Malcolm N. McFarland

Evidence for ubiquitous preferential particle orientation in representative oceanic shear flows

Abstract In situ measurements were undertaken to characterize particle fields in undisturbed oceanic environments. Simultaneous, co‐located depth profiles of particle fields and flow characteristics were recorded using a submersible holographic imaging system and an acoustic Doppler velocimeter, under different flow conditions and varying particle concentration loads, typical of those found in coastal oceans and lakes. Nearly one million particles with major axis lengths ranging from ∼14 μm to 11.6 mm, representing diverse shapes, sizes, and aspect ratios were characterized as part of this study. The particle field consisted of marine snow, detrital matter, and phytoplankton, including colonial diatoms, which sometimes formed “thin layers” of high particle abundance. Clear evidence of preferential alignment of particles was seen at all sampling stations, where the orientation probability density function (PDF) peaked at near horizontal angles and coincided with regions of low velocity shear and weak turbulent dissipation rates. Furthermore, PDF values increased with increasing particle aspect ratios, in excellent agreement with models of spheroidal particle motion in simple shear flows. To the best of our knowledge, although preferential particle orientation in the ocean has been reported in two prior cases, our findings represent the first comprehensive field study examining this phenomenon. Evidence of nonrandom particle alignment in aquatic systems has significant consequences to aquatic optics theory and remote sensing, where perfectly random particle orientation and thus isotropic symmetry in optical parameters is assumed. Ecologically, chain‐forming phytoplankton may have evolved to form large aspect ratio chains as a strategy to optimize light harvesting.

Bio-optical Properties of Cyanobacteria Blooms in Western Lake Erie

There is a growing use of remote sensing observations for detecting and quantifying freshwater cyanobacteria populations, yet the inherent optical properties of these communities in natural settings, fundamental to bio-optical algorithms, are not well known. Towards bridging this knowledge gap, we measured a full complement of optical properties in western Lake Erie during cyanobacteria blooms in the summers of 2013 and 2014. Our measurements focus attention on the optical uniqueness of cyanobacteria blooms, which have consequences for remote sensing and bio-optical modeling. We found the cyanobacteria blooms in the western basin during our field work were dominated by Microcystis, while the waters in the adjacent central basin were dominated by Planktothrix. Chlorophyll concentrations ranged from 1 to over 135

Ploidy Distribution of the Harmful Bloom Forming Macroalgae Ulva spp. in Narragansett Bay, Rhode Island, USA, Using Flow Cytometry Methods.

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Biological thin layers: history, ecological significance and consequences to oceanographic sensing systems

across the study area with the highest concentrations associated with Microcystis in the western basin. We observed large, amorphous colonial Microcystis structures in the bloom area characterized by high phytoplankton absorption and high scattering coefficients with a mean particle backscatter ratio at 443nm greater than 0.03, which is higher than other plankton types and more comparable to suspended inorganic sediments. While our samples contained mixtures of both, our analysis suggests high contributions to the measured scatter and backscatter coefficients from cyanobacteria. Our measurements provide new insights into the optical properties of cyanobacteria blooms, and indicate that current semi-analytic models are likely to have problems resolving a closed solution in these types of waters as many of our observations are beyond the range of existing model components. We believe that different algorithm or model approaches are needed for these conditions, specifically for phytoplankton absorption and particle backscatter components. From a remote sensing perspective, this presents a challenge not only in terms of a need for new algorithms, but also for determining when to apply the best algorithm for a given situation. These results are new in the sense that they represent a complete description of the optical properties of freshwater cyanobacteria blooms, and are likely to be representative of bloom conditions for other systems containing Microcystis cells and colonies.

Using observation networks to examine the impact of Lake Okeechobee discharges on the St. Lucie Estuary, Florida

Macroalgal blooms occur worldwide and have the potential to cause severe ecological and economic damage. Narragansett Bay, RI is a eutrophic system that experiences summer macroalgal blooms composed mostly of Ulva compressa and Ulva rigida, which have biphasic life cycles with separate haploid and diploid phases. In this study, we used flow cytometry to assess ploidy levels of U. compressa and U. rigida populations from five sites in Narragansett Bay, RI, USA, to assess the relative contribution of both phases to bloom formation. Both haploid gametophytes and diploid sporophytes were present for both species. Sites ranged from a relative overabundance of gametophytes to a relative overabundance of sporophytes, compared to the null model prediction of √2 gametophytes: 1 sporophyte. We found significant differences in cell area between ploidy levels for each species, with sporophyte cells significantly larger than gametophyte cells in U. compressa and U. rigida. We found no differences in relative growth rate between ploidy levels for each species. Our results indicate the presence of both phases of each of the two dominant bloom forming species throughout the bloom season, and represent one of the first studies of in situ Ulva life cycle dynamics.

Individual particle measurements to monitor ecological processes in the Indian River Lagoon, FL

Thin layers are water column structures that contain concentrations of organisms (or particles) that occur over very small vertical scales (a few meters or less), but with large horizontal scales (e.g. kilometers). Thin layers are now known to be common phenomenon in a wide variety of environments and can be a critical componant in marine ecosystem dynamics and functioning. While knowledge about their dynamics is important to our basic understanding of oceanic processes, thin layers can have significant impacts on both oceanographic and defense related sensing systems, e.g. thin layers can affect underwater visibility, imaging, vulnerability, communication and remote sensing for both optical and acoustic instrumentation. This paper will review the history of thin layers research, their ecological significance, innovations in oceanographic instrumentation and sampling methodologies used in their study, and the consequences of their occurence to oceanographic sensing systems.

