Effects of Didymosphenia geminata massive growth on stream communities: Smaller organisms and simplified food web structure

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.

Lentification in Alpine rivers: patterns of diatom assemblages and functional traits

“…This result was probably due to the fact that we never sampled completely dried sites, since we monitored the lentification process, not completely dry riverbeds. The pattern followed by mucous-forming taxa, such as those belonging to the genus Encyonema, also seems related to the presence of Didymosphenia geminata, confirming previous results (Spaulding et al 2010;Ladrera et al 2018).…”

Community signals of the effect of Didymosphenia geminata (Lyngbye) M. Schmidt on benthic diatom communities in Chilean rivers

“…The main effect found in this study was the homogenization of the benthic diatom communities, which is reflected in the increase in density of small stalked diatoms and the reduction of species turnover of invaded rivers among basin. Although we did not detect signals of species exclusion, D. geminata favors a particular group of diatom species which in the long run may displace others and thus trigger different types of impacts, including alterations in the fluvial trophic webs [44]. The effect of D. geminata may be expressed in different components of diversity and may surpass the effect of spatial scale.…”

Freshwater conservation planning informed and validated by public participation: The Ebro catchment, Spain, as a case study

“…For example, in the Ebro catchment (where this study was performed) the invasions by the mussel Dreissena polymorpha and the crayfish Procambarus clarkii have led to drastic habitat modifications, such as the loss of aquatic macrophytes (Durán, Lanao, Anadón, & Touyá, 2010;Oscoz, Tomds, & Duron, 2010;Souty-Grosset et al, 2016). Similarly, the massive growth of the invasive diatom Didymosphenia geminata in some reaches of the Ebro catchment have significantly affected the structure and functioning of the macroinvertebrate communities (Ladrera, Gomà, & Prat, 2018;Ladrera, Rieradevall, & Prat, 2015). Given the recognized importance of invasive species for disrupting ecosystem processes, they should be explicitly considered when designing protected areas (Decker, Linke, Hermoso, & Geist, 2017;Turak et al, 2018).…”