ENV102: Rivers and Wetlands
Rivers and Wetlands: An Introduction to Environmental Science
This document provides a comprehensive overview of rivers and wetlands, detailing their characteristics, origins, ecological functions, threats, and management strategies, as presented by Assoc. Prof. Belinda Robson for ENV102.
Distinguishing Rivers and Wetlands
Rivers
Unidirectional Flow: Rivers are defined by their one-way current.
Strong Catchment/Riparian Zone Connections: Their health and function are intimately linked to the surrounding land (catchment) and the vegetation along their banks (riparian zone).
Substratum Dependence: In their upper reaches, river ecosystems are highly dependent on the type of material forming their bed (e.g., rocky or sandy).
Ecosystem Function Varies by Position: The specific ecological role of a river changes significantly with its location in the catchment. For example, upland rivers and lowland rivers exhibit distinct ecosystem functions.
Wetlands
Water Source Dependence: Wetlands rely on various water sources, including groundwater, surface runoff, or direct river flow.
Strong Catchment Connections: Similar to rivers, wetlands have strong ties to their surrounding catchments.
Ecosystem Function by Depth: The ecological processes within a wetland are largely determined by its depth. This creates functional differences between shallow wetlands and deep lakes.
Biodiversity and Habitat Complexity: The variety of life (biodiversity) in a wetland is directly related to the complexity of its habitats.
Lake Types and Origins
Lakes, often considered a type of wetland (especially shallow ones), vary significantly in their characteristics based on depth and origin.
Deep Lakes
Euphotic Zone Definition: Deep lakes are characterized by being deeper than their euphotic zone, the layer of water where sufficient sunlight penetrates for photosynthesis to occur.
Three Main Origins:
Glaciation: Formed by glacial activity.
Tectonics: Resulting from movements of the Earth's crust.
Vulcanism: Created by volcanic processes.
Shallow Lakes
Euphotic Zone Extent: In shallow lakes, the euphotic zone extends all the way to the bed, meaning sunlight can reach the bottom.
High Productivity Potential: This allows the entire volume of the lake to be potentially productive, supporting photosynthesis throughout.
Origins: Shallow lakes typically have either riverine or coastal origins.
Biological Assemblages in Standing Waters (Figure 4.2)
Zonation: Standing waters exhibit distinct zones that support different biological communities, as illustrated by Chambers et al. (2009).
Euphotic Zone: The illuminated surface layer where P > R (photosynthesis exceeds respiration).
Profundal Zone: The deeper, darker layer where P < R (respiration exceeds photosynthesis), dominated by microbial decomposers.
Littoral Zone: The nearshore zone with rooted aquatic plants.
Fringing Zone: The immediate terrestrial area bordering the water.
Open Water Zone: The main body of the lake away from the shore.
Benthic Zone: The bottom sediments, further divided into littoral benthos and profundal benthos.
Associated Organisms:
Pleuston: Organisms floating on the water surface.
Nekton: Free-swimming animals (e.g., fish).
Plankton: Microscopic organisms suspended in the water column (phytoplankton and zooplankton).
Aquatic Plants: Macrophytes rooted or floating.
Biofilm: Microbial communities adhering to any submerged surface.
Psammon: Organisms living among sand grains.
Microbial Decomposers: Break down organic matter, especially in the profundal zone.
Lake Mixing (Epilimnion) (Page 6)
Solar Radiation: Drives surface water warming.
Wind Mixing: Physical force that mixes the upper layers of a lake.
Epilimnion: The uppermost, warmest, and well-mixed layer of a stratified lake, directly influenced by wind and solar radiation. Beneath this is often a thermocline and hypolimnion in deep lakes.
Specific Lake Origins and Their Characteristics
Glaciation Lakes
Age: These lakes are geologically young, typically less than years old.
Typical Characteristics: They often possess clarity, rocky beds, cold water temperatures, are permanent (non-intermittent), oligotrophic (low nutrient content), and may be ice-covered for part of the year.
Threats: Primary threats include climate change (affecting ice cover and water temperature) and pollution.
Tectonic Lakes
Rarity in Australia: Tectonic lakes are uncommon in Australia.
Lake Eyre Example: The most renowned example is Lake Eyre, which is situated in a slumped area of the continent, indicating its tectonic origin.
Volcanic Lakes (Caldera and Maar Lakes)
Morphology: These lakes are typically steep-sided.
Catchments: They possess small catchments.
Nutrient Levels: Generally low in nutrients.
Fauna: Limited diversity of fauna.
Age Variability: Volcanic lakes can range from very young (less than years old) to relatively old.
Hydrological Connections: Some have strong hydrological connections to the broader landscape, while others are hydrologically isolated.
Threats: They may be threatened by groundwater extraction, which can alter their water levels and quality.
Threats and Solutions for Shallow Lakes (Wetlands)
Shallow lakes are vulnerable to a range of environmental stressors, necessitating careful management.
