2.1 Light penetration and attenuation

Factors affecting light penetration

Light penetration controls where photosynthesis can happen in a lake or river, which in turn shapes the entire food web. The depth light reaches depends on conditions both above and below the water surface.

Angle of incidence

The angle of incidence is the angle at which sunlight strikes the water surface. When the sun is directly overhead (close to 90° relative to the surface), most light enters the water. As the sun drops toward the horizon, more light reflects off the surface instead of penetrating, following Fresnel's equations for reflectance.

• At solar angles below about 20° above the horizon, reflection increases sharply
• Light penetration peaks at midday and during summer months when the sun is highest
• Wind-driven surface waves can scatter incoming light in multiple directions, further complicating the picture

Wavelength of light

Water itself is a selective filter. Different wavelengths (colors) of light are absorbed at very different rates.

• Red and orange light (longer wavelengths, ~600–700 nm) is absorbed within the first few meters, even in clear water
• Blue and green light (shorter wavelengths, ~400–550 nm) penetrates much deeper
• In very clear open water, blue light can reach beyond 200 m, but most freshwater lakes are far less transparent due to dissolved and particulate matter
• This selective absorption is why underwater environments often look blue-green at depth

Dissolved substances

Dissolved organic matter (DOM) and other dissolved compounds absorb and scatter light as it passes through the water column.

• Colored dissolved organic matter (CDOM) is the most important dissolved light absorber in many lakes. It preferentially absorbs short-wavelength light (blue and UV), which shifts the underwater light spectrum toward green and yellow.
• High CDOM concentrations give water a brown or tea-stained appearance. This is common in lakes draining wetlands or boreal forests.
• Dissolved minerals and ions contribute to attenuation as well, though usually less than CDOM in freshwater systems.

Suspended particles

Particles suspended in the water column, including sediment, phytoplankton cells, and detritus, both scatter and absorb light.

• High particle loads reduce clarity dramatically. A river carrying glacial flour or eroded soil can have a euphotic zone only centimeters deep.
• The effect depends on particle size, shape, and composition. Fine clay particles scatter light efficiently; phytoplankton cells both scatter and absorb (especially at chlorophyll absorption peaks).
• Turbidity is a quick measure of water clarity related to suspended particle concentration. It's measured in nephelometric turbidity units (NTU) using the amount of light scattered at 90°.

Measuring light in water

Quantifying how much light is available at different depths is essential for estimating primary production and characterizing habitat. Several complementary methods exist.

Photosynthetically active radiation (PAR)

PAR is the slice of the electromagnetic spectrum (400–700 nm) that photosynthetic organisms can actually use. PAR sensors (quantum sensors) measure photon flux density, reported in units of $\mu mol \; photons \; m^{-2} \; s^{-1}$.

• By lowering a PAR sensor through the water column and recording values at set depth intervals, you build a vertical PAR profile
• This profile reveals how quickly usable light drops off and where the euphotic zone ends
• PAR at the surface varies with time of day, cloud cover, and season, so profiles are often normalized to surface irradiance

Secchi disk transparency

The Secchi disk is the simplest and oldest tool for estimating water clarity.

1. Lower a 20 cm black-and-white disk horizontally into the water on a calibrated line
2. Record the depth at which the disk just disappears from view
3. That depth (in meters) is the Secchi depth

Secchi depth gives a rough estimate of euphotic zone depth: multiply Secchi depth by approximately 2.5 to 3. For example, a Secchi depth of 4 m suggests a euphotic zone around 10–12 m. It's subjective (it depends on the observer's eyesight and ambient light), but its simplicity makes it invaluable for long-term monitoring datasets.

Vertical light attenuation

Light intensity decreases exponentially with depth. The rate of that decrease is captured by the vertical diffuse attenuation coefficient, $K_d$ (units: $m^{-1}$).

• $K_d$ is calculated from PAR measurements at two or more depths using the Lambert-Beer relationship
• A low $K_d$ (e.g., 0.1 $m^{-1}$) means light penetrates deep (clear water)
• A high $K_d$ (e.g., 2.0 $m^{-1}$) means light is snuffed out quickly (turbid water)

Euphotic zone depth

The euphotic zone ($Z_{eu}$) is the depth at which PAR falls to 1% of its surface value. Below this depth, net photosynthesis can't sustain itself.

$Z_{eu}$ relates to $K_d$ by:

$Z_{eu} = \frac{4.6}{K_d}$

The 4.6 comes from $\ln(100) \approx 4.605$, since you're solving for the depth where light drops to 1/100 of the surface value. A lake with $K_d = 0.5 \; m^{-1}$ has a euphotic zone about 9.2 m deep.

Optical properties of water

Optical properties describe how light interacts with water and everything dissolved or suspended in it. They fall into two categories depending on whether they're intrinsic to the medium or also depend on the light field.