On plankton distributions and biophysical interactions in diverse coastal and limnological environments

The St. Lucie River Estuary (SLE) in southeast Florida has a very large watershed comprised of several natural rivers and a network of artificial canals used for water supply and flood control. One of the largest and most critical of these canals is the C-44, which connects Lake Okeechobee to the South Fork of the SLE and is one of the primary means by which excess water is drained from the lake. Major discharges from the lake at the start of the 2016 summer wet season resulted in one of the most severe harmful algal blooms in the SLE in recent history, causing millions of dollars of economic losses in the area. Despite similar discharges from Lake Okeechobee in 2017 following Hurricane Irma, no such bloom occurred. However, algae blooms are not the only hazard associated with lake discharges. Large influxes of freshwater harm organisms adapted to life in a brackish estuary. Observation networks, augmented with ad hoc sampling, can speak volumes about the status of the ecosystem and the impact of water management decisions. Examination of historical data can begin to reveal the causes of negative ecological events in the SLE and the conditions correlated to the termination of the event. Stakeholders can use this data to inform choices about potential discharges, including timing and volume, and verify expected outcomes via real-time network data, thereby mitigating ecological and economic harm to the SLE.

In situ particle characterization and evidence of ubiquitous particle orientation in the ocean using a submersible holographic imaging system (Conference Presentation)

Suspended particles are important components of coastal marine ecosystems that are often the target of environmental sensing efforts (e.g. harmful algae blooms, suspended sediments). Automated measurements of individual particles provide advantages over traditional manual methods of particle analysis and sensors that measure bulk water properties commonly used for coastal ecosystem monitoring. However, the large, multidimensional data sets provided by automated particle measurement techniques can be difficult to analyze and interpret without the use of automated algorithms to classify large numbers of particles. In this paper we demonstrate efficient methods for classifying particles using an unsupervised, watershed transform based, clustering algorithm. The methods were applied to samples collected from the Indian River Lagoon, Banana River Lagoon, and St. Lucy Estuary located along the eastern coast of Florida. Samples were analyzed by flow cytometry and by imaging in flow (FlowCam). Results of analyses reveal patterns of distribution for distinct particle populations over space and time, and in relation to environmental characteristics. These methods represent a highly efficient strategy for monitoring coastal waters that can improve our understanding of ecosystem structure and function.

Layered organization in the coastal ocean: An introduction to planktonic thin layers and the LOCO project

Digital holography provides a unique perspective towards studying aquatic particles/organisms. The ability to sample particles in undisturbed conditions, coupled with the ability to generate 3-D spatial distributions is currently unmatched by any other technique. To leverage these advantages, field experiments with the goal of characterizing aquatic particle properties in situ, were conducted using a submersible holographic imaging system. Diverse aquatic environments were sampled over 3 separate deployments between 2014 and 2017. The areas included: (a) The Gulf of Mexico (GoM), in the vicinity of the Mississippi river plume; (b) Lake Erie; and (c) East Sound in the US Pacific Northwest. A database of more than two million different types of particles in the 10-10000 m size range, was created after processing > 100,000 holograms. Particle size distributions (PSDs) exhibited a Junge-type distribution when characterized by size grouping into logarithmically spaced bins. Particles/plankton were also classified into different groups (e.g. diatoms, copepods). Results presented will be broadly grouped into two parts: (a) PSDs at different depths within the water column during the occurrence of a Microcystis bloom at Lake Erie and individual cell counts within these colonies; and (b) Vertical structure of plankton in East Sound, specifically the presence of diatom thin layers. Finally, the rich diversity in species composition in the GoM and successful data collection towards creating a training set to implement automated classification routines will be briefly discussed.

Thin layers and species-specific characterization of the phytoplankton community in Monterey Bay, California, USA.

Field experiments with the goal of characterizing aquatic particle properties, including size distributions and orientations in their natural environment, were conducted using a submersible holographic imaging system (HOLOCAM). Digital holography is a non-intrusive technique that allows particle fields to be mapped within a 3-D sampling volume at high resolution. The HOLOCAM was deployed at East Sound, a fjord in the US Pacific Northwest, and Lake Erie over three separate deployments from 2013 to 2015. A database of more than a million particles in the 100-10000 µm size range of varying shape and orientation was created after processing < 50,000 holograms. Furthermore, simultaneous, co-located acoustic Doppler velocimeter measurements of small-scale shear and turbulence structure were used to study the effects of the ambient flow field on particle orientation. Several interesting features presented themselves, with a Microcystis bloom dominating the surface layer of Lake Erie, while ‘thin layers’ of high particle concentrations dominated by colonial diatoms were seen in East Sound. Particle size distribution (PSD) slopes in the 50-250 µm size range were ~1.7-1.9, while for particles < 250 µm, the slopes were significantly higher. Clear evidence of ubiquitous particle alignment to the horizontal flow field in regions of low shear and turbulent dissipation was seen. This result, obtained under flow conditions representative of coastal and open oceans, can have significant consequences to ocean optics as random particle orientation is inherently assumed in theory and models. Preferential alignment can increase/decrease optical properties such as backscattering and attenuation relative to random distributions.