Preface: understanding fine sediment dynamics in aquatic systems

This Special Issue of Journal of Soils and Sediments (JSS) celebrates Dr Ian Droppo’s significant contributions in the field of sediment science to mark his retirement in 2021 from Environment and Climate Change Canada (ECCC). In a career spanning more than three decades, Ian has published over 100 international peer-reviewed and highly cited articles in leading science journals (> 7500 citations and an H-index of 43; Google Scholar) and a further 65 peer-reviewed publications comprising reports, book chapters and edited books. The cutting-edge quality and value of his research are also exemplified through an impressive funding record including a series of Natural Sciences and Engineering Research Council of Canada (NSERC) grants. The title of this Special Issue Understanding fine sediment dynamics in aquatic systems captures Ian’s wide-ranging research and, in this Preface, we reflect on Ian’s career and research legacy and introduce a series of papers that encapsulate many of his research interests.

1 Reflections on Ian’s career

Ian’s career has been characterised by impressive interdisciplinary and international collaboration which has underpinned new insights and novel approaches, and fostered strong research networks that continue to flourish. His academic life started at McMaster University in Hamilton, Ontario, Canada, with his BA (1987) and MSc (1990) and his links with McMaster continued through his appointment as an Adjunct Professor (2009–2018). Moreover, he holds the honour of being inducted into the McMaster University Alumni Gallery in 2008, recognising the outstanding contribution of his work to society which reflects a theme throughout his career of combining an interest in fundamental science with an ever-present awareness of practical applications. Throughout his career, Ian has also held Adjunct Professor positions at the Universities of Windsor, Toronto (Scarborough), Ryerson (all Canada), and Buffalo State, SUNY, USA. Ian has also enjoyed successful collaborations with the University of Exeter (UK), through an International Research Fellowship funded by the Leverhulme Trust (2001–2002) working with Professor Des Walling who supervised his PhD (2000) on the Characterisation of suspended and bottom sediment in natural and engineered systems. In 2016, Ian was a Visiting Research Scientist at the University of Grenoble Alpes, France, working with Cédric Legout and his team on understanding solid transport in the Alps in catchments exhibiting very high suspended sediment fluxes. In particular, he participated in the development of the SCAF device measuring the settling distribution of particles and flocculation indices for highly concentrated suspensions of up to several tens of grams per litre (Wendling et al. 2015; Legout et al. 2018). In the intervening years, Ian collaborated with researchers at Queen Mary University of London (QMUL, UK) working on the erodibility of fine sediments in chalk streams and combining field measurements in Dorset, UK, with laboratory experiments at ECCC (Grabowski et al. 2010, 2011, 2012) and developing novel tracer techniques for applications in urban water management (Spencer et al. 2011). More recently, research with QMUL focused on novel microscopy to characterise the structure of natural flocs (Wheatland et al. 2020; Spencer et al. 2021).

It was at ECCC from 1989 to 2021 that Ian enjoyed a highly successful career in the public service as a Physical Scientist, Research Scientist and Senior Scientist. During his time at ECCC, Ian was instrumental in delivering novel research and innovation towards policy development as they relate to sediment and contaminant dynamics within the aquatic ecosystem. This included work on the source, fate and effect of sediments and associated contaminants (e.g. Droppo et al. 2001), and aquatic pathogen and sediment dynamics with implications for human health and improved engineering practices for wastewater treatment (e.g., Amos et al. 2003). His research in the Athabasca Oil Sands monitoring programme, including 2 years as the Water component lead of the Joint Oil Sands Monitoring Program, has been integral to a better and more holistic understanding of the role of sediments in this complex, industrially influenced system (e.g. Culp et al. 2018; Droppo et al. 2018, 2019; Reid et al. 2020). Most recently, Ian worked on understanding how climate change is impacting the mobilisation and transport of sediments in the Canadian Arctic (Droppo et al. 2022). This research included simulating the effects of rainfall on retrogressive thaw slump scars and the washoff of particulates into adjacent water bodies. His research lives on in various forms within the Hydroecology Monitoring and Research Facility (HMReF) at ECCC’s Canada Center for Inland Waters, where current and ongoing research programmes continue to study the effects of the increasing pressures of climate change in the Canadian Arctic, Great Lakes and other priority areas within Canada.

