Browsing by Author "Rohrlack, Thomas"
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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 Microcystin concentrations in Nile tilapia (Oreochromis niloticus) caught from Murchison Bay, Lake Victoria and Lake Mburo: Uganda(Hydrobiologia, 2010) Semyalo, Ronald; Rohrlack, Thomas; Naggawa, Christine; Nyakairu, George W.Catches of the economically important Nile tilapia (Oreochromis niloticus L.) from two eutrophic tropical lakes in Uganda, Lake Mburo and Murchison Bay, Lake Victoria, were examined to determine the presence of microcystins (MCs) in gut, liver and muscle of the fish. Analysis for MCs (RR, LR and YR) in both fish and water samples was by liquid chromatography coupled with mass spectroscopy (LC-MS) method. Physico-chemical parameters were also measured to establish the status of both lakes. MC-RR was the most prominent MC detected in Lake Mburo and Murchison Bay samples, there was no evidence of significant seasonal variation in the concentration of MCs in fish tissue. MCs were detected in all water samples from both study lakes. The mean concentration of MCs in water was found higher in dry times for Lake Mburo (P\0.05) and higher in wet times for Murchison Bay (P\0.05). MC concentrations in the fish guts were positively related with MC concentrations in water samples from Murchison bay (P\0.05), no such correlation was found in Lake Mburo. In eutrophic tropical lakes, fish seem to have a high tolerance to the toxicity of cyanotoxins including MCs. However, there is a possibility of accumulating these toxins in their tissue with the threat of transferring them higher up in the food chain. Due to a low sample size and short sampling period, the results can only serve to highlight the potential risk of MC accumulation in Nile tilapia in these lakes. Further studies are needed for the purpose of risk assessment.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