Browsing by Author "Larsson, Petter"
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Item Diel Vertical Migration of zooplankton in a eutrophic bay of Lake Victoria(Hydrobiologia, 2009) Semyalo, Ronald; Nattabi, Juliet K.; Larsson, PetterWe studied the Diel Vertical Migration (DVM) of several zooplankton taxa and an important zooplanktivore Rastrineobola argentea in a eutrophic bay of Lake Victoria for a total of 3 months during wet and dry seasons. Zooplankton were sampled twice a month at full moon and new moon. The zooplankton community of this lake was numerically dominated by cyclopoid copepods ([80%) of which 80% were juveniles. The composition of zooplankton was not significantly different (P[0.05) for the different sampling months (November 2000, March 2001 and July 2001), but total zooplankton abundance was significantly lower (P\0.05) in July (dry season) than during March or November (wet seasons). DVM behaviour was observed in some of the zooplankton groups including Tropocyclops spp., Thermocyclops spp., Thermodiaptomus galeboides and Diaphanosoma excisum. In these groups, we also observed that they were significantly (P\0.05) more abundant at full moon than at new moon in some months, a pattern not seen in non-DVM groups. The amplitude of migration tended to be higher at new moon than full moon for all DVM zooplankton but not significantly so (P[0.05). The zooplanktivore R. argentea exhibits typical DVM behaviour, perhaps to avoid visual predation itself, but it also increases the predation pressure on zooplankton in deeper waters. Feeding times for R. argentea peak just after dusk and dawn. For the copepod calanoid T. galeboides, there was a strong relationship (P\0.05) between the amplitude of migration and midday (12:00) water transparency, suggesting that these zooplankton may have descended deeper on days with higher water transparency. In the absence of temperature and food gradients but with predation pressure from a migrating zooplanktivore, the depth of descent of zooplankton may be well associated with water transparency.Item Growth and survival responses of a tropical Daphnia (Daphnia lumholtzi) to cell-bound microcystins(Journal of plankton research, 2009) Semyalo, Ronald; Rohrlack, Thomas; Larsson, PetterEutrophic tropical freshwater bodies often have cyanobacterial blooms that last throughout the year and some of these blooms may produce compounds that are toxic to Daphnia. Nevertheless, tropical species like Daphnia lumholtzi continue to remain abundant in the presence of such blooms. We conducted an experiment on the growth and survival of D. lumholtzi. We fed a toxic cyanobacteria Microcystis aeruginosa PCC 7806 to study the response of D. lumholtzi to cell-bound microcystins. Daphnia lumholtzi showed poor growth and survival in the presence of M. aeruginosa PCC 7806 both with and without microcystins. Survival and growth improved significantly when M. aeruginosa comprised ,50% of the food offered. However, regardless of the amounts of M. aeruginosa in the food, we did not find any significant difference in the growth or survival between the microcystin-containing and the microcystin-lacking treatments. We observed a significant difference in age at maturity between the microcystin-containing and the microcystin-lacking groups when the amount of M. aeruginosa in the food offered was ,50%. The toxic cyanobacteria M. aeruginosa PCC 7806 contains substances other than microcystins that reduce the growth and survival of the tropical D. lumholtzi. However, under low cyanobacteria concentrations and in the absence of microcystins, D. lumholtzi shows improved reproduction rates.Item On the diet of Nile tilapia in two eutrophic tropical lakes containing toxin producing cyanobacteria(Limnologica, 2011) Semyalo, Ronald; Rohrlack, Thomas; Kayiira, Duncan; Kizito, Yusuf S.; Byarujali, Stephen; Nyakairu, George; Larsson, PetterDue to the production of toxins, cyanobacteria may adversely affect economically important fish such as Nile tilapia Oreochromis niloticus in tropical lakes. We studied the diet composition and factors affecting the diet of Nile tilapia in two tropical lakes where cyanotoxins were present. Particle-bound microcystins were present in all analysed water samples, ranging in concentration from 0.00012 to 1.11 and from 0.006 to 0.254 mg L1 in Murchison Bay in Lake Victoria and Lake Mburo, respectively. Detritus and phytoplankton were the main dietary components of the Nile tilapia, with phytoplankton contributing to over 30% by volume of stomach contents. The cyanobacteria Microcystis spp., which are also the most likely source of microcystins in the lakes, accounted for more than 80% of ingested phytoplankton. Microcystis spp. were also the most abundant cyanobacteria in both lakes (460%). We found no significant relationship between the contribution of phytoplankton in Nile tilapia diet and the concentration of microcystins in the water but we found a close association between water transparency and the contribution of insects to Nile tilapia diets in Murchison Bay. Our results further show that none of the other measured environmental variables was a good predictor of diet items in Nile tilapia. Adult Nile tilapia in our study lakes, rely heavily on filter feeding, particularly under conditions of low water transparency, trapping detritus and phytoplankton cells especially colonies. They can ingest more mobile prey like insects and insect larvae when the water transparency and visibility increases.Item Phytoplankton dynamics and cyanobacterial dominance in Murchison Bay of Lake Victoria (Uganda) in relation to environmental conditions(Limnologica, 2011) Haande, Sigrid; Rohrlack, Thomas; Semyalo, Ronald P.; Brettum, Pal; Edvardsen, Bente; Lyche-Solheim, Anne; Sørensen, Kai; Larsson, PetterMurchison Bay is a shallow embayment in the north-western part of Lake Victoria, strongly influenced by urban pollution from the Ugandan capital Kampala. Two stations, representing the semi-enclosed innermost part of the bay and the wider outer part of the bay, were sampled in the period from April 2003 to March 2004, in order to assess the phytoplankton community and the nutrient status in the bay. Murchison Bay was highly eutrophic with average concentrations (n¼25) of total phosphorous 490 mg L1 and total nitrogen 41100 mg L1 in the inner part of the bay. The phytoplankton community was dominated by a variety of cyanobacterial species and diatoms. Cyanobacteria were dominant in the whole bay, whereas diatoms were more abundant in the outer part of the bay. Moreover, the proportion of N-fixing species like Anabaena sp. was higher in the outer part of the bay, whereas species like Microcystis sp. were more abundant in the inner part of the bay. The phytoplankton community, especially in the outer part of the bay, may be influenced by light limitation. Low NO3–N concentrations in the bay may also indicate a possible N-limitation, thus favouring growth of N-fixing cyanobacteria. The open bay is, however, a complex system, and additional environmental factors and loss processes most likely affect the phytoplankton community