Comprehensive Study Notes: Aquatic Ecosystems, Exam Format, and Limnology

Exam Format and Logistics

  • Question Quantity and Types:

    • Total number of questions: Approximately 25 questions25\text{ questions} or slightly more.
    • Objective Questions (1–2 points1\text{--}2\text{ points} each): True/false, multiple choice, matching, and fill-in-the-blank.
    • Short Answer Questions (3–4 points3\text{--}4\text{ points} each): Requires a response ranging from a couple of words to a single sentence.
    • Medium Answer Questions (5–8 points5\text{--}8\text{ points} each): Requires a response consisting of a couple of sentences.
    • Essay Questions (8–12 points8\text{--}12\text{ points} each): 1–21\text{--}2 comprehensive essay questions.

  • Time Allocation and Testing Rules:

    • Duration: Students are allowed the full 60 minutes60\text{ minutes} (1 hour1\text{ hour}) class period to complete the exam.
    • Timing & Environment: A clock will be visible in the room. Cell phone usage is strictly prohibited.
    • Testing Expectations: Content is entirely memory-based (matching serves as the only form of a word bank). Answers must accurately describe the concept or problem asked without relying on fluff.
    • Calculators: Allowed for mathematical calculations (e.g., stream discharge). Formulas must be memorized; all necessary raw data will be provided on the test.

  • Scope and Course Schedule:

    • Exam Material Cutoff: Covers all content discussed up through Wednesday's class.
    • Lab Notes Integration: Lab material is fair game for the exam, with the exception of the current week's lab (which has not had proper pen placement/coverage yet).
    • Wednesday Class Plan: Includes dedicated review time. Students should review notes to identify gaps, request alternative examples, or clarify complex concepts.
    • Group Assignments: Presentation and forum group dates (e.g., short reports, periphyton sign-ups) are being finalized for upcoming groups.

Lake Zonation and Morphometry Review

  • Life / Habitat Zones vs. Thermal Zones:

    • Critical Distinction: Read exam questions carefully to differentiate spatial life/habitat zones from thermal stratification layers.

  • Spatial Habitat Zonation:

    • Pelagic Zone: The open surface water of a lake seen when looking across the body of water.
    • Photic Zone: The upper water column extending from the surface down to the depth where sunlight can penetrate, supporting photosynthesis.
    • Aphotic Zone: The deep water layer beneath the photic zone where sunlight cannot penetrate.
    • Benthic Zone: The physical bottom floor or substrate of the lake.
    • Littoral Zone: The region extending from the lakeshore to the maximum depth where rooted submerged plants can no longer grow.

  • Plant Morphological Categorizations:

    • Free-floating Plants: Possess roots that suspend directly in the water column to absorb nutrients without contacting the bottom substrate.
    • Rooted Floating Plants: Anchored in the bottom substrate with shoots extending upward to display floating leaves on the surface to capture sunlight (e.g., water primrose, white water lily, pond lilies).
    • Emergent Plants: Rooted in the substrate with the vast majority of their shoot system rising completely above the water surface (e.g., bulrushes, cattails).
    • Rooted Submerged Plants: Anchored in the bottom substrate and growing entirely underwater; their outer boundary defines the extent of the littoral zone.

  • Biological Grouping Classifications:

    • Neuston: Surface-dwelling organisms residing in the surface film / epipelagic zone.
    • Nekton: Free-swimming organisms inhabiting the water column (e.g., fish, zooplankton).
    • Phytoplankton: Photosynthetic organisms located strictly within the photic zone. They are autotrophs and are not classified as nekton.
    • Benthos: Organisms residing on or within the benthic lake floor.
    • Periphyton: Organisms and non-rooted plants attached to or living on emergent vegetation like cattails (e.g., dragonflies, tree frogs, snails, and red-winged blackbirds nesting on emergent stalks).

  • Thermal Stratification (Summer Profiles):

    • Epilimnion: The upper, warmest, well-mixed water layer.
    • Thermocline: The middle layer characterized by a rapid, dramatic decline in temperature with depth.
    • Hypolimnion: The deep, coldest water layer at the lake bottom.

  • Lake vs. Pond Classification Threshold:

    • Surface Area Cutoff: A surface area threshold of 5 acres5\text{ acres} generally separates a pond from a lake.
    • Physical Characteristics: Lakes are typically characterized by distinct beaches formed by both wind mixing and convective (temperature) mixing.

Lotic Ecosystems: Streams and Rivers

  • Fundamental Definition:

    • Lotic: Refers specifically to running or flowing water systems.

  • Dissolved Oxygen (DODO):

    • Lotic systems consistently display higher concentrations of dissolved oxygen (DODO) compared to lentic (still) ecosystems.
    • Continuous water movement and physical aeration over obstacles—such as waterfalls, fallen logs, and rocks—mechanically mix atmospheric oxygen into the water.

  • Temperature Dynamics:

    • Stream vs. Pond Comparison: Headwater streams (e.g., Cold Brook Creek) maintain lower average temperatures than nearby standing water bodies (e.g., Wet Pond).
    • Drivers of Cold Stream Temperatures:
    1. Canopy Cover: Dense riparian forest canopies shield the water surface from solar radiation. Cool water temperatures are essential for species like trout, which require cold water and gravelly substrates for spawning.
    2. Groundwater Inputs: Many streams originate from groundwater sources, entering at low baseline temperatures.
    • Thermal Instability: Because streams have a smaller volume of water than large ponds or lakes, their water temperature fluctuates more rapidly and dramatically in response to ambient ambient conditions.

