Estuaries: A Unique and Dynamic Ecosystem

Introduction to Estuaries

Estuaries are unique ecosystems defined by the interaction of rivers and the sea. The definition has evolved over time to encompass their dynamic and often challenging nature, especially in Mediterranean climates:

Lyell (1883): Famously defined an estuary as "Where the river meets the sea."
Pritchard (1967): Provided a more scientific definition: "An estuary is a semi-enclosed body of water which has a free connection with the open sea and within which seawater is measurably diluted with fresh water derived from land drainage." In Mediterranean climates, this definition faces two problems: estuaries can become disconnected from the ocean and can become hypersaline.
Potter et al. (2010): Offered a comprehensive modern definition: "A partially enclosed coastal body of water that is either permanently or periodically open to the sea and which receives at least periodic discharge from a river(s), and thus, while its salinity is typically less than that of natural sea water and varies temporally and along its length, it can become hypersaline in regions when evaporative water loss is high and freshwater and tidal inputs are negligible."

Salinity Scale and Gradients

Salinity is a fundamental characteristic influencing estuarine ecosystems:

Freshwater: Contains 0 ext{ g per litre} of solutes.
Seawater: Contains approximately 35 ext{ g per litre} of solutes.
Composition: Mainly sodium chloride ( ext{NaCl}), accounting for approximately 85 ext{%} of solutes. Other significant solutes include sulfate ( ext{SO}_4^{-2}), magnesium ( ext{Mg}^{+2}), calcium ( ext{Ca}^{+2}), and potassium ( ext{K}^{+}).
Salinity Gradients: Salinity within estuaries varies significantly along their length and depth, influenced by factors such as rainfall and tidal input.

Hypersaline Systems

Hypersaline systems are a particular type of estuary with elevated salinity:

Prevalence: Found worldwide, with prominent examples including 60 lagoons, 34 estuaries, and 7 embayments. They are common in tropical, arid, and warm temperate climates characterized by low and/or highly seasonal rainfall.
Characteristics: Generally shallow (< ext{2 m} deep) and located in microtidal regions, meaning they have a small tidal range (< ext{2 m}).
Causes of Hypersalinity: Typically results from a reduced or closed connection to the ocean, high evaporation rates, and low freshwater input.
Local Examples: The Vasse-Wonnerup (Busselton) and Hamersley Inlet (Hopetoun) in Western Australia are notable examples, sometimes setting world records for hypersalinity.

Salinity and Mixing Patterns in Estuaries

Estuaries exhibit various salinity patterns and vertical stratification depending on freshwater discharge, evaporation, and marine exchange:

Positive Estuary (Macrotidal): Characterized by large marine exchange, significant mixing, internal circulation, and freshwater discharge (e.g., Picture A on slide 8).
Positive Estuary (Microtidal): Features tidal mixing and freshwater discharge (e.g., Picture B on slide 8).
Negative Estuary (Microtidal): Defined by high evaporation, hypersaline outflow, and marine exchange. This type can experience upwelling and internal circulation (e.g., Picture C, D, F on slide 8).
Salt-Plug Estuary (Microtidal): Involves a sandbar and a distinct body of hypersaline water (e.g., Picture E on slide 8).

Vertical Stratification

The mixing of fresh and salt water is crucial for classifying estuaries:

Vertically Mixed or Well-Mixed: High mixing due to strong tidal currents or low freshwater input, leading to uniform salinity from surface to bottom.
Slightly Stratified or Partially Mixed Estuary: Moderate mixing with a gradual increase in salinity with depth.
Salt Wedge: Occurs when a strong freshwater flow overrides a denser saltwater wedge, resulting in a distinct layer of freshwater atop saltwater with minimal mixing.
Highly Stratified: Similar to a salt wedge but with even less mixing, often found in deep estuaries with a large freshwater input.

Horizontal Stratification

Changes in salinity, temperature, and other parameters also occur along the length of the estuary, from the freshwater riverine end to the marine mouth. This creates distinct zones horizontally.

Estuaries Are Dynamic

Estuaries are constantly changing environments, as exemplified by systems like the Swan-Canning Estuary (Western Australia), which can be monitored via virtual observatories (e.g., https://swan.science.uwa.edu.au/overview.html). Data collected includes:

Salinity (psu): Variations from surface to bottom and along different sections (lower, mid, upper).
Temperature (C): Gradients across the estuary.
Oxygen (mg/L): Levels differing by depth and location.
Tide: Measured in meters above Australian Height Datum (mAHD), e.g., 1.04 ext{ (mAHD)}.
Inflow: Freshwater discharge from rivers, e.g., 7.24 ext{ (m}^3 ext{/s)}.

Sediment Patterns

Sediment dynamics are vital for estuarine morphology and function:

Sediment Transport: Sediment enters estuaries from both riverine and marine environments.
Velocity Influence: Water velocity plays a significant role in scouring (erosion) and deposition of sediments.
Microtidal Estuaries: Often act as sediment sinks, accumulating sediments over time due to relatively weaker tidal currents compared to macrotidal systems.

Estuarine Habitats

Estuaries provide critical habitats, with seagrasses being a prime example of their ecological importance:

Seagrass Benefits:
- Food Source: For species like the Black Swan.
- Nutrient Removal: Seagrasses absorb nutrients (e.g., nitrogen and phosphorus - ext{N+P}) from the water, preventing algal overgrowth and maintaining water quality.
- Sediment Stabilization: They reduce sediment re-suspension, leading to clearer water.
- Oxygenation: They oxygenate both the sediment and the water, breathing life into the estuary.
- Biodiversity: Provide habitat for numerous fish and invertebrate species, thereby increasing overall biodiversity.

