Okavango Research Institute, University of Botswana, P/Bag 285, Maun, Botswana,
Environ Manage. 2013 Dec;52(6):1386-99. doi: 10.1007/s00267-013-0183-9. Epub 2013 Nov 7.
Water extraction from floodplain river systems may alter patterns of inundation of adjacent wetlands and lead to loss of aquatic biodiversity. Water reaching the Okavango Delta (Delta), Botswana, may decrease due to excessive water extraction and climate change. However, due to poor understanding of the link between inundation of wetlands and biological responses, it is difficult to assess the impacts of these future water developments on aquatic biota. Large floods from 2009 to 2011 inundated both rarely and frequently flooded wetlands in the Delta, creating an opportunity to examine the ecological significance of flooding of wetlands with widely differing hydrological characteristics. We studied the assemblages of small fishes and microcrustaceans, together with their trophic relationships, in temporary wetlands of the lower Delta. Densities of microcrustaceans in temporary wetlands were generally lower than previously recorded in these habitats. Microcrustacean density varied with wetland types and hydrological phase of inundation. High densities of microcrustaceans were recorded in the 2009 to 2010 flooding season after inundation of rarely flooded sites. Large numbers of small fishes were observed during this study. Community structure of small fishes differed significantly across the studied wetlands, with poeciliids predominant in frequently flooded wetlands and juvenile cichlids most abundant in rarely flooded wetlands (analysis of similarity, P < 0.05). Small fishes of <20 mm fed largely on microcrustaceans and may have led to low microcrustacean densities within the wetlands. This result matched our prediction that rarely flooded wetlands would be more productive; hence, they supported greater populations of microcrustaceans and cichlids, which are aggressive feeders. However, the predominance of microcrustaceans in the guts of small fishes (<20 mm) suggests that predation by fishes may also be an important regulatory mechanism of microcrustacean assemblages during large floods when inundated terrestrial patches of wetlands are highly accessible by fish. We predict that a decline in the amount of water reaching the Delta will negatively affect fish recruitment, particularly the cichlids that heavily exploited the rarely flooded wetlands. Cichlids are an important human food source, and their decline in fish catches will negatively affect livelihoods. Hence, priority in the management of the Delta's ecological functioning should be centred on minimising natural water-flow modifications because any changes may be detrimental to fish-recruitment processes of the system.
从洪泛平原河流系统中抽取水分可能会改变相邻湿地的淹没模式，导致水生生物多样性丧失。由于过度抽取水分和气候变化，到达博茨瓦纳奥卡万戈三角洲（三角洲）的水量可能会减少。然而，由于对湿地淹没与生物响应之间联系的了解不足，难以评估这些未来水资源开发对水生物群的影响。2009 年至 2011 年的大洪水淹没了三角洲中很少和经常被洪水淹没的湿地，为研究具有广泛不同水文特征的湿地洪水的生态意义提供了机会。我们研究了小型鱼类和微型甲壳类动物的集合，以及它们的营养关系，在三角洲下游的临时湿地。临时湿地中微型甲壳类动物的密度通常低于以前在这些栖息地记录的密度。微型甲壳类动物的密度随湿地类型和淹没的水文阶段而变化。在 2009 年至 2010 年洪水季节，很少被淹没的地点被淹没后，记录到了高的微型甲壳类动物密度。在这项研究中观察到大量的小鱼。在研究的湿地中，小型鱼类的群落结构差异显著，其中脂鲤科鱼类在经常被淹没的湿地中占优势，幼年慈鲷在很少被淹没的湿地中最丰富（相似性分析，P < 0.05）。小于 20 毫米的小鱼主要以微型甲壳类动物为食，这可能导致湿地内微型甲壳类动物密度较低。这一结果符合我们的预测，即很少被淹没的湿地将更具生产力；因此，它们支持更多的微型甲壳类动物和慈鲷的种群，而慈鲷是具有侵略性的食者。然而，小鱼（<20 毫米）肠道中微型甲壳类动物的优势表明，鱼类的捕食也可能是洪水期间湿地被淹没的陆地斑块高度可被鱼类利用时，对微型甲壳类动物组合的重要调节机制。我们预测，到达三角洲的水量减少将对鱼类的繁殖产生负面影响，特别是对大量利用很少被淹没的湿地的慈鲷。慈鲷是人类的重要食物来源，其鱼类捕捞量下降将对生计产生负面影响。因此，三角洲生态功能管理的重点应放在最大限度地减少自然水流的变化上，因为任何变化都可能对系统的鱼类繁殖过程造成不利影响。
