Limnology of southern African coastal lakes - new vistas from Mozambique

摘要

Fourteen mostly small, isolated, and relatively deep coastal inundation lakes were surveyed during the cool season (August) of 1996. No thermal or chemical stratification existed, but near-bottom hypoxia was evident in several systems. Ionic conductivity varied quite widely, but was highest in isolated systems, suggesting evaporative dominance. Low water transparency, attributable to biogenic turbidity, was reflected in high chlorophyll levels (mean = 28.3mug l-1) in most lakes. However, maximal penetration of red light in six lakes implies the influence of mineral turbidity - dystrophy being excluded by the alkaline character (pH > 8) of all 14 takes. Unusually high standing stocks of zooplankton (mean = 604mg m-2 DW), mostly small-bodied freshwater species (rotifers, cyclopoids, cladocerans) as well as larger calanoid and cyclopoid copepods were present, consistent with algal indications of eutrophy. Characteristically relict estuarine/marine fauna (a calanoid copepod, crown crab, amphipod and fish) were present in two oligotrophic lakes, including the largest - putatively a drowned-valley lake. Significant contrasts between these Chidenguele lakes and other southern African coastal lakes are evaluated and discussed. Artisanal fisheries, targetting two indigenous freshwater cichlids and a clariid, as well as an estuarine ambassid and gobiid, were active. Survey findings were used to assess the suitability of these lakes as prospective habitats for exotic grass and silver carp. As these lakes support artisanal fisheries based on indigenous taxa the need for and wisdom of alien introductions is challenged, and we call for parallel summer surveys to precede any such decision. Evidence at hand indicates the potential suitability of more lakes for silver than for grass carp, but four lakes were identified as meriting special conservation status, in view of certain unique attributes and/or values.