How Many Estuaries Are There?

Estimates highlight the global abundance and the significant number that experience disconnection from the sea:

Global Estimates: McSweeny et al. (2017) estimated approximately 54,000 estuaries worldwide.
Disconnected Estuaries: Of these, approximately 2.8 ext{%} ( ext{~}1,500) become disconnected from the sea (referred to as Intermittently Closed and Open Lakes and Lagoons, or ICOLLs).
Australian Estuaries: A National Audit identified 979 estuaries in Australia, with 166 located in Western Australia.

Isolation from the Sea

Many estuaries, particularly in regions with highly seasonal rainfall, can become isolated from the open ocean. This isolation is driven by several factors:

Sandbar Formation: Sandbars build up at the mouth of the estuary, blocking the connection.
Low Tidal Range: Microtidal regions (< ext{2 m} tidal range) lack the strong tidal flushing needed to keep mouths open.
Highly-Seasonal Rainfall / Freshwater Flow: Periods of low freshwater input from rivers reduce the scouring action that would otherwise keep the estuary mouth open.
Along and Onshore Sediment Transport: Coastal processes can deposit sediment at the mouth.
Artificial Management: Human intervention is often required to manage the opening and closing of estuary mouths.

Classification of Estuaries by Isolation

Hodgkin & Hesp (1998) classified estuaries based on their extent of isolation from the sea:

Permanently-Open (PO): Maintain a continuous connection with the ocean (e.g., Peel-Harvey Estuary).
Intermittently-Open (IO): Periodically open and close to the sea (e.g., Vasse-Wonnerup Estuary).
Seasonally-Open (SO): Open during specific high-flow seasons and close during dry periods (e.g., Broke Inlet).
Normally-Closed (NC): Typically closed but may open during extreme events (e.g., Stokes Inlet).
Permanently-Closed (PC): Have no permanent or periodic connection to the sea; essentially become saline lakes.

Artificial breaching, such as at Wilson Inlet, is a common management practice to re-establish connection and manage water levels.

Biological Importance of Estuaries

Estuaries are ecologically vital due to their high productivity and role as critical habitats:

High Productivity: Support a rich diversity of life, including phytoplankton, seagrasses, salt marshes, mangroves, zooplankton, benthos, and nekton.
Feeding Grounds: Essential feeding grounds for a wide array of fish and birds, including migratory species.
Nursery Areas: Provide critical nursery habitats for many fish and crustacean species, supporting marine populations.
Coastal Ecosystem: An indispensable coastal ecosystem upon which numerous marine species depend for various life stages.

Humans and Estuaries

Estuaries have historically been, and continue to be, foci for human activity, leading to both benefits and challenges:

Colonisation Sites: Attractive locations for human settlements due to access to water and resources.
Source of Food/Water: Provide fish, shellfish, and freshwater resources.
Support Industry: Facilitate industries such as fishing, aquaculture, and port operations.
Transport: Serve as vital waterways for navigation and shipping.
Agriculture: Support agricultural practices in surrounding catchment areas.
Urban Development: Often sites of urban areas and infrastructure.
Recreation: Used for various recreational activities like boating, fishing, and swimming.
Site of Waste Disposal: Unfortunately, historically and sometimes currently, used for the discharge of waste, including factory effluent.

Health and Management of Estuaries in WA

The health of estuaries, particularly in Western Australia, is significantly impacted by human activity and requires complex management:

Human Impact: Estuaries can be severely impacted by various human activities.
National Survey: A national survey of approximately 1,000 estuaries highlighted prevalent health issues.
Measuring Health: Estuarine health is assessed using various biological and ecological indicators, such as species richness, Shannon-Wiener diversity index, number of trophic species, proportion of detritivores, proportion of benthic organisms, and proportion of estuarine spawners.
Increasing Pressures: Estuaries face growing pressures from human development and climate change, necessitating more management actions.
Management Complexity: Estuarine management is a complex task involving the use and care of the interface between land, rivers, and the sea. This is particularly challenging in south-west WA due to the lower resilience of these systems.

Management Issues

Management issues can be broadly categorized:

Proximal Issues: Directly impacting the estuary, such as urban development, port development, bank stabilization, stormwater runoff, marinas, jetties, boat traffic, fishing pressure, and artificial bar breaching.
Catchment Issues: Originating from the surrounding land, including water extraction by dams, salinization of catchments, nutrient run-off from agriculture, erosion, and pollution.

Competing Usage and Complex Management Frameworks

Estuarine environments often face conflicts due to competing human uses, making management intricate:

Activity Conflict Assessment: Tools like the "Humber Estuary Activity Conflict Assessment" are used to evaluate the effect of various uses (e.g., dredging, recreational access, defence setbacks) on other uses or environmental features (e.g., high-value landscape features, protected subtidal areas).
Multi-Jurisdictional Management: Management involves a complex web of international law, European directives (in the case of the Humber Estuary example), and national/local implementations. This includes numerous conventions, directives (e.g., Water Framework Directive - ext{WFD}, Habitats Directive - ext{HD}, Marine Strategy Framework Directive - ext{MSFD}), regulations, and various stakeholder groups.
Swan-Canning Estuary Management Agencies: In Western Australia, the Swan-Canning Estuary involves numerous management agencies, highlighting the complexity: Department of Water & Environmental Regulation, Department of Primary Industries & Regional Development, Department of Biodiversity, Conservation & Attractions, Department of Transport, Department of Jobs, Tourism, Science & Innovation, Department of Premier and Cabinet, Environmental Protection Authority, Water Corporation, Fremantle Port Authority, and 27 local councils, plus many other interest groups.

Estuaries: A Unique and Dynamic Ecosystem