The role of Potamogeton crispus L. in the Pongolo River floodplain ecosystem.

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.