Phytoplankton community structure and cyanobacterial blooms in reservoirs in the semi-arid highlands of Tigray, Ethiopia

Ethiopia is the third largest country in Africa with an area of over one million km2. It is one of the most populous countries in Africa (probably second), with more than 80 million inhabitants. The Ethiopian highlands represent about 43% of the country but support about 88% of the population. The highlands account for 95% of the regularly cropped land, more that 70% of the livestock population, and 90% of the economic activities of the country. They are considered to be amongst the most degraded lands in Africa by some authors. Rain fed agriculture is the main stay for most farmers. The frequent rainfall anomalies suggest that there are recurrent periods of drought every 3- 5 years in the northern parts of Ethiopia and every 6-8 years over the whole country. Part of the variability in the seasonal and annual rainfall across time and space is known to be associated with the El Nino-Southern Oscillation (ENSO) phenomenon. The conditions in Tigray are worse. Extreme spatial and temporal variation in rainfall is characteristic for this region. To tackle the problem associated with the rain fall pattern, several small reservoirs have been constructed over the last two decades. Given the population intensity and long history of agriculture in the highlands of Ethiopia, massive erosion linked to land degradation is a prominent problem. This is expected to bring excessive nutrient to the reservoirs. And many of the reservoirs are expected to be characterized by high nutrient loads and phytoplankton blooms, including cyanobacteria blooms. This has indeed observed in a field survey of reservoirs, most of them suffer from heavy blooms of cyanobacteria. In this study we started with a field survey of a set of 32 shallow semi-arid sub-tropical reservoirs in the highlands of Tigray, Ethiopia. This survey was carried out in both the wet and dry season to capture seasonal variations of phytoplankton communities and associated environmental variables. We assessed seasonal variation Summary 240 in more details by monitoring eight selected reservoirs (sub sets of the 32 reservoirs) on a monthly basis during a whole year. We also carried out field enclosure experiments in an effort to better understand the trophic structure of the reservoirs and identify mechanisms that potentially lead to cyanobacterial blooms. First we tested the impact of fish on abiotic conditions in the water column as well as the dynamics of phytoplankton species composition and cyanobacteria biomass. In the second experiment we assessed the potential top-down effects of zooplankton on the phytoplankton communities including the toxic cyanobacteria. The studied reservoirs were characterized by high nutrient concentrations and high turbidity. Most of the reservoirs harbor the riverine fish Garra. Overall, the local phytoplankton richness was low with most reservoirs dominated by a single genus of cyanobacteria (mostly Microcystis), chlorophytes, euglenophytes, cryptophytes or dinophytes. Similarly the bacterial community richness in the studied reservoirs was also low. Lower bacterial taxon richness was encountered in reservoirs with Microcystis blooms than bacterial communities in reservoirs without blooms. High altitude reservoirs were more nutrient-rich and associated with high abundances of green algae, euglenophytes or cyanobacteria other than Microcystis. Microcystis was associated with high pH in the rainy and high conductivity in the dry season. Additional factors correlated with Microcystis biomass were Daphnia biomass and possibly altitude and fish biomass. Environmental factors explained the bacterial community composition differently among season. Percentage contribution of Microcystis to the total phytoplankton biomass and copepod biomass showed significant association with the bacterial community composition in the wet season whereas variation in bacterial community composition was associated with total nitrogen (TN), total phosphorus (TP), oxygen, the number of cattle frequenting the Summary 241 reservoir, and fish biomass in the dry season. Pronounced temporal variation was observed for both biotic and abiotic variables in our study systems. This variation involved both the intra-annual and interannual variations. For the intra-annual variation, the main limnological changes were associated with seasonal differences in rainfall, while also water temperature differed strongly between winter (sub-tropics) and the rest of the year. We observe two minima for phytoplankton biomass: one in winter and a more pronounced one during August. We also observed two main bloom periods for cyanobacteria: one in September-October and a more pronounced one in May-June. Seasonal variation in total phytoplankton and cyanobacterial biomass was, however, not significant. The first field enclosure experiment was the experiment with fish. The results of this experiment showed that the presence of Garra in general increased the amount of suspended matter, nutrient concentrations (total nitrogen and total phosphorus), phytoplankton and to some extent also Microcystis biomass (including the proportion of Microcystis in the phytoplankton community), and reduced water transparency. The second experiment was carried out to study the effect of zooplankton grazing on phytoplankton community structure, including the relative abundance of toxic cyanobacteria. From this experiment top-down regulation by zooplankton was observed for some of the phytoplankton taxa, including Anabaena, Euglenoids, Chlorophytes and Cryptomonads, whereas the impact of the presence of zooplankton on Microcystis and Peridinum biomass was limited. From the same experiment we observed negative correlation between Anabaena and calanoid copepods and Daphnia carinata. We also detected microcystin from all experimental units in the second experiment; and higher concentrations were detected in the treatment with than without zooplankton. Summary 242 We draw some important associations from the field observations and field enclosure experiments for Microcystis in our system. The results indicate an inter-play between bottom-up (possibility of Microcystis affecting the zooplankton composition) and top-down (zooplankton grazing) regulating the Microcystis. Negative association between Microcystis and Daphnia has been observed mainly from the field survey and to some extent the enclosure experiments with fish and fishless treatment demonstrated a top-down regulation. From these results we can conclude that the zooplankton grazing can not fully regulate Microcystis. But, the possibility remains, and is in fact quite realistic, that Daphnia also control Microcystis, mainly at lower biomasses of Microcystis until it reaches a certain biomass and Microcystis “escapes” potential control by zooplankton after which it may poison the major grazer zooplankton due to the high densities. Based on our results of the present study, we put some general suggestions and recommendations for sustainable utilization, maintaining the ecological integrity of the reservoirs and protecting water quality deterioration. The recommendations follow in the following statements. 1) Cyanobacterial monitoring and survey for hazardous effects to assess if the toxins of the organisms are translated into problems of animal or human health should be set-up. 2) Reduction of nutrient loading and sediment input to the reservoirs to curb the eutrophication of the reservoirs. Catchment treatment with reforestation and setting up of exclosures can serve the purpose. 3) Reduction of cattle trampling by restricting cattle access to the reservoirs at selected sites of the reservoir. 4) Reduction of fish (mainly Garra). Here we recommend the use of methods to reduce the riverine fish with a high level care to protect the reservoirs from unpredictable consequences like the introduction of exotic fish

