Aerial colonization and growth: rapid invertebrate responses to temporary aquatic habitats in a river floodplain

Floodplains of unregulated rivers alternate between aquatic and terrestrial phases, resulting in temporary aquatic habitats. If aquatic invertebrates are to take advantage of such habitats, they must be capable of rapid colonization and growth. Aerial immigration is one means of colonization and may include active dispersal by reproductive and nonreproductive adult insects and passive dispersal of other invertebrates. The aquatic invertebrate assemblage that could develop from aerial colonization during the time of a flood was investigated with floating colonization trays containing natural detritus and water on the water surface of a southeastern USA floodplain during 5 flooded periods in a year. Density and biomass of aquatic invertebrates that resulted from aerial colonization were measured after 17 d. Minimum growth rates and potential secondary production of several chironomid taxa were measured based on increases in larval size during this time period. Egg-mass collection trays, similar to colonization trays, were placed in the floodplain to estimate an egg-laying flux of chironomids (egg masses m(-2) d(-1)) during the same 5 periods. Aquatic insects, including dipterans, ephemeropterans, and odonates, oviposited in trays during April, June, August, and November, but not January. Adult coleopterans and hemipterans colonized by nonreproductive immigration during June, August, and April. Noninsect invertebrates, such as micro-crustaceans, nematodes, and water mites, colonized the trays during all months, apparently by hitchhiking or wind dispersal. The highest density of insects occurred in August (78,600 ind./m(2)) and the lowest in November (159 ind./m(2)). Chironomids had the highest relative abundance of insects in June and August (> 75%), but did not colonize in November or January. Chironomids had minimum daily growth rates between 0.271 and 0.515/d, and chironomid secondary production ranged between 0 (November and January) and 230.9 mg m(-2) d(-1) (August). Egg-laying fluxes were similar in June (12 egg masses m(-2) d(-1)) and August (6 egg masses m(-2) d(-1)) but no chironomid egg masses were collected in November, January, or April. Aquatic invertebrates may arise from several sources (including recovery from dormancy or drift) in inundated floodplains, but aerial colonization and rapid growth rates alone can reestablish a diverse assemblage quickly, at least during warmer parts of the year. However, floodplains of regulated rivers managed for biological productivity should be inundated for a time period that enables colonization and completion of life cycles, a minimum of 2 wk for the fastest growing taxa.