Observing Life Processes in the Sanctuary

Nowhere is the complexity of ocean processes or the need for advanced observation methods greater than in the coastal ocean. This complexity demands investigation of physical, chemical, geological, and biological realms across disciplinary boundaries. Within the rapidly changing fluid environment of the coastal ocean, capturing snapshots of environmental structure can be a powerful tool in the effort to understand processes that shape the environment of marine life. How can we take a snapshot that has conceptual breadth and depth? We need to move an interdisciplinary spectrum of sensors rapidly through large regions of the fluid environment. Ideally, the platform carrying these sensors would be smart enough to respond to the environment it is sampling. Wouldn’t that be nice? …With help of AUVs, it sure is!

An autonomous underwater vehicle (AUV) is an unmanned, untethered, robotic submarine about the size of a large porpoise (or two aligned end-to-end). Programmed with a mission, the onboard computer directs the AUV to fly a course, sense the environment, and record what it senses. AUVs are becoming important research platforms in the Sanctuary. They were used extensively during a major field experiment last summer that brought together researchers from twelve institutions in a multidisciplinary study of processes in the Sanctuary (MUSE; see Ecosystem Observations 2000, page 7 and http://www.mbari.org/MUSE). Observations from the AUV provided remarkably clear definition of complex processes that influence life in the Sanctuary.

Figure 1: High-resolution vertical section of properties measured via autonomous underwater vehicle (AUV) adjacent to Monterey Canyon on August 30, 2000. Particle concentrations are contoured; the highest levels were in the phytoplankton bloom and in a plume of non-living suspended material.

A Bloom and a Plume
During the MUSE 2000 field study, researchers were studying details of a harmful algal bloom (HAB) in Monterey Bay, including species identification with visual and molecular methods and toxin measurement within and around the microbes. In a concerted effort, researchers on another ship deployed an AUV to map the distribution of the bloom. In two hours the AUV surveyed at high-resolution an eight-kilometer section over the continental shelf in the bloom region, oscillating between the surface and bottom and using its ability to detect the bottom to follow the bottom topography. The resulting map (Figure 1) showed that the bloom was concentrated in a subsurface layer approximately five to ten meters thick that closely followed the seawater density distribution. (In Figure 1 the bloom is shaded, and the black contour is a constant density surface that the lower boundary of the bloom followed.)

Physical and biological layers exist across a wide range of scales in marine and estuarine ecosystems. Physical layers structure the fluid environment of ocean life through control of physical and chemical properties, and ocean life processes are concentrated in these layers. There is thus strong motivation to understand the physical-biological coupling underlying these relationships as well as the effects of layers on ecosystem structure and function (e.g., biogeography of marine organisms, survival of larval fish, and bioaccumulation of harmful algal bloom toxins).

We expected to map the structure of the harmful algal bloom, but we were surprised by another dimension of the marine environment sensed by the AUV. Beneath the bloom was a plume of suspended particulate material emanating from Monterey Canyon. The gray contours in Figure 1 delineate regions of high particle concentrations. The particles in the upper water column were largely living particles (phytoplankton), but those in the lower water column were non-living. The shallow phytoplankton layer was flowing opposite the deep plume of suspended material. Between these two flow regimes, particles were being transported from the bottom to the top. Particulate material from the bottom can contain iron-bearing sediments that fertilize productivity of the pelagic ecosystem as well as resting spores of harmful algal bloom species. Did the plume start the bloom by seeding the upper water column with resting spores? Did the plume fertilize the bloom with iron? Is transport from canyon to shelf a persistent influence on the ecology of Monterey Bay? What physical processes forced the transport? These and other questions are being pursued to further our understanding of ecology in the Sanctuary.

Beneath a Surface Slick
Have you ever been out on the bay and observed a surface slick, where the small wind-forced ripples are damped? If so, you have observed environmental structure that extends well beneath the surface. During MUSE, we set out to map an oceanic front on the northern Monterey Bay shelf. Fronts are regions where physical, chemical, and biological variability are concentrated and enhanced, and they are very important to marine ecosystem dynamics. We identified the location and orientation of a front from the ship’s underway mapping system, then deployed the AUV to see what was happening beneath the surface. As we passed through a surface slick, which extended as far as the eye could see, we knew that a new window on complexity was opening because our AUV was flying a high-resolution, three-dimensional sampling pattern. Taking nearly 60,000 measurements from each of six instruments, we thoroughly surveyed a volume of ocean, 7 kilometers x 3 kilometers x 70 meters.

Figure 2: View from above Moss Landing, looking NW down into the ocean. Volume visualization of a subsurface phytoplankton bloom layer (mesh isosurface) on the northern Monterey Bay shelf relative to a constant density surface (gray). The bloom closely followed the density surface and was interrupted at a ridge in the density field. Chlmax identifies the highest chlorophyll fluorescence observed. Only the upper twenty-two meters of the seventy meters surveyed are shown.

Similar to observations south of the canyon (Figure 1), north of the canyon there was a subsurface layer abundant in phytoplankton (Figure 2). The outer boundary of the layer is defined by the black mesh surface. Within the volume inside this boundary, phytoplankton abundance was equal to or greater than that along the outer boundary. The gray surface in Figure 2 is a constant density surface, or isopycnal. We can view the density field as a kind of topography of the ocean interior that illustrates environmental structure and processes. Throughout the domain, the phytoplankton layer closely followed the density field, and a break in the biological layer was aligned with a ridge in the density field. The greatest concentrations of phytoplankton within the layer (Chlmax) were in a trough in the density field, south of the ridge. Internal waves deform the density field, create surface slicks, and concentrate plankton in their troughs. Thus the environmental structure observed beneath a surface slick suggests internal wave processes shaping life in the Sanctuary.

The complex and rapidly changing coastal ocean challenges our investigations of ecology. We seek to enter that complexity and to extract knowledge that can not only guide environmental decision making but also advance methods of study across disciplinary realms. The processes we study in the Sanctuary are important in marine systems around the world.

John Ryan
Monterey Bay Aquarium Research Institute