Influences of Pacific Island human communities on benthic coral reef functioning and resilience

A multitude of local and global stressors are threatening the diversity and productivity of coral reef ecosystems within the current era of the Anthropocene. While the effects of global stressors on coral reefs are relatively well understood, the role of various local human impacts and their interaction with global stressors remains under debate. By using a combination of observational-, theoretical- and secondary data-based approaches, this thesis aimed to improve understanding of relationships between local human impacts and benthic coral reef communities in the understudied Pacific Island region. Particularly, it addressed how various levels and types of local impacts can directly and indirectly influence benthic coral reef functioning and in turn future resilience to global stressors

Climate change impacts on lakes: an integrated ecological perspective based on a multi-faceted approach, with special focus on shallow lakes

Freshwater ecosystems and their biodiversity are presently seriously threatened by global development and population growth, leading to increases in nutrient inputs and intensification of eutrophication-induced problems in receiving fresh waters, particularly in lakes. Climate change constitutes another threat exacerbating the symptoms of eutrophication and species migration and loss. Unequivocal evidence of climate change impacts is still highly fragmented despite the intensive research, in part due to the variety and uncertainty of climate models and underlying emission scenarios but also due to the different approaches applied to study its effects. We first describe the strengths and weaknesses of the multi-faceted approaches that are presently available for elucidating the effects of climate change in lakes, including space-for-time substitution, time series, experiments, palaeoecology and modelling. Reviewing combined results from studies based on the various approaches, we describe the likely effects of climate changes on biological communities, trophic dynamics and the ecological state of lakes. We further discuss potential mitigation and adaptation measures to counteract the effects of climate change on lakes and, finally, we highlight some of the future challenges that we face to improve our capacity for successful prediction

Profiling an invader - is the invasive cyanobacterium Raphidiopsis raciborskii on the path to ecological dominance in Australia in the context of environmental change?

Biological invasions are commonly reported ecological phenomena and are universally accepted as symptoms of the Anthropocene. Microbial invaders are particularly difficult to study, but potentially represent the most serious group of invaders, due to their extremely rapid responses to changing conditions. In this group, phytoplankton are perhaps the most significant; they are responsible for most of the primary production in aquatic ecosystems and are therefore the cornerstone of aquatic food webs. However, they can also produce toxins that pose threats to water quality, particularly when they form extensive blooms. One species exhibiting invasive behaviour is Raphidiopsis raciborskii. Known for its production of potentially lethal toxins, and flexible physiology, it favours warming temperatures and stratification regimes. In Australia, these conditions are becoming more prevalent, and with the species already having a foothold in many Australian ecosystems, it may continue to spread and impact ecosystem servies and stability. In this thesis, we use a combination of historical, theoretical, laboratory, and monitoring data studies to examine the factors behind the success of R. raciborskii in the context of Australian ecosystems. This research provides novel contributions to the field by demonstrating that ‘invasiveness’ is a symptom linked to environmental change, that R. raciborskii exhibits metabolic plasticity under various conditions, that R. raciborskii may be implicated in changing the bacterial community structures of freshwater systems, and that the historic presence of R. raciborskii in Australian environments reinforces its theoretically and experimentally purported environmental niche. This knowledge facilitates discussion of Australian water security and ecological health, and the critical importance of phytoplankton communities, in the face of an increasing population and cascading environmental change

