SPIE Newsroom
10.1117/2.1200601.0093
Satellites capture first glimpse of
‘milky-sea’ glowing-water
phenomenon
Steven Miller and Steven Haddock
Rare glowing-water effects known as ‘milky seas’ are generally attributed to bacterial bioluminescence, but their causes are still a mystery. Low-light satellite measurements have now provided the first
overview of a milky sea, observed off the Somali coast and spanning
∼15,000km2 . The findings offer new hope for learning about these extraordinary displays.
‘Milky seas’ are rare nocturnal events where the ocean gives off
a radiant glow that is reminiscent of a snow field and bright
enough to read by. Witnessed over the centuries by mariners
crossing the Indian Ocean,1 milky seas have been associated
more with legends of the sea than scientific knowledge.
More recently they have been attributed to bacterial
bioluminescence2 but their cause, distribution, and role remain
mysteries. In particular, the environmental circumstances supporting the seemingly unrealistic bacterial populations needed
to produce milky seas have been a topic of debate in the microbial ecology community for decades. Some evidence points to
bacterial colonization upon surface slicks of organic matter,3 but
observations of milky seas under higher wind conditions (where
slicks would break apart) challenge this hypothesis.
One point of consensus is the fundamental importance of additional in situ observations to help understand the cause of
these elusive macro-scale processes. The problem has always
been finding a practical way to locate them. Satellite sensors represent a previously unexplored tool for this study.4
Perhaps surprisingly, the night-time hours feature a rich diversity of visible light, both natural (moonlight reflection, lightning, fires, and aurora) and artificial (cities).5 The DefenseMeteorological-Satellite-Program6 (DMSP) series of military
weather satellites carries the operational linescan system (OLS).
This instrument is capable of detecting very weak light sources
(roughly one million times fainter than the solar disk). Despite
this sensitivity, the OLS cannot detect the more common bio-
Figure 1. (A) Raw digital data for 25 January 1995 at 1836 GMT, collected by the Defense Meterorological Satellite Program’s operational
linescan system. (B) Noise-filtered image with the reported entry (a)
and exit (b) coordinates of the British merchant ship S. S. Lima overlaid. Adapted from Miller et al. 2005.
luminescence events associated with disturbed-water (breaking
waves, ship wakes) due to their, typically small, extent. HowContinued on next page
10.1117/2.1200601.0093 Page 2/3
SPIE Newsroom
ever, the steady, widespread glow of milky seas makes them a
better candidate for OLS detection.
Correlating with ship reports
We searched a collection of ship reports for milky sea sightings
and found an encounter by the British merchant vessel S. S. Lima
in 1995 off the Somali coast that provided sufficient detail to pursue. An archive of OLS data collected since 1992 is available at
the National Geophysical Data Center. We used this to match the
ship observations to any discernable coherent feature in the OLS
night-time imagery (after pre-processing to remove instrument
noise). As the OLS signal-to-noise characteristics vary greatly
across its swath, some luck was needed to capture the low-light
feature in the right place and at the right time.
An OLS pass collected at around 30min into the S. S. Lima
encounter revealed a faint anomaly in a patch of water east of
Somalia. The data were digitally processed by subtracting the
mean noise, applying a ∼ 10 × 10km (3 × 3 pixel) coherency
filter (requiring two-thirds of the data to exceed a noise-floor
threshold), and running a ∼ 20km (6 pixel) boxcar smoothing
process. Ship positions for entry/exit of the glowing waters were
overlaid, revealing a close match to the OLS observations (see
Figure 1). The bright feature persisted over three nights, displaying a rotational evolution consistent with local sea-surfacecurrent analyses.
