support various trophic levels.

Borders: Estuaries are frequently bordered by salt marshes or mangroves, which provide additional ecosystem services, such as habitat for wildlife, carbon sequestration, and protection against coastal erosion.

Types of Estuaries (Geomorphology)

Influence of Geology: The geomorphology and geological history of a region significantly determine the types of estuaries formed. Certain areas may produce geologically ephemeral estuaries that can change over extensive geological timescales, affecting local ecosystems.

Types of Estuaries

1. Coastal Plain Estuary: Formed by the glacial melt that results in sea level rise, flooding low-lying rivers and plains. They are characterized by a shallow, wide mouth that gradually narrows, allowing significant mixing of fresh and saltwater.

   • Example: Chesapeake Bay, one of the largest estuaries in the United States, which harbors diverse ecosystems and significant fisheries.

2. Tectonic Estuary: Created when tectonic activities such as earthquakes cause land to sink, allowing seawater to inundate the area. These estuaries can be more complex due to geological structures.

   • Example: San Francisco Bay (referenced as Levinton Fig. 2.26), featuring various sub-estuaries and an array of habitats.

3. Bar-built Estuary: Occurs when sediments deposited at the river mouth form a barrier between the river and the ocean, creating shallow lagoons or ponds prone to significant evaporation changes.

   • Example: Pamlico Sound, known for its varied habitats and wildlife.

4. Fjord: These estuaries are carved by glaciers that create deep valleys, which are later filled with seawater following glacial retreat. Fjords are often characterized by steep sides and unique ecosystems.

   • Examples: Saanich Inlet and Bonne Bay are notable fjords supporting distinct marine life.

Salinity Gradients in Estuaries

Salinity Range: The salinity levels in estuaries can vary significantly, ranging from 35 parts per thousand (psu) in ocean water to nearly 0 psu in riverine environments. This gradient is crucial for the survival of various aquatic species.

Mixing Factors: Several factors influence salinity, including river discharge, tidal actions, and the particular morphology of the estuary basin. These factors contribute to the diverse habitats supported within the estuary.

Basic Estuarine Flow: The typical flow pattern sees low-density river water flowing downstream and high-density saline water moving upstream, creating unique ecological niches.

Types of Estuaries (Salinity Gradients)

1. Highly Stratified: Characterized by distinctly separate layers of salinity, often due to high river flows overwhelming tidal influence.

2. Moderately Stratified: Features mixing caused by both wind and tide, leading to inclined isohalines that reflect a gradual transition of salinity levels.

3. Vertically Homogeneous: Occurs in shallow estuaries where strong mixing causes little variation in salinity, resulting in a uniform distribution of salt concentrations (referenced as Levinton Fig. 2.27).

Salinity Structure

• The salinity structure is influenced by various factors, including topographic features, the size of the river and estuary channel, and tidal dynamics.

• River Discharge: Exhibits seasonal variations; typically, spring floods bring fresher water, while lower flows in summer contribute to nutrient influx (important for primary productivity).

Other Physical Features

• Substrates: Estuarine environments typically consist of soft mud, leading to unique biological communities.

• Temperature Variability: Generally higher temperatures are experienced in estuarine waters compared to adjacent coastal waters, affecting metabolic rates of organisms.

• Wave Action: Reduced wave action commonly occurs in estuaries, setting the stage for a calmer environment that supports diverse biota.

• Current Concentration: Currents are often concentrated in specific channels, leading to erosion in those areas, while the sediment is typically deposited in calmer zones.

• Turbidity: Estuaries are often turbid due to suspended sediments, which can influence light penetration and consequently affect photosynthetic organisms.

• Oxygen Levels: While oxygen is abundant in the water column, it can be limited in sediments due to high Biological Oxygen Demand (BOD), particularly in areas rich in organic matter.

Estuarine Biology

Productivity: Estuaries represent some of the most productive habitats on Earth, with high biological production typically concentrated at higher trophic levels, including benthic invertebrates, fish, and bird populations.

Diversity: Generally low diversity is observed, with many species serving as generalist feeders due to the fluctuating nature of available resources.