General Threats (Page 11 & 20)
Climate Change: Leads to altered water volumes and changes in the timing of water availability (low, infrequent rainfall, increased fire risk).
Eutrophication: Excessive nutrient input, leading to algal blooms and oxygen depletion.
Salinisation: Increased salt concentrations, often from land-use changes or reduced freshwater inflow.
Declining Groundwater or River Flow: Reduced water availability due to over-extraction or altered flow regimes.
Clearing of Fringing and Catchment Vegetation: Loss of buffer zones, leading to increased runoff, erosion, and reduced habitat.
Urbanization and Pollution: Runoff from urban areas introduces pollutants and excess nutrients.
Solutions
Land Use and Drainage Management: Implementing practices that minimize pollutant runoff and manage water flow.
Limitations on Groundwater Extraction: Regulating pumping to maintain water tables.
Groundwater Replenishment: Actively recharging groundwater aquifers.
Catchment Revegetation: Restoring natural vegetation in the catchment to improve water quality and habitat.
Water Replenishment: Direct addition of water to wetlands where feasible.
Riverine Lakes
Two Main Types:
Billabongs (or oxbow lakes): Formed when a meander of a river is cut off.
Floodplain Lakes: Lakes that form on river floodplains and are intermittently connected to the main river channel.
Intermittency: Frequently dry out for periods.
Water Quality Extremes: May experience wide fluctuations in water quality (e.g., oxygen levels, temperature) due to their intermittent nature.
Threats:
Changes to River Flow Regimes: Alterations from dams, irrigation, or reduced rainfall can severely impact their hydrology.
Blackwater Events: Episodes of very low dissolved oxygen, often caused by the decomposition of large amounts of organic matter during floods.
Climate Change: Exacerbates altered flow regimes and water availability.
Coastal Lakes
Age: Often very young, typically originating in the Quaternary period.
Formation: Created through the complex interaction between coast-building processes (e.g., dune formation) and groundwater or river flow (Page 13).
Example: The wetlands of the Swan Coastal Plain in Western Australia are a prime example of coastal, groundwater-fed wetlands (Page 14 & 15).
Threats:
Sea-level Rise: Can inundate or increase the salinity of coastal lakes.
Drops in Groundwater Tables: Reduces the freshwater input that sustains these systems, often due to over-extraction.
Swan Coastal Plain Example (Page 16 & 17)
Quaternary Lakes: These lakes are relatively young and are located east of the Spearwood Dunes.
Groundwater Interaction (Figure 9.8):
(a) Wet Winter-Spring: Rainfall recharges groundwater, raising the water table and filling wetlands situated between ancient dune ridges.
(b) Hot, Dry Summer-Autumn: Groundwater levels decline naturally, leading to the drying of wetlands.
(c) Current Situation: In many areas, groundwater levels are now permanently lowered. Wetlands only fill if they receive surface inputs, such as stormwater, rather than consistent groundwater supply. This is a critical issue illustrated by images of South Lake wet (Page 18) and dry (Page 19).
Nutrients in Shallow Lakes
Managing nutrients in shallow lakes presents a significant environmental challenge due to complex recycling mechanisms.
Nutrient Entry Points: Nutrients primarily enter from surface runoff or groundwater inflow.
Nitrogen Fixation: Biological nitrogen fixation is also a common internal source of nitrogen.
Continuous Recycling from Sediments: Nitrogen (N) and Phosphorus (P) are continuously recycled from lake sediments via three main mechanisms:
Macrophytes: Aquatic plants (macrophytes) absorb nutrients from sediments and then remobilize them into the water column and detrital food chain upon decomposition.
Stratification & Anoxia: Short periods of water column stratification can lead to anoxia (lack of oxygen) at the sediment-water interface, releasing nutrients. When the lake mixes again, these nutrients are rapidly distributed throughout the water column.
Resuspension: Wind action can stir up bottom sediments, directly lifting nutrient-rich particles into the water column (Figure from Atkinson et al., 2021).
Lake States: Macrophyte-Dominated vs. Phytoplankton-Dominated
Shallow lakes can exist in different ecological states, with significant implications for their health and ecosystem services. For example, Lake Forrestdale in July (winter) 2012 (Page 24) could illustrate one of these states.
Macrophyte-Dominated State (Page 25)
Characteristics: Features several species of macrophytes, exhibiting seasonal changes.
Benefits:
Aesthetics: Visually appealing.
Pre-Impact State: Often considered the natural or desired state.
Water Clarity & Quality: Promotes clear water and high dissolved oxygen (DO).
Extensive Habitat: Provides complex habitat structure for diverse fauna.
Complex Ecosystem: Supports a more intricate food web and ecological interactions.
Nutrient Uptake/Storage: Macrophytes can perform 'luxury uptake' and storage of nutrients, preventing them from fueling algal blooms.
Algae Inhibition: Can actively inhibit the growth of phytoplankton (algae).
