Biology 20: Aquatic Ecosystems

Aquatic Ecosystems: Physical Factors

  • Depth
    • Determines the amount of light that penetrates the water column and reaches the soil.
    • Aphotic Zones: Deeper bodies of water have lightless zones that cannot sustain photosynthetic organisms in their food chain. They rely on external or surface-derived organic matter.
    • Photic Zones: Shallow waters where light can reach the bottom. These areas can be highly productive due to abundant light for photosynthesis.
  • Turbidity
    • A measure of water clarity.
    • Caused by suspended particles such as dirt, silt, phytoplankton, and algae.
    • In turbid areas, cellular respiration (decomposition) is likely to outpace photosynthesis because light penetration is reduced.
    • Measured using a Secchi disk, which assesses the depth at which an object can no longer be seen.
  • Soil and Aquatic Zones
    • Regions with both light and soil support a greater diversity and abundance of organisms.
    • Aquatic plants can root into the soil, obtaining nutrients and providing a greater diversity of habitats for other organisms.
    • Benthic Zone: The soil region located in the aphotic zone at the bottom of a body of water.
    • Benthos: Organisms that live in the benthic zone.
    • Littoral Zone: The shallow, near-shore zone where light reaches the soil (a photic zone), allowing rooted aquatic plants to thrive.
    • Limnetic Zone: The open-water zone in the photic region, away from the shore, where light penetrates but the bottom is too deep for rooted plants.
  • Motion
    • Refers to the flow rate, rapids, and ripples that stir up the water.
    • While choppy water can be difficult for some organisms to live in, increased motion also significantly increases the amount of dissolved gases (like oxygen) in the water, which is beneficial for many aquatic organisms.
  • Density
    • Water is most dense at 4^{\circ}C.
    • Due to this property, bodies of water become stratified (layered) as different regions experience varying temperatures and, consequently, different densities.
    • Water at 0^{\circ}C (ice) is less dense than cooler liquid water, causing it to float. This allows life to be supported underneath the ice during winter, providing insulation and a stable environment.
  • Temperature
    • Cooler waters generally contain more dissolved gases (e.g., oxygen) and tend to be more nutrient-rich and productive.
    • Warmer waters are often more hospitable to a wider range of organisms and favor biological reactions occurring within organisms' bodies, but nutrients can become depleted more easily.
    • Bodies of water are stratified due to differences in temperature and density.
    • The layers are separated by a region of steep temperature change known as the thermocline.
    • Summer Stratification: Warmer water lies above the thermocline in the epilimnion, while nutrients often become locked at the bottom of lakes in the cooler, denser hypolimnion.
    • Mixis or Turnover: This crucial event occurs in the fall and spring seasons. During turnover, the water column mixes, pushing nutrients from the hypolimnion up and around, providing essential nutrients to limnetic organisms (in the open water) and delivering oxygen to benthic organisms (at the bottom).

