THE RIO GUAPORE BRAZIL Biotope No 1
The Rio Guapore has its source in the Brazilian state of Mato Grosso, near the border with Bolivia. Its riverbanks are the only place where primary forests are still found. The starting point for the excursion in August 1987 was the little town of Vila Bela on the upper course of the Guapore (about 60°W, 15°S), where the river features a wide range
of species and dense plant populations in its flood areas and tributaries.
Ecological Data of the Upper Rio Guapore
In its upper course the Rio Guapore is a clear water river. In this area the wet season extends from mid September to the end of April. The high-water period accordingly falls into our European 6-month-long winter; the low-water period coincides with our summer. The highest temperatures are reached during the wet season. For the city of San Luiz de Caceres, 250 km away, the average daily maximum in the warmest month (March) measures 32.8 °C, and the average daily minimum during the coldest month (July) measures only 14.6 °C (absolute minimum 3.8 °C). The data helps to explain the surprisingly low water temperature of the Rio Guapore of 20 °C recorded in August. It can be concluded from the water analysis (Table 3, Biotope No. 1) that the Guapore is a very soft, nutrient-deficient (oligotrophic) waterway in which aquatic plants are nevertheless able to find a sufficient nutrient supply to sustain growth.
At many locations on the riverbed, the soil base is washed out and consists of fine white sand. Emersed plants of the riverbank and flood areas, however, grow mainly on muddy loamy soil. Dense plant populations are found mainly on open and shallow parts of the riverbank where intense solar radiation is also possible.
During low tides, the water level of the Rio Guapore drops during the dry period by a maximum of 1.5 m. Plant species located on the banks of the waterway, for example, the large-growing Echinodorus species E. pani-culatus and E. grandiflorus subsp. grandi-florus, are found emersed only a few centimeters underwater. During high water, these species will temporarily grow in a depth of 1-1.5 m and are partially able to adapt to the raised water level and the associated reduc tion in lightly developing longer petioles. It is assumed that plants growing several meters away from the riverbank, for example, the smaller Echinodorus species E. tenellus, E. bolivianus and E. grisebachii, are found in relatively shallow water during high water periods where they develop submersed leaves. These light-requiring and small-remaining sword plants grow in those locations in nature where they will still continue to receive sufficient light to support adequate growth even during high-water periods, i.e., in an area which is only just reached by the river during maximum high tides. These small to medium-sized species thus occupy a different ecological niche compared, for example, to the larger Echinodorus species.
Plant Societies and Species Diversity
The diversity of aquatic and marsh plants in the vicinity of this river is exceptionally high. Alongside 5 Echinodorus species, 3 of which belong to the smaller to medium-sized species and 2 to the larger species, 15 other species from other genera were located: 11 in the river and 4 in the nearby flood areas.
Under closer scrutiny, the river displays a number of different living environments in which certain species tend to be found more often, e.g., species which have adapted to the prevailing ecological conditions to a specific degree. In the following, these living environments will be portrayed in order to convey an insight into the ecology and sociology of several aquatic and marsh plants which are of interest to aquaristics. The sedge belt, often several meters wide, found at many places in the Rio Guapore and its flood areas, is not included.
The zone accommodating the amphibious growing species constitutes an important living environment. On the Guapore, the five Echinodorus species already mentioned are among these, as well as Limnophila indica
(introduced) which, on the one hand, can be found in up to 50 cm of depth during low water, and at the same time produces emersed flowering populations.
Adjacent to this zone is a plant community made up of submersed plants which flower above the water surface. Among these are Cabomba furcata which is often found in association with Limnophila indica. Cabomba furcata grows in the placid sunny border areas of the river in depths of more than 50 cm. Larger populations are found mainly in subsidiary branches and quiet coves with slow-moving water. A species which is found quite often in the Guapore is Ottelia brasi-liensis, which also only grows submersed and only sends its flowers to the water surface. During low water it can be located in both very shallow water, for example on sunny sand banks, as well as in depths of almost 2 m. It can be assumed that these plants will stand in more than 3 m of water during high-water periods. According to the author's observations, Ottelia brasiliensis occupies a similar ecological niche to Limnophila indica and Cabomba furcata; Ottelia brasiliensis seems to prefer locations with strong currents and far deeper water, whereas Limnophila and Cabomba are more likely to be found in more placid zones of shallow water. This does not preclude all three species occasionally occurring next to each other at the same location under appropriate conditions. Common to all these species obviously is their preference to populate spots that are fully sunlit. The overall picture of the river landscape on the Guapore is essentially defined by a belt of Eichhornia, consisting of
Eichhornia azurea, Echinodorus particulates and Echinodorus grandiflorus grow on this riverbank on the Rio Guapore (Brazil).
