The influence of Monterey Bay's seafloor topography on phytoplankton blooms
Synoptic mapping with autonomous underwater vehicles (AUVs) is an effective tool in coastal oceanography and will be necessary to carry out fieldwork to determine the evolution of microbial communities.
Researchers would not have been able to make an important connection between Monterey Bay's seafloor topography and phytoplankton blooms without synoptic maps. Synoptic means to have a comprehensive or wide breadth of view. In this case, synoptic mapping would mean to map a large section of the seafloor in order to produce a detailed and comprehensive image. Since this 2005 study, MBARI has made great technological improvements to AUVs with regards to autonomy for navigation, decision making, and sensing capabilities.
A major coastal upwelling system lies along the eastern margin of the North Pacific. Recently upwelled waters can be distinguished by their relatively cold sea surface temperature (SST) as well as increased productivity due to an increase in nutrients. Within the productive coastal upwelling system lies Monterey Bay in the state of California.
The influence of seafloor topography on the ecology of Monterey Bay is marked by the presence of Monterey Canyon, which extends from the center of the bay to the deep ocean. The canyon and the shelf break in Monterey Bay play an important role on the ecology of adjacent shelf waters through the movement of deep nutrient rich waters. Studies have shown that amplified internal wave activity over canyons can influence shelf ecology by increasing horizontal and vertical fluxes of nutrient rich waters.
The large internal wave movement in the Monterey Bay Canyon is said to account for one-third of the primary production in Monterey Bay during a non-upwelling season. It is difficult to observe continental shelf waters adjacent to the canyon since there are rapidly changing conditions and widely different ecosystem processes.
High-resolution, multidisciplinary observations in diverse marine environments are being enhanced by autonomous underwater vehicles (AUVs).
Autonomous underwater vehicle surveys were made in the region of a coastal upwelling filament north of Monterey Bay, across a frontal zone outside Monterey Bay, and on the Monterey Bay shelf. For each AUV mission, control parameters were programmed on the research vessel and then radioed to the AUV in the ocean. They were able to keep track of the AUV's location in the ocean by using a global positioning system (GPS) as well as a Trackline II baseline system. The AUV was equipped to measure pressure, conductivity, temperature, oxygen, chlorophyll fluorescence, and optical backscatter. A three dimensional track of where the submerged vehicle was traveling through the ocean was determined by "dead-reckoning".
Dead reckoning uses information from the vehicle's propeller rotational speed, crossbow attitude, and the heading reference system. AUVs can be programmed to travel in a straight line, such as "A" shown in the image to the right. They can also be programmed to travel in a "lawnmower" type pattern, such as "B" shown in the image to the right.
*Note- Due to continuous cloud cover, over the Monterey Bay Region, there was no clear satellite sea surface temperature (SST) imagery of the bay during the period of the shelf AUV surveys. Instead SST coverage was accomplished by low-altitude aircraft overflights.
The AUV survey that mapped the head of the Monterey Canyon revealed a plume of relatively cold ocean water that indicated waters that were recently upwelled or vertically mixed to the surface This plume contained a phytoplankton layer and within the wedge of the high-density water was a plume of suspended particulate material. However, the sensors on the autonomous underwater vehicle as well as the low chlorophyll fluorescence of the plume indicated that the particles were not active plankton.
This pattern observed by the AUV indicated that canyon-influenced tidal flow, which is known to force the transport of deep waters from the Monterey Bay Canyon onto the shelf, was the primary cause. The AUV survey suggests that there is a strong canyon-shelf exchange of the upwelling of deep ocean waters. Surface and water column observations from the AUV survey indicate that there was a steering of the plume up the canyon, an upwelling maximum along the downstream of the canyon, as well as the formation of a cold band along the inner shelf downstream of the canyon.
Physical-biological coupling in a frontal zone
The AUV volume survey in the northern shelf waters reveal how the topography of Monterey Bay can have a strong influence on phytoplankton distributions. The image below shows how density and chlorophyll fluorescence are represented within the selected volume of the Monterey Bay. The AUV was able to successfully map 10 meters above the bottom of the seafloor in order to show the chlorophyll fluorescence isosurfaces which define a patchy phytoplankton layer as shown below.
Topographic influence on phytoplankton ecology is supported by these observations of physical–biological coupling through the frontal zone. The isopycnal ridge observed on the northern shelf where the AUV mapped is consistent with internal wave generation in that particular region. Internal waves are associated with surface slicks where an increased concentration of phytoplankton occur. The presence of the highest chlorophyll fluorescence over the isopycnal trough support the role of internal wave dynamics in the physical–biological coupling observed through this frontal zone.
In this case study, flow–topography interactions are indicated as strong influences on the environment and spatial distribution of phytoplankton. These represent processes that can have a consistent influence on the Monterey Bay's ecology. The flux of suspended particles from the canyon/shelf represent the transport of plankton and nutrients. Most importantly, iron is a limited nutrient that helps to regulate phytoplankton productivity in the coastal upwelling system. High concentrations of phytoplankton layers also play a viable role in feeding fish larvae in the Monterey Bay.
These studies may have important implications for harmful algal bloom (HAB) ecology in the Monterey Bay. In one instance the pennate diatom, (P. australis) produces domoic acid, a neurotoxin that harms marine vertebrates and causes amnesic shellfish poisoning in humans. In 1998, a bloom of P. australis resulted in widespread mortality of marine mammals and seabirds in coastal waters of central California (Scholin et al. 2000). The benthic–pelagic coupling observed in southern shelf waters represents a process that may influence sediment and iron flux in the euphotic zone and thus toxin dynamics in P. australis.
Developing a better understand of flow-topograph interactions may allow researchers to:
- Have an early event detection (using Environmental Sample Processors, and moorings) to detect phytoplankton blooms
- Use further synoptic surveillance by using AUV mapping
- To track vast bodies of water in order to capture the ecosystem dynamics
- To be able to predict where future harmful algal blooms (HABs) may occur.
The information above is from the following reference:
Ryan, J.P., F.P. Chavez, and J.G. Bellingham (2005). Physical-biological coupling in Monterey Bay, California: Topographic influences on phytoplankton ecology. Marine Ecology Progress Series, 287: 23-32.