Aquatic macrophytes in a tropical African reservoir: diversity, communities and the impact of reservoir-level fluctuations

ABSTRACT

Variation in species composition, distribution and abundance of macrophytes in the Malilangwe reservoir, a tropical African reservoir, was investigated at four sites in three seasons; March (hot-wet season), June (cool-dry season) and September (hot-dry season) over a nine-month period. Thirteen macrophyte species, representing nine families, were recorded during the study period. Submerged macrophytes had a patchy distribution and the mean transect cover per species was below 3% with the exception of Ceratophyllum demersum and Potamogeton pusillus. Canonical Correspondence Analysis (CCA) identified four distinct macrophyte groups closely associated with the three seasons; hot-wet, cool-dry and hot-dry and environmental factors; pH, dissolved oxygen, phosphorus and water level. Change of seasons and subsequent fluctuations in water levels resulted in changes in macrophyte community structure from the dominant emergent macrophytes (Cyperus involucratus, Panicum repens, Ludwigia stolonifera, Phragmites mauritianus and Schoenoplectus corymbosus) in the hot-wet season to submerged macrophytes (Najas horrida, Potamogeton crispus, Potamogeton pusillus, Potamogeton tricarinatus and Persicaria decipiens) in the hot-dry season. Although changes in water level variations seemed to influence macrophyte structure and composition changes, further research is needed to evaluate to what extent water level fluctuations interact with other seasonal factors acting independently.

Keywords:

• Malilangwe
• macrophytes
• dissolved oxygen
• water level fluctuations
• pH

ACKNOWLEDGEMENTS

This study was made possible through the financial support of the DAAD (A/10/02914) and Malilangwe Trust Research Grant. Special thanks go to Philemon Chivambu, Patrick Mutizamhepo, Clemence Chakuya and Pandeni Chitimela who assisted during the field programme. Our appreciation goes to Mrs Elizabeth Munyoro and the technical staff of the Department of Biological Sciences, University of Zimbabwe, for all their technical support during the study. We thank Mark Hyde and Bart Wursten of Zimbabwe Flora for help with macrophyte identification.