Impressive dense populations of Eichhoritia diversifolia in a floating plant zone on the Rio Guapore.
Diagram 1 Schematic representation of the distribution of plant populations in different living environments in the Rio Guapore at low-water levels (excluding the marsh zone). The following species were selected to represent the different living environments and their respective plant populations: Echinodorus tenellus (1), E. grisebachii (2) and E. paniculatus (3) are typical of the river-bank zone; Eichhornia azurea (4) is common in the Eichhornia belt; Limnophila indica (5) represents amphibious species; Cabomba furcata (6) and Ottelia brasiliensis (7) represent those submersed plants flowering on the water surface; Eichhornia diversifolia (8) typifies the floating plant belt; Egeria najas (9) is characteristic of those submersed plants floating freely in the water; and Salvinia auriculata (10) is an example of those species inhabiting the floating plant zone.
Diagram 2 Simplified graphic representation of the living environments of aquatic and marsh plants in the Rio Guapore at low-water levels (excluding species inhabiting the marsh zone).
In reference to Diagram 1 the continuous lines represent the most important area or zone of presence of the respective species, which is dependent on the river's water levels and the degree of moisture in the substrate, thereby limiting it accordingly.
The arrows indicate presence also in adjoining living environments. The broken lines point to a rare, limited occurrence in the respective environment. The vertical dividing line marks the border between the dry zone during low-water levels and the permanent water zone.
-Dry zone during low-water periods-
Cabomba furcata (6)
Echinodorus boiivianus l-
Echinodorus grandiflorus Echinodorus grisebachii (2) Echinodorus paniculatus (3) Echinodorus tenellus (1) Egeria najas (9) Eichhornia azurea (4) Eichhornia crassipes Eichhornia diversifolia (8) Limnophiia indica (5) Limnobium iaevigatum Ludwigia sedoides Najas confería Ottelia brasiliensis (7) Saivinia auriculata (10)
- Zone permanently carrying water-
Eichhornia azurea and Eichhornia crassipes and which stretches along the embankment zones of the waterway. The prevailing ecological conditions, however, don't seem to be optimal for E. crassipes since the plants were unusually frail for tropical conditions and found to exist only in small groups. The Eichhornia azurea specimens, in contrast, displayed a surprisingly strong habit. The meter-long, tangled shoots were anchored to the bank and stretched from there into the , river. In areas with a strong current, the populations are smaller compared to the waterways' quieter fringe zones. The belt of Eichhornia often borders on the belt of floating leaf plants or the area of floating plants.
The belt of floating leaf plants in the Guapore can be found primarily in the numerous shallow and quiet fringe waters as well as in the current-deficient coves on the banks. Next to the real-floating leaf plants it can, however, also be interspersed with genuine aquatic plants, for example, Egeria najas and Najas conferta. In the Guapore's belt of floating leaf plants Eichhornia diversifolia and Ludwigia sedoides species can be found, species which shaped this habitat during the Brazilian winter with their floating leaf shoots carrying numerous flowers. In contrast to Eichhornia diversifolia, Ludwigia sedoides populates the free-water zone as well as those areas of the riverbank having no currents. It is also able to produce emersed leaves.
The free-water zone with submersed, free-floating plants in the water is made up mainly of Egeria najas and Najas conferta. Whereas the strong current areas in the center of the Guapore are free of any plant growth, one can often find mass groups of both these species in tranquil subsidiary arms or current-deficient bays which have the characteristics of stagnant waterways. The tender shoots of Egeria najas and Naja$ conferta float in dense groups underwater in locations fully exposed to the sun.
Besides the Eichhornia populations on the riverbanks, the author found only two floating plant species in the actual riverbed of the Guapore: Limnobium laevigatum and Sal-vinia auriculata. Both floating plants are rare in the Guapore and will occur only occasionally, so the ecological conditions for both species are obviously not optimal.
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