CHAPTER 18: Trophic State and Eutrophicatio

CHAPTER 18: Trophic State and Eutrophication

Definition of Trophic State

• Trophic state refers to the classification of aquatic ecosystems based on their productivity and nutrient status.

• Categories of trophic state:

  • Oligotrophic:

  • Defined by low nutrient concentrations, low algal biomass, and high water clarity.

  • Commonly supports coldwater fisheries.

  • Mesotrophic:

  • Intermediate nutrient levels, algal productivity, and water clarity.

  • Eutrophic:

  • Characterized by higher nutrient levels, algal blooms, low oxygen levels, and possible fish kills.

  • Nuisance species like cyanobacteria may be prevalent.

• Historical perspectives emphasized primary producer biomass driven by nutrient availability, correlating with algal bloom incidents.

• Current views expand on heterotrophic productivity and CO₂ production in ecosystems, considering full system metabolism.

• Traditional measurements include water clarity, phytoplankton biomass, and nutrient concentrations, often leading to complex classifications.

Advanced: Determining Reference Nutrient Conditions in Freshwater Environments

• Reference conditions are essential for understanding natural nutrient distributions without human impact.

  • Identifying minimally impacted sites represents the baseline understanding for conservation and management.

• Stratification of regions for sampling based on geology and soil types is necessary to understand reference conditions accurately.

• Tears from agriculture (e.g., the Corn Belt) and atmospheric deposition complicate the establishment of reference conditions.

• Various modeling techniques inform estimations of nutrient conditions, suggesting anthropogenic influences increase TN and TP concentrations significantly higher than reference states.

Why Does Alteration of Trophic State by Nutrient Pollution Matter in Lakes?

• Increased nutrient loading due to human activities causes rising incidents of algal blooms, which are exacerbated by climate changes and population growth.

  • Algal blooms lead to aesthetic issues, such as unpleasant odors and visual impairments in water quality.

  • Increased costs for water purification due to inefficiencies in establishing potable water sources due to algal blooms.

  • Toxic effects of certain algal species (e.g., cyanobacteria) can threaten livestock and human health, including potential risks for chronic diseases like liver cancer.

• Fish kills are often linked to hypoxic conditions in eutrophic lakes, especially during periods of temperature fluctuation or after prolonged cloudy weather.

• Historical evidence shows patterns of decreased biodiversity linked with cultural eutrophication, which can irreparably modify local ecosystems, affecting the food web.

Natural and Cultural Processes of Eutrophication

• Lakes naturally evolve from oligotrophic to eutrophic states over thousands of years through sediment accumulation and nutrient dynamics.

• Human activities speedup eutrophication, often leading to significant ecological shifts.

• Paleolimnological evidence indicates past conditions in many lakes, showing that anthropogenic nutrient inputs have resulted in dramatic changes in ecosystem function.

Relationships Among Nutrients, Water Clarity, and Phytoplankton: Managing Eutrophication in Lakes

• A clear relationship exists between nutrient loading, aquatic productivity (i.e., algal biomass), and water clarity, as outlined by empirical data by Vollenweider et al. (1976).

• Effective management of eutrophication requires recognizing these correlations using comprehensive models that consider external nutrient sources and in-lake dynamics.

• Proposed methods aim to assess nutrient constraints and implement localized practices for sustainable nutrient management.

Advanced: Empirical Relationships Used to Predict Control of Eutrophication

• Using mathematically based models can help predict in-lake nutrient concentrations and subsequently algal productivity.

• The general steady-state equation for in-lake TP concentration is given by: TP = L imes (\rho + \sigma) where:

  • TP = total phosphorus (mg/m³)

  • L = phosphorus loading (kg/m²/year)

  • \rho = flushing rate (year⁻¹)

  • \sigma = sedimentation rate (year⁻¹)

• Additional equations help relate chlorophyll concentrations to phosphorus levels and assess conditions conducive to algal bloom events.

Mitigating Lake Eutrophication

Control of nutrient sources

• Nutrient inputs may be controlled at the source (point & nonpoint sources), including agricultural practices and waste management strategies.

• Various methods include banning phosphates in household products, managing fertilizer applications in agriculture, and enhancing sewage treatment processes.

Treatment in the lake

• In-lake treatments can include oxygenation, sediment dredging, chemical applications (e.g., alum), and biomanipulation efforts to maintain water quality and reduce algal blooms.

