Impacts of climate variability and hydrological modifications on cyanobacteria potentials in aquaculture systems in the Ngerengere Catchment, Morogoro, Tanzania

Abstract

The scientific consensus is that the global increase in cyanobacterial harmful algal blooms (CyanoHABs) is attributed to eutrophication, hydrological variations, and anthropogenic activities compounded by climate change. Cyanobacteria are known to produce a wide range of toxins (cyanotoxins) and hypoxia conditions that can alter the food web. Besides the fact that Tanzania is rich in water resources such as the ocean, lakes, rivers, dams, and ponds and shreds of evidence of environmental constraints, climate and hydrological variations (episodic events and altered river flows), on HABs, much is yet to be done. Their growth is not spatially homogenous, and that there is an information gap on their dynamics. CyanoHABs have registered impacts on food security, tourism, water resources, fishery, and human health. The only recorded incidences (recurrence) are the mass fatalities of Lesser flamingo in Lake Manyara, Arusha. Some recent reports, for instance, gauged the rapid growth of the aquaculture sector as a result of livelihood diversification, but the sector is facing management and environmental constraints. Since the aquaculture farms/fishponds are hydrologically connected with the domestic water reservoirs in the Ngerengere catchment, there is a need for a thorough analysis. The study explored the impacts of climate variability and hydrological modification on cyanobacteria potentials in aquaculture systems in the Ngerengere catchment in Morogoro, in the United Republic of Tanzania. In this study, a review of the literature (i.e., historical reconstruction) on CyanoHABs occurrences in Tanzania and their link with climate and hydrological variation was conducted. A stakeholders’ perception and experience (questionnaire, focus group discussions, and key informant interviews) and anecdotal observations were also conducted and examined. Water samples were collected and analyzed for the identification and characterization of common species of cyanobacteria and in-situ measurements of physicochemical characteristics. Also, case studies were also diagnosed to examined how CyanoHABs link with other key environmental observations. The study also involved obtaining online data for chlorophyll-a to analyze their trends and how they are teleconnected with climate and hydrological variation in Mindu, Dam situated in the Ngerengere catchment. The methods and procedures followed in the study are detailed under the specific chapters. The study found that on HABs, the field is still at its nascent stage in Tanzania, and research in the field is lagging. Most studies are events driven, for example, post-mortem analysis and that there is no limnological data to perform time series analysis. Regarding the survey on stakeholders’ perception of CyanoHABs in the Ngerengere catchment, 95% of the respondents could recognize blooms as displayed to them, with 70% noting that algal blooms proliferate more during the dry season. On the other hand, respondents were uncertain about any health effects associated with blooms. During the survey, farmers revealed that they sometimes feel itching during and after fishing, which is linked to toxic effects from CyanoHABs. The morphological assessment shows the occurrences of common species of CyanoHABs in the Ngerengere catchment, such as Microcystis, Cylindrospermopsis, Anabaena, Lyngbya, as well as other species such as diatoms and Euglenophytes, which at times might be a nuisance to the environment. It was further observed that colony-forming cyanobacteria dominated the fishponds while filamentous species were dominant in Mindu Dam (a domestic water supply in the catchment). However, the eutrophic state of the sampling points varied from eutrophic to hypereutrophic state for all the sampling schedules. Moreover, Mindu Dam was eutrophic, becoming hypereutrophic toward the end of sampling (September 2018, usually the dry season). During the study period, two unique cases, namely, heavy rainfall, which caused flash flood (in this study defined as an episodic hydrological event) and unusual observation of reddish colouration (red algae) fishponds at Kingolwira National Fish Farming Centre were gauged and examined. The heavy rainfall event happened between the first (October 2017) and the second (February 2018) sampling phases, which showed to be affecting the equivalent spherical diameter (ESD) and area-based diameter (ABD) of the community structure. The heavy rain phenomenon was localized (enhanced by the orographic nature of the place) but also steered by the presence of the Intertropical Convergence Zone (ITCZ) and the tropical cyclone Berguitta (which was the dominant weather over the Indian Ocean). Regarding the unusual observation of reddish bloom in the fishpond, the associated physicochemical characteristics during the event were also studied. To the best of our knowledge, the observed red algae is the first to be reported in fishponds in Tanzania. These results show that the fishponds were significantly (p<0.05) different from each other except for water temperatures. Microcystis and euglenophytes species respectively dominated non-reddish and reddish fishponds. The study findings demonstrate the dynamics of harmful cyanobacterial blooms in iv the study area. The use of case studies and observations synergistically can be utilized in understanding the local context hence studying the dynamics of CyanoHABs. A retrospective analysis of chlorophyll-a was conducted in Mindu Dam, which demonstrated increasing Chlorophyll-a trends (defined as a chlorophyll-a index and a ratio between bands characteristics using Landsat 7 surface reflectances), minimum and maximum temperatures, and solar radiation. While rainfall trends were neutral, wind speed and directions and water levels for the Mindu Dam showed a significant decreasing trend with time. However, there was a strong correlation between wind speed and maximum temperature but weak with minimum temperature. There was also a significant weak correlation between Nino-3.4 monthly rainfall as well as water levels. The study area is to the leeward side of the Uluguru mountains. The patterns suggest links and causality between the CyanoHABs variations and meteorological parameters such as temperatures, solar radiations, and water levels. Findings in this study contribute to the understanding of CyanoHABs with climate and hydrological variation in a region in the face of data paucity. The study further provides an insight into state of the art, cause of CyanoHABs, and their link with climate and hydrological variation hence informing policies and practices and professional development. The findings also provide a basis for the development of the CyanoHABs management framework and health risk assessment.