Plenty of plants live their whole lives underwater, and many of them are true flowering plants whose ancestors moved back into the water from land. Others are mosses and quillworts, plus stoneworts, which are algae that look like plants. Water carries far less carbon dioxide and oxygen than air, and it dims sunlight with depth. Most of what makes these plants look strange, from thin ribbon leaves to spongy stems, is a workaround for those shortages.
The Main Kinds of Underwater Plants
- Freshwater water weeds: waterweed (Elodea), watermilfoil, eelgrass (Vallisneria) and pondweeds grow tall stems toward the light.1Freshwater Biology. Depth limits and minimum light requirements of freshwater macrophytes2Environmental Engineering Science. Purification Mechanism of Low-Pollution Water in Three Submerged Plants and Analysis of Bacterial Community Structure in Plant Rhizospheres
- Seagrasses: flowering plants of the sea that form meadows in shallow coastal water.3PubMed Central. Seagrass Restoration Enhances “Blue Carbon” Sequestration in Coastal Waters
- Quillworts: small rosette plants (Isoetes) of clear, nutrient-poor lakes, from an ancient non-flowering lineage.4The Botanical Review. CAM photosynthesis in submerged aquatic plants
- Stoneworts: large algae that look like plants and can form low carpets on lake beds.5Freshwater Biology. Decline of charophytes during eutrophication: comparison with angiosperms
- Aquatic mosses: they grow attached in lakes and streams.6Freshwater Biology. Depth limits and minimum light requirements of freshwater macrophytes
Seagrasses are easy to confuse with seaweeds, but they are different kinds of living things. Seagrasses have roots, flowers and seeds like land plants. Seaweeds are algae that cling with holdfasts instead of true roots, and their tissues are built differently.
Plants with floating leaves breathe through pores on the upper leaf surface only, while fully submerged leaves usually have no pores at all.7The Open University. Leaf Structure as Environment Indicator The lily pad sits at the boundary between air and water, so it keeps pores on its upper face and a waxy top that sheds water, much like a land leaf. A submerged leaf has no air to reach, so it is thin and soft, often only a few cells thick, and gases soak straight through. Many species make both kinds on one plant: finely cut or ribbon-like leaves below, broader floating leaves at the surface.
How Underwater Plants Make Food
Underwater leaves have a very thin waxy coating or none at all.8AoB PLANTS. A perspective on underwater photosynthesis in submerged terrestrial wetland plants Land plants need thick wax to save water, and submerged plants don’t have that problem.9The Open University. Leaf Structure as Environment Indicator10AoB PLANTS. A perspective on underwater photosynthesis in submerged terrestrial wetland plants
Carbon is the harder part. In seawater and many lakes, bicarbonate, the dissolved form of carbon in water, is several times more plentiful than carbon dioxide.11PubMed Central. Underwater Photosynthesis of Submerged Plants – Recent Advances and Methods Many underwater plants have evolved to use it, and this ability arose many times independently.12PubMed Central. Underwater Photosynthesis of Submerged Plants – Recent Advances and Methods About half of submerged plant species in northwest European fresh waters can use bicarbonate.13Freshwater Biology. Acclimation of photosynthesis to supersaturated CO 2 in aquatic plant bicarbonate users Some pump acid onto the lower leaf surface to turn bicarbonate into carbon dioxide, which can leave a white crust of lime on the upper leaf.14PubMed Central. Underwater Photosynthesis of Submerged Plants – Recent Advances and Methods Even these plants still grow faster when carbon dioxide is plentiful.15Freshwater Biology. Acclimation of photosynthesis to supersaturated CO 2 in aquatic plant bicarbonate users
Still water makes all of this harder. A thin layer of unmoving water clings to every leaf and slows gas exchange.16PubMed Central. Underwater Photosynthesis of Submerged Plants – Recent Advances and Methods Currents thin that layer and bring fresh carbon, and thin, finely divided leaves help shrink it.
Some quillworts add a night shift. They take in carbon dioxide at night and store it as acid, then use it for photosynthesis by day.17Aquatic Botany. Carbon acquisition and carbon dynamics by aquatic isoetids They are the oldest known plants using this cactus-like trick, which probably began in seasonal wetlands.18The Botanical Review. CAM photosynthesis in submerged aquatic plants
If you keep an aquarium or pond, you have probably seen plants fizz with tiny bubbles in bright light. Photosynthesis releases oxygen, and a submerged leaf in strong light can make it faster than the water can absorb it. The excess leaves as streams of bubbles, often along leaf edges and cut stems. They usually slow or stop when the light dims or carbon runs short.