  • Habitat Structure and Microhabitats:

    • Streams possess greater structural complexity and microhabitat diversity than ponds.
    • Microhabitat Features: Deep pools, shallow riffles, riprap, submerged logs, calm backwaters, and high-velocity current channels.
    • Ecological Function: Deep pools and obstructions (behind logs and boulders) create crucial low-velocity resting zones where fish can escape main currents to conserve energy.
    • Channel Morphology: Sinuosity and channel bends alter water velocity patterns and dictate local sediment deposition.

  • Nutrient Dynamics in Lotic Systems:

    • First-Order Headwaters: Typically feature low baseline nutrient levels.
    • Mechanisms Maintaining Low Nutrients:
    1. Minimal Runoff Contact: Water in lower-order streams has traveled a short distance over the surrounding landscape, limiting terrestrial nutrient collection.
    2. Artesian Sources: Streams frequently originate as artesian wells (groundwater flowing naturally to the surface), which naturally contain negligible nutrients.
    • Anomalies: Anthropogenic inputs can severely disrupt normal low-nutrient baselines (e.g., elevated phosphate readings in Cold Brook Creek exceeding detection limits).
    • Downstream Gradient: As stream order increases and the river traverses more land area, cumulative terrestrial landscape impacts steadily elevate nutrient loads.

Wetlands: Definitions, Classifications, and Dynamics

  • Legal/Working Definition of a Wetland:

    • Definition: "A wetland is characterized by the presence of water, which is sufficient to support and normally supports wetland vegetation and aquatic life."
    • Critical Evaluation of Definition:
    • Uses the term being defined ("wetland") within the body of the definition.
    • Uses the word "characterized" rather than establishing strict structural boundaries.
    • Contains ambiguous terms such as "normally supports" without defining specific timeframes, hydroperiods, or seasonal durations.
    • Legal and Practical Implications: The inherent ambiguity of this definition creates ongoing legal friction between land developers (seeking to drain or fill land) and conservationists (seeking to protect ecosystem services and endangered species habitat).

  • Major Wetland Classifications:

    • Marshes: Wetlands dominated by herbaceous (grassy) vegetation. While scattered woody plants may exist, non-woody species occupy the dominant space.
    • Swamps: Wetlands dominated by woody vegetation (trees and shrubs; e.g., the Great Cypress Swamp dominated by cypress trees).
    • Bogs: Peat-accumulating wetlands lacking significant surface inflow or outflow of water, dominated by Sphagnum moss.

  • Peat Characteristics and Uses:

    • Definition: Compressed plant material—predominantly Sphagnum moss—accumulated over thousands of years.
    • Coal Precursor: Represents the initial stage of coal formation (peat compresses over thousands of years; coal compresses over millions of years). Used directly as a burnable fuel source in regions like Ireland and Scotland.
    • Horticultural Applications: Utilized in agriculture/gardening due to its high nutrient storage capacity and exceptional water retention capability.
    • Quaking Bogs: Active bogs featuring a thin (a few inches\text{a few inches}) top mat of living Sphagnum moss floating over deep accumulated peat and saturated water, historically developing from open-water lakes.

  • Water Residence Times:

    • Definition: The length of time a specific volume/drop of water remains within an ecosystem before exiting.
    • Relative Ranking: Bogs>Marshes>Swamps\text{Bogs} > \text{Marshes} > \text{Swamps}
    • Bogs: Possess the longest residence time due to the lack of significant surface water inflow and outflow; water primarily leaves via evaporation.
    • Marshes: Possess intermediate residence time because dense matrices of herbaceous vegetation impede water flow.
    • Swamps: Possess the shortest residence time among the three due to open water channels flowing continuously past trees and shrubs.

Estuarine Systems and Salt Wedge Dynamics

  • Definition of an Estuary:

    • Coastal zones where freshwater rivers flow into the ocean, producing brackish water.

  • Salt Wedge Mechanics:

    • Saltwater is denser than freshwater. Consequently, lighter river water flows over the top of the underlying ocean water, forming a distinct, curved "salt wedge" boundary rather than a vertical partition.

  • Salting Out (Chemical Precipitation):

    • Definition: The physical-chemical process wherein dissolved solutes transition from soluble form into solid precipitate particles upon contacting high-salinity/high-density water.
    • Precipitating Ions: Elements common in freshwater, such as calcium (Ca2+\text{Ca}^{2+}) and magnesium (Mg2+\text{Mg}^{2+}), precipitate out of solution at the salt wedge interface.
    • Ecological Impact: Salting out traps essential nutrients (e.g., nitrates and phosphates) within the estuary, driving extremely high primary/net productivity, high rates of carbon dioxide (CO2\text{CO}_2) fixation, and extensive biodiversity.

  • Stratification and Spatial Dynamics:

    • Thermal Disruption: The intense density gradient of the salt wedge completely overrides and disrupts any local thermal stratification.
    • Fluctuating Salt Wedge Position: The precise spatial location of the salt wedge shifts continuously based on:
    1. River Discharge: Fluctuations in river flow velocity and volume.
    2. Tidal Forces: Ocean tides cause massive movements of the salt wedge (e.g., the Bay of Fundy exhibits tidal range variations of up to 10 m10\,\text{m} or approximately 30 ft30\,\text{ft} between low and high tide).