Photosynthetically Active Radiation and San Diego Kelp Forest Health
San Diego’s kelp forests are among the most productive ecosystems on the planet. From the shallow reefs off La Jolla to the deeper forests around Point Loma and the Coronado Islands, these underwater canopies support everything from garibaldi to sea urchins to the occasional great white. But what drives kelp growth isn’t just nutrient-rich water or cold temperatures—it’s light. Specifically, photosynthetically active radiation, or PAR.
If you’ve noticed changes in kelp density during your dives or when checking conditions before paddling out, PAR is often the invisible factor at work. Understanding how light penetrates San Diego’s water column matters if you want to predict kelp health, plan better dives, and understand the broader conditions that make our local ocean thrive.
What Is PAR and Why It Matters
Photosynthetically active radiation is the portion of the electromagnetic spectrum (wavelengths between 400–700 nanometers) that photosynthetic organisms can actually use. It’s not the same as total light—it’s the usable light that drives photosynthesis in kelp, seagrass, and phytoplankton.
Kelp, particularly the giant kelp (Macrocystis pyrifera) that dominates San Diego’s underwater landscape, is a voracious photosynthesizer. It needs sufficient PAR to grow rapidly, repair damage from storms and sea urchin grazing, and maintain the forest structure that younger kelp and understory species depend on.
The more PAR that reaches the kelp fronds, the faster the growth. The less PAR available—due to murky water, suspended sediment, or depth—the slower the growth and the more vulnerable the forest becomes to collapse.
How Water Clarity Affects PAR in San Diego
San Diego’s water clarity varies dramatically by season and location. In winter, stormwater runoff from coastal canyons and rivers reduces visibility and PAR penetration. The Tijuana River, in particular, can dump sediment into the southern San Diego coast during heavy rains, murking visibility from Imperial Beach to the Coronado Islands for weeks.
Summer brings clearer water, especially offshore. By July and August, visibility often exceeds 60 feet at exposed reefs like Sunset Cliffs or the kelp forests near Point Loma, allowing PAR to penetrate deeper and support kelp growth across a wider depth range.
Inland bays like Mission Bay and San Diego Bay naturally have lower clarity due to shallower depths, sediment resuspension, and urban runoff. This limits PAR penetration and keeps kelp forests in these areas sparse or absent—a key reason why the open ocean reefs and deeper offshore locations support the healthiest, most extensive forests.
PAR and Kelp Forest Collapse
San Diego experienced a catastrophic kelp forest decline between 2014 and 2016, driven primarily by a combination of warm water, sea star wasting disease, and an explosion of voracious purple sea urchins. But PAR played a supporting role in the collapse.
As kelp forests thinned, less light was blocked from reaching the seafloor. Purple urchins, which had previously been controlled by sunflower sea stars, multiplied unchecked. These urchins grazed down any remaining kelp and prevented recovery—even when PAR conditions were favorable.
Recovery has been slow and patchy. In areas where urchin populations remain high (like parts of the Coronado Islands and offshore San Clemente), kelp cannot establish itself even when light and nutrients are adequate. In other areas, like the kelp forests off La Jolla Shores and Torrey Pines, forests have rebounded more robustly, suggesting that a combination of lower urchin density and adequate PAR allows recovery.
What This Means for Divers and Ocean Enthusiasts
If you’re planning a dive at a local kelp forest, PAR conditions affect what you’ll see. Winter dives at Sunset Cliffs or La Jolla Caves will have lower visibility and less vibrant kelp canopy—both due to reduced PAR and increased surge. Summer dives offer clearer water, deeper PAR penetration, and healthier-looking kelp.
Monitoring kelp health also tells you something about broader ocean conditions. Thick, fast-growing kelp indicates good light, low urchin density, and stable water temperature. Thin, sparse kelp suggests stress—either from poor light, warm water, or urchin grazing pressure.
Looking Ahead
As climate change alters water temperature and coastal upwelling patterns, PAR availability will shift too. Warmer water tends to be clearer but also reduces the nutrients that fuel both kelp and the plankton that support the entire food web. San Diego’s kelp forests will continue to depend on the balance between light, temperature, and herbivore control.
By paying attention to light conditions—both above and below the waterline—you’re tuning into one of the most fundamental drivers of ocean health in San Diego.
Track real-time water clarity, light conditions, and kelp forest status with the Element app to plan your dives and sessions when conditions are optimal.