Storms turn coastal water murky, dimming the light kelp depends on. How turbidity works, where the sediment comes from, and how long forests stay in the dark.
A week before, the reef water was a clear green dive at twenty metres. Then the storm came through — three days of hard rain, a swollen river, two metre-plus swells — and now a diver at eight metres can barely see the kelp above her own head. The water has gone the colour of weak tea, then closer to chocolate milk near the river mouth. What the storm did to the seafloor is one thing; what it did to the light is another, and for a kelp forest the second may matter more.
Turbidity is the cloudiness of water caused by suspended particles — sediment, organic matter, plankton. It is one of the least glamorous parameters in ocean science and one of the most consequential in a coastal forest, because kelp runs on sunlight. This piece is about where those particles come from, what they do to the light budget, and how a forest rides out the murk.
Turbid water is water carrying a load of suspended material fine enough not to settle quickly — clay and silt grains a few thousandths of a millimetre across, organic detritus, resuspended microalgae. Each particle absorbs some light and scatters the rest, sending photons off in directions that no longer serve a photosynthesising blade below.
Scientists measure turbidity a few ways. The oldest is beautifully simple: the Secchi disk, a white disc lowered until it vanishes — its disappearance depth is a one-number measure of water clarity that limnologists have used for over a century. Modern field instruments use turbidity sensors, which shine light at a sample and measure side-scatter in nephelometric turbidity units (NTU), and thus distinguish sediment-borne haze from colour. Satellites measure it too, from the colour of reflected sunlight — the same ocean-colour data used to map plankton can map sediment plumes, since phytoplankton greens and inorganic muds tint water differently.
One distinction matters for kelp ecology: turbidity is about particles, not colour. Water stained brown by dissolved humic acids (think tannin-rich river runoff) can dim light without being particularly turbid. Both reduce the light a kelp blade sees.
A storm attacks water clarity from three directions at once.
The river plume. Heavy rain drives rivers into flood, and floodwater carries the watershed's loosest material: fine silt and clay stripped from bare soil, farmed land, burnt hillsides and construction sites. Along farmed or urbanised coasts a big rain can turn months of land into hours of sediment in the sea, and plumes of it drift alongshore on the coastal current for tens of kilometres, sometimes reaching reefs far from any river mouth.
The resuspended bottom. Big waves — focused onto shallows by wave refraction — stir the seafloor where their orbits reach down. Sand gets tumbled into a bedload rolling along the bottom; finer silt previously settled in calm pockets lifts into the water column. On a shallow continental shelf, a single heavy swell can re-suspend more sediment than a whole wet season of river flow delivers to the same coast.
The bloom (sometimes). Storm runoff brings nitrogen and phosphorus with the dirt. If a warm, calm spell follows, nutrient-fed phytoplankton can bloom on top of the sediment haze — the water goes from tea to pea soup. That layered haze has two particle types with different fates: sediment settles out; plankton die off, are eaten, or sink slowly.
Kelp is a plant-dimension organism with an algal metabolism: every blade is a solar panel. The water column taxes each photon. Even in utterly clear seawater, light attenuates exponentially with depth; our explainer on light underwater covers the spectrum — blues penetrate deepest, reds vanish by ten metres. Turbidity adds a heavy second tax, and crucially it taxes the light throughout the water column, not just at a fixed depth.
Two numbers rule a kelp forest's dark days:
For the deeper winter story — kelp forests already run on short, low, slanted light — see water clarity and light.
Light is only the first casualty. When the storm passes and particles finally fall, they land on every horizontal surface: kelp blades, coralline-encrusted rock, grazed-open settlement space. A millimetre or two of silt does several kinds of harm:
A healthy grazer community — urchins held in balance, limpets, chitons, the cleanup crew of the kelp forest food web — keeps the rock cropped between storms so that the next cohort of settlers finds clean surface. This is one quiet reason biodiversity pays for itself after disturbance.
The settling rate depends on particle size, salinity and continued wave energy. Sand falls in minutes. Silt may take days; clays, in protected bays where they stay unmoved by swell, can remain a floating fines haze for weeks, and can re-suspend every spring tide. River flood plumes drift out to sea or get diluted alongshore over days to weeks depending on the coastal currents.
A useful mental model: a storm injects a pulse; the coast processes it on a timescale set by particle fineness, tides, and whether another storm reloads the plume. One isolated storm is a dark fortnight. Back-to-back winter systems can make a whole season dim — the difference between kelp losing a little growth and kelp losing a recruitment year. This is also why the forest's own storm ecology is double-edged: storms prune and renew, but on coasts already close to a light or sediment tipping point they can tip it.
You don't need instruments to watch storms change the water:
For the instrumental version, ocean-colour satellite products — and the guides to reading ocean colour from space and reading buoy data — translate all this into numbers, and sediment traps and turbidity loggers on reefs tell us the local dose a particular forest received.
Looks similar, but resuspended sediment near a wave-tossed coast is sand-and-silt dominated and settles quickly, while river plume water carries fine clay that stays up for days to weeks. Plume water also arrives fresher (lower salinity) and carries nutrients.
The surface mixed layer warms slightly (sunlight absorbed near the surface rather than passing through), but the light lost to the kelp far outweighs any small temperature blip — kelp needs those photons many times more than it needs a smoother temperature.
Somewhat. Populations on naturally murky coasts often grow shallower and have adapted physiologies, but there are limits; beyond them the community shifts to sediment-tolerant turf algae. Acute storm pulses are different — evolved systems handle pulses; the danger is raising the background murk.
Usually no. Brown is sediment from land and seafloor; green arriving over days is often a nutrient-fed phytoplankton bloom riding on the runoff. Satellites separate the two by colour — the same way we monitor plankton (chlorophyll from space).
Site surveys log water clarity across seasons and storm recovery; sites that stay dim after storms are deprioritised or paired with catchment work, and juvenile outplants are timed to post-storm clear windows. Tidewright's own monitoring practice on illustrative projects treats turbidity as a first-class planning variable alongside wave energy and temperature.
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