Satellite sensor requirements for monitoring essential biodiversity variables of coastal ecosystems

© The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Ecological Applications 28 (2018): 749-760, doi: 10.1002/eap.1682.The biodiversity and high productivity of coastal terrestrial and aquatic habitats are the foundation for important benefits to human societies around the world. These globally distributed habitats need frequent and broad systematic assessments, but field surveys only cover a small fraction of these areas. Satellite‐based sensors can repeatedly record the visible and near‐infrared reflectance spectra that contain the absorption, scattering, and fluorescence signatures of functional phytoplankton groups, colored dissolved matter, and particulate matter near the surface ocean, and of biologically structured habitats (floating and emergent vegetation, benthic habitats like coral, seagrass, and algae). These measures can be incorporated into Essential Biodiversity Variables (EBVs), including the distribution, abundance, and traits of groups of species populations, and used to evaluate habitat fragmentation. However, current and planned satellites are not designed to observe the EBVs that change rapidly with extreme tides, salinity, temperatures, storms, pollution, or physical habitat destruction over scales relevant to human activity. Making these observations requires a new generation of satellite sensors able to sample with these combined characteristics: (1) spatial resolution on the order of 30 to 100‐m pixels or smaller; (2) spectral resolution on the order of 5 nm in the visible and 10 nm in the short‐wave infrared spectrum (or at least two or more bands at 1,030, 1,240, 1,630, 2,125, and/or 2,260 nm) for atmospheric correction and aquatic and vegetation assessments; (3) radiometric quality with signal to noise ratios (SNR) above 800 (relative to signal levels typical of the open ocean), 14‐bit digitization, absolute radiometric calibration <2%, relative calibration of 0.2%, polarization sensitivity <1%, high radiometric stability and linearity, and operations designed to minimize sunglint; and (4) temporal resolution of hours to days. We refer to these combined specifications as H4 imaging. Enabling H4 imaging is vital for the conservation and management of global biodiversity and ecosystem services, including food provisioning and water security. An agile satellite in a 3‐d repeat low‐Earth orbit could sample 30‐km swath images of several hundred coastal habitats daily. Nine H4 satellites would provide weekly coverage of global coastal zones. Such satellite constellations are now feasible and are used in various applications.National Center for Ecological Analysis and Synthesis (NCEAS); National Aeronautics and Space Administration (NASA) Grant Numbers: NNX16AQ34G, NNX14AR62A; National Ocean Partnership Program; NOAA US Integrated Ocean Observing System/IOOS Program Office; Bureau of Ocean and Energy Management Ecosystem Studies program (BOEM) Grant Number: MC15AC0000

Satellite Sensor Requirements for Monitoring Essential Biodiversity Variables of Coastal Ecosystems

The biodiversity and high productivity of coastal terrestrial and aquatic habitats are the foundation for important benefits to human societies around the world. These globally distributed habitats need frequent and broad systematic assessments, but field surveys only cover a small fraction of these areas. Satellite-based sensors can repeatedly record the visible and near-infrared reflectance spectra that contain the absorption, scattering, and fluorescence signatures of functional phytoplankton groups, colored dissolved matter, and particulate matter near the surface ocean, and of biologically structured habitats (floating and emergent vegetation, benthic habitats like coral, seagrass, and algae). These measures can be incorporated into Essential Biodiversity Variables (EBVs), including the distribution, abundance, and traits of groups of species populations, and used to evaluate habitat fragmentation. However, current and planned satellites are not designed to observe the EBVs that change rapidly with extreme tides, salinity, temperatures, storms, pollution, or physical habitat destruction over scales relevant to human activity. Making these observations requires a new generation of satellite sensors able to sample with these combined characteristics: (1) spatial resolution on the order of 30 to 100-m pixels or smaller; (2) spectral resolution on the order of 5 nm in the visible and 10 nm in the short-wave infrared spectrum (or at least two or more bands at 1,030, 1,240, 1,630, 2,125, and/or 2,260 nm) for atmospheric correction and aquatic and vegetation assessments; (3) radiometric quality with signal to noise ratios (SNR) above 800 (relative to signal levels typical of the open ocean), 14-bit digitization, absolute radiometric calibratio

