Aerial plankton detected by radar

SIR - For many years, meteorologists using sensitive research radars have observed widespread echoes in precipitation-free areas of the diurnal atmosphere. Although the source of these clear-air echoes has been controversial, recent studies have demonstrated that they are attributable to weakly flying insects that drift with the wind1-3. Such insects, whose dispersal in the atmosphere is governed largely by the wind, have been referred to collectively as the "aerial plankton"2-5.

We used powerful C-band Doppler radars6 to study relationships between wind patterns and the spatial distribution of aerial plankton during the Convection and Precipitation/Electrification (CaPE) experiment, which was conducted on the east coast of Florida from 8 July to 18 August 1991. On most days during the summer, a mesoscale sea-breeze circulation7 develops in this area as a consequence of the different thermal properties of land and water. Low-level winds early in the morning are typically light and from the west. Daytime heating causes warmed air over the land to expand and rise, generating a high-altitude outflow from the land and a compensating low-level inflow of cooler air (the sea breeze) from over the Atlantic Ocean.

The effects of sea-breeze development on the distribution of aerial plankton were dramatic. The figure shows a time series of radar images for 11 August 1991. In the morning, when the winds blew towards the water, the density of aerial plankton was visibly enhanced along the coastline. Following the onset of the sea breeze, the coastal concentrations were advected westward and dispersed rapidly. Densities of aerial plankton became depressed overall in the afternoon compared with the morning throughout the study area.

The coastal concentrations, together with the absence of overwater echoes under all wind conditions, suggest that the radar targets responded behaviourally to landscape features to avoid being drifted out over the ocean. The behavioural mechanisms involved in the avoidance response require further study, but the selective advantage of this response is clear. Terrestrial insects that have drifted to sea probably deplete their fuel stores rapidly and fall to the sea surface. Extensive mortality in these insects should impose strong natural selection for flight behaviours that result in avoidance of overwater drift.

Although largely ignored to date by terrestrial biologists, sensitive Doppler radars have tremendous potential as platforms for conducting basic research on the ecology of aerial plankton. The US National Weather Service is modernizing its radar network, replacing 1950s-vintage units with new NEXRAD Doppler radars8 which have similar capabilities to the radars used in the CaPE experiment. The availability of this technology9 should promote interest in the aeolian ecology of aerial plankton, and will eventually allow comparisons of the planktonic faunas of the ocean and the atmosphere.

Footnote

1. WIlson, J. W. et al. J. atmos. oceanic Tech. 11, 1184-1206 (1994). 2. Russell, R. W. thesis (Univ. California, Irvine, 1994). 3. Russell, R. W. & Wilson, J. W. Boundary-Layer Met. (in press). 4. Johnson, C. G. Migration and Dispersal of Insects by Flight (Methuen, London, 1969). 5. Drake, V. A. & Farrow, R. A. Trends Ecol. Evol. 4, 381-385 (1989). 6. Keeler, R. J. et al. 25th int. Conf Radar Met. 859-862 (Am. Met. Soc., Boston, 1991). 7. Simpson, J. E. Sea Breeze and Local Winds (Cambridge Univ. Press, Cambridge, 1994). 8. Heiss, W. H. et al. Microwave J. 33, 79100 (1990). 9. Crum, T. Bull. Am. met. Soc. 76, 2485-2487 (1995). 10. Brock, F. V. et al. J. atmos. oceanic Tech. 3, 573-582 (1986). 11. Battan, L. J. Radar Observation of the Atmosphere (Univ. Chicago Press, 1973).