During his career, Ian has been a key and enthusiastic member of the International Association for Sediment Water Science (IASWS) serving as Editor of the Newsletter (1996–2002), Secretary and Treasurer (1996–2002), Vice President (2002–2005), President (2005–2008) and Past Present (2008–2011). He played an important role in facilitating the organisation of conferences, ensuring that these events became hallmark gatherings, fostering knowledge exchange and collaboration among experts in sediment water science. His dedication to the preparation of Special Issues within the association’s publications helped disseminate cutting-edge research to a wider audience, furthering the field’s advancement. His presidency left an indelible mark on the organisation, fostering a legacy of excellence that continues to inspire and benefit the scientific community to this day.

Ian was also on the editorial board of JSS between 2009 and 2021, representing a considerable service to his discipline. During this period, he handled many manuscripts, providing advice that significantly improved those manuscripts that were accepted.

2 Ian’s contributions to sediment science

Ian’s pioneering work at ECCC and through the many academic collaborations outlined above has advanced our understanding of fine-grained sediment in freshwater ecosystems with pioneering work providing the evidence for the importance of flocculation of fine-grained sediment as a fundamental control on the erodibility (Liss et al. 1996; Droppo 2001, 2004; Droppo et al. 2007, 2008, 2015, 2016, Grabowski et al. 2011, 2012), transport and deposition of sediments and the environmental fate of contaminants bound to sediments (e.g. Droppo et al. 1997, 2004, 2018; Droppo and Amos 1998; Petticrew and Droppo 2000; Guo and He 2011; Krishnappan 2022; Lamb et al. 2020). Understanding floc structure and the importance of flocculation as controls on the transport of particle-bound contaminants was the focus of Ian’s research career and his body of work, encompassing a wide range of freshwater environments and sediment sources, underpins our collective understanding of this important phenomenon.

In Ian’s seminal paper ‘Rethinking What Constitutes Suspended Sediment’ (Droppo 2001), which built on earlier collaborative work (e.g. Liss et al. 1996), a novel and comprehensive conceptual model of floc formation and behaviour was presented to describe the dynamic interrelationships between the hydrodynamic forces and the biological, chemical and physical floc formation processes. This conceptual model continues to inform current research (e.g. Spencer et al. 2011; Walch et al. 2022) including papers in this Special Issue. It also underpins the re-examination of the factors that influence the erodibility of fine sediment. The refocusing of research onto the floc as the building block of fine sediment deposits led to significant advances in our understanding of the influence of hydrological forces (Lau and Droppo 2000; Droppo et al. 2001; Lau et al. 2001) and microbiological activity on bed stability (Amos et al. 2003; Droppo et al. 2007; Grabowski et al. 2011, 2012).

Ian’s research has demonstrated the ubiquitous nature of flocculated sediments in freshwater systems and hillslope sediment sources (e.g. Droppo et al. 1998, 2016; Grangeon et al. 2014), and perhaps more importantly, the significance of flocculation as a major control on the fate of inorganic sediment and sediment-associated contaminants and pathogens for human health and catchment management (e.g. Droppo et al. 2000). For example, with respect to sediments derived from urban environments, Droppo et al. (2010) demonstrated that the particle size and settling properties of sediments changed significantly from source to sink as a consequence of dynamic flocculation through the sediment pathway. In field-based studies, Droppo and Stone (1994), Stone and Droppo (1994) and Droppo and Amos (1998) demonstrated the importance of flocculation as a mechanism for the development of surficial fine-grained lamina (SFGL) and the potential role of SFGL as a vector for contaminant transport in river systems. In laboratory-based analysis, Droppo et al. (2010) demonstrated the dependence of pathogen dynamics on the erosion, transport and fate of flocculated particles in river systems with significant implications for water quality monitoring programmes, a theme explored further with respect to lake sediments by Van Mensel et al. (2023). In the unique context of the hydrophobic sediments of the Athabascan oil sands region in Alberta, Canada, Droppo and Krishnappan (2016) demonstrated through a modelling-based investigation that entrapment within the bed structure, rather than sedimentation of low-density flocculated sediments, was likely to be the dominant mechanism controlling the transport and deposition of fine-grained sediment and oil-based contaminants. Further work by Droppo et al. (2015, 2016) regarding the Athabascan oil sands provided evidence of a complex inter-dependency between the microbial communities, the particle size of the sediments and the ambient hydrodynamic regimes as determinants of the potential for sediment to control the erosion, transport and fate of oil-based contaminants. Droppo et al. (2018) provided new insights into the potential significance of airborne anthropogenic sources of Polycyclic Aromatic Compounds (PACs) in the Lower Athabasca River, concluding that the fluvial erosion of PACs bound to bed and bank sediments was the principal source reinforcing the importance of understanding sediment properties and associated pathways for contaminants that contribute to catchment scale environmental stress.

