Using cell size to represent phytoplankton diversity in studies of nitrogen dynamics in the southern Benguela

Phytoplankton are a key component in the functioning of marine ecosystems and play a central role in the cycling of nitrogen and other elements. Metrics that can adequately represent the biogeochemical processes associated with phytoplankton diversity are needed in order to make use of remote sensing and modeling platforms. A single-value size proxy, effective diameter (Deff ), represents the mean volume to surface area ratio across the nano and micro plankton size fraction (2-200µm) in the southern Benguela, but has yet to be tested regarding its biogeochemical relevance. Cell size imposes overarching constraints on phytoplankton metabolism; there are therefore strong grounds for evaluating the usefulness of the metric (Deff ) in studies of nitrogen dynamics in diverse, natural assemblages. Three case studies were used to explore the nitrogen dynamics in naturally occurring assemblages and to evaluate the relationships between Deff and the uptake of the different sources of nitrogen. Two of the case studies comprised high biomass, harmful algal blooms observed off Lamberts Bay during an upwelling/downwelling cycle. The third case study used bi-monthly sampling over a full year in Saldanha Bay. The Lamberts Bay case studies involved blooms occasionally dominated by HAB-forming species: a mixotrophic ciliate, Myrionecta rubra, and a dinoflagellate, Prorocentrum triestinum. The nitrogen uptake rates followed the well observed pattern of high nitrate uptake by large cells and regenerated nitrogen uptake by small cells. Myrionecta rubra had a wide range of nitrate (O₃⁻ ) uptake rates (0.02-0.3 µmol N L⁻¹ h⁻¹). Prorocentrum triestinum showed slower rates of O₃⁻ uptake (0.01-0.2 µmol N L⁻¹ h⁻¹) and dominated in low O₃⁻ , stratified conditions. Diatoms were the most efficient utilisers of O₃⁻ and total nitrogen in these cases. The effective diameter was significantly related to the uptake rates of ammonium (NH₄⁺ ) (r=-0.54, p<0.005) and urea (r=-0.59, p<0.005), but not O₃⁻ (r=0.27, p=0.11). This was attributed to some instances of bi-modality in observed size distributions as well as potentially specialist nutrient uptake strategies employed by diatoms. The year-round data from Saldanha Bay indicated the system was diatom-dominated and was used to assess 1 how well Deff could represent the nitrogen uptake strategies employed by the diverse diatom assemblages. The Saldanha Bay system has O₃⁻ limited surface waters during summer, and light-limited bottom waters during winter. No significant relationship was found between Deff and the mass-specific uptake rates of the different nitrogen species in this data set. This was attributed to the complex shapes of the size distributions and the comparatively low biomass observed. Uptake kinetic experiments revealed high variability for maximum uptake rates (Vmax) and half saturation values (Ks) for both O₃⁻ and NH₄⁺ . For O₃⁻ : Vmax ranged 0.007-0.17 µmol N L⁻¹ h⁻¹, and Ks ranged between 0.2-42.5 µmol N L⁻¹. For NH₄⁺ Vmax was observed between 0.02-2.7 µmol N L⁻¹ h⁻¹; and Ks values ranged 0.1- 14.02 µmol N L⁻¹. Variability was observed in association with the availability of the ambient sources of nitrogen, but some variation was accounted for by the presence of different diatom species. From these three case studies it was concluded that the single-value size proxy was an adequate metric to quantify the uptake of regenerated nitrogen in scenarios of high biomass algal blooms. Such blooms are a pervasive feature in the southern Benguela Ecosystem. For lower biomass blooms, however, Deff did not adequately represent the nutrient dynamics of diverse diatomdominated assemblages. The variable shape of the size spectrum is an important factor in determining the rates of nutrient uptake and, in cases of bi- or multi-modality, this information could be lost when represented by a single descriptor such as Deff . It was subsequently hypothesised that size spectra could be used to accurately represent the nitrogen dynamics in diverse phytoplankton assemblages. This was tested by comparing the observed uptake rates of the three case studies to estimated uptake rates based on size spectra. Observed particle size distributions were used to estimate the uptake of O₃⁻ and NH₄⁺ , based on theoretical relationships to calculate size-dependent values of Vmax and Ks. Michaelis-Menten models were applied to measured ambient nutrient concentrations and particle size distributions, generating size-integrated estimates of O₃⁻ , NH₄⁺ and total N uptake rates. The variability in the estimated uptake rates was similar to that of the measured values. It was thus concluded that the representation of phytoplankton diversity by size spectra allowed modification of model parameters, such that improved estimates of uptake rates of O₃⁻ and NH₄⁺ could be obtained for a dynamic eutrophic environment

Youth visions in a changing climate: Emerging lessons from using immersive and arts-based methods for strengthening community-engaged research with urban youth

Despite increasing efforts, youth perspectives remain largely excluded from decision- making processes concerning their future and the social-ecological challenges they&nbsp;are set to inherit. While youth are a critical and powerful force for social change, many youths in underserved communities have limited access to appropriate information on the root causes and consequences of environmental change, in addition to an array of other complex social injustices. To address this, we embarked on a participatory action research process which focused on democratising research, science and the arts by facilitating experiential, immersive learning opportunities with the intention of eventually co-producing artifacts (in the form of participatory murals) in public spaces to facilitate longer term engagement with human nature futures. This article outlines and shares reflections on our process and offers insights for future engagement activities that seek to mobilise youth imaginaries and agency. We found participants were better engaged when conversations were (1) facilitated by other participants; (2) were outdoors and centred on public art; and (3) were happening in parallel with a hands-on activity. This contrasted with asking interview-type questions, or asking participants to write down their answers, which felt more like a test than a conversation, minimising participation. Key learnings included: the need to co-develop knowledge around enhancing climate literacy that is based on local realities; that multiple capacities and hives of activity already exist in communities and need to be mobilised and not built; that creative visioning and futuring can help identify options for change; and that many youths are seeking creative, immersive and safe spaces for co-learning and connection. Initiatives that aim to engage diverse voices should therefore be well- resourced so as to carefully co-design processes that start by acknowledging contextual differences and capacities within those contexts, and co-create immersive dialogues, in order to move away from test-like engagements which perpetuate power imbalances and discourage participation

In situ measurements and model estimates of NO3 and NH4 uptake by different phytoplankton size fractions in the southern Benguela upwelling system.

