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Daniel M. Hanes, Colin Jago, Ole Mikkelsen, and Yogesh C. Agrawal

With the recent announcement that a fifth “Particles in Europe” (PiE) conference will be held in Budapest in Fall, 2016, it would be useful to the community of potential participants to review the events of the previous PiE conference.

The fourth PiE conference was held in Esbjerg, Denmark on 7-9 October, 2014. The conference was attended by 42 participants who hailed from 10 countries. The PiE conference series is focused on the characterization and dynamics of aquatic particles in the broadest sense, with an emphasis on field observations. The meeting consisted of two days of technical presentations, a group discussion, and a full day field trip. The field trip included a visit to the Skallingen Peninsula in the Wadden Sea National Park, a tour of the MacArtney company production facility, and a tour and banquet at the Fisheries and Maritime Museum. The technical sessions were focused on: 1) new instruments and methods; 2) suspended sediment dynamics; and 3) environmental impacts.

The presentations covered technological and recent scientific advancements achieved using laser diffraction, holography, and acoustic backscatter instruments. Many of the presentations demonstrated how the laser diffraction based LISST particle sizer has revolutionized our ability to characterize suspended particles in aquatic environments. The LISST provides information on suspended particulate matter (SPM) that was, until recently, impossible to obtain. Knowledge of the particle size spectra of SPM has generated insights into the transport, flux and dispersal of the size components of SPM. Papers presented at the conference showed how the size components of SPM are deferentially dispersed vertically and laterally within the River-Estuary Transition Zone, RETZ (Howlett et al.; Jackson & Jago), across a river-fjord interface (Marussen et al.), in a lagoon (Anderson et al.), on the inner shelf within a Region of Freshwater Influence, ROFI (Jago et al.), and across the continental shelf seaward of a major ROFI (Many et al.). Papers described studies on time scales from tidal cycles to seasonal (Fettweis et al.; Larcombe). Such knowledge is particularly important in a biogeochemical context since the major biogeochemical components (e.g. carbon, as well as important pollutants and pathogens) are attached to SPM; modelling biogeochemical fluxes across globally significant boundaries such as the RETZ and ROFI and determining the fate of biogeochemical components in the marine environment requires information on the differential dispersal of particles of different size.

Moreover, the LISST provides volumes concentrations of discrete particle size classes and this information has added considerably to our understanding of the temporal and spatial scales of flocculation and deflocculation of SPM in response to physical and biological drivers (e.g. Anderson et al.; Jago et al.; Fettweis & Baeye), which in turn determine the ever-changing particle size spectra of SPM. Such insights allow identification of different modes of SPM transport along a transport path such as washload and bed-suspended loads in rivers (Agrawal).

However, there are uncertainties about the ability of the LISST to characterize accurately particles of irregular shape, heterogeneous composition, and variable density. This is generally not a significant issue when dealing with transport of marine silicic and carbonate sands which are approximately unidimensional and of fixed (and known) composition and density. Thus an oolite sand grain is highly spherical with an easily definable and measurable volume, area, and diameter based on the definitions for a sphere. A quartz grain is hardly spherical but is sufficiently unidimensional to be treated as a sphere. Problems arise with biogenic grains that are whole organisms (e.g. radiolarians) or fragments of organisms that are three-dimensionally highly variable and depart radically from a sphere, but at least generally their composition and density are known. In such cases, the grains are given a conceptual ‘equivalent diameter’ which is dependent on the method of size analysis (e.g. sieving, laser diffraction, image analysis, acoustic imaging, etc.). From these methods a ‘mean diameter’ is often determined even though the value depends on the method used. Despite this uncertainty, sediment dynamicists routinely use a ‘mean diameter’ to quantify sediment size.

The challenge of SPM is that the particles exhibit a huge size range with flocs exhibiting complex three dimensional shapes that are impossible to predict. Studies of LISST outputs in comparison with other instruments and technologies (e.g. Winter et al.), particularly with the LISST-HOLO (Nimmo Smith et al.), show that the LISST has limitations especially when confronted with floc sizes that are beyond its designated range. Moreover, the instrument can be fooled by ‘schlieren’ effects when deployed in waters with large density gradients (Karageorgis and Mikkelsen). It is known that bubbles in the water can also confuse the instrument but, interestingly, Davies et al. showed how this can be turned to advantage by using the instrument to characterise oil droplets released in sub-sea blowouts. Several contributors showed how other optical techniques (Nimmo Smith et al.; Winter & Herbst) and acoustic techniques (Agrawal et al.; Decrop; Felix, Fromant et al.; Hanes, Haun et al.) add to our knowledge of SPM properties, especially when deployed in concert with a LISST. These presentations showed that optical and acoustic techniques alike have difficulties in characterizing the complex flocs of SPM. Studies which employ several different methods simultaneously can resolve some of these difficulties (e.g. Haun et al.; Many et al.; Winter & Herbst).

In practice, the most meaningful particle characterizations depend upon the application or question being addressed. Fundamental research, applied research, and monitoring typically require different particles properties to me measured. For sediment (transport) studies it can easily be argued that the most useful particle parameter is the (mass) settling velocity, since this ultimately decides how far a particle can be transported before being deposited, as well as the sedimentation rate. Consequently, development of instrumentation and methodologies that can measure the mass settling velocity directly without any assumptions about particle shape, composition etc. should be prioritized. A challenge is to develop a method that provides high resolution data on settling velocity – existing methods are extremely time consuming in terms of observation and/or computation with the result that we have very limited knowledge of how much settling velocity varies on the scales of, for example, turbulence. We also need much greater knowledge of the variability of floc strength in response to turbulence, biological controls, etc., on time scales from a tidal cycle to seasons since floc strength mediates flocculation and deflocculation and hence the behavior and fate of SPM in the marine environment.

Discussion at the meeting also covered perceived needs for new instrumentation to measure several particle properties. Most important amongst these are the composition of flocs, biogenic interactions, particle settling velocity, and particle measurements in highly concentrated systems. Here it should be noted that some of the most highly concentrated systems are generally storm-water systems, forest streams after forest fires, and sludge – for example wastewater plants. For future PiE workshops it would be useful to include scientists and technicians from these disciplines to discuss their experiences with these highly concentrated particle systems.