ALPS in Coastal Oceanography

Nicholas Nidzieko, Catherine Edwards, and Robert Todd

Coastal ecosystems contain energetic and diverse habitats that are a challenge to observe. An overarching goal for researchers working in the coastal zone is to understand the interaction between continents and the global oceans. What are the fates of terrestrial materials in the ocean? How do open-ocean processes affect the physics, chemistry, and biology of the coastal margin? What ecological and evolutionary processes are at work in these habitats? The range of spatial and temporal scales that must be sampled to answer these questions can be difficult to achieve with traditional sampling methods. Moorings under-​resolve processes that vary across complex bathymetry, whereas shipboard sampling can be prohibitively expensive and limited by adverse weather and the ability to sample close to shore. The novel observational capabilities of ALPS have made them indispensable research tools for coastal scientists (Schofield et al., 2010; Boicourt et al., 2012). To date, coastal ALPS research applications have skewed toward studying ocean physics, but emerging sensor technologies are enabling biologists and biogeochemists to pioneer new techniques for ALPS-driven sampling. We expect that ALPS will have a major impact on coastal interdisciplinary studies, combining ocean physics, chemistry, biology, and ecology as new sensors, imaging techniques, vehicle capabilities, and sampling practices mature. In this chapter, we provide examples of how ALPS have been used in coastal research, describe some of the challenges for their operation, and consider how these opportunities and limitations might evolve in the future.

In the middle to inner continental shelf, common research questions focus on cross-shore and alongshore fluxes of momentum and materials, air-sea momentum transfer, benthic fluxes of sediment and organic matter, and fisheries ecology. Many platforms are used for such studies, including drifters, gliders, and propeller-driven autonomous underwater vehicles (AUVs). For all of these platforms, the main operational risks include collision with both commercial and recreational traffic, as well as entanglement, damage, or accidental bycatch from fisheries activity. With increasing distance from shore, communications become limited by satellite bandwidth, and recovery challenges increase.

There is a long history of using Lagrangian drifters to track coastal and nearshore circulation (Stevenson et al., 1969, 1974; Davis, 1985). Coastal drifters are typically deployed at a fixed depth in a small array, often with the expectation of recovery. The movement and deformation of the array is used to calculate mean flows, dispersion, and submesoscale features (Winant et al., 1999; Rypina et al., 2016; Ohlmann et al., 2017). Drifters are particularly well suited to study the Lagrangian evolution of scalar fields such as temperature, chlorophyll, and dissolved oxygen, and quantify habitat connectivity (Carlson et al., 2016). Miniaturized Lagrangian drifters with buoyancy control (Jaffe et al., 2017) allow the vehicle to mimic behavior of larvae and other nearshore and coastal plankton.

In the past decade, gliders (and to a lesser extent AUVs) have become the primary means of mapping coastal shelf hydrographic structure (Castelao et al., 2008; Todd et al., 2009; Rudnick, 2016), harmful algal blooms (Schofield et al., 2008; Zhao et al., 2013), and hypoxia (Adams et al., 2016; Perry et al., 2013) on time scales of days to weeks. At present, the utility of buoyancy-driven gliders typically decreases as bottom depths shoal in the inner shelf: peak through-water horizontal speeds of 20–50 cm s–1 may be insufficient to deal with strong coastal currents. Strong stratification (e.g., due to river plumes) reduces the power of the buoyancy engine, and gliders typically require a few meters vertically to transition from descending to ascending flight. The integration of auxiliary propellers in “hybrid” gliders and ongoing work focused on adaptive path planning (Smith et al., 2010; Chang et al., 2015; Smedstad et al., 2015) will likely reduce some of these constraints in the near future.

Propeller-driven AUVs are readily capable of operating in these shallower areas as their peak speeds of more than 2 m s–1 are sufficient for overcoming most coastal currents. These speed gains come at the cost of deployment duration, however, and AUVs are typically deployed for hours to days. Owing to the battery requirements for propeller-driven vehicles, AUVs can carry a heavier instrument payload, including Doppler velocity logs or inertial motion units that greatly aid navigation. These advanced navigational capabilities are well matched to the need for more precise measurements of features as depths become shallower or the features of interest become smaller or more dynamic, such as thin layers (Wang and Goodman, 2009, 2010).

Figure 1.  High-resolution observations of the buoyant Chesapeake Bay plume made with a REMUS 600 during the transition to upwelling. Four cross-shore transects over 32 hours highlight the ability of coastal ALPS to carry a suite of sensors. Here, the cross-shore salt flux (positive onshore) is computed from the onboard current profilers and vehicle profiles of salinity. The offshore extent of the transects were determined adaptively during the mission via an onboard computer.

 Closer to shore, buoyant coastal plumes from rivers and estuaries can occupy variable portions of the shelf. Because these coastal plumes can rapidly transport terrestrial material tens to hundreds of kilometers along the coast, their fate and the mechanics that drive their variability are of great interest. A variety of ALPS have been used to study these features including drifters (Warrick et al., 2007), gliders (Schofield et al., 2010, 2013) and AUVs (Rogowski et al., 2012; Figure 1).