How Roots Get Oxygen in Airless Mud
Many wetland and water plants have air channels in a spongy tissue running from leaves to root tips.19Aquatic Botany. Aerenchyma development and oxygen transport in the estuarine cordgrasses Spartina alterniflora and S. anglica Oxygen from the air or from photosynthesis reaches roots buried in mud.20Aquatic Botany. Aerenchyma development and oxygen transport in the estuarine cordgrasses Spartina alterniflora and S. anglica Experiments show it moves through these channels all the way out into the soil around the roots.21PubMed Central. Primary and secondary aerenchyma oxygen transportation pathways of Syzygium kunstleri (King) Bahadur & R. C. Gaur adventitious roots in hypoxic conditions
You can see the tissue yourself. Many water plant stems feel spongy or hollow, and cutting one shows honeycomb-like holes. Those holes also help the plant float upright.
Oxygen the roots don’t use leaks into the surrounding mud.22Ecological Engineering. Effects of sediment anoxia on growth and root respiratory metabolism of Iris pseudacorus: Implications for vegetation restoration of eutrophic waters in China A thin oxidized zone forms, which keeps out toxic iron, manganese and sulfide.23Ecological Engineering. Effects of sediment anoxia on growth and root respiratory metabolism of Iris pseudacorus: Implications for vegetation restoration of eutrophic waters in China That matters because mud without oxygen produces sulfide gas, the source of the rotten-egg smell of pond mud. Black mud around roots often has rusty-orange streaks where oxygen leaked out.
How Roots and Leaves Share the Work of Feeding
Leaves of water weeds can absorb phosphate from the water.24American Journal of Botany. THE ROLE OF ROOTS IN THE NUTRITION OF AQUATIC VASCULAR PLANTS Over longer periods, though, most phosphorus in new shoots came from the roots, from 59% to over 90% depending on species.25American Journal of Botany. THE ROLE OF ROOTS IN THE NUTRITION OF AQUATIC VASCULAR PLANTS
That has practical consequences for aquariums and ponds. Rooted plants like eelgrass benefit from root tabs or a rich substrate, and planting them in bare gravel can starve them. Free-floating stems rely more on nutrients in the water.
How Underwater Plants Reproduce and Spread
Water-pollinated flowers have separate male and female flowers, reduced petals and feathery stigmas.26PubMed Central. Experimental evidence of pollination in marine flowers by invertebrate fauna Their pollen is often thread-like or released in slimy strands.27PubMed Central. Experimental evidence of pollination in marine flowers by invertebrate fauna In one water nymph species, male flowers release pollen in clouds lasting 5–10 minutes.28Aquatic Botany. Mechanism of underwater pollination in Najas marina (Najadaceae) The pollen starts growing tubes before reaching a stigma, which raises its chance of being caught.29Aquatic Botany. Mechanism of underwater pollination in Najas marina (Najadaceae) In seagrasses, small animals can help carry pollen.30PubMed Central. Experimental evidence of pollination in marine flowers by invertebrate fauna
Not every submerged plant leaves pollination to the water. Many pondweeds and milfoils send flower spikes up into the air even though the rest of the plant stays underwater. At the surface, wind or insects can carry pollen, which is far more reliable than drifting through water. A pond that looks flowerless may actually bloom briefly in summer.
Plants of open water often spread by broken fragments and by winter buds.31Aquatic Botany. Distribution of clonal growth forms in wetlands Winter buds are dormant shoots formed when conditions turn bad, and they sink and regrow later.32PubMed Central. Hormonal profiles in dormant turions of 22 aquatic plant species: do they reflect functional or taxonomic traits? That explains why pond weeds vanish in winter and return.
Fragments matter when a boat cuts weeds. Each broken piece can root and form a new plant, so raking can spread a weed rather than clear it. Removing cut pieces from the water keeps them from starting new beds.
How Deep Underwater Plants Can Grow
In moderately clear lakes, plant depth limits rise and fall with how clear the water is.33Freshwater Biology. Depth limits and minimum light requirements of freshwater macrophytes In murky lakes, tall plants escape by growing up toward the surface.34Freshwater Biology. Depth limits and minimum light requirements of freshwater macrophytes Leafier, thinner plants need less light to survive deep down.35Freshwater Biology. Depth limits and minimum light requirements of freshwater macrophytes In some clear lakes, grazing by crayfish keeps plants shallower than light allows.36New Zealand Journal of Marine and Freshwater Research. Analysis of relationships between maximum depth limits of aquatic plants and underwater light in 63 New Zealand lakes
Light also changes in color. Water filters out red light with depth, leaving blue-green. One invasive milfoil made more pigment under blue-heavy light.37Global Ecology and Conservation. Underwater light quality selection of invasive and native Myriophyllum species in China: Invasive species prefer blue light, while native species prefer red light
You can estimate your own lake’s plant depth by seeing how deep a white object stays visible. Plants often grow to roughly a few times that depth, though this is only a rough guide, and murky years shrink the planted zone.
