Emma J. Rosi

The influence of a semi-arid sub-catchment on suspended sediments in the Mara River, Kenya

The Mara River Basin in East Africa is a trans-boundary basin of international significance experiencing excessive levels of sediment loads. Sediment levels in this river are extremely high (turbidities as high as 6,000 NTU) and appear to be increasing over time. Large wildlife populations, unregulated livestock grazing, and agricultural land conversion are all potential factors increasing sediment loads in the semi-arid portion of the basin. The basin is well-known for its annual wildebeest (Connochaetes taurinus) migration of approximately 1.3 million individuals, but it also has a growing population of hippopotami (Hippopotamus amphibius), which reside within the river and may contribute to the flux of suspended sediments. We used in situ pressure transducers and turbidity sensors to quantify the sediment flux at two sites for the Mara River and investigate the origin of riverine suspended sediment. We found that the combined Middle Mara—Talek catchment, a relatively flat but semi-arid region with large populations of wildlife and domestic cattle, is responsible for 2/3 of the sediment flux. The sediment yield from the combined Middle Mara–Talek catchment is approximately the same as the headwaters, despite receiving less rainfall. There was high monthly variability in suspended sediment fluxes. Although hippopotamus pools are not a major source of suspended sediments under baseflow, they do contribute to short-term variability in suspended sediments. This research identified sources of suspended sediments in the Mara River and important regions of the catchment to target for conservation, and suggests hippopotami may influence riverine sediment dynamics.

Organic matter loading by hippopotami causes subsidy overload resulting in downstream hypoxia and fish kills

Organic matter and nutrient loading into aquatic ecosystems affects ecosystem structure and function and can result in eutrophication and hypoxia. Hypoxia is often attributed to anthropogenic pollution and is not common in unpolluted rivers. Here we show that organic matter loading from hippopotami causes the repeated occurrence of hypoxia in the Mara River, East Africa. We documented 49 high flow events over 3 years that caused dissolved oxygen decreases, including 13 events resulting in hypoxia, and 9 fish kills over 5 years. Evidence from experiments and modeling demonstrates a strong mechanistic link between the flushing of hippo pools and decreased dissolved oxygen in the river. This phenomenon may have been more widespread throughout Africa before hippopotamus populations were severely reduced. Frequent hypoxia may be a natural part of tropical river ecosystem function, particularly in rivers impacted by large wildlife.Hypoxic (low oxygen) water conditions are generally thought to be uncommon in rivers and result from human impacts. However, Dutton and colleagues show here that waste from hippos in the Mara River contributes to frequent hypoxic events, suggesting hypoxia is a natural aspect of this system.

Nutrient Limitation and Uptake

Abstract In this chapter, we describe nutrient limitation in stream ecosystems, the problems associated with elevated nutrient loading caused by human activities in the watershed, and how streams process and transform nutrients prior to downstream export. We provide a basic method for quantifying nutrient limitation of stream biofilms using nutrient diffusing substrata. We also detail two advanced methods that can be used to quantify nutrient spiraling within streams using experimental addition approaches: a short-term nutrient release and a 15N stable isotope tracer addition. Finally, we describe several modifications to the various methods that could expand their application to various research questions or to allow for customization depending on specific research questions. Understanding the nutrient limitation status and quantifying nutrient uptake in streams is vital because these are key components of ecosystem function, providing information on how streams retain nutrients from watersheds prior to downstream export.

Defining Extreme Events: A Cross‐Disciplinary Review

Extreme events are of interest worldwide given their potential for substantial impacts on social, ecological, and technical systems. Many climate-related extreme events are increasing in frequency and/or magnitude due to anthropogenic climate change, and there is increased potential for impacts due to the location of urbanization and the expansion of urban centers and infrastructures. Many disciplines are engaged in research and management of these events. However, a lack of coherence exists in what constitutes and defines an extreme event across these fields, which impedes our ability to holistically understand and manage these events. Here, we review 10 years of academic literature and use text analysis to elucidate how six major disciplines--climatology, earth sciences, ecology, engineering, hydrology, and social sciences--define and communicate extreme events. Our results highlight critical disciplinary differences in the language used to communicate extreme events. Additionally, we found a wide range in definitions and thresholds, with more than half of examined papers not providing an explicit definition, and disagreement over whether impacts are included in the definition. We urge distinction between extreme events and their impacts, so that we can better assess when responses to extreme events have actually enhanced resilience. Additionally, we suggest that all researchers and managers of extreme events be more explicit in their definition of such events as well as be more cognizant of how they are communicating extreme events. We believe clearer and more consistent definitions and communication can support transdisciplinary understanding and management of extreme events.

