Course: Biol 3711 - Winter 2025
Estuaries are inlets of the ocean extending into river valleys, marked by the upper limit of tidal influence.
They serve as transition zones between freshwater and seawater ecosystems.
Recognized for their high productivity and constant environmental changes.
Often bordered by salt marshes or mangroves, contributing to biodiversity.
Variations in geomorphology and geological history result in distinct types of estuaries.
Some estuaries are geologically ephemeral, meaning they are temporary and may evolve over time.
Coastal Plain Estuary: Formed by glacial melt, flooding low-lying rivers and plains.
Example: Chesapeake Bay
Tectonic Estuary: Created when earthquakes cause land to sink, allowing seawater influx.
Example: San Francisco Bay
Bar-built Estuary: Formed by sediment deposits at river mouths, creating barriers between river and ocean.
Example: Pamlico Sound
Fjord: Deep valleys cut by glaciers, filled by seawater upon glacier retreat.
Examples: Saanich Inlet, Bonne Bay
Salinity can range from 35 ppt (parts per thousand) to near 0 ppt.
Movement and mixing of water are influenced by river inputs, tides, and the shape of the estuarine basin.
Basic flow dynamics:
Low-density river water flows downstream, while high-density saline water flows upstream.
Highly Stratified Estuary: Distinct layers due to high river flow.
Moderately Stratified Estuary: Mixing layers due to wind and tides, inclined isohalines present.
Vertically Homogeneous Estuary: Strong mixing in shallow estuaries leads to more uniform salinity.
Factors affecting salinity:
Topography, size of river and estuary channel, tidal flow.
River discharge varies seasonally:
Spring: Increased flow from freshwater.
Summer: Lower flow, with nutrient additions influencing biological oxygen demand (BOD).
Salinity varies with tidal changes and river discharge.
Point A: Affects by varying salinity levels during low tide.
Point B: Inundated only at high tide, receives high-salinity water, unaffected by low-salinity water during low tide.
Soft sediments (mud) dominate the substrate.
Higher temperature variability compared to the open sea.
Reduced wave action leads to focused currents, causing erosion.
High turbidity present, with abundant oxygen in the water column but limited in sediments due to high BOD.
Estuaries are among the most productive habitats with high fertility.
Supporting higher trophic levels: abundance of benthic invertebrates, fishes, and birds, although diversity may be low.
Home to significant fisheries with many human impacts influencing health.
Nutrient runoff (nitrogen, phosphorus, silicate) and detritus from freshwater contribute to productivity.
Ocean water enhances nutrient availability, supporting primary productivity in photic waters through phytoplankton, diatom mats, seaweeds, and seagrasses.
Detritus forms the foundational element of estuarine food webs due to high detritivore biomass.
Key components involve marine, freshwater, and brackish water species.
Salinity tolerances vary:
Marine species: tolerate >10-15 ppt.
Freshwater species: <5 ppt.
Brackish species: 5 to 18 ppt.
Critical salinity range for various species: 3-8 ppt.
Species within salinity of 5-18 ppt, not found in pure freshwater or true seawater.
Exclusion from marine habitats may result from biological interactions.
Infaunal species exhibit less salinity variation in their habitats.
Epifaunal species constitute most abundant varieties, possessing effective osmotic regulation (e.g., crustaceans), allowing upstream colonization beyond planktonic species.
Species such as salmon and striped bass are notable for their osmoregulation capabilities when migrating from freshwater to seawater.
Estuaries serve as vital spawning grounds or nurseries due to nutrient-rich waters and lower predation risks.
Many aquatic species utilize estuaries during part of their life cycle before migrating to the continental shelf for other activities, creating distinct breeding and feeding grounds.
High populations of suspension-feeding bivalves (like oysters) play a crucial role in controlling phytoplankton levels in estuaries.
Restoration of oyster reefs can enhance overall ecosystem health.
Detritus, colonized by microbes, forms a base for estuarine food webs.
