Aquatic Plants

Aquatic Plants

Floating and truly aquatic macrophytes make up a most remarkable community and show the most unexpected diversity in várzea plant life. In contrast, in forest streams, higher aquatic plants are almost absent. Three types of plants are represented in the várzea. Firstly, there are plants typically rooted on the bottom but with floating parts that for some or most of the flood-cycle photosynthesise at or above the water surface. Secondly there are plants always submerged but floating unrooted. Thirdly, there are plants floating at the surface and photosynthesising above it.

The first class includes the large grasses. 'Floating meadows' are often tens to hundreds of hectares in extent but they contain only very few species. Paspalum repens and Echinochloa polystachya are equally and by far dominant, but Hymenachne amplexicaule, and Oryza glumipatula are present locally. A less robust grass, Leersia hexandra, forms smaller floating mats in less eutrophic (but not 'black') water bodies. When water is present these grow extremely fast, easily keeping pace with the rising flood and spreading rapidly laterally across the water surface at the same time. When dropped back on the mud at the end of the flood season they are greatly reduced by disease, decay and consumption by fish but do not necessarily die or cease growing completely. Only Oryza (later when floods return to vie with Hymenachne as the least buoyant floater) grows substantially during the 'terrestrial' phase. Less common but similarly semi-aquatic are Polygonum acuminatum and Caperonia castanaeifolia. Fully aquatic and requiring a pool that persists even at low water, is the characteristic and magnificent várzea water lily, Victoria amazonica. Because it re-grows its leaves and flowers every year, it grows at a rapid rate. The leaves are rapidly attacked by herbivores. Dubiously also fully aquatic is a species of Callitriche starwort and a Riccia liverwort. The second class, that of free-floating but submerged plants includes Ceratophyllum, Najas, Wolffiella and Utricularia. The paucity of submerged aquatics arises because such plants are unusually at the mercy of

the currents that affect most habitats at one season or another - they are less likely to be held amongst attached vegetation than full floaters
the abundance, diversity, specialism and hunger of the low-water fish population.

Herbivorous fish such as anostomids are to be observed eating leaves and shoots of Paspalum. Fish may act against any fully exposed floating plants like piranhas against ducks, which are another at first sight surprisingly under-represented group.

The third class, the surface floaters, make up the most immediately striking community. It is remarkable in the following respects:

While the community comprises at least 23 species, about a half dozen are so abundant that at the margins of any open area of still water they can usually be found in a few minutes search.
In a 'floating lawn' as we prefer to call an assemblage of these free-floating small species, plants often exist interspersed as a true community in which the smaller fill gaps between the larger. Thus, in descending size, may be observed: Eichhornia > Pistia > Limnobium > Salvinia > Azolla > Ricciocarpus > Spirodela > Lemna. The 'floater' genus Wolffia (Lemnaceae) containing the smallest of all angiosperms, is recorded from Amazonia. Lawns of single species occur but are less common than mixtures.
On all floating lawns phytophagous insects are abundant and cause obvious damage. Fungal and microbial diseases are also evident.
So far as can be judged by presence of sexual structures, all plants of these floating lawns reproduce sexually.
All the species also reproduce prolifically by vegetative means.
While some species may have long been pantropical, several from the community have been spread by human agency from seemingly just these kinds of communities in tropical South America where in natural waters they are virtually never pests, to other continents where they quickly become pests. These include: Gymnocoronis, Eichhornia, Pistia, Utricularia, Salvinia, Azolla.

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A floating lawn in a varzea lake showing the yellow flowers of Utricularia	A patch of Pistia stratiotes	A photograph of a lawn of floating plants in an Amazonian floodplain lake. In this picture can be seen a number of characteristic floating plants including Pistia, Azolla and Limnobium	Flowering water hyacinth, Eichhornia crassipes
The giant Amazonian water lily Victoria amazonica. The leaves have been heavily eaten by insects. The photograph was taken in a small pond on the edge of a channel in the River Amazon floodplain	The giant amazonian water lily, Victoria amazonica, in a floodplain pool showing a single flower about to open	The leaves of giant Amazonian water lily Victoria amazonica	The giant Amazonian water lily Victoria amazonica. The leaves have been heavily eaten by insects
Paspalum repens growing along the edge of a varzea channel		A floating fern, Ceratopteris, photographed in a white water floodplain lake