Scaling Dissolved Nutrient Removal in River Networks: A Comparative Modeling Investigation

Along the river network, water, sediment, and nutrients are transported, cycled, and altered by coupled hydrological and biogeochemical processes. Our current understanding of the rates and processes controlling the cycling and removal of dissolved inorganic nutrients in river networks is limited due to a lack of empirical measurements in large, (non-wadeable), rivers. The goal of this paper was to develop a coupled hydrological and biogeochemical process model to simulate nutrient uptake at the network scale during summer baseflow conditions. The model was parameterized with literature values from headwater streams, and empirical measurements made in 15 rivers with varying hydrological, biological, and topographic characteristics, to simulate nutrient uptake at the network scale. We applied the coupled model to 15 catchments describing patterns in uptake for three different solutes to determine the role of rivers in network-scale nutrient cycling. Model simulation results, constrained by empirical data, suggested that rivers contributed proportionally more to nutrient removal than headwater streams given the fraction of their length represented in a network. In addition, variability of nutrient removal patterns among catchments was varied among solutes, and as expected, was influenced by nutrient concentration and discharge. Net ammonium uptake was not significantly correlated with any environmental descriptor. In contrast, net daily nitrate removal was linked to suspended chlorophyll a (an indicator of primary producers) and land use characteristics. Finally, suspended sediment characteristics and agricultural land use were correlated with net daily removal of soluble reactive phosphorus, likely reflecting abiotic sorption dynamics. Rivers are understudied relative to streams, and our model suggests that rivers can contribute more to network-scale nutrient removal than would be expected based upon their representative fraction of network channel length.

Organic matter and nutrient inputs from large wildlife influence ecosystem function in the Mara River, Africa.

Animals can be important vectors for the movement of resources across ecosystem boundaries. Animals add resources to ecosystems primarily through egestion, excretion, and carcasses, and the stoichiometry and bioavailability of these inputs likely interact with characteristics of the recipient ecosystem to determine their effects on ecosystem function. We studied the influence of hippopotamus excretion/egestion and wildebeest carcasses, and their interactions with discharge, in the Mara River, Kenya. We measured nutrient dissolution and decomposition rates of wildlife inputs, the influence of inputs on nutrient concentrations and nutrient limitation in the river and the influence of inputs on biofilm growth and function in both experimental streams and along a gradient of inputs in the river. We found that hippopotamus excretion/egestion increases ammonium and coarse particulate organic matter in the river, and wildebeest carcasses increase ammonium, soluble reactive phosphorus, and total phosphorus. Concentrations of dissolved carbon and nutrients in the water column increased along a gradient of wildlife inputs and during low discharge, although concentrations of particulate carbon decreased during low discharge due to deposition on the river bottom. Autotrophs were nitrogen limited and heterotrophs were carbon limited and nitrogen and phosphorus colimited upstream of animal inputs but there was no nutrient limitation downstream of inputs. In experimental streams, hippo and wildebeest inputs together increased biofilm gross primary production (GPP) and respiration (R). These results differed in the river, where low concentrations of hippo inputs increased gross primary production (GPP) and respiration (R) of biofilms, but high concentrations of hippo inputs in conjunction with wildebeest inputs decreased GPP. Our research shows that inputs from large wildlife alleviate nutrient limitation and stimulate ecosystem metabolism in the Mara River and that the extent to which these inputs subsidize the ecosystem is mediated by the quantity and quality of inputs and discharge of the river ecosystem. Thus, animal inputs provide an important ecological subsidy to this river, and animal inputs were likely important in many other rivers prior to the widespread extirpation of large wildlife.

Occurrence, leaching, and degradation of Cry1Ab protein from transgenic maize detritus in agricultural streams

The insecticidal Cry1Ab protein expressed by transgenic (Bt) maize can enter adjacent water bodies via multiple pathways, but its fate in stream ecosystems is not as well studied as in terrestrial systems. In this study, we used a combination of field sampling and laboratory experiments to examine the occurrence, leaching, and degradation of soluble Cry1Ab protein derived from Bt maize in agricultural streams. We surveyed 11 agricultural streams in northwestern Indiana, USA, on 6 dates that encompassed the growing season, crop harvest, and snowmelt/spring flooding, and detected Cry1Ab protein in the water column and in flowing subsurface tile drains at concentrations of 3-60ng/L. In a series of laboratory experiments, submerged Bt maize leaves leached Cry1Ab into stream water with 1% of the protein remaining in leaves after 70d. Laboratory experiments suggested that dissolved Cry1Ab protein degraded rapidly in microcosms containing water-column microorganisms, and light did not enhance breakdown by stimulating assimilatory uptake of the protein by autotrophs. The common detection of Cry1Ab protein in streams sampled across an agricultural landscape, combined with laboratory studies showing rapid leaching and degradation, suggests that Cry1Ab may be pseudo-persistent at the watershed scale due to the multiple input pathways from the surrounding terrestrial environment.