Macrophyte removal

• Management of excessive macrophyte populations may involve physical, chemical, or biological control. Strategies may include manual harvesting, chemical treatments, or introducing herbivorous species.

Managing Eutrophication in Streams and Rivers

• Eutrophication drives detrimental changes in stream ecology, affecting benthic algae and overall water quality.

• Nutrient control methodologies must address both point and nonpoint sources to mitigate these effects effectively.

Case Studies of Eutrophication in Lakes and Lotic Systems

Lake Washington

• Demonstrated recovery from eutrophication through concerted community efforts to divert sewage inputs, showcasing the successful application of scientific knowledge to environmental management.

Lake Trummen

• Success noted through sediment dredging efforts which directly resulted in improved water quality metrics and restoration of recreational values.

Lake Tahoe

• The ecological monitoring report indicates pressures from urban development leading to deteriorating water clarity, necessitating urgent intervention strategies to conserve its oligotrophic nature.

Lake Okeechobee

• Challenges remain with persistent nutrient loading and viability as a water supply; managing ongoing agricultural runoff has been a significant focus of restoration efforts.

Clark Fork River & Murray-Darling River

• Demonstrated linkages between nutrient management practices and ecological outcomes; both cases underscore the complexities of nutrient dynamics and watershed management, particularly concerning local agricultural practices and hydrological conditions.

Managing Eutrophication in Wetlands

• Wetlands function as crucial nutrient sinks; however, they are also vulnerable to eutrophication pressures, necessitating targeted conservation and restoration strategies.

• Strategies for nutrient control in wetlands mirror those applied in lakes and streams, including landscape management to reduce nonpoint source inputs.

Wetlands as Nutrient Sinks

• Wetlands are integral to nutrient cycling within landscapes, acting to capture and store nutrients during flooding events, but face risks from increased nutrient loading.

• Restoration efforts aim to enhance the capacity of wetlands for nutrient retention while balancing ecological health and functional integrity.

Summary

1. Aquatic systems function along a trophic gradient influenced by both autotrophic and heterotrophic activities.

2. Cultural eutrophication significantly impacts water quality, leading to ecological degradation and socio-economic implications.

3. Management of eutrophication necessitates comprehensive strategies addressing both nutrient inputs and ecosystem responses.

Key Terms and Definitions

• Trophic State: The classification of aquatic ecosystems based on their productivity and nutrient status.

• Oligotrophic: An aquatic ecosystem characterized by low nutrient concentrations, low algal biomass, and high water clarity, often supporting coldwater fisheries.

• Mesotrophic: An aquatic ecosystem with intermediate nutrient levels, algal productivity, and water clarity.

• Eutrophic: An aquatic ecosystem characterized by higher nutrient levels, frequent algal blooms, low oxygen levels, and possible fish kills, with a prevalence of nuisance species like cyanobacteria.

• Reference Conditions: Essential baselines for understanding natural nutrient distributions in freshwater environments without human impact, used for conservation and management.

• Cultural Eutrophication: The acceleration of the natural eutrophication process due to human activities, leading to significant ecological shifts over a much shorter timescale.

• Algal Blooms: Rapid increases in the population of algae (often cyanobacteria) in an aquatic system, typically caused by excess nutrients, leading to aesthetic issues, water purification challenges, and potential toxicity.

• Hypoxic Conditions: Low oxygen levels in water, often a consequence of eutrophication, where the decomposition of large amounts of organic matter (from algal blooms) consumes oxygen, which can lead to fish kills.

• Phosphorus Loading (L): The amount of phosphorus entering an aquatic ecosystem per unit area per year, expressed in kg/m²/year.

• Flushing Rate (ρ): The rate at which water is replaced in a lake, expressed in year⁻¹.

• Sedimentation Rate (σ): The rate at which particles settle out of the water column to the sediment, expressed in year⁻¹.

• Biomanipulation: An in-lake treatment method involving the manipulation of food web components (e.g., introducing piscivorous fish) to control algal blooms and improve water quality.

• Point Sources: Identifiable, localized sources of nutrient pollution, such as treated sewage effluent or industrial discharges.

• Nonpoint Sources: Diffuse sources of nutrient pollution, such as agricultural runoff, urban stormwater, and atmospheric deposition, which are harder to control.

• Wetlands as Nutrient Sinks: The role of wetlands in capturing and storing nutrients, particularly during flooding events, thereby reducing nutrient loads to downstream aquatic systems.