On the inner shelf and in the nearshore, the diversity of habitats increases as benthic topography becomes more varied in composition and form as a result of, for example, kelp forests, rocky reefs, deep coral reefs, and sand flats; sediment composition varies from sand to mud with proximity to rivers. AUVs have been readily employed to map these benthic habitats (Raineault et al., 2012) and their flow structures (Jones and Monismith, 2008) and to understand how fish utilize habitats (Grothues et al., 2008; Haulsee et al., 2015). Drifters are commonly used to understand surf zone dynamics (Ohlmann et al., 2012; Herdman et al., 2017). A growing area of research uses tagged animals to carry sensors through these environments (see Roquet and Boehme, 2018, in this report). In more protected coastal waters—estuaries, fjords, barrier lagoons, and mangrove swamps—currents are swifter, bathymetry is more complex, and the risks posed by recreational and commercial vessels are more acute. Despite these challenges, AUVs have been used to study the evolution of estuarine hydrographic structure (Giddings et al., 2012; Figure 2)

Figure 2. Repeat transects of salinity structure taken with a REMUS 100 during ebb tide in Elkhorn Slough, California, highlight the ability of autonomous underwater vehicles to operate in complex, energetic coastal environments. (left panel) Salinity observations mapped onto vehicle location, during transect in panel (e). (right panels) Time evolution of salinity deviation from mean transect salinity, S’ = S − mean(S). Time of transects is shown in top panel, with moored salinity traces from 1 m and 3 m depth.

Lagrangian drifters have been used to measure circulation and dispersion (Spencer et al., 2014). As with nearshore subtidal habitats, these ecosystems are ripe for rapid innovation of ALPS in support of scientific questions. With properly equipped vehicles or drifters, we will see measurements connecting biogeochemical fluxes between adjoining marshes and open channels and research that brings new insights into how estuaries are linked to open coasts. Modeling across these domains is challenging due to the need for very high grid resolution, and ALPS will provide important validation and assimilation data.

A common theme for all of the habitats and platforms mentioned above is that the energetics of these environments pose operational challenges. Drifters may remain in an area of interest for only a short period of time, gliders may be swept off course, and the ability to drive a vehicle to keep up with these currents comes at the cost of endurance. But it is this same dynamic environment that will drive innovation in the use of ALPS as part of a suite of measurement and modeling tools.

We speculate that the continuing development of ALPS technologies coupled with the emergence of low-cost electronics and sensors will drive innovation in the use of ALPS. The most significant innovations that enable new research directions will be related to operating software, vehicle design, and the development of new sensors.

Coastal research will benefit significantly from smart mission and path planning. It will become commonplace for ALPS to use numerical forecasts in order to optimize the goals of the researcher, not just with regard to power efficiency but also with regard to scientific data collection. While large-scale experiments have been conducted that incorporate planning along these lines (Curtin et al., 1993; Leonard et al., 2010), it seems likely that such capabilities will become built-in features of the next class of robot operating systems. Current research in swarm capabilities will be extended into the realm of heterogeneous fleets, which will facilitate the development of networks of ALPS (and UAVs) that can sample cooperatively, leveraging the strengths of different platforms.

More robust operating suites will enable the development new vehicle forms. One such vehicle could be a hybrid drifter/lander, in which the vehicle can selectively use forecast cur- rents to move throughout the ecosystem, alternating between collecting moored time series at the bed, vertical profiles, and Lagrangian tracks. Another possible vehicle could follow the “flying fin” form factor of the Sentry vehicle. While existing AUVs tend to be torpedo-shaped for efficient forward travel, shortening this form and stretching the vehicle vertically increases mobility and stability, particularly with the inclusion of ducted thrusters. Such a platform would be equipped with advanced imaging equipment and capable of tracking and studying individual organisms, profiling vertically in complex terrain, and performing detailed bottom mapping. These and other ALPS will be further advanced as researchers and engineers repurpose existing and emerging sensors into oceanographic applications. These innovations will be enabled, in part, by the popularity of low-cost electronics (e.g., Arduino) and technologies developed for mobile computing and smart phones. Technological developments in the self-driving car industry will lead to a rapid expansion of biological and ecological studies where benthic imaging is important, owing to advances in image processing and new applications of machine learning. Collectively, these new capabilities will enable advanced animal behavior studies using vehicles that would typically only be possible with scientific diving.

Taken together, these technological changes may have the greatest impact on research in nearshore subtidal habitats. The maturation of image processing and recognition software will drive new research in benthic studies, and biologists and ecologists will be an important driving force of AUV capabilities. Likewise, because vegetation and steep, complex terrain pose navigational challenges to underwater vehicles in these environments, innovations that enable vehicles to better cope with these challenges will significantly advance research applications. Finally, these nearshore habitats are also ideal locations for UAVs to be used for surfzone dynamics, water sampling, low-level remote sensing applications, and wildlife surveys.

 

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Authors

Nicholas Nidzieko, Department of Geography, University of California, Santa Barbara, CA, USA, nidzieko@ucsb.edu

Catherine Edwards, Skidaway Institute of Oceanography, University of Georgia, Savannah, GA, USA, catherine.edwards@skio.uga.edu

Robert Todd, Physical Oceanography Department, Woods Hole Oceanographic Institution, Woods Hole, MA, USA, rtodd@whoi.edu