Satellite sensor requirements for monitoring essential biodiversity variables of coastal ecosystems

The biodiversity and high productivity of coastal terrestrial and aquatic habitats are the foundation for important benefits to human societies around the world. These globally distributed habitats need frequent and broad systematic assessments, but field surveys only cover a small fraction of these areas. Satellite-based sensors can repeatedly record the visible and near-infrared reflectance spectra that contain the absorption, scattering, and fluorescence signatures of functional phytoplankton groups, colored dissolved matter, and particulate matter near the surface ocean, and of biologically structured habitats (floating and emergent vegetation, benthic habitats like coral, seagrass, and algae). These measures can be incorporated into Essential Biodiversity Variables (EBVs), including the distribution, abundance, and traits of groups of species populations, and used to evaluate habitat fragmentation. However, current and planned satellites are not designed to observe the EBVs that change rapidly with extreme tides, salinity, temperatures, storms, pollution, or physical habitat destruction over scales relevant to human activity. Making these observations requires a new generation of satellite sensors able to sample with these combined characteristics: (1) spatial resolution on the order of 30 to 100-m pixels or smaller; (2) spectral resolution on the order of 5 nm in the visible and 10 nm in the short-wave infrared spectrum (or at least two or more bands at 1,030, 1,240, 1,630, 2,125, and/or 2,260 nm) for atmospheric correction and aquatic and vegetation assessments; (3) radiometric quality with signal to noise ratios (SNR) above 800 (relative to signal levels typical of the open ocean), 14-bit digitization, absolute radiometric calibration \u3c2%, relative calibration of 0.2%, polarization sensitivity \u3c1%, high radiometric stability and linearity, and operations designed to minimize sunglint; and (4) temporal resolution of hours to days. We refer to these combined specifications as H4 imaging. Enabling H4 imaging is vital for the conservation and management of global biodiversity and ecosystem services, including food provisioning and water security. An agile satellite in a 3-d repeat low-Earth orbit could sample 30-km swath images of several hundred coastal habitats daily. Nine H4 satellites would provide weekly coverage of global coastal zones. Such satellite constellations are now feasible and are used in various applications

Satellite sensor requirements for monitoring essential biodiversity variables of coastal ecosystems.

The biodiversity and high productivity of coastal terrestrial and aquatic habitats are the foundation for important benefits to human societies around the world. These globally distributed habitats need frequent and broad systematic assessments, but field surveys only cover a small fraction of these areas. Satellite-based sensors can repeatedly record the visible and near-infrared reflectance spectra that contain the absorption, scattering, and fluorescence signatures of functional phytoplankton groups, colored dissolved matter, and particulate matter near the surface ocean, and of biologically structured habitats (floating and emergent vegetation, benthic habitats like coral, seagrass, and algae). These measures can be incorporated into Essential Biodiversity Variables (EBVs), including the distribution, abundance, and traits of groups of species populations, and used to evaluate habitat fragmentation. However, current and planned satellites are not designed to observe the EBVs that change rapidly with extreme tides, salinity, temperatures, storms, pollution, or physical habitat destruction over scales relevant to human activity. Making these observations requires a new generation of satellite sensors able to sample with these combined characteristics: (1) spatial resolution on the order of 30 to 100-m pixels or smaller; (2) spectral resolution on the order of 5 nm in the visible and 10 nm in the short-wave infrared spectrum (or at least two or more bands at 1,030, 1,240, 1,630, 2,125, and/or 2,260 nm) for atmospheric correction and aquatic and vegetation assessments; (3) radiometric quality with signal to noise ratios (SNR) above 800 (relative to signal levels typical of the open ocean), 14-bit digitization, absolute radiometric calibration <2%, relative calibration of 0.2%, polarization sensitivity <1%, high radiometric stability and linearity, and operations designed to minimize sunglint; and (4) temporal resolution of hours to days. We refer to these combined specifications as H4 imaging. Enabling H4 imaging is vital for the conservation and management of global biodiversity and ecosystem services, including food provisioning and water security. An agile satellite in a 3-d repeat low-Earth orbit could sample 30-km swath images of several hundred coastal habitats daily. Nine H4 satellites would provide weekly coverage of global coastal zones. Such satellite constellations are now feasible and are used in various applications