Aquatic Environments

Photic and Aphotic Zones in Aquatic Environments

• The aquatic environment is divided into zones based on depth and distance from shore.

• Key zones to know:

  • Photic zone: receives light and supports photosynthesis.

  • Aphotic zone: does not receive light.

  • Benthic zone: bottom of the lake, river, or ocean.

  • Pelagic zone: open water column (includes Neritic and Oceanic zones).

  • Neritic zone: coastal water over the continental shelf.

  • Oceanic zone: the open-ocean water away from the shelf.

  • Intertidal zone: area that alternates between dry and wet with tides.

  • Continental shelf: shallow region over which the continent shelves out; depth up to about 200\ \mathrm{m}.

  • Abyssal zone: deep-ocean zone typically between 2{,}000\ \text{m} and 6{,}000\ \mathrm{m}.

  • Spherically, oceans have zones defined by dep /th and distance from shore, including intertidal, neritic, and oceanic components.

• Global context:

  • The oceans cover a large portion of Earth and contain distinct zones such as intertidal, corals, abyssal, pelagic, and benthic regions depending on depth and proximity to shore.

  • The map reference includes latitude markers (e.g., Tropic of Cancer at \approx 23.5^{\circ}\mathrm{N}, Tropic of Capricorn at \approx -23.5^{\circ}\mathrm{S}, equator) to show global distribution of aquatic zones.

• Distinctions by depth and distance help categorize habitats and their typical organisms.

Marine Environments: Intertidal, Benthic, and Coral Reefs

• Intertidal zone

  • Alternates between dry and wet with tides.

• Benthic zone

  • Organisms living on the ocean bottom.

• Coral reefs

  • Coral is an animal that grows in large communities.

  • Supports high biodiversity.

  • Very sensitive to pollution, temperature fluctuations, and disturbances.

  • Ecological note: small changes in temperature or water quality can drastically affect coral health and associated communities.

Estuaries

• Estuary: where river meets ocean.

• Important transitional habitats often with brackish water and high productivity.

Freshwater Aquatic Environments and Zones

• Freshwater environments include:

  • Lakes and Ponds

  • Rivers and Streams

  • Wetlands (Marshes, Swamps, Bogs, Fens)

• Key zones in freshwater systems:

  • Littoral zone: nearshore shallow waters where rooted vegetation is common.

  • Limnetic zone: open water away from the shore where light penetrates and photosynthesis occurs.

  • Photic zone: illuminated layer where photosynthesis can occur.

  • Pelagic zone: the water column away from the bottom.

  • Benthic zone: bottom layer of the water body.

  • Aphotic zone: deeper layers with little to no light.

• Lake nutrient status (productivity):

  • Oligotrophic lake: low nutrients and typically clear, well-oxygenated water.

  • Eutrophic lake: high nutrients, often more productive with higher algal biomass.

  • Related terminology can be used to describe nutrient-richness and biological productivity.

Thermoclines and Lake Turnover

• Concept of seasonal stratification:

  • In temperate lakes, a thermocline forms in the warmer months, creating distinct thermal layers.

  • Epilimnion: warm, well-oxygenated surface layer.

  • Thermocline (metalimnion): rapid temperature decrease with depth.

  • Hypolimnion: cooler, deeper water.

• Turnover (spring and fall mixing):

  • During turnover, the water column mixes, redistributing oxygen and nutrients.

  • For many lakes, the water at or near 4^{\circ}\mathrm{C} is at maximum density, enabling uniform mixing during turnover.

  • Consequences:

  • Oxygen from the surface can reach deeper layers.

  • Nutrients from the bottom can enter the photic zone to support primary production.

• Example temperature references (typical seasonal pattern):

  • Winter: surface layers may approach cold temperatures; hypolimnion can remain around 4^{\circ}\mathrm{C}.

  • Spring/Fall turnover: vertical mixing occurs around the neutral density depth, promoting homogeneous conditions.

