The role of Potamogeton crispus L. in the Pongolo River floodplain ecosystem.
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Date
1984
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Abstract
1. Aquatic macrophytes generally dominate energy and nutrient
transfers in wetland ecosystems. They provide a diverse
habitat for other organisms but each species or life-form,
has a different role in determining ecosystem structure and
function. A multidisciplinary research project on the Pongolo
River Floodplain, Zululand, South Africa, provided a good
opportunity to study the role of a submerged species, Potamogeton
crispus L. in a wetland ecosystem.
2. The role of a plant species in ecosystem structure and functioning
is contingent primarily upon its adaptations to the
interactive biotic and abiotic components of the multifactorial
environment. This role is manifest in the contribution the
species makes both to the sum total of transfers of resources
within the ecosystem and to the nature of the physical and
chemical environment. These contributions can be to both
the conservative energy-matter transfers of the ecosystem and
to the information network which regulates ecosystem functioning.
3. This study set out, using Tete pan as the main field site, to:
i) Characterise the major physical, chemical and biotic
variables of the floodplain ecosystem which are most likely
to affect the acquisition of resources by P. crispus.
ii) Determine the way the species influences the environment.
iii) Assess the species adaptations to environmental characteristics.
iv) Assess transfers of resources from P. crispus to other
ecosystem components, the factors affecting them and the
contributions they make to ecosystem structure and
functioning.
v) Outline the implications of the findings for management of
the ecosystem which is threatened by the construction of an
upstream dam. Annual flooding of the Pongolo river and resultant poor light
climate and fluctuating water levels create an unfavourable
habitat for submerged plant growth in Tete pan during summer.
During autumn through spring, however, stable water levels and
good light penetration are favourable for plant growth. The
duration of favourable and unfavourable periods varies annually,
and on a longer time scale periodic droughts cause the pan to dry
up completely. Nutrient concentrations and salinity of the water
were favourable for freshwater plant growth during the study.
5. Potamoget on crispus on the Pongolo r.iver floodplain is a winter
producing annual which reproduces both sexually (achenes) and
asexually (turions). Winter growth is initiated by turion
germination each year and achenes ensure survival after drought.
If floods continue into autumn young plants remain dormant until
the light climate improves. Individual plants have a life span
of 4 - 5 months but staggered germination of turions from April
to June ensured that the population was present for up to 8
months. Pot amogeton crispus was classified as a competitiveruderal
with multiple regenerative strategies. This life history
enables the species to exploit the favourable period between floods
and survive both summer flooding of unpredictable duration and
intermittent droughts.
6. The major source of nutrfents for P. crispus is the sediment pool
and plant growth brings large amounts of nitrogen and phosphorus
into circulation each year. The rapid completion of the life
cycle ensures that in most years Nand P released to the water
during decay are taken up by other components of the ecosystem
before the next summer floods. Thus, despite a potential for
flushing of nutrient rich water by floods, pans such as Tete
experienced a net gain in nutrients between flood periods during
the study period.
7. Potamogeton crispus was the major primary producer in Tete pan
during this study and although production was seasonal it was
high (126 gm¯³ in comparison to other species reported in the
literature. The presence of an abundant and nutritious food source in the
form of P. crispus turions, attracts a large and diverse
waterfowl population to the floodplain each winter. During
feeding the waterfowl uproot P. crispus plants and remove almost
90% of the turions. This represents a potentially unstable
grazing system as the birds utilize the "residual growth potential"
of the plant population. The hypothesis was formulated that
stability of the grazing system and maintenance of P. crispus
reproduction may be achieved by production of small turions
which are energetically unrewarding for the waterfowl but
form viable propagules. Overall, transfer of nutrients and
organic matter to this grazing food chain represents a small
(<10 % ) proportion of P. crispus production.
9. The growth of P. crispus in Tete pan provides a large surface
area for epiphyton growth which increases in density and species
diversity as plants age. Experiments showed that the host
plant epidermis was invaded by necrotrophic bacteria but grazing
of epiphyton by snails retarded this process in young leaves.
However, as leaves aged the action of the bacteria softened
(conditioned) the host tissues and promoted consumption of the
leaves by snails. The co-existence of epiphyton and host plant
is therefore achieved through a complex set of interacting
attributes of the epiphyton community, the host plant and
grazing animals. Comparisons with previously published
hypotheses of the macrophyte-host relationship indicated that
attempts to find a single controlling factor may be misleading
and that adaptation for co-existence should be sought in a number
of interacting variables.
10. Since conditioning of P. crispus by epiphytes and consumption
of plants by snails follows a succession which reaches a climax
during host senescence, it was important to develop a method of
studying decomposition which included these processes. Experiments
showed that both snails and bacterial conditioning had marked
effects on the rates and pathways of dry matter and nutrient
transfer during decomposition and that no single technique
(eg. litter bags) will permit a thorough understanding of
decomposition processes. Conservative transfers of organic matter, nitrogen and phosphorus
in Tete pan are dominated by the detrital food web with only a
small proportion going to the grazing food chain. The consumption
of P. crispus and its epiphyton by snails in winter and
spring provides the major pathway of allochthonous production to
the benthic detrital aggregate and detrital food web. These
transfers are separated in both time and space from the allochthonous
inputs of C. dacty lon detritus which occur as a result
of decay in the flooded pan margins during summer. The major
input of phosphorus to the pans appears to occur by deposition
of silt during floods but fixation of di-nitrogen by the
epiphyton and in the detrital aggregate greatly exceed flood
related inputs of nitrogen.
12. The continued functioning of the detritus dominated foodweb of
the floodplain pans is dependent upon a sequence of resource
transfers which are dominated, during periods between floods,
by P. crispus and its epiphyton. The processes which regulate
these conservative transfers show characteristics typical of the
information network of cybernetic ecosystem functioning and
hence it was hypothesised that the Pongolo floodplain ecosystem
has the potential to buffer changes in resource input which will
occur now that a dam has been constructed upstream. While the
hydrological regime of the Pongolo river forms the primary
determinant of the floodplain ecosystem, structure and function
management of the system must also account for the "managerial"
role of animals and temperature which initiate and rate regulate
major ecosystem processes.
Description
Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1984.
Keywords
Ecology--KwaZulu-Natal--Pongolo River Watershed., Aquatic plants--KwaZulu-Natal., Potamogeton., Theses--Botany.