Calculating potential bacterial population
Under the assumption that bacteria were indeed responsible for
milky seas, we estimated the minimum population that would
be detectable by the OLS sensor. Lab cultures of bioluminescent
bacteria with emission properties that are similar to commonlyfound free-living species were grown. Their spectrum and percell photon emission were recorded. Given the OLS minimum
detectable power per unit area (adjusted to account for atmospheric attenuation and partial overlap between the OLS response and the bacterial spectra), the total equivalent bacteria
per unit area was computed. This allowed for extrapolation to
the total population based on the glowing surface area that was
observed by the satellite.
Our conservative estimate of 4×1022 cells participating in this
event is roughly the same as the estimated total background freeliving bacteria present in the upper 200m of all the oceans of the
world.7
Satellite remote sensing has provided our first objective view
of the scale, structure, and—in some respects—integrity of a
maritime legend. Indeed, the space perspective has enabled
an ‘infusoria’ population estimate dismissed as impossible by
the knowledgeable Professor Aronnax, protagonist of the Jules
Verne’s classic novel 20,000 Leagues Under the Seas.8 More importantly, this capability instills hope within a research community
hindered by the elusive nature of milky seas.
Information available from the satellite observations is
inherently limited so additional details on milky seas must await
in situ observations by research vessels properly equipped for
depth-resolved chemistry, organics, and radiometry. The next
step in our research is to follow several new leads that have
emerged since the announcement of our findings. Our focus
will then shift towards the National Polar-orbiting Operational
Environmental Satellite System (NPOESS), which is due to come
into operation in around 2010. This system is scheduled to
carry low-light sensors, which are currently being examined to
determine their potential for bioluminescence detection. These
could hold promise for future observations of milky seas and
possibly the first satellite-directed ship excursions into the
surreal.
Author Information
Steven Miller
Naval Research Laboratory
Monterey, CA
http://www.nrlmry.navy.mil/NEXSAT.html
Dr. Steven Miller is a satellite applications meteorologist at the
Naval Research Laboratory in Monterey. He received his M.S.
(1997) and Ph.D. (2000) degrees in atmospheric science from Colorado State University. His current interests center around design, development, and implementation of value-added environmental characterization algorithms based on near real-time
multispectral/multisensor satellite observations.
Steven Haddock
Monterey Bay Aquarium Research Institute
Moss Landing, CA
http://www.mbari.org/staff/haddock
Dr. Steven Haddock is a biologist at the Monterey Bay
Aquarium Research Institute in Moss Landing, CA. After undergraduate studies in engineering, he received his Ph.D. from the
University of California, Santa Barbara. He studies bioluminescence, fluorescence and deep-sea gelatinous zooplankton observations.
References
1. P. J. Herring and M. Watson, Milky seas: a bioluminescent puzzle, Mar. Obsvr. 63,
pp. 22–30, 1993.
2. K. H. Nealson and J. W. Hastings, Bacterial bioluminescence: its control and ecological significance, Microbiological Reviews 43, pp. 496–518, 1979.
3. D. Lapota et al, 1988, Observations and measurements of planktonic bioluminescence
in and around a milky sea, J. Mar. Exp. Mar. Biol. Ecol. 119, pp. 55–81, 1988.
4. S. D. Miller, S. H. D. Haddock, C. Elvidge, and T. F. Lee, Detection of a bioluminescent milky sea from space, Proc. Nat. Acad. Sci. 102 (40), pp. 14181–14184, 2005.
Continued on next page
10.1117/2.1200601.0093 Page 3/3
SPIE Newsroom
5. T. A. Croft, Burning waste gas in oil fields, Nature 245, pp. 375–376, 1973.
6. R. W. Lieske, DMSP primary sensor data acquisition, Proc. Int. Telemetering Conf.
17, pp. 1013–1020, 1981.
7. W. B. Whitman, D. C. Coleman, and W. J. Wiebe, Prokaryotes: the unseen majority,
Proc. Nat. Acad. Sci. 95, pp. 6578–6583, 1998.
8. J. Verne, 20,000 Leagues Under the Sea, William Butcher (trans.), Oxford University Press, New York, 1998 (originally published 1870).
c 2006 SPIE—The International Society for Optical Engineering