BIO 105 CH3 (Part 2) - Introduction to Ecology Lecture

Overview

The lecture connects wetland, river, and lake ecology to hydrology, climate, watershed structure, and seasonal change. It explains how river communities shift from headwaters to downstream reaches, how dams and nutrient runoff alter aquatic systems, and how the Index of Biological Integrity (IBI) uses biological traits such as fish richness and environmental tolerance to assess ecosystem health.

Key takeaways

Prolonged saturation makes wetland soils hypoxic by filling soil pores with water and slowing gas exchange, creating conditions that shape both vegetation and biological communities.
Watersheds determine where water, sediment, nutrients, and contaminants travel; an upstream input can affect downstream floodplains, estuaries, and coastal waters.
River communities change along the river continuum: headwaters depend heavily on coarse terrestrial organic matter, while wider reaches support more algal production and large rivers receive fine particles and planktonic inputs.
High organic inputs can increase biochemical oxygen demand as microbes decompose waste, reducing dissolved oxygen and imposing physiological stress on aquatic organisms.
Lake turnover in spring and fall mixes the water column and redistributes oxygen and nutrients, while summer stratification separates warm surface water from colder deep water.
The IBI evaluates aquatic ecosystem health through biological responses, including fish richness, tolerance, trophic composition, abundance, and physical condition, rather than relying only on a one-time water chemistry sample.

Chapters

0:00 Wetland Hydrology and River Networks Across Watersheds

Wetland soil saturation slows gas exchange, depletes oxygen, and selects for vegetation able to tolerate hypoxic root conditions.
Salt marshes are common at temperate and higher latitudes, while mangrove forests occupy tropical and subtropical shorelines; both stabilize coasts, provide habitat, and affect nutrient cycling.
A river basin includes the land drained by a river and its tributaries, with watersheds along high points such as the Rocky Mountain divide separating drainage networks.
Rivers connect upstream tributaries to downstream floodplains, estuaries, and coastal systems, and vary longitudinally, laterally, and vertically across channels, riparian zones, and subsurface habitats.

4:00 River Continuum, Oxygen Dynamics, and Dam Impacts

Shaded headwaters receive coarse organic matter from terrestrial vegetation, which shredding invertebrates process; wider downstream reaches support more algal production and grazers.
In larger rivers, fine particulate matter transported from upstream and planktonic production become more important, driving changes in feeding strategies and community composition.
Warm water holds less dissolved oxygen, while microbial decomposition of organic waste raises biochemical oxygen demand (BOD) and can stress aquatic organisms.
Dams transform flowing rivers into reservoirs, alter discharge and temperature, trap sediment and nutrients, and obstruct migratory organisms; these effects can propagate downstream.

8:12 Lake Stratification, Eutrophication, and Biological Integrity

The Great Lakes contain about 20% of the world's fresh water; lake habitats include the nearshore littoral zone, open-water limnetic zone, and depth layers called the epilimnion, metalimnion, and hypolimnion.
Summer stratification limits vertical mixing, while spring and fall turnover redistribute oxygen and nutrients; winter ice can form above denser water near 4°C.
Oligotrophic lakes have low nutrients and primary production but tend to stay oxygenated, whereas eutrophic lakes produce more organic matter whose decomposition can deplete deep-water oxygen.
Runoff, recreation, and non-native species such as zebra mussels affect lakes through watershed connections, so management must consider the surrounding drainage area.
The IBI assesses biological integrity using measures including fish species richness, environmental tolerance, trophic composition, abundance, and physical condition.

Transcript excerpt

0:00 Wetlands occupy the transition between aquatic and terrestrial environments. Their defining characteristic is prolonged soil saturation. When soil pores fill with water, gas exchange slows dramatically. Oxygen becomes depleted and the soil can become hypoxic. That creates a selective environment very different from ordinary terrestrial soils. Wetland vegetation must tolerate saturated conditions and reduced oxygen availability around the roots. So, wetlands are not simply areas that happen to be wet. Their hydrology fundamentally alters soil chemistry and biological community structure. Different hydrologic and climatic conditions produce different wetland types.

0:47 Salt marshes are common in temperate and higher latitudes. Mangrove forests dominate many tropical and subtropical shorelines. Estuaries occur where rivers enter marine systems and therefore experience considerable salinity variation. Freshwater marshes form where water accumulates in low-lying terrestrial areas and are typically dominated by grasses and other herbaceous vegetation. These systems are also especially vulnerable to human disturbance because they frequently occupy coastal and lowland areas desirable for agriculture, urban development, and aquaculture. The geographic pattern here is controlled largely by temperature. Salt marshes occupy many middle and high latitude coastlines.

1:36 Mangrove forests are restricted largely to tropical and subtropical environments where minimum sea surface temperatures remain sufficiently warm. What's interesting is that these systems can perform many comparable ecological functions. Both stabilize coastlines. Both provide habitat. Both influence nutrient cycling. But climate determines which plant community performs those functions. Now, let's move into flowing freshwater systems. A river basin includes the entire area drained by a river and its tributaries. Basins are separated by watersheds, generally along topographic high points. The Rocky Mountains provide a good North American example.

2:23 Precipitation falling on opposite sides of the divide can enter completely different drainage networks and eventually reach different oceans. So, topography determines hydrologic connectivity. And that connectivity determines how water, sediments, nutrients, and

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