Spatial dispersion of aerial plankton over east-central Florida: Aeolian transport and coastline concentrations

Abstract

Based on previous radar observations that diurnal 'aerial plankton' (i.e., the ensemble of small, weakly flying insects that drift with the wind ) is usually absent over the ocean, we hypothesized that the insect targets employ flight tactics to resist being drifted overwater. We expected that if aerial insects actively resist overwater drift, then (1) radar-observed clear-air echoes should accumulate along the coastline when winds blow offshore, but they should be advected inland rapidly when winds blow onshore; and (2) accumulations of aerial plankton should be greatest along stretches of coastline most nearly perpendicular to the ambient wind flow. We tested these predictions by comparing the distribution of clear-air radar reflectivity at different sites and under different regimes of wind during the Convection and Precipitation/Electrification (CaPE) Experiment, which was conducted on the east coast of Florida in the summer of 1991. Elevated levels of clearair reflectivity were evident along the coast on mornings when winds were from the west, but not on mornings with easterly winds. On days with westerly winds, this coastal enhancement occurred only in the morning, prior to the onset of the sea breeze. Following sea breeze development, coastal concentrations of clear-air echoes were advected westward and they dispersed rapidly; levels of clear-air reflectivity became depressed overall throughout the study area in the afternoon compared to the morning. Clear-air reflectivity was higher along a north-facing coastline compared to a south-facing coastline when winds had a northward component, with a mirror-image reversal of this pattern on days with southward winds. In contrast, clear-air echoes were uniformly nearly absent from the airspace over the adjacent ocean at all times of the day on all days. These findings support the hypothesis that the organisms comprising the aerial plankton respond behaviourally to coastlines to avoid being blown out to sea. Persistent concentrations of aerial plankton along coastlines may have important consequences for insect population dynamics and local ecosystem structure in coastal regions.