Work on soil erodibility and the transformation of eroded soil into riverine flocs (e.g. Grangeon et al. 2014) has provided further significant advances. Through combining research on soils and sediments, Ian has promoted a more integrated and interdisciplinary catchment-based perspective in the understanding of sediment and sediment-bound contaminant pathways (see Dowdeswell-Downey et al. 2023 in this Special Issue for further discussion). The main themes of Ian’s research are reflected in the contributions to this Special Issue which cover flocculated sediments and erodibility (Falk et al. 2023; Grangeon et al. 2023; Haddad et al. 2023; Maltauro et al. 2023; Rasmus et al. 2023); sediment-bound contaminants (Irvine et al. 2023; Owens and Rutherford 2023) and soil stability and erosion (Dowdeswell-Downey et al. 2023; Porto and Callegari 2023) (Fig. 1).

Fig. 1

Ian fully outfitted in helicopter gear heading out on one of many sampling trips to the Fort McMurray region, Canada, to assess the transport and fate of sediments, and the biogeochemical activity within them (Photo taken Oct 2017; kindly supplied by Thomas Reid)

3 Contributions to the special issue: short summaries

Falk et al. (2023) ‘Integrating microbial DNA community analyses into time-integrated suspended sediment sampling method’ present the results of the first reported application of a time-integrated suspended sediment sampler for the purpose of collecting a sufficient sample size for the microbiological characterisation of suspended sediment. The composition of microbial communities is an important variable in the geochemical processing that takes place within flocculated sediments, and this paper considers how deposition and retention within the sampler may influence the composition of microbial communities. Furthermore, the paper investigates the potential for sediment-associated microbial communities to aid in discriminating between sediment sources. The findings have implications for sediment sampling programmes intended to characterise in situ biogeochemical processes.

Maltauro et al.’s (2023) contribution ‘Effect of shear-dependent flocculation on the multimodality of effective particle size distributions in a gravel-bed river during high flows’ provides valuable new field data on the dynamic relationship between in situ hydrodynamic forces and floc formation and break up. Measurement of the in situ or ‘Effective Particle Size Distribution’ (EPSD) of suspended sediments is technically challenging but scientifically invaluable for the development of realistic sediment transport models, as these measurement techniques minimise the confounding effects on the data of artificially changing ambient shear stress and chemistry. This paper focuses on the significance of ambient shear stress in the river as a determinant of the EPSD and provides evidence of shear stress-driven changes in discrete groups of particles and aggregates (primary particles, flocculi, microflocs and macroflocs) comprising the EPSD with implications for existing methods modelling the transport of flocculated sediments.

The paper by Haddad et al. (2023) ‘Spatial variability of erodibility of fine sediments deposited in gravel river beds: from field measurements to 2D numerical models’ presents an original dataset of measurements of critical shear stresses and erosion rates of fine sediment deposits in gravel river beds. Due to the high spatial and temporal variability of the erodibility of these deposits, a sensitivity analysis is carried out using a 2D hydrosedimentary numerical model in order to assess its impact on modelling outputs such as suspended sediment fluxes. The results of this analysis are used to propose recommendations for future studies on the minimum number, location and optimum periods for erodibility measurements to feed the numerical models and reduce uncertainties in modelling outputs.

In Grangeon et al. (2023) ‘Catchment‑scale variability and driving factors of fine sediment deposition: insights from a coupled experimental and machine‑learning‑based modelling study’, the analysis of an extensive and unique data set from three temperate catchments to investigate the driving factors of fine sediment deposition is described. A random forest model was developed and evaluated to predict sediment deposition at the catchment scale, providing valuable insights into sediment connectivity and supporting decision making around the management of fine sediments in catchments.

