Abstract
This study aims to contribute to the understanding of the impact of Didymosphenia geminata massive growths upon river ecosystem communities’ composition and functioning. This is the first study to jointly consider the taxonomic composition and functional structure of diatom and macroinvertebrate assemblages in order to determine changes in community structure, and the food web alterations associated with this invasive alga. This study was carried out in the Lumbreras River (Ebro Basin, La Rioja, Northern Spain), which has been affected by a considerable massive growth of D. geminata since 2011. The study shows a profound alteration in both the river community composition and in the food web structure at the sites affected by the massive growth, which is primarily due to the alteration of the environmental conditions, thus demonstrating that D. geminata has an important role as an ecosystem engineer in the river. Thick filamentous mats impede the movement of large invertebrates—especially those that move and feed up on it—and favor small, opportunistic, herbivorous organisms, mainly chironomids, that are capable of moving between filaments and are aided by the absence of large trophic competitors and predators -prey release effect-. Only small predators, such as hydra, are capable of surviving in the new environment, as they are favored by the increase in chironomids, a source of food, and by the reduction in both their own predators and other midge predators -mesopredator release-. This change in the top-down control affects the diatom community, since chironomids may feed on large diatoms, increasing the proportion of small diatoms in the substrate. The survival of small and fast-growing pioneer diatoms is also favored by the mesh of filaments, which offers them a new habitat for colonization. Simultaneously, D. geminata causes a significant reduction in the number of diatoms with similar ecological requirements (those attached to the substrate). Overall, D. geminata creates a community dominated by small organisms that is clearly different from the existing communities in the same stream where there is an absence of massive growths.
Highlights
The high transport capacity of our globalized society has allowed invasive species to become one of the main threats to biodiversity around the world, especially with relation to inland aquatic ecosystems [1,2]
Among the various invasive species described in Iberian rivers, we found the alga Didymosphenia geminata, which has been recently included in the Spanish Invasive Species Catalog (RD 630/2013)
The specific objectives of this work are: i) to assess the degree of change of the taxonomic composition and functional structure of diatom and macroinvertebrate assemblages related to massive growths of D. geminata; and ii) to determine if these changes led to large-scale trophic and community structure alterations in the river food webs
Summary
The high transport capacity of our globalized society has allowed invasive species to become one of the main threats to biodiversity around the world, especially with relation to inland aquatic ecosystems [1,2]. D. geminata is a diatom that, under certain environmental conditions [6,7,8,9,10], is capable of producing a large amount of extracellular stalks, creating massive growths. These biological episodes can cover the river bed for several kilometers, profoundly altering the environmental river conditions. In line with this observation, many papers note that D. geminata has a considerable impact on aquatic ecosystems based on the assumption that the large biomass of this species will have negative consequences for other species [11]
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