Observing a Red Tide with Multiple Sensors
by John Ryan, Heidi Dierssen, Raphael Kudela, Christopher Scholin, Kenneth Johnson, James Sullivan, Andrew Fischer, Erich Rienecker, Patrick McEnaney, and Francisco Chavez; Monterey Bay Aquarium Research Institute
Oceanic phytoplankton consists of many species, and dinoflagellates are among those phytoplankton species that cause reddish discoloration of surface waters when they bloom and accumulate near the surface. These conspicuous blooms, known as red tides, are of interest because they can have significant negative consequences for coastal marine life, fisheries and human health.
The adverse consequences of a red tide result primarily from introduction of toxins into the food web, direct physical harm to vital tissues (e.g., fish gills), and from oxygen depletion when the dense algae accumulations respire and decay. These impacts and the associated monitoring efforts can have significant economic consequences. Investigations of dinoflagellate ecology are central to understanding and predicting red tides. Such investigations are extremely challenging due to the complexity of rapidly changing coastal ocean environments and the rudimentary understanding of phytoplankton ecology.
Monterey Bay lies in the central California Current (CC) upwelling system where phytoplankton productivity and abundance are greatly augmented by windforced up-welling of deep, nutrient-rich waters to the shallow sunlit layer. During fall 2002, an intense and widespread red tide suddenly developed throughout the bay. Th e physical oceanography underlying this bloom was investigated using multidisciplinary from satellites, aircraft, ships and moorings.
Observed from the deck of a ship, the red tide was characterized by highly concentrated patches having sharp boundaries. Same-day high-resolution remote sensing from the Airborne Visible-Infrared Imaging Spectrometer (AVIRIS) and satellite SAR during the peak of the red tide detailed the extremely patchy nature of the bloom as well as processes that created the patchiness. We obtained some unique insights provided by this combination of high-resolution remote sensing at the height of the red tide.
Satellite ocean color and temperature imagery and observations from moorings described rapid and dramatic change when a filament of the CC swept through the bay. Observations from a ship-towed undulating vehicle, carrying a multidisciplinary instrument suite to monitor the bay also revealed extensive changes permeating the bay within a fi ve-day period. Conditions became favorable for dinoflagellates, which led to red tide inception in the northern bay. Transport in coastal currents strongly influenced spread of the bloom throughout the bay and out into the adjacent sea.
Understanding patchiness of red tides is central to understanding their impacts, which are modulated by cell concentrations. One process creating patchy aggregations of dinoflagellates is the interaction of swimming with convergent circulation. They move in vertical migrations that enable access to the
resources they need to grow: light that increases in intensity toward the surface, and nutrients that increase in concentration with depth.
They may also use motility to stay near the surface in the presence of downward vertical currents. Where horizontal currents converge and create downward flow, dinoflagellates will be concentrated near the surface if they can swim upward at a rate greater than that of the downward currents.
Internal waves (IW), which vertically displace and propagate along subsurface density layers, are a ubiquitous physical forcing in coastal ocean environments, and they create convergence zones oriented parallel to and overlying the waves. IW are evident in SAR imagery because they modify ocean surface roughness in bands along the wave fronts. The SAR image on this page revealed an IW packet in northern bay waters. The white, 1km scale bar at 36.9°N defi nes an IW wavelength of ~1km. The same scale was evident in the wavelike spatial pattern of a red tide patch in this region, supporting the influence of IW on red tide patchiness.
An earlier SAR image (not presented here) showed no evidence of IW in the region of this patch, thus it is possible that the dinoflagellate aggregations retained an imprint of IW influence from the wave packet shown in the figure. Combining these high-resolution measurements with coarser resolution satellite sea surface temperature and mooring observations showed that at larger scales than the IWs, convergence created by the confluence of water masses also influenced bloom patchiness.
Multiple physical phenomena occurring across a wide range of scales forced initiation and development of this red tide in Monterey Bay. Such biological-physical interactions can only be discerned through multidisciplinary, multi-scale sensing. In situ observations from moorings, ships and autonomous underwater vehicles in the bay provided a rich interpretive framework upon which to build with advancing capabilities in remote sensing.
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