Nutrient Cycling and Agricultural Watersheds

Every cyanobacterial bloom is built from nutrients, and most of those nutrients arrive from the land. Nitrogen and phosphorus wash off fields, lawns and roads, seep through soil and groundwater, flow out of wastewater and septic systems, and are recycled within lakes from their own sediments [1, 2]. Enrichment with these nutrients — eutrophication — is the main reason blooms have become more frequent and intense worldwide [3, 4].

Two ideas in this chapter guard against the most common errors in watershed reasoning. First, blooms rarely have a single nutrient cause: which nutrient limits growth depends on the water body and season, co-limitation by nitrogen and phosphorus is common, and scientists still debate when nitrogen as well as phosphorus must be reduced [5, 6, 7, 8]. Second, being near farmland is not the same as receiving its runoff. Whether nutrients from a field reach a stream depends on where water actually goes — over the surface, through soil, through pipes, or, in Kentucky's karst, through underground conduits that ignore surface divides.

Planned sections

4.1 Forms of nitrogen

Nitrate (NO₃⁻), ammonium (NH₄⁺), dissolved organic nitrogen and particulate nitrogen; nitrogen fixation, nitrification and denitrification. Nitrate is highly soluble and moves readily with water through soils and groundwater, which is why it dominates nitrogen export in many agricultural and karst basins [9, 10].

4.2 Forms of phosphorus

Orthophosphate (soluble reactive phosphorus), organic phosphorus and particulate phosphorus bound to soil particles. Because much phosphorus travels attached to sediment, soil erosion during storms is a major delivery route [1].

4.3 Limitation and co-limitation

Experiments across many ecosystems show that nitrogen and phosphorus limitation are about equally common in fresh water and that adding both often produces the largest response [5]. A 37-year whole-lake experiment showed that reducing nitrogen alone did not control eutrophication, because nitrogen-fixing cyanobacteria made up the shortfall [6]; other syntheses argue that dual reductions are needed in many systems [7, 8]. Low nitrogen-to-phosphorus ratios have long been associated with cyanobacterial dominance [11]. Kentucky and Cumberland River microcosm studies illustrate that the limiting nutrient can change with place and time [12, 13].

4.4 Eutrophication and its consequences

Increased algal growth, reduced clarity, oxygen depletion, shifts toward cyanobacteria, and economic costs [14, 2]. Streams respond to nutrient enrichment too, through attached algae and altered oxygen dynamics [15].

4.5 External sources: fertilizer, manure, soil, wastewater and stormwater

Diffuse (nonpoint) sources from cropland, pasture and urban land, and point sources such as wastewater treatment plants [1]. Statewide, Kentucky streams draining agricultural basins have higher total nitrogen and phosphorus than streams draining forested basins [9]. Kentucky adopted a Nutrient Reduction Strategy in 2014 and has since updated it [16].

4.6 Internal loading

Sediments store phosphorus and can release it, especially when bottom water loses oxygen, sustaining blooms even after external inputs fall [17].

4.7 Storms, flow and timing

Most nutrient and sediment transport happens during a small number of high-flow events, so a sampling program that visits only in fair weather can miss most of the load. In a Kentucky karst basin, nutrient concentrations were generally higher in spring and summer, coinciding with fertilizer application [18].

4.8 Proximity versus hydrologic connectivity, and karst

Surface drainage divides and hydrologic units describe where water would flow over the land surface. In karst, sinkholes and conduits can carry water quickly underground and across those divides. Intensely karst basins in Kentucky have higher median nitrogen and phosphorus concentrations than basins with little karst [9], and in a Logan County karst spring basin, nitrate stayed essentially unchanged after best-management practices were widely adopted [10]. Whether a particular field drains to a particular stream is a question for field evidence — flow observation, chemistry, and in karst, dye tracing — not for a map alone.

Connection to local research

Key terms

Eutrophication

Nutrient enrichment and its consequences.

Limiting nutrient

The nutrient whose addition increases growth.

Co-limitation

Strongest growth response when two nutrients are added together.

Orthophosphate

Dissolved inorganic phosphate, readily used by producers.

Internal loading

Release of nutrients from a lake's own sediments.

Hydrologic connectivity

Whether and how water actually moves between two places.

Karst

Landscape of dissolved limestone with sinkholes, springs and conduits.