  • Summer: epilimnion may be warm (e.g., 22^{\circ}\mathrm{C} at the surface); deeper layers remain cooler.

• Density reference (illustrative):

  • Water has maximum density at 4^{\circ}\mathrm{C}, which drives turnover dynamics.

• Consequences for dissolved gases and nutrients:

  • Usually, high oxygen concentration near the surface (from atmospheric exchange and photosynthesis).

  • Nutrient concentrations often higher in the bottom sediments and become available during turnover.

• Notes on the figure (described):

  • Density vs. temperature curves show the layering and how turnover distributes
    oxygen and nutrients; the numbers and curves illustrate how layering changes with seasons.

Rivers and Freshwater Wetlands: Ecological Roles

• Rivers

  • Central to the hydrological cycle and return water to the ocean.

  • Large source of biodiversity.

  • Highly sensitive to development, pollution, and dam construction.

• Marshes, Swamps, and Bogs (Wetlands)

  • Very high biodiversity.

  • Ecosystem services:

  • Important breeding and migration sites for wildlife.

  • Purify water and store flood water, reducing flood risk.

  • Highly sensitive to development and pollution.

• Real-world examples (from the transcript):

  • Myrick Marsh (Fall 2001).

  • Cyprus swamps in southern U.S.

Quick Quiz Review (from pages 17–19)

• Question 4: The bottom of a lake is called

  • A. Photic zone

  • B. Aphotic zone

  • C. Benthic zone

  • D. Oligotrophic

  • E. Eutrophic

  • Answer: \text{C: Benthic zone}

• Question 5: A lake with low levels of nutrients is called

  • A. Photic zone

  • B. Aphotic zone

  • C. Benthic zone

  • D. Oligotrophic

  • E. Eutrophic

  • Answer: \text{D: Oligotrophic}

• Question 6: A lake turning over in the spring and fall is important for

  • A. Distribute light in the lake

  • B. Stopping algal growth

  • C. Mix fish in the lake

  • D. Remove ice from the lake

  • E. Carrying sediments and nutrients up from the benthic zone

  • Answer: \text{E: Carrying sediments and nutrients up from the benthic zone}

Connections and Real-World Relevance

• Zonal organization informs habitat suitability for different organisms (e.g., coral reefs in shallow photic zones; abyssal organisms in deep dark zones).

• Pollution, climate change, and damming can disrupt zonation, oxygen balance, and nutrient cycles, with cascading effects on biodiversity.

• Wetlands provide crucial ecosystem services (water purification, flood mitigation, wildlife habitat) and are often among the most biodiverse systems, yet face intense development pressures.

• Understanding lake turnover helps explain seasonal patterns in oxygen availability for aquatic life and supports freshwater management strategies.

Notation and Key Terms Summary

• Photic zone: ext{zone that receives light, enabling photosynthesis}

• Aphotic zone: ext{zone that receives little to no light}

• Benthic zone: ext{bottom habitat in a water body}

• Pelagic zone: ext{open water column away from the bottom}

• Neritic zone: \text{coastal water over the continental shelf}

• Oceanic zone: \text{open ocean beyond the neritic zone}

• Intertidal zone: \text{area exposed to air at low tide and covered at high tide}

• Continental shelf: d \approx 0 \text{ to } 200\ \mathrm{m}

• Abyssal zone: 2{,}000\ \mathrm{m} \leq d \leq 6{,}000\ \mathrm{m}

• Oligotrophic: ext{low nutrient levels}

• Eutrophic: \text{high nutrient levels}

• Littoral: \text{nearshore shallow zone}

• Limnetic: \text{open-water zone away from shore}

• Thermocline: \text{layer with rapid temperature change with depth}

• Turnover: \text{seasonal mixing of the water column, redistributing oxygen and nutrients}

Note: All numerical values in this summary are taken from the transcript and are presented here for study purposes in LaTeX-formatted expressions where applicable.