ENVSCI 201: Lecture 3 - Agricultural Landscapes & Freshwater

Agriculture Development on Streams

It involves a lot of changes such as land disturbances, water diversion, nutrient enrichment, and animal stocking

Agriculture & Land Disturbances

The development of agriculture typically means the removal of streams' riparian zones.

This means changes to shading, detritus/energy incoming, and sediments within the stream (typically becoming homogenised).

The woody debris that is taken away is important for shaping the physical habitat & channel as well as allows for variable flow.

Channels tend to be alrtered, straightened, or even may be diverted. This also includes modification of the floodplain.

Agriculture & Hydrology: Irrigation

Typically, water from streams are diverted and used as irrigation for crop land. Water is typically harvested, stored, and distributed. Or, they may pump groundwater using wells. All of which means less flow due to less water being in streams.

At times, tile drains are used to quickly collect terrestrial surface water, pumped through pipes, and then drained into nearby streams

Agriculture & Hydrology: Groundwater Pumping

Excessive groundwater pumping also lowers the water table. As we dig wells and pump, the water table lowers and then we'd have to dig even lower. It can reach a point where instead of groundwater leaching into streams, the stream leaches into the ground instead (loses input of water).

Environmental Flow

Defined as the amount of water input required over tiem to maintain river health in a particular state

Agriculture & Nutrient Enrichment: Nitrogen

Fertiliser-use and nearby animals tend to increase the amount of nitrogen ending in streams. Nitrogen can be swept into streams through runoff or leached into the ground and eventually into streams.

Excessive nitrogen levels can kill species. However, eutrophication is the main issue caused with nutrient enrichment.

Agriculture & Nutrient Enrichment: Phosphorus

There is typically also phosphorus in fertilisers that may end up in streams too. Most of the phosphorus comes from excess erosion due to higher magnitude flows. Phosphorus can also lead to eutrophication.

When lakes become anoxic, phosphorus is released and ressurects algal blooms. In typical oxygenated conditions, phosphates react with iron to create sediments and sink, making it unavailable for algae.

Eutrophication

Also known as algal blooms, is when algae populations explode due to excess nutrients.

It greatly disrupts aquatic ecosystems for various reasons:

• It blocks out the sun, preventing other aquatic plants from photosynthesising
• As algae die and sink, they undergo aerobic decomposition. Excess algae leads to anoxic conditions for waters below.
• These anoxic conditions greatly impact aquatic ecosystems, food webs, and so on.
• Temperatures will also greatly decrease as you go down. These waters below don't mix with waters above