Water clarity and kelp productivity are linked by the fundamental requirement of sunlight for marine plant growth. Water clarity measures how far light can penetrate through the water column, often quantified by turbidity or light attenuation. Kelp productivity is the rate at which these large brown algae convert light energy into biomass through photosynthesis. Kelp forests rely on light to fuel their growth, making water transparency a primary limiting factor for their distribution and biomass output.
Light Penetration and Kelp Growth
Kelp are primary producers in the coastal ocean, meaning they form the base of the food web by generating their own energy from light. For kelp to grow, the rate of photosynthesis must exceed the rate of respiration, a balance strongly controlled by the amount of light reaching the plant. When water clarity is high, sunlight is able to penetrate deeper into the water, extending the habitable zone for kelp growth.
Reduced water clarity increases light attenuation, which is the process of light intensity decreasing rapidly with depth due to scattering and absorption by suspended particles. This scattering of photons effectively raises the depth limit at which kelp can survive and grow. For some Arctic kelp species, this compensation irradiance—the minimum light level required for net positive growth—can be as low as 0.1–0.3 mol photons per square meter per day.
A decrease in transparency shrinks the underwater area where light levels are sufficient, restricting kelp to shallower depths and reducing the overall size of the forest. Studies have shown that increased turbidity can lead to a drastic reduction in kelp productivity, with one example demonstrating a 95% productivity loss following a 63% reduction in light availability. The loss of light limits the depth range and slows the growth rate of kelp, lowering the ecosystem’s total biomass production.
Sources of Reduced Water Clarity
The primary cause of reduced water clarity in coastal zones is the introduction of suspended materials and dissolved substances that absorb and scatter incoming light. Runoff from land is a significant driver, carrying fine sediments and particulate matter into the ocean. Activities such as construction, agriculture, and forestry accelerate soil erosion, increasing the sediment load that clouds the water.
Another major source of clarity loss is nutrient pollution, often from wastewater effluent and agricultural fertilizers, which causes eutrophication. This influx of nutrients fuels excessive growth of microscopic algae, known as phytoplankton. These algal blooms significantly increase the biological material in the water column, which absorbs and scatters light, creating a self-shading effect that limits light penetration to the kelp below.
Natural events and human activities can also re-suspend bottom sediments, contributing to transient or persistent turbidity. Storms and strong currents can stir up fine particles from the seafloor, while human actions like dredging or commercial trawling can have a similar effect.
Broad Ecological Impacts of Kelp Decline
When kelp productivity is diminished by low water clarity, the consequences affect the entire coastal ecosystem. Kelp forests are recognized as foundational ecosystems, providing structure and habitat for diverse marine communities. They act as shelter, foraging ground, and nursery habitat for hundreds of species of fish, invertebrates, and marine mammals.
The decline of kelp productivity leads to a loss of this physical structure, which results in reduced biodiversity and a shift in community composition. Opportunistic species may replace the long-lived kelp, changing how food and nutrients flow through the system. This disruption affects local fisheries, as the nursery function for commercially valuable species is compromised.
Kelp forests also play a role in coastal protection by dampening wave energy, which reduces shoreline erosion and protects infrastructure. These highly productive ecosystems contribute to global carbon sequestration. As kelp photosynthesize, they absorb carbon dioxide, and when their biomass sinks to the deep ocean floor, the carbon can be stored in marine sediments for centuries.