Aquatic Ecosystems: Chemical Factors

  • Eutrophication
    • Excessive runoff of nitrogen (N2) and phosphorus (P4) from fertilizers into aquatic ecosystems triggers eutrophication.
    • This leads to an overgrowth of aquatic plants and algae. As these organisms die, their bodies settle and decompose at the bottom of the lake, reducing the lake's depth.
    • Shallower lakes become even more productive and accumulate more decomposing organic matter.
    • Over time, eutrophication transforms lakes into shallow, murky, anaerobic (oxygen-depleted) environments that are less biodiverse.
  • Key Chemical Parameters and Testing
    • Nitrate (NO_3)
      • Nitrogen is a crucial nutrient that acts as a fertilizer for aquatic plants. High nitrate levels lead to excessive plant and algae growth, causing water quality problems.
      • Sources include human and animal waste, decomposing organic matter, and runoff from agricultural (lawn and crop) fertilizers.
      • Nitrogen exists in water in various forms, including Nitrate (NO3), Nitrite (NO2), and Ammonia (NH_3).
      • Unpolluted waters typically have nitrate levels below 4 \text{ ppm}. Levels above 40 \text{ ppm} are considered unsafe for drinking water.
      • Test Procedure Snippet (LaMotte): Fill test tube to 5 \text{ mL}, add Nitrate #1 TesTab, mix. Add Nitrate #2 CTA TesTab, mix, wait 5 minutes. Compare color to chart and record as ppm Nitrate.
    • Phosphate (PO_4)
      • Phosphorus is another vital nutrient that fertilizes aquatic plants. High phosphate levels result in excessive plant and algae growth, leading to water quality issues.
      • In natural waters, phosphorus primarily occurs as phosphates (PO_4).
      • Over half of the phosphates found in lakes, streams, and rivers originate from detergents.
      • Phosphate levels exceeding 0.03 \text{ ppm} significantly contribute to increased plant growth.
      • Test Procedure Snippet (LaMotte): Fill test tube to 5 \text{ mL}, add Phosphorus TesTab, mix. Wait 5 minutes. Compare color to chart and record as ppm Phosphate.
    • pH
      • pH is a measurement of the activity of hydrogen ions (H^+) in a water sample, indicating its acidity or alkalinity.
      • The pH scale ranges from 0 to 14. Water samples with a pH below 7.0 are acidic, those above 7.0 are basic, and 7.0 is considered neutral.
      • A pH range of 6.5 to 8.2 is optimal for the majority of aquatic organisms.
      • Rapidly growing algae and vegetation remove carbon dioxide (CO_2) from the water during photosynthesis, which can lead to a significant increase in pH.
      • Most natural waters have pH values ranging from 5.0 to 8.5.
      • Acidic, freshly fallen rainwater may have a pH of 5.5 to 6.0.
      • Alkaline soils and minerals can raise the pH to 8.0 to 8.5.
      • Seawater typically has a pH value close to 8.0.
      • pH is influenced by geologic surroundings, runoff, plant life, and pollutants such as sulfur compounds.
      • Only a narrow pH range can support most aquatic life. Low pH can be damaging to tissues.
      • Test Procedure Snippet (LaMotte): Fill test tube to 10 \text{ mL}, add pH Wide Range TesTab, mix. Compare color to chart and record as pH.
    • Dissolved Oxygen (DO)
      • The amount of oxygen (O_2) available in the water for organisms to perform cellular respiration.
      • DO levels are influenced by the motion of the water (e.g., rapids increase O2), temperature (cooler water holds more O2), and the amount of producers (e.g., algae, plants) that release O_2 during photosynthesis.
    • Biological Oxygen Demand (BOD)
      • The amount of oxygen required by organisms (primarily decomposers) to break down organic matter in the water.
      • BOD is higher when there is a large amount of decomposing material present. Decomposers rapidly consume oxygen, thereby reducing the amount of dissolved oxygen available for other aquatic organisms.

Aquatic Ecosystems: Biological Indicators of Health

  • Coliform Bacteria
    • The presence of coliform bacteria in water indicates a relatively large amount of feces in the water, as these bacteria reside in the digestive systems of animals (including humans).
    • Coliform contamination is a strong indicator of broader water contamination problems, potentially including the presence of more pathogenic disease-causing microorganisms.
  • Amphibian Indicators (e.g., frogs, salamanders)
    • Amphibians are highly susceptible to changes in their environment, making them excellent biological indicators of ecosystem health.
    • Gas Exchange: They complete gas exchange (breathing) through their highly permeable skin, making them directly exposed and very vulnerable to changes in the chemical composition of their aquatic environment.
    • Life Stages: Their life cycle includes both aquatic (larval) and terrestrial (adult) forms. If the quality of either habitat declines, amphibians will be significantly affected, reflecting the health of both ecosystems.
    • Dietary Diversity: Amphibians eat different foods at their larval stage (e.g., herbivores) compared to their adult stage (e.g., carnivores). This means they are influenced by materials at different points in a food chain, making them sensitive to various contaminants or changes in food availability across trophic levels.
    • External Development: Their eggs and larval stages develop externally in the water, exposing them directly to any contaminants present in the aquatic environment.
    • Moisture Reliance: Amphibians rely heavily on moisture for survival. Many habitats are experiencing drying and warming trends, which not only directly threatens them but also makes them more prone to fungal infections, contributing to their global decline (e.g., the "Disappearing Frogs" phenomenon).