Linkage between aquatic and terrestrial habitats aroused scientific interest on ecological subsidies, i.e. how the transfer of matter and energy between adjacent ecosystems can modify their own functioning. Recent attention was paid to subsidies associated with aquatic insects emerging from streams and rivers in pristine areas as they are deeply involved in adjacent terrestrial food webs. Freshwater ecosystems are widespread in agricultural landscapes, however the possible implication of aquatic subsidies is rarely considered. Substantial evidences in literature shown that agriculture practices significantly altered production and community of benthic macroinvertebrates of streams. These changes may resulted both in changes in magnitude and dispersal of subsidies impacting terrestrial ecosystems, but data are missing. Hence, measuring emerging biomass of winged aquatic insects and their dispersal is crucial to better understand or anticipate cascading effects originating from stream toward adjacent agricultural ecosystems. To fill this knowledge gap, we carried out a field work to study winged aquatic insects emergence and dispersal from twelve second-to-third order lowland streams in an intensive agricultural landscape in the northwest of France for one year.
Results/Conclusions
Three dominant taxonomic groups (Chironomidae, Ephemeroptera, Trichoptera) emerged and reached a mean total drymass of 4 g DM. m-2.yr-1. Chironomidae were the most abundant (>90%) and emerged almost constantly through the year (~ 1.4 g DM. m-2.yr-1). Trichoptera (~ 2 g DM. m-2.yr-1) and Ephemeroptera (~ 0.6 g DM. m-2.yr-1) emergence occurred mainly in spring. Locally, habitat parameters enhancing light received by stream (e.g. channel width, riparian density) promote emerging biomass of all three groups therefore denoting the importance of autotrophic pathways for agricultural stream. Conversely, incised channel, which is typical of agricultural landscape, lessened Trichoptera and Ephemeroptera emergence while having no effect on Chironomidae. Proportion of agricultural land is moreover positively correlated with emergence of Chironomidae. Primary results on dispersal highlighted the significant presence of aquatic insect biomass up to 50 meters away from stream channel in crops. Seasonal patterns and habitat parameters variously affected dispersal of each taxonomic groups but further works are needed. Therefore, our study provide valuable data on how time, landscape and habitat affect both magnitude and dispersal of winged aquatic insects in agriculture landscape. These results should guide further work on the effect of such subsidies on terrestrial food webs in agricultural landscape.