Fisheries: They function as significant fisheries zones, commonly serving as breeding and nursery grounds for many commercially important fish species, although they are increasingly impacted by human activities such as overfishing and habitat destruction.

Estuarine Productivity

Nutrient Sources: Freshwater runoff is responsible for transporting essential nutrients such as nitrogen, phosphorus, and silica, along with organic detritus, into estuaries.

Ocean Contributions: Ocean water also plays a critical role as it is typically rich in phosphorus and other nutrients that support productivity.

Mixing Effect: Shallow photic waters allow for substantial primary productivity to occur. Organisms such as phytoplankton, diatom mats, seaweeds, and seagrasses thrive in these nutrient-rich environments, forming the base of the food web.

Food Web Base: Most estuarine food webs rely on detritus as a major energy source, which promotes high detritivore biomass and sustains higher trophic levels.

Estuarine Fauna

Components: The fauna of estuarine environments consists of a mixture of marine, freshwater, and brackish species that have adapted to the unique salinity variations.

Salinity Tolerance: Different species exhibit varying tolerances to salinity levels:

• Marine Species: Typically require salinities greater than 10-15 psu.

• Freshwater Species: Prefer salinities under 5 psu.

• Brackish Species: Thrive in salinity ranges between 5 to 18 psu.

• The most sustainable conditions for many species exist within a critical salinity range of 3-8 psu (referenced as Levinton Fig. 16.50).

Benthic Fauna

• Brackish-water Benthic Fauna: Found in salinities ranging from 5 to 18 psu. These species are often limited in their habitats by biological interactions that exclude them from purely marine environments.

• Infauna and Epifauna:

  • Infaunal Species: Residing within sediments, they experience less variation in salinity than surface-dwelling organisms.

  • Epifaunal Species: Predominantly crustaceans, these organisms are capable of osmoregulation, enabling them to inhabit areas with greater salinity extremes than those of planktonic species (referenced as Levinton Fig. 16.49).

Estuarine Fish

• Many estuarine fish species (e.g., salmon, striped bass) are effective osmoregulators, allowing them to migrate between freshwater and saltwater environments quickly.

• Estuaries often serve as vital spawning grounds or nurseries for fish due to the abundance of nutrients and scarcity of predators, aiding in the survival of juvenile fish.

Estuaries & Shelf: Two-phase System

• Numerous fish and invertebrate species alternate their life cycles between estuarine habitats and oceanic shelf areas for breeding, feeding, and larval development, which enhances genetic diversity and population stability (referenced as Levinton Fig. 7.26).

Estuarine Suspension-Feeders

• High populations of suspension-feeding organisms, particularly bivalves such as oyster reefs, play a crucial role by controlling phytoplankton density within shallow estuarine waters.

• The restoration of oyster reefs can significantly enhance overall ecosystem health by improving water clarity and quality (referenced as Levinton Fig. 16.53).

Estuarine Food Webs

• The base of the food web in estuaries is formed by detritus, which is then colonized by microbial life, playing a key role in nutrient cycling.

• Feeding Process: Detritus can be consumed directly by suspension feeders or can settle on substrates, where it is consumed by deposit feeders.

• Predator Dynamics: Higher trophic levels consist of diverse fish and invertebrates that prey upon bivalves, annelids, and other lower trophic level organisms abundant in the ecosystem.

Human Impacts on Estuaries

• Nutrient Inputs: Elevated nutrient levels from agricultural runoff can negatively affect eelgrass beds by increasing epiphyte loads and turbidity, leading to habitat degradation.

• Oxygen Dynamics: Seasonal occurrences of hypoxia or anoxia in sediments can have detrimental effects on benthic organisms, ultimately impacting higher trophic levels within the food web.

• Overfishing: Overexploitation of molluscs and top predators can result in trophic cascades, disrupting the delicate balance of estuarine ecosystems.

• Invasive Species: The introduction of invasive species, such as Corbula and Dreissena, can outcompete and disrupt native populations, leading to further ecological imbalance.