measuring-structure-composition-change-underwater-environments-with

Measuring the Structure, Composition, and Change of Underwater Environments with Large-area Imaging

Digital imaging and processing technology have evolved to facilitate the expansion of large-area imaging surveys, which increase our capacity to study the ...

University of Miami

The recent decline in the condition of coral reef communities worldwide has fueled the need to develop innovative assessment tools to document coral abundance and distribution rapidly and effectively. While most monitoring programs rely primarily on data collected in situ by trained divers, digital photographs and video are used increasingly to extract ecological indicators, provide a permanent visual record of reef condition, and reduce the time that divers spend underwater. In this study, we describe the development and application of a video-based reef survey methodology based on an algorithm for image registration and the estimation of image motion and camera trajectory. This technology was used to construct two-dimensional, spatially accurate, high-resolution mosaics of the reef benthos at a scale of up to 400 m(2). The mosaics were analyzed to estimate the size and percent cover of reef organisms and these ecological indicators of reef condition were compared to similar measurements collected by divers to evaluate the potential of the mosaics as monitoring tools. The ecological indicators collected by trained divers compared favorably with those measured directly from the video mosaics. Five out of the eight categories chosen (hard corals, octocorals, Palythoa, algal turf, and sand) showed no significant differences in percent cover based on survey method. Moreover, no significant differences based on survey method were found in the size of coral colonies. Lastly, the capability to extract the same reef location from mosaics collected at different times proved to be an important tool for documenting change in coral abundance as the removal of even small colonies (<10 cm in diameter) was easily documented. The two-dimensional video mosaics constructed in this study can provide repeatable, accurate measurements on the reef-plot scale that can complement measurements on the colony-scale made by divers and surveys conducted at regional scales using remote sensing tools.

Development and application of a video-mosaic survey technology to document the status of coral reef communities.

Neutral points are specific directions in the light field where the three Stokes parameters Q, U, V, and thus the degree of polarization simultaneously go to zero. We have made the first measurement of non-principal-plane neutral points in the upwelling light field in natural waters. These neutral points are located at approximately 40°- 80° nadir angle and between 120° - 160° azimuth to the sun which is well off of the principal plane. Calculations show that the neutral point positions are very sensitive to the balance in the incident light between the partially polarized skylight and the direct solar beam.

Observation of non-principal plane neutral points in the in-water upwelling polarized light field.

Simulated bidirectional reflectance distribution functions (BRDF) were compared with measurements made just beneath the water's surface. In Case I water, the set of simulations that varied the particle scattering phase function depending on chlorophyll concentration agreed more closely with the data than other models. In Case II water, however, the simulations using fixed phase functions agreed well with the data and were nearly indistinguishable from each other, on average. The results suggest that BRDF corrections in Case II water are feasible using single, average, particle scattering phase functions, but that the existing approach using variable particle scattering phase functions is still warranted in Case I water.

Detailed validation of the bidirectional effect in various Case I and Case II waters.

Emerging technology has immense potential to increase the scale and efficiency of marine conservation. One such technology is large-area imaging (LAI), which relies on structure-from-motion photogrammetry to create composite products, including 3-dimensional (3-D) environmental models, that are larger in spatial extent than the individual images used to create them. Use of LAI has become widespread in certain fields of marine science, primarily to measure the 3D structure of benthic ecosystems and track change over time. However, the use of LAI in the field of marine conservation appears limited. We conducted a review of the coral reef literature on the use of LAI to identify research themes and regional trends in applications of this technology. We also surveyed 135 coral reef scientists and conservation practitioners to determine community familiarity with LAI, evaluate barriers practitioners face in using LAI, and identify applications of LAI believed to be most exciting or relevant to coral conservation. Adoption of LAI was limited primarily to researchers at institutions based in advanced economies and was applied infrequently to conservation, although conservation practitioners and survey respondents from emerging economies indicated they expect to use LAI in the future. Our results revealed disconnect between current LAI research topics and conservation priorities identified by practitioners, highlighting the need for more diverse, conservation-relevant research using LAI. We provide recommendations for how early adopters of LAI (typically Global North scientists from well-resourced institutions) can facilitate access to this conservation technology. These recommendations include developing training resources, creating partnerships for data storage and analysis, publishing standard operating procedures for LAI workflows, standardizing methods, developing tools for efficient data extraction from LAI products, and conducting conservation-relevant research using LAI.

Closing the gap between existing large-area imaging research and marine conservation needs.

Regularly patterned reef ridges develop in the lagoons of at least one-third of Earth's coral reefs. The interactions between corals and their environment, occurring at scales from millimeters to meters, can lead to self-organized spatial patterns spanning hundreds of meters to kilometers. To understand the mechanism behind pattern formation, we first characterize these spatial patterns using satellite imagery from 63 sites across the Atlantic, Pacific, and Indian Oceans. Next, we develop a generalized Turing morphogenesis model. Corroborated by observed spatial patterns, results from our numerical model suggest that patterned ridges develop through a four-phase trajectory, dictated by changes in the lagoon's hydrodynamic regime. Initially, after an atoll lagoon forms, the first colonizing reefs establish as isolated pinnacles. These pinnacles then evolve into low-relief ridges and eventually form semi-enclosed inter-ridge ponds. In the terminal phase, a dense interconnected, branching, and rejoining ("anastomosing") pattern of reef ridges develop into a network, fully enclosing the ponds. Once enclosed, wind- and tide-induced currents are significantly reduced. Since corals rely on flow for feeding and shedding metabolites, ridge development stalls, and the pattern stabilizes. By combining empirical observations from around the world with a theoretical model, our study reveals the mechanism of reef pattern formation. Such a mechanistic understanding enables the use of emergent reef patterns to identify reef stress at the coral colony scale.

Arthur C. R. Gleason

Physics Department

University of Miami

Measuring the Structure, Composition, and Change of Underwater Environments with Large-area Imaging

Arthur C. R. Gleason

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Physics Department

University of Miami

Measuring the Structure, Composition, and Change of Underwater Environments with Large-area Imaging

Physics Department