Absorption vs. scattering

These are the two fundamental processes that remove light from a downward-traveling beam.

• Absorption converts light energy into heat or chemical energy. The photon is gone.
• Scattering redirects light in a different direction without destroying it. The photon still exists but is no longer traveling downward.

Both contribute to overall attenuation. Their relative importance shifts with wavelength and with what's in the water. Pure water absorbs strongly in the red; CDOM absorbs strongly in the blue; particles tend to scatter across a broad range of wavelengths.

Inherent optical properties (IOPs)

IOPs depend only on the water and its constituents, not on the direction or intensity of the light field. They're the building blocks of optical models.

• Absorption coefficient ($a$): how strongly the medium absorbs light per unit path length ($m^{-1}$)
• Scattering coefficient ($b$): how strongly the medium scatters light per unit path length ($m^{-1}$)
• Beam attenuation coefficient: $c = a + b$

Because IOPs are independent of illumination conditions, they can be measured in a lab with a known light source and used to predict how light will behave under any natural lighting scenario.

Apparent optical properties (AOPs)

AOPs depend on both the medium and the ambient light field (sun angle, cloud cover, surface conditions). They're what you actually measure in the field under natural conditions.

• Diffuse attenuation coefficient ($K_d$): the rate of decrease of downwelling irradiance with depth
• Irradiance reflectance ($R$): the ratio of upwelling to downwelling irradiance just below the surface
• Remote sensing reflectance ($R_{rs}$): the ratio of water-leaving radiance to downwelling irradiance above the surface

AOPs are measured with in-water radiometers or derived from satellite imagery. They connect the physics of light in water to what we can observe from above.

Spectral absorption coefficients

Absorption isn't uniform across wavelengths. Each component in the water has a characteristic absorption spectrum.

• Pure water absorbs strongly in the red and near-infrared (>600 nm), which is why even clear water looks blue
• Phytoplankton pigments (especially chlorophyll-a) absorb in the blue (~440 nm) and red (~675 nm), creating a distinctive two-peaked signature
• CDOM absorbs most strongly in the UV and blue, with absorption declining exponentially toward longer wavelengths
• Non-algal particles (detritus, minerals) absorb in the blue, with a spectrum similar to CDOM but usually weaker

By measuring the total absorption spectrum and decomposing it into these components, you can estimate concentrations of chlorophyll, CDOM, and suspended sediment from optical data alone.

Biological implications

Light availability shapes where organisms live, how much food the system produces, and how aquatic communities are structured.

Photosynthesis and primary production

Phytoplankton need light to drive photosynthesis. Within the euphotic zone, production is often co-limited by light and nutrients. Below the euphotic zone, light is too dim for net carbon fixation, so heterotrophic processes dominate.

The relationship between light and photosynthesis follows a characteristic curve: production increases with light up to a saturation point, then levels off or even declines at very high intensities (photoinhibition near the surface).

Vertical distribution of phytoplankton

Phytoplankton don't distribute evenly through the water column. They respond to gradients of both light and nutrients.

• In thermally stratified lakes, nutrients are often depleted near the surface (where light is abundant) and concentrated below the thermocline (where it's dark). Phytoplankton frequently form a deep chlorophyll maximum (DCM) at the depth where light and nutrients overlap favorably.
• Motile species like dinoflagellates can migrate vertically, moving toward the surface during the day for light and descending at night to access nutrients.
• Vertical mixing events (wind, convective cooling) redistribute phytoplankton through the water column, changing their average light exposure.

Phytoplankton adaptations to light

Phytoplankton have evolved several strategies to cope with variable light environments.

• Pigment adjustment: Cells in low-light environments increase their chlorophyll content and may produce accessory pigments (phycocyanin in cyanobacteria, fucoxanthin in diatoms) to capture wavelengths that penetrate deepest.
• Buoyancy regulation: Some cyanobacteria use gas vesicles to control their vertical position, rising toward light when needed.
• Photoprotection: Under excessive light, xanthophyll cycle pigments dissipate surplus energy as heat, preventing damage to the photosynthetic machinery.

UV radiation effects on aquatic organisms

UV radiation, particularly UV-B (280–320 nm), penetrates the upper water column and can damage DNA, proteins, and lipids in exposed organisms.

• Many phytoplankton and zooplankton produce mycosporine-like amino acids (MAAs), which act as natural sunscreens by absorbing UV
• Some zooplankton (e.g., Daphnia) exhibit diel vertical migration partly to avoid UV exposure near the surface during midday
• Increased UV-B reaching Earth's surface due to stratospheric ozone depletion has raised concerns about impacts on surface-dwelling plankton communities

Modeling light in water

Mathematical models translate optical measurements into predictions about light fields, primary production, and water quality. They range from simple analytical equations to complex numerical simulations.

Lambert-Beer law

This is the foundational model for light attenuation in water.

