Spatial Patterns of Zooplankton Distribution and Abundance
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SPATIAL PATTERNS OF ZOOPLANKTON DISTRIBUTION AND ABUNDANCE IN RELATION TO PHYTOPLANKTON, FISH CATCH AND SOME WATER QUALITY PARAMETERS AT SHIRATI BAY, LAKE VICTORIA-TANZANIA Revania K. Waya1§, Samwel M. Limbu3, Godfrey W. Ngupula2, Chacha J. Mwita3 and Yunus D. Mgaya3* 1Tanzania Fisheries Research Institute, P. O. Box 46, Shirati, Tanzania 2Tanzania Fisheries Research Institute, P. O. Box 475, Mwanza, Tanzania 3Department of Aquatic Sciences and Fisheries, University of Dar es Salaam, P. O. Box 35064, Dar es Salaam, Tanzania *Corresponding author, e-mail: [email protected] §The writing of this paper was completed posthumously. ABSTRACT Spatial patterns and abundance of zooplankton in aquatic habitats are important determinants for production of fish species, invertebrates and availability of phytoplankton. Weekly monitoring for zooplankton abundance was conducted in Shirati Bay, Lake Victoria, to explore their spatial patterns in relation to phytoplankton, fish catch and some water quality parameters. The vertical distribution of zooplankton was generally higher close to the bottom as compared to surface waters of the lake. Zooplankton vertical distribution positively correlated with water transparency (r = 0.680, p = 0.011). The horizontal abundance of zooplankton was not significantly different amongst the three stations (p = 0.5143). While Copepoda was the dominant group in terms of composition, Rotifera had the highest diversity indices of all the zooplankton groups obtained. The abundance of nauplius larvae was significantly higher than that of the copepodites (p = 0.022). Nile perch, Lates niloticus dominated the total catches (47%) followed by Nile tilapia, Oreochromis niloticus (29%) and haplochromines (21%). The abundance of haplochromines and juvenile fishes correlated significantly with the abundance of zooplankton (r = 0.856, p = 0.002 and r = 0.58, p = 0.038, respectively). The current study revealed that zooplankton vertical distribution at Shirati Bay is mainly controlled by water transparency and predation by juvenile Nile perch, Nile tilapia and haplochromines. Keywords: chlorophyll-a; calanoid; cyclopoid; nauplius larvae; juvenile fish INTRODUCTION (Mwebaza-Ndawula 1994, Sitoki et al. 2010, Lake Victoria has received an exceptional Vincent et al. 2012, Ngupula 2013). consideration because of its changes in Eutrophication and pollution are known to limnological conditions caused by nutrients affect the structure of the lake communities inputs and regional climate change (Waya and by modifying its invertebrate species Mwambungu 2004, Ojwang et al. 2014). The composition, distribution and abundance bays of the lake are highly impacted by patterns which in turn affect their overall eutrophication and pollution due to the ecological functions (Mwebaza-Ndawula chronic release of water containing higher 1994, Ngupula et al. 2010). levels of nutrients from urban/industrial effluents and agricultural activities conducted Zooplankton are a major constituent of along the shoreline and in nearby catchments freshwater ecosystems, interceding the energy 20 Tanz. J. Sci. Vol. 40, 2014 flow between phytoplankton communities and flourishing fish and invertebrate productivity higher trophic levels, such as fish in pelagic of the lake. Unpacking the effects of different habitats (Kelly et al. 2013). The distribution factors that control spatial distribution and and abundance of zooplankton are abundance of zooplankton remains an heterogeneous (Castro et al. 2007, Mulimbwa important challenge for the ecological et al. 2014). In most aquatic ecosystems, the scientists working in freshwater ecosystems spatial distribution and abundance of including Lake Victoria (Lévesque et al. zooplankton are characterized by both vertical 2010). This study, determined the patterns of and horizontal patterns (Yurista and Kelly spatial distribution (vertical and horizontal) 2009). These patterns are important indicators and abundance of zooplankton at Shirati bay, of the ecological status of aquatic ecosystems Lake Victoria (Tanzania) in relation to and thus directly affecting human interests phytoplankton, fish catch, water temperature, (Dietzel et al. 2013). Notably, spatial patterns conductivity and transparency. Understanding and zooplankton abundance determine the the distribution patterns and abundance of production of fish species, invertebrates and zooplankton is an important step in predicting availability of phytoplankton (Casper and their influence on phytoplankton and fish Thorp 2007). production under the changing lake environments. The zooplankton patterns and abundance in large lakes vary spatially (Lévesque et al. MATERIALS AND METHODS 2010). They are affected by many factors Study area including water depth (Irvine 1995), water Water quality parameters, chlorophyll-a, transparency (Semyalo et al. 2009), zooplankton and fish samples were collected conductivity, wastewater discharged into on a weekly basis (March 2005 to March lakes, herbicides and nutrients produced from 2006) from three stations (Nyangoge, 15 m agricultural activities which diffuse into lakes deep; Nyasobu, 5 m deep; and Michire, 7 m either directly through run-off or indirectly deep) at Shirati Bay, located at latitude 1° 09' via polluted rivers (Dietzel et al. 2013). 