Phytoplankton-Dominated State (High Biomass) (Page 26)
Problems Associated with High Biomass:
Toxicity: Certain algal species produce toxins harmful to wildlife and humans.
Low Dissolved Oxygen (DO): Decomposition of large algal blooms can lead to severe oxygen depletion, causing fish deaths.
Bad Odor: Algal blooms often produce unpleasant smells.
Macrophyte Deaths & Inhibition: Dense algal growth blocks sunlight, leading to the death of macrophytes and preventing their recruitment.
Turbidity: Makes the water cloudy.
No Nutrient Storage: Unlike macrophytes, phytoplankton have limited long-term nutrient storage capacity, leading to rapid cycling.
No Predation Refuges: Lack of structured habitat means grazers (e.g., zooplankton) have no refuges from predators.
Midge or Mosquito Problems: High algal biomass can fuel increased populations of nuisance insects.
Fringing Vegetation
Fringing vegetation, found at the water's edge, plays crucial ecological roles in wetlands (Page 27).
Composition: Typically consists of emergent rushes and sedges, along with shrubs and trees.
Habitat: Provides essential habitat, especially for birds and web-spinning spiders (Page 29).
Ecological Services:
Traps Biomass: Captures emerging insect biomass, which becomes food for terrestrial predators.
Terrestrial Subsidies: Wetlands subsidize the adjacent terrestrial environment with carbon and nutrients through this vegetation.
Midge Control: Can help obscure artificial lights, reducing midge attraction (midges cannot see porch lights if vegetation is present).
Social Implications: While beneficial ecologically, residents may feel that extensive fringing vegetation obstructs their view of the water.
Problems with Chironomids (Nuisance Midges)
Chironomids, non-biting midges, can become a nuisance, particularly in eutrophic environments (Page 30).
Rapid Life Cycle: Their life cycle is very fast in warm temperatures.
Eutrophication Impact: Eutrophication significantly increases their food base (algae), leading to higher population densities.
Adult Behavior: Adults are crepuscular (active at dawn/dusk) or nocturnal, swarm for mating, and are attracted to artificial lights.
Health Concerns: Although non-biting, mass inhalations of midges can cause respiratory problems.
Factors Contributing to Nuisance Midge Problems (Pinder et al. 1992, Page 31)
A cascade of factors, often linked to human activity, leads to midge outbreaks:
Urban Development, Septic Tanks, Fertilizer and Detergent Use
Nutrients and Pollutants in Run-off and Groundwater (direct consequence of item 1)
High Concentrations of Nutrients Enter Wetlands (from item 2)
Excessive Algal Growth in Wetlands (fueled by item 3)
High Production of Midges in Wetlands (due to increased food base from item 4)
Additional Factors:
High Water Temperature
Fewer Predators
Loss of Fringing Vegetation & Woodland (reducing habitat for predators and obscuring lights)
Salinization and Natural Salt Lakes
Salinization is a significant threat, but some salt lakes occur naturally. In the Western Australian wheatbelt, shallow lakes of differing salinity exist in distinct states (Figure 4.12, Jenny Davis, Page 32):
Submerged Plants Dominated
Phytoplankton Dominated
Benthic Microbial Community Dominated (microbial mats)
These different states can occur in close proximity despite varying community assemblages.
Dryland Salinity (Page 33)
Mechanism: A major environmental problem in dryland agricultural areas.
Natural State: Deep-rooted native vegetation uses substantial water via evapotranspiration, keeping the shallow freshwater aquifer robust and deep saline groundwater well below the surface.
Human Impact: Conversion to shallow-rooted crops (e.g., wheat) results in less water loss by evapotranspiration. This allows more rainwater to infiltrate, causing the shallow freshwater aquifer to become waterlogged and the deep saline groundwater to rise to the surface, leading to saline conditions.
Conclusions: Management Strategies
Effective management of aquatic ecosystems requires targeted approaches for different water body types.
Wetlands Management (Page 34)
Management efforts for wetlands typically focus on:
Controlling Drainage: Maintaining or restoring appropriate water levels and limiting pollution inputs.
Groundwater Management and Replenishment: Regulating extraction and actively recharging aquifers.
Maintaining/Restoring Fringing Vegetation: Protecting and re-establishing the crucial buffer zone around wetlands.
Controlling Nutrients and Salinity in Catchments: Implementing land-use strategies and, where necessary, using barrages to limit nutrient and salt ingress.
Controlling Mosquitoes and Nuisance Midges: Employing various methods to manage insect populations, often linked to nutrient control and habitat restoration.
Rivers & Wetlands General Management (Page 35)
Rivers and wetlands face different primary threats and thus require distinct management options:
Rivers: Management is predominantly focused on maintaining natural flow regimes (e.g., preventing over-extraction, regulating dams) and restoring riparian vegetation to enhance water quality and habitat.
Wetlands: Management primarily targets controlling catchment nutrients, salinity, and water regimes (e.g., ensuring adequate water supply, preventing pollution).