$I_z = I_0 \cdot e^{-K_d \cdot z}$

Where:

• $I_z$ = light intensity at depth $z$
• $I_0$ = light intensity just below the surface
• $K_d$ = diffuse attenuation coefficient ($m^{-1}$)
• $z$ = depth (m)

The model assumes a vertically homogeneous water column with a constant $K_d$. That assumption holds reasonably well in well-mixed lakes but breaks down when there are strong vertical gradients in particles or CDOM (e.g., a dense phytoplankton layer at a specific depth).

Radiative transfer theory

For more accurate modeling, radiative transfer theory accounts for the full angular distribution of light, multiple scattering events, and depth-varying optical properties.

• Radiative transfer models (RTMs) solve the radiative transfer equation numerically
• They predict the complete underwater light field, not just downwelling irradiance
• RTMs are computationally intensive but necessary for interpreting remote sensing data and modeling complex water columns
• The widely used Hydrolight software is one example of an RTM applied to aquatic systems

Bio-optical models

Bio-optical models link IOPs to the concentrations of optically active substances (phytoplankton, CDOM, suspended sediment). They work in both directions:

• Forward: Given known concentrations, predict what the water's optical properties and reflectance will look like
• Inverse: Given measured reflectance or IOPs, estimate concentrations

Examples include the Garver-Siegel-Maritorena (GSM) model and the Quasi-Analytical Algorithm (QAA). These are widely used to convert satellite ocean/lake color data into maps of chlorophyll, CDOM, and particle concentrations.

Remote sensing applications

Satellite and airborne sensors measure the color of water from above, providing spatial coverage that in-water sampling can't match.

• Sensors like MODIS, Sentinel-2, and Landsat measure $R_{rs}$ at multiple wavelengths
• Bio-optical algorithms convert $R_{rs}$ into estimates of chlorophyll-a, CDOM, turbidity, and $K_d$
• Remote sensing is especially powerful for tracking phytoplankton blooms, seasonal CDOM dynamics, and sediment plumes across entire lake basins or regions
• Limitations include cloud cover, shallow-water bottom reflectance, and the need for ground-truth calibration with in-water measurements

Anthropogenic influences

Human activities alter the light environment of lakes and rivers in ways that cascade through the entire ecosystem.

Eutrophication and light attenuation

Eutrophication is the over-enrichment of water with nutrients, primarily nitrogen and phosphorus, from agricultural runoff, wastewater, and urban stormwater.

• Excess nutrients fuel phytoplankton blooms, which increase turbidity and raise $K_d$
• The euphotic zone shrinks. In severely eutrophic lakes, it may be only 1–2 m deep.
• Reduced light favors cyanobacteria, which can regulate buoyancy to stay near the surface, outcompeting other phytoplankton that depend on mixing to access light
• Loss of submerged aquatic vegetation (macrophytes) often follows because plants rooted on the bottom no longer receive enough light

Colored dissolved organic matter (CDOM)

CDOM is a major controller of underwater light in many freshwater systems, particularly in boreal and humic lakes.

• CDOM originates from the breakdown of terrestrial plant material (leaves, soil organic matter) and in-lake biological processes
• Land use changes like deforestation, wetland drainage, and peatland disturbance can increase CDOM export to downstream lakes and rivers
• Many northern lakes have experienced "browning" over recent decades, with rising CDOM concentrations linked to recovery from acid rain, increased precipitation, and warming soils
• Higher CDOM shifts the underwater light spectrum away from blue toward green/yellow, which can alter phytoplankton community composition

Climate change impacts on water clarity

Climate change affects light availability through multiple pathways.

• Stronger thermal stratification from warmer air temperatures reduces vertical mixing, which can concentrate phytoplankton near the surface and alter nutrient supply from deeper water
• Changing precipitation patterns affect how much CDOM and sediment wash into lakes from their catchments. More intense storms can deliver pulses of turbidity.
• Shorter ice cover duration extends the growing season but also increases the period of wind-driven sediment resuspension in shallow lakes
• Permafrost thaw in high-latitude regions releases stored organic carbon, increasing CDOM in Arctic and subarctic lakes

Management strategies for improving transparency

Restoring water clarity typically requires reducing the inputs that cause light attenuation.

1. Reduce nutrient loading: Upgrade wastewater treatment, implement agricultural best management practices (cover crops, buffer strips, precision fertilization)
2. Riparian buffers and wetland restoration: Vegetated zones along shorelines intercept sediment and nutrients before they reach the water
3. In-lake interventions: Biomanipulation (removing planktivorous fish to allow zooplankton to graze down phytoplankton) and chemical treatments (alum dosing to bind phosphorus in sediments) can improve clarity
4. Monitoring and adaptive management: Long-term Secchi depth records, PAR profiles, and satellite remote sensing help track whether management actions are working and guide adjustments