Why Underwater Plants Disappear From Polluted Water
Stoneworts decline in polluted lakes mainly from poor light, not toxic phosphorus or fish.38Freshwater Biology. Decline of charophytes during eutrophication: comparison with angiosperms Large stoneworts vanish from the murkiest lakes, while small ones survive only in shallow water.39Freshwater Biology. Decline of charophytes during eutrophication: comparison with angiosperms Flowering weeds cope better by stretching shoots and growing early in spring.40Freshwater Biology. Decline of charophytes during eutrophication: comparison with angiosperms In rivers, above about 100 micrograms of phosphate per liter, algae can coat plants and starve them of light.41PubMed Central. Responses of Aquatic Plants to Eutrophication in Rivers: A Revised Conceptual Model Brown water, silt and shade from bank or floating plants make losses more likely.42PubMed Central. Responses of Aquatic Plants to Eutrophication in Rivers: A Revised Conceptual Model
Algae block light in three places: floating in the water as a green haze, in a film on the leaves, and in thick mats at the surface. A coating on the leaf also blocks gas uptake.
A pond shifting toward algae shows it. Weeds get coated in slimy green fuzz, the water turns green or brown, and low-growing plants disappear first. To protect submerged plants, cut fertilizer runoff near the shore, keep a planted buffer strip, and avoid stirring up sediment.
Why Some Underwater Plants Become Invasive
Each hydrilla tip can grow up to an inch a day.43UF/IFAS Entomology and Nematology Department. Hydrilla Integrated Management One nine-inch shoot produced over 3,200 inches of growth in five weeks.44UF/IFAS Entomology and Nematology Department. Hydrilla Integrated Management It grows in as little as 1% of full sunlight, letting it start early and shade others out.45UF/IFAS Entomology and Nematology Department. Hydrilla Integrated Management It spreads by winter buds, tubers, fragments and seeds, and has no native enemies in the U.S.46UF/IFAS Entomology and Nematology Department. Hydrilla Integrated Management
Growth and reproduction traits are the ones most often tied to invasiveness.47Acta Limnologica Brasiliensia. A global review on invasive traits of macrophytes and their link to invasion success Brazilian and Canadian waterweeds tolerate an especially wide range of conditions.48Acta Limnologica Brasiliensia. A global review on invasive traits of macrophytes and their link to invasion success Nuttall’s waterweed actually grows better in low light, giving it a spring head start.49Aquatic Invasions. Can invasive aquatic plants thrive in cold water or low light conditions? Implications for control – an experimental study
Most spread is human-made, so prevention is simple. Clean, drain and dry boats and trailers, check bait buckets and gear for fragments, and never dump aquarium plants in ponds.
How Underwater Plants Help Water and Climate
In planted test ponds, hydrilla removed up to 93% of nitrogen and 83% of phosphorus from mildly polluted water.50Environmental Engineering Science. Purification Mechanism of Low-Pollution Water in Three Submerged Plants and Analysis of Bacterial Community Structure in Plant Rhizospheres Native submerged plants can likewise keep algae in check. They compete with algae for nutrients, and their stems slow the water so suspended particles settle. Their roots hold mud in place, so wind and waves are less likely to stir it back up. They also shelter fish and insects. Because plants do all this, clearing all the weeds from a pond often leads to green water.
Seagrasses do similar work in the sea. Dense meadows hold seabed mud in place and trap sediment.51PubMed Central. Seagrass Restoration Enhances “Blue Carbon” Sequestration in Coastal Waters52PubMed Central. High variability of Blue Carbon storage in seagrass meadows at the estuary scale Seagrass sediments hold more carbon than bare seabed.53Ecosystems. Blue Carbon Storage in Tropical Seagrass Meadows Relates to Carbonate Stock Dynamics, Plant–Sediment Processes, and Landscape Context: Insights from the Western Indian Ocean A restored meadow buried about 37 grams of carbon per square meter a year after 10 years, close to natural meadows.54PubMed Central. Seagrass Restoration Enhances “Blue Carbon” Sequestration in Coastal Waters Young sparse meadows of 4 years did not yet steady the seabed.55PubMed Central. Seagrass Restoration Enhances “Blue Carbon” Sequestration in Coastal Waters