00" S and longitude 33° 58' 00" E in the Predation by fishes and other invertebrates, as Tanzanian waters of Lake Victoria near the well as availability of algal biomass further border with Kenya (Figure 1). On each modify zooplankton patterns, especially in sampling occasion, almost similar stations shallow lakes that experience constant mixing were visited using the aid of a Global and have uniform temperature and food Positioning System (GPS) model GPSMAP gradients (Semyalo et al. 2009), therefore, the 62S, made in USA. Within each sampling zooplankton distribution patterns and station, samples were taken from three points abundance are a function of physico-chemical and afterwards their mean was estimated as a and biological processes (Semyalo et al. 2009, single value for the station on each sampling Oyoo-Okoth et al. 2011). occasion. The choice of the above sites was done strategically to include varying depths to Spatial patterns and abundance of capture zooplankton variation with depths. zooplankton distribution are an inherent The climate of Lake Victoria basin is characteristic of pelagic freshwater characterized by two distinct seasons; a dry ecosystems. Advancement in understanding season from July to September and a wet spatial patterns of zooplankton distribution in season with two rainfall maxima, i.e., short relation to changes in Lake Victoria rains from October to December and long ecosystem (Ngupula et al. 2010) has lagged rains from March to May. behind despite their role in supporting the 21 Waya et al. – Spatial patterns of zooplankton distribution … Figure 1: A map of Shirati Bay showing the three sampling stations. Sampling of water quality parameters transportation to the Tanzania Fisheries Water temperature, pH and conductivity were Research Institute (TAFIRI) laboratory at measured using a multi-meter probe deployed Sota Centre for analysis. In the laboratory, at the positions where the zooplankton chlorophyll-a pigment was extracted samples were collected. Water transparency according to procedures recommended by was measured using a Secchi disc with a Wetzel and Likens (2000). diameter of 20 cm. Estimation of zooplankton abundance and Determination of chlorophyll-a diversity To obtain the concentration of chlorophyll-a Water samples for determination of (an indirect measure of phytoplankton zooplankton vertical distribution were taken abundance), 150 mL of water sample was at each site during the day using a one-litre filtered through membrane filters of 0.45 µm van Dorn water sampler at five-metre depth pore size. Three to four drops of magnesium intervals between 0800 and 1000 hrs. carbonate slurry were added to aid filtration Replicates of three samples from each site and prevent acid formation during extraction. were sieved through 60 µm mesh size. After filtration, the filter papers were Samples for determination of zooplankton transferred onto absorbent pads, folded in horizontal distribution were taken by making labelled aluminium foil, kept in darkened three vertical hauls at each site. Hauls were desiccators and subsequently stored in frozen made from one metre below the lake’s water conditions using a cool box prior to surface, using an open conical plankton net 22 Tanz. J. Sci. Vol. 40, 2014 with a 29 cm mouth diameter, 60 μm mesh During counting, adult copepod males and and one metre in length. The samples were females, ovigerous and non-ovigerous preserved in four percent formaldehyde and females as well as copepodite stages one to transported in a cool box to TAFIRI five and the nauplii stages were counted laboratory at Sota Centre. In the laboratory, separately in order to obtain an insight into fixed water samples were diluted and at least the different life stages of zooplankton. 10 mL of water from each sample were Abundance as individuals per cubic metre of examined for zooplankton under an inverted water and as individuals in a given one litre of microscope at 40x magnification. water are analysed using the following Zooplankton were identified using keys and formulae (IFMP 2003-2008) according to manuals by Ruttner-Kolisko (1974), implementation of the fisheries management Korovchinsky (1993) and Maas (1993). plan:- Fish composition and