Floating World

 

A mixed group of floating water plants.  
Palm Beach County, Florida

Afloat on the water, plants experience two dramatically different worlds.  One is bright, hot and dry, the other is dim, cool and wet. Some floating plants are rooted to the bottom and others sail about, moved by wind or current.  Many floating plants survive and thrive at this odd interface and we will look at just a few. 

Sargassum fluitans, Broad-leaf Gulfweed, floating in the clear blue water
of the Gulf Stream off Cape Hatteras, North Carolina. 

One marine floating plant is a seaweed in the genus Sargassum.   This brown alga floats at the surface of the ocean carrying out photosynthesis.  Sargassum never attaches to the ocean floor and can drift for long distances.  Sargassum supports many species of animals and microbes and will often wash up on Atlantic or Gulf beaches.  The North Atlantic Gyre is in the center of the ocean and is bounded on all sides by currents.  In the gyre, an area called the doldrums, have calm winds and little current.  Here vast amounts of Sargassum accumulates to form the Sargasso Sea.  Unfortunately, plastic trash also accumulates in the gyre and makes the North Atlantic Garbage Patch. 

 

A fresh-water swamp with a mix of floating plants. 
Colleton County, South Carolina. 

Freshwater ecosystems have many floating plants.  They generally grow in still or slow-moving water found in swamps, ponds, marshes and lakes. Fragrant Waterlily, Nymphaea odorata, grows throughout Central and North America.  This plant has a rhizome, an underground stem, rooted in the bottom of the body of water.  Long petioles bear the large, round, notched leaves that float on the surface.  Nymphaea odorata makes beautiful, fragrant white flowers that give the plant its name. The upper surface of the floating leaf has a waxy layer to prevent water loss.  Stomates, pores that allow CO₂ to enter and O₂ to exit the leaves are also on the upper leaf surface.  The lower leaf surface has neither stomates nor a waxy layer since that side of the leaf is in the water.    

Fragrant Waterlily (Nymphaea odorata).
Rowan County, North Carolina.

Another rooted water plant with floating leaves is Nuphar advena, Spatterdock.  This plant is found in both the old and new worlds and has large leaves that are deeply cut at the base.  Spatterdock makes a compact yellow flower that extend above the surface of the water.  

Spatterdock (Nuphar advena). Collier County, Florida.

Water-shield, Brasenia schreberi, is yet another water plant that is rooted to the bottom.  Its elliptical, shield-shaped leaves are smaller than those of Fragrant Waterlily and Spatterdock.  Water-shield leaves can grow in dense clusters in their still water habitat.  The leaves produce mucilage on the lower surface that may protect the plant from herbivores and prevent algal growth.  All three of these rooted water plants have air spaces in their leaves to give them buoyancy and to help transfer oxygen to the roots that may be in the anaerobic mud bottom of a pond.   

Water-shield (Brasenia schreberi).
Georgetown County, South Carolina.

Many floating plants are not rooted to the bottom but float freely on the surface.  One plant with this way of life is Water Lettuce, Pistia stratiotes. This pan-tropical plant is in found in the Southeastern United States and may be an introduced species.  Water Lettuce has a rosette of leaves that resembles a head of lettuce. Pistia stratiotes is a fast-growing plant and may cover the entire surface of ponds or canals and cause fish kills.  

Water Lettuce (Pistia stratiotes) floating in a pond.
Palm Beach County, Florida.

Water Lettuce and the water fern Salvinia minima. 
Palm Beach County, Florida. 

Common Duckweed, Lemna minor, a tiny, free-floating plant has leaves that are just a few millimeters in diameter with each plant having a single root. It is found in ponds and ditches worldwide.  Common Duckweed rarely flowers and the plant reproduces by fragmentation.  Lemna minor grows rapidly and has a number of uses.  Common Duckweed is used as an animal feed, it is important in wastewater treatment because it removes inorganic nutrients and it accumulates toxic heavy metals.  

Common Duckweed (Lemna minor) in a swamp forest. 
Rowan County, North Carolina.

Salvinia minima is a floating fern.  Its common name, Water Spangles, describes its sparkling appearance on the surface of a pool.  Salvinia minima has round floating leaves that are ¾ inch in diameter and hairs on the upper surface that repel water.  This fern has no roots but it does have highly branched underwater leaves that play the role of a root system.  Salvinia minima is native to Central America and the West Indies.  It is considered and invasive species throughout the Southern United States. 

Water Spangles (Salvinia minima) a water fern growing in a cypress swamp.
Palm Beach County, Florida.

Golden Club, Oronitum aquaticum is our final floating plant. It grows in slow moving streams and ponds in the Eastern United States and is rooted in the mud.  Another common name for this plant is Never-wet because its leaves repel water.  Golden Club has unique flowers that gives the plant its name. The flowers have white stalks tipped with tiny golden blooms.  

Leaves and flowers of Golden Club (Oronitum aquaticum).

Rowan County, North Carolina. 

A drop of water beaded up on the leaf of Golden Club. 
This water repulsion shows why this plant is also called Never-wet. 
Rowan County, North Carolina.

Plants in the floating world range from the oceans to freshwater ponds.  They may be as tiny as Common Duckweed or as large as Fragrant Waterlily.  But all these plants share adaptations that allow them to live at this intersection of air and water.