Bulk measurements can be made of phytoplankton standing stocks on a quasi-synoptic scale but it is more difficult to measure rates of production and nutrient uptake. We present a method to estimate nitrogen uptake rates in productive coastal environments. We use observed phytoplankton cell size distributions and ambient nitrogen concentrations to calculate uptake rates of nitrate, ammonium and total nitrogen by different size fractions of diverse phytoplankton communities in a coastal upwelling system. The data are disaggregated into size categories, uptake rates are calculated and these uptake rates are reaggregated to obtain bulk estimates. The calculations are applied to 72 natural assemblages for which nitrogen uptake rates and particle size distributions were measured textit in situ . The calculated values of total N uptake integrated across all size classes are similar to those of textit in situ bulk measurements (N slope=0.90), (NH _ 4 slope=0.96) indicating dependence of NH _ 4 and total N uptake on ambient N concentrations and cell size distributions of the phytoplankton assemblages. NO _ 3 uptake was less well explained by cell size and ambient concentrations, but regressions between measured and estimated rates were still significant. The results suggest that net nitrogen dynamics can be quantified at an assemblage scale using size dependencies of Michaelis-Menten uptake parameters. These methods can be applied to particle size distributions that have been routinely measured in eutrophic systems to estimate and subsequently analyse variability in nitrogen uptake

The urban water metabolism of Cape Town: Towards becoming a water sensitive city

To improve its resilience to increasing climatic uncertainty, the City of Cape Town (the City) aims to become a water sensitive city by 2040. To undertake this challenge, a means to measure progress is needed that quantifies the urban water systems at a scale that enables a whole-of-system approach to water management. Using an urban water metabolism framework, we (1) provide a first city-scale quantification of the urban water cycle integrating its natural and anthropogenic flows, and (2) assess alternative water sources (indicated in the New Water Programme) and whether they support the City towards becoming water sensitive. We employ a spatially explicit method with particular consideration to apply this analysis to other African or Global South cities. At the time of study, centralised potable water demand by the City amounted to 325 gigalitres per annum, 99% of which was supplied externally from surface storage, and the remaining ~1% internally from groundwater storage (Atlantis aquifer). Within the City’s boundary, runoff, wastewater effluent and groundwater represent significant internal resources which could, in theory, improve supply efficiency and internalisation as well as hydrological performance. For the practical use of alternative resources throughout the urban landscape, spatially explicit insight is required regarding the seasonality of runoff, local groundwater storage capacity and the quality of water as it is conveyed through the complex urban landscape. We suggest further research to develop metrics of urban water resilience and equity, both of which are important in a Global South context. Significance: This research provides the initial groundwork of quantifying the magnitude of the urban water cycle of the City of Cape Town at an annual timescale, in relation to becoming a water sensitive city. The urban water metabolism framework used in this study provides important insight to assess whole-of-system urban water dynamics and to benchmark progress towards becoming water sensitive. By quantifying the magnitude of flows into and out of the urban system, this research sheds light on the opportunities to improve circularity in the urban water cycle. The spatial approach adopted here provides a platform to interrogate the urban landscape and its role in the urban water cycle. By using data products that are available via national data sets or remote sensing, this approach can be applied to other African or Global South where data is characteristically scarce. Further work is required to establish metrics that can adequately describe urban water resilience and equity

The urban water metabolism of Cape Town : towards becoming a water sensitive city

CITATION: Atkins, F., Flugel, T. & Hugman, R. 2021. The urban water metabolism of Cape Town : towards becoming a water sensitive city. South African Journal of Science, 117(5/6):8630, doi:10.17159/sajs.2021/8630.The original publication is available at https://sajs.co.zaTo improve its resilience to increasing climatic uncertainty, the City of Cape Town (the City) aims to become a water sensitive city by 2040. To undertake this challenge, a means to measure progress is needed that quantifies the urban water systems at a scale that enables a whole-of-system approach to water management. Using an urban water metabolism framework, we (1) provide a first city-scale quantification of the urban water cycle integrating its natural and anthropogenic flows, and (2) assess alternative water sources (indicated in the New Water Programme) and whether they support the City towards becoming water sensitive. We employ a spatially explicit method with particular consideration to apply this analysis to other African or Global South cities. At the time of study, centralised potable water demand by the City amounted to 325 gigalitres per annum, 99% of which was supplied externally from surface storage, and the remaining ~1% internally from groundwater storage (Atlantis aquifer). Within the City’s boundary, runoff, wastewater effluent and groundwater represent significant internal resources which could, in theory, improve supply efficiency and internalisation as well as hydrological performance. For the practical use of alternative resources throughout the urban landscape, spatially explicit insight is required regarding the seasonality of runoff, local groundwater storage capacity and the quality of water as it is conveyed through the complex urban landscape. We suggest further research to develop metrics of urban water resilience and equity, both of which are important in a Global South context.https://sajs.co.za/article/view/8630Publisher's versio