Rasmus et al. (2023) in their paper ‘The seasonal movement of sediment‑associated marine‑derived nutrients in a morphologically diverse riverbed: the influence of salmon in an Interior British Columbia river’ report on a 12-month study investigating the effect of spawning salmon in freshwater streams on fine sediment and nutrient dynamics. The study examined how flocculation and the hydrological and morphological characteristics of the river controlled the seasonal storage of marine-derived nutrients (MDN) as compared to other nutrients stored on the riverbed. Whilst most sediment and nutrient inputs in these streams come from terrestrial riparian sources, Pacific salmon are shown to have an important role through their spawning activities (building redds), and ultimate decay (95% of salmon body mass is accumulated in the marine environment). The study shows how spawning salmon can transfer external nutrients and create the conditions for flocculation between fine sediments and organic matter adding further evidence on the importance of flocculation in aquatic systems.

The paper by Owens and Rutherford (2023) ‘Concentrations and total mass storage of fine sediment, potentially toxic elements (PTEs) and phosphorus in the channel bed of an urban river: a multi‑year study’ continues earlier work by Owens, Droppo and others (Owens et al. 2011) investigating the effect of urban development on the quality and quantity of sediment on the road network and in river systems in the city of Prince George, British Columbia, Canada. Here, they determined the quality and amount of sediment stored in an urban river that supports Pacific salmon. They show that some elements decreased in concentration in a downstream direction, reflecting agricultural inputs from upstream sources, whilst other elements increased in a downstream direction reflecting inputs from industrial and urban sources in the lower part of the watershed. For those elements that increased downstream, they show that a large road culvert was an important source, contributing road-deposited sediment (RDS) and other urban materials that were enriched in contaminants like As, Mn and Zn. These sources of sediment and contaminants were important in controlling the amounts and locations of storage in the channel bed.

Irvine et al.’s paper (2023) ‘Nature‑based solutions to manage particle‑bound metals in urban stormwater runoff: current design practices and knowledge gaps’ builds on earlier work by Droppo et al. (2002) in Hamilton, Ontario, Canada, to consider the benefits of nature-based solutions for wastewater management. Bed sediments from two constructed wetlands in Geelong, Australia, were sampled and analysed for total Cd, Cr, Cu, Pb and Zn and benthic macroinvertebrate populations. The findings showed encouraging results on sediment-bound metal retention, and the authors make recommendations for management options and further research on bioretention cell mitigation for metal treatment.

Porto and Calegari (2023) ‘Relating ¹³⁷Cs and sediment yield from uncultivated catchments: the role of particle size composition of soil and sediment in calculating soil erosion rates at the catchment scale’ studied erosion processes in two small headwater catchments using ¹³⁷Cs. The study based on soil sampling, discharge and sediment monitoring since 1978 investigates the challenging question of how to account for particle size selectivity in theoretical models used to assess erosion rates from measurements of radiotracers such as ¹³⁷Cs. Their results suggest that traditional relationships used to account for particle size in conversion models may not be dependent only on soil and sediment texture. Given the good agreement between the long-term monitored sediment yields and the results of the Diffusion and Migration Model, this model seems to be an effective means to assess long-term soil erosion rates without considering corrections for particle size effect.

Dowdeswell-Downey et al. (2023) ‘Do temperature and moisture conditions impact soil microbiology and aggregate stability?’ provides new empirical evidence on the influence of environmental conditions on the stability of soil aggregates, a predictor of soil erodibility. Previous research has suggested that the microbiological community may be responsible for spatial and temporal variations in erodibility. Using laboratory microcosm experiments, the study found that aggregate stability is affected by soil moisture and temperature, but the response is dependent on soil texture. The research findings demonstrate the dynamic and responsive nature of aggregate stability and emphasisise the need to further study the stabilising function of the microbiological community.

4 Some personal reflections

Of equal significance to his research impact, Ian’s career is widely recognised for creating a legacy of respectful and kind-hearted friendships throughout both the government and academic worlds (Fig. 2). There are researchers around the globe who have enjoyed stimulating collaborations and a large academic ‘family’ of current and former graduate students who gained so much from Ian’s love of science and genuine interest in their research. Ian exemplifies what it means to be a true mentor, colleague, friend and scientist, and this Special Issue is a recognition and celebration of all Ian’s contributions.

Fig. 2

Eager crew of researchers at one of many stream locations in the Fort McMurray region, assessing the impacts of natural bitumen outcrops on river water and sediment biogeochemical processes (Ian, third from right) (Photo kindly supplied by Thomas Reid)