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Please use this identifier to cite or link to this item: http://hdl.handle.net/123456789/394

Title: Seasonal and Interannual Variability of Pelagic Zooplankton Community Structure and Secondary Production in Lake Bosumtwi Impact Crater, Ghana
Authors: Sanful, Peter Osam
Issue Date: 18-Jul-2008
Abstract: The seasonal and interannual variability of the community structure, biomass and production of zooplankton taxa in the pelagic zone of Lake Bosumtwi were investigated from August 2004 to October, 2008. Field studies occurred biweekly at a fixed central station throughout 2005 and 2006. Diurnal vertical distribution of zooplankton was assessed in relation to vertical gradients in light, temperature, dissolved oxygen, food and predation risk. Diel vertical migration was also studied on three dates corresponding to the seasonal deep mixing, weak mixing and the stratified period which was also characterized by shifts in the maximum depth distribution of chlorophyll a concentration. The zooplankton community was species poor and comprised nine taxa namely, a dominant sole copepod, the cyclopoid Mesocyclops bosumtwii, which is a novel species and perhaps endemic to the lake. The other species included the sole cladoceran, Moina micrura, and six rotifers, Brachionus calyciflorus, Brachionus dimidiatus, Keratella cochlearis, Filinia pejleri, Filinia camascela and Hexarthra intermedia. The principal invertebrate predator was the larvae of the dipteran Chaoborus ceratopogones which possessed fairly stable population sizes in 2005 due to low pelagic fish predation pressure inferred from the low pelagic fish abundance. Adults and developmental stages of M. bosumtwii constituted 68 % on average of monthly total zooplankton abundance and together with H. intermedia contributed over 80 % of annual total community abundance. There was a weak and insignificant statistical correlation between chlorophyll a concentration and total herbivore abundance (R2=0.10, P > 0.05). Similarly, correlation analyses between grazeable phytoplankton biomass (GPB) and the biomass of major herbivores revealed that maximum herbivore biomass occurred between low to high GPB and that predation was likely holding down herbivore populations below their carrying capacities based on GPB. Total zooplankton abundance was higher during the mixing season relative to the stratified. Stage-specific species length-dry weight relationships were developed for adults and copepodites of the cyclopoid copepod, M. bosumtwii, adults of Moina micrura and larval instars and pupae of C.ceratopogones. There was no significant difference in length-dry weight relationships between the stratified and mixing seasons for Chaoborus instars (slope: P=0.78, n = 4; intercepts: P= 0.36, n = 4) and all other taxa except differences in intercepts (slopes: P= 0.43, n = 4, intercepts: P < 0.05, n =11). Mean total annual community biomass and production were 11.19 gdwm-3 and 1.59 gdwm-3y-1 respectively with M. bosumtwii and Chaoborus both contributing about 99 %. Mean community biomass was 0.5 gdwm-3 whereas mean production recorded was 0.92 gdwm-3y-1. Likewise, M. bosumtwii formed 68 % of annual community production but the contribution to community biomass was shared almost equally with Chaoborus in successive years. Mean annual community P/B ratio (biomass turnover rates) measured was 23.40, which is lower than those reported for most tropical lakes. Community P/B ratio was highest during the deep mixing period and Chaoborus contributed the least to community P/B ratio. Maximum epilimnetic chlorophyll a concentration was about 25µg/L whereas levels reached ~ 99 µg/L in the metalimnion during the seasonal deep water chlorophyll a maximum from April to July of each year. Over 90 % of each taxon with the exception of copepod nauplii (80 %) occurred in the epilimnion throughout the year except during the deep seasonal mixing when a broader vertical distribution occurred, nonetheless, high epilimnetic densities of taxa were maintained. The zooplankton in Lake Bosumtwi have individually distinct vertical distribution patterns. Vertical migration was apparent in some taxa and developmental stages of M. bosumtwii, but only Chaoborus migrated en masse, ascending into surface waters at night and descending into deeper and darker depths below 20 m during the day. The zooplankton community responds appreciably to climatic changes regarding wind generated water column mixing and precipitation. Seasonal increases in community abundance are interrupted by short term fluctuations in abundance. The intensity of mixing and the onset of seasonal maximum precipitation both influenced short term, seasonal and interannual differences in community abundance. The stable patterns in zooplankton vertical distribution may enhance effective predictions about community structure and dynamics of zooplankton in the lake. The effect of seasonal rainfall and wind generated lake mixing as prime physical factors influencing zooplankton community abundance, biomass, production and vertical distribution characterizes the zooplankton communities of many tropical lakes. A large proportion of zooplankton production may be unutilized at higher consumer levels and much of this may enter detrital pathways to feed a potentially rich microbial food web, but potentially, production is mostly lost to the deeper anoxic waters of Lake Bosumtwi. Understanding of the ecological role of zooplankton in the functioning of lake ecosystems will effectively enhance management of fisheries and additionally inform policy and decision making regarding the sustainable use and management of lake ecosystems and their resources.
Description: A Thesis submitted to the Department of Theoretical and Applied Biology, Kwame Nkrumah University of Science and Technology in partial fulfillment of the requirements for the award of Doctor of Philosophy
URI: http://hdl.handle.net/123456789/394
Appears in Collections:College of Science

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