Suspension-feeders consume detritus directly or indirectly after its deposition as sediment.
Bivalves and annelids are abundant, serving as food sources for various invertebrate and vertebrate predators.
Primary components of the food web include:
Detritus from river and sea.
Feeding interactions include:
Detritus feeders
Plant grazers (e.g., Gammarus, Corophium)
Birds (e.g., shorebirds, ducks)
Predators (e.g., various fish species).
Significant nutrient inputs can adversely affect eelgrass beds due to increased turbidity and epiphyte loading.
Seasonal hypoxia or anoxia can occur in sediments, severely impacting habitat quality.
Overfishing, especially of molluscs and apex predators, can result in trophic cascades.
Invasive species present additional challenges to estuarine ecosystems (e.g., Corbula, Dreissena).
Estuaries are dynamic waterways where freshwater from rivers meets and mixes with saltwater from the ocean. These unique ecosystems are characterized by the upper limit of tidal influence, making them vital transition zones between freshwater and marine environments. Known for their remarkable productivity, estuaries support diverse biological communities and exhibit constant environmental changes due to tidal fluctuations, seasonal variations, and human impacts. They often host coastal ecosystems, including salt marshes and mangroves, which provide essential habitat and contribute significantly to biodiversity.
Estuaries can be classified based on variations in geomorphology and geological history, leading to distinct types that reflect their formation processes and ecological functions. Some estuaries are geologically ephemeral, meaning they are temporary and may evolve over time due to natural processes such as sedimentation or sea-level changes.
Coastal Plain Estuary: Formed primarily by glacial melt and subsequent flooding of low-lying areas. These estuaries often feature gentle slopes and are characterized by broad river mouths.Example: Chesapeake Bay, one of the largest estuaries in the United States, renowned for its rich biodiversity and important fisheries.
Tectonic Estuary: Created as tectonic activity leads to subsidence, resulting in depressions that fill with seawater. These estuaries tend to have steep sides and can be significantly influenced by geological events.Example: San Francisco Bay, known for its varied habitats and immense ecological significance.
Bar-built Estuary: Formed when sediment deposits at river mouths create natural barriers, such as sandbars, separating the river from the ocean.Example: Pamlico Sound, characterized by shallow waters and rich biodiversity amidst a complex system of tidal creeks and marshes.
Fjord: These estuaries are deep and narrow, typically formed by glacial activity, which carves out valleys that are later inundated by seawater as glaciers retreat.Examples: Saanich Inlet and Bonne Bay, both exhibiting unique geological features and rich marine life.
Estuaries exhibit a wide range of salinity levels, typically spanning from 35 ppt in open ocean waters to near 0 ppt at river inputs. Various factors shape salinity patterns, including river discharge, tidal movements, and the physical structure of the estuarine basin. Understanding the basic flow dynamics is critical: low-density freshwater flows downstream, while denser saline water can move upstream, creating a complex layering of water types.
Highly Stratified Estuary: Characterized by distinct layers due to a significant river flow, leading to a pronounced difference between the saltwater below and the freshwater above.
Moderately Stratified Estuary: Displays a mix of water layers influenced by wind, tidal action, and varying isohalines, promoting biological diversity.
Vertically Homogeneous Estuary: In these estuaries, shallow conditions promote strong mixing, resulting in a more uniform salinity distribution.
Several crucial factors affect salinity in estuaries, including topography, the size of river channels, and tidal flows. Additionally, the seasonal variability in river discharge leads to distinct ecological patterns:
Spring: Increased freshwater flow occurs due to snowmelt, which can support higher nutrient loads.
Summer: Reduced freshwater flow leads to greater concentrations of salinity, with additional nutrient contributions potentially increasing biological oxygen demand (BOD).
Salinity levels can fluctuate significantly with tidal changes and variations in river discharge.
Point A: Experiences fluctuating salinity levels during low tide, impacting local flora and fauna.
