Nicole Stockley

Assessing uncertainties in scattering correction algorithms for reflective tube absorption measurements made with a WET Labs ac-9

In situ absorption measurements collected with a WET Labs ac-9 employing a reflective tube approach were scatter corrected using several possible methods and compared to reference measurements made by a PSICAM to assess performance. Overall, two correction methods performed best for the stations sampled: one using an empirical relationship between the ac-9 and PSICAM to derive the scattering error (ε) in the near-infrared (NIR), and one where ε was independently derived from concurrent measurements of the volume scattering function (VSF). Application of the VSF-based method may be more universally applicable, although difficult to routinely apply because of the lack of commercially available VSF instrumentation. The performance of the empirical approach is encouraging as it relies only on the ac meter measurement and may be readily applied to historical data, although there are inevitably some inherent assumptions about particle composition that hinder universal applicability. For even the best performing methods, residual errors of 20% or more were commonly observed for many water types. For clear ocean waters, a conventional baseline subtraction with the assumption of negligible near-IR absorption performed as well or better than the above methods because propagated uncertainties were lower than observed with the proportional method.

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

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

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

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.

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

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.

Combined Atmospheric and Ocean Profiling from an Airborne High Spectral Resolution Lidar

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.

Airborne and shipborne polarimetric measurements over open ocean and coastal waters: Intercomparisons and implications for spaceborne observations

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.