Point B: Remains inundated only at high tide, receiving predominantly high-salinity water and becoming minimally influenced by low-salinity conditions during periods of low tide.
Estuarine environments are often dominated by soft sediments (mud), resulting in habitats with higher temperature variability than open sea environments. Reduced wave action allows focused currents that can lead to erosion and other changes over time. Moreover, high turbidity levels can lead to a diverse yet challenging habitat, offering abundant dissolved oxygen in the water column, although this can be limited within sediments due to high BOD levels.
Estuaries are among the most productive habitats on Earth, exhibiting high fertility that supports various trophic levels. While they host abundant populations of benthic invertebrates, fish, and birds, overall species diversity may be relatively lower compared to other ecosystems. Estuaries also play a crucial role in global fisheries, with numerous human activities posing threats to their health and sustainability.
Nutrient runoff from terrestrial sources (nitrogen, phosphorus, silicate) and detrital material from freshwater systems greatly enhance estuarine productivity. Ocean water further boosts nutrient availability in photic zones, fostering primary productivity through phytoplankton, diatom mats, seaweeds, and seagrasses. Detritus serves as a critical foundational element within estuarine food webs due to the high biomass of detritivores that rely on this resource for energy.
The fauna of estuaries comprises a mix of marine, freshwater, and brackish species, with different species showing varied tolerance levels to salinity:
Marine species: Typically tolerate salinity levels greater than 10-15 ppt.
Freshwater species: Thrive in salinity below 5 ppt.
Brackish species: Occupy a range of 5 to 18 ppt salinity. Critical salinity ranges for successful growth and reproduction of various species tend to lie within 3-8 ppt.
Species that thrive in brackish conditions (5-18 ppt salinity) do not typically inhabit pure freshwater or full seawater environments. Their absence from marine habitats can arise from interactions such as competition and predation, which shape community composition within estuaries.
Infaunal species are found within sediment layers and often exhibit reduced salinity variation in their habitats. In contrast, epifaunal species, which dwell on the sediment surface, represent a significant proportion of faunal varieties, with many possessing effective osmotic regulation (e.g., certain crustaceans) that enables them to migrate upstream, beyond the limits of planktonic distributions.
Important fish species, such as salmon and striped bass, are recognized for their exceptional osmoregulation capabilities, allowing them to transition between freshwater and seawater successfully. Estuaries serve as vital spawning grounds or nurseries, providing nutrient-rich environments where young fish can flourish with reduced predation risks.
Many aquatic organisms utilize estuaries at various life stages before migrating to the continental shelf, where they engage in foraging and breeding activities, creating distinct and critical habitats for their evolution and survival.
The presence of high populations of suspension-feeding bivalves, such as oysters, plays an essential role in controlling phytoplankton populations within estuaries. Restoration of oyster reefs is increasingly recognized as beneficial for the overall health and resilience of estuarine ecosystems.
Detritus, often colonized by microbes, forms the foundational basis of estuarine food webs. Suspension-feeders consume detritus directly or benefit indirectly when it accumulates as sediment. Abundant bivalves and annelids serve as important food resources for diverse invertebrate and vertebrate predators, supporting the overall food chain.
Key components of estuarine food webs include:
Detritus originating from both riverine and oceanic sources.
Feeding relationships feature:
Detritus feeders
Herbivores or plant grazers (e.g., Gammarus, Corophium)
Shorebirds and other avian species
Predatory fish species that exploit the various prey populations.
Human activities have significantly impacted estuarine health, with excessive nutrient inputs leading to algal blooms and degradation of eelgrass beds through increased turbidity and epiphyte overload. Seasonal occurrences of hypoxia or anoxia in sediments can drastically reduce habitat quality and biodiversity. Overfishing, particularly of economically significant molluscs and apex predators, can create trophic cascades that disrupt ecological balance. Additionally, the introduction of invasive species further complicates the challenges faced by estuarine environments, highlighting the need for effective management strategies to protect these critical ecosystems.