Hydrological intermittency drives diversity decline and functional homogenization in benthic diatom communities

Abstract

Over the last decades, the combined effects of global climate changes and severe land use modifications have been exacerbating river hydrological alterations and habitat fragmentation in many Mediterranean rivers. This trend is predicted to intensify, with expected significant impacts on taxonomic and functional diversity of benthic communities in the next future. The present research aims at investigating the long-term combined effects of flow intermittency, climate and land use changes on benthic diatom communities, by analysing data collected over 11 years in Mediterranean streams of the NW-Italy. We demonstrated that the ongoing global changes and local environmental pressures determined a significant decline in diatom species diversity at both local and regional scales. More in detail, flow intermittency affected both diatom diversity and life history traits, with communities of intermittent reaches taxonomically and functionally different and less heterogeneous than assemblages characterizing perennial ones. Communities inhabiting intermittent sections showed high percentages of small, mainly stalked and pioneer taxa belonging to the low profile guild, highlighting the strong environmental pressure exerted by the hydrological alterations. Conversely taxa colonizing permanent reaches were bigger, belonging to the high profile guild and able to produce colonies, denoting environmental stability. The results we obtained could be ascribed to the long-term effects of drying in Mediterranean streams and, as first in the literature, we highlight that diatoms are able to provide long-terms responses to environmental changes caused by water stress, when hydrological disturbance is persistent.

Diatom communities and ecological status classification in the upper Po River basin

“…Our data showed a strong homogenization of diatom species composition along the increasing disturbance gradient represented by the LIMECO classification (see the PCoA), confirming results obtained by Pillsbury et al (2019). Recently, homogenization of diatom communities has also been observed in response to hydrological alterations and urbanization in Mediterranean rivers which also led to significant species loss at both local and regional scales (Falasco et al, 2021a) and a reduced role of diatoms in the total benthic chlorophyll-a (Piano et al, 2016). Moreover, the present study confirmed the high trophic value of some species, mainly collected in the R-C macrotype, such as N. gregaria (Hicks and Taylor, 2019;Hausmann et al, 2016;Szczepocka et al, 2016), Nitzschia palea (Licursi et al, 2016;Szczepocka et al, 2016;Yang et al, 2015), Gomphonema parvulum (Lu et al, 2020;Szczepocka et al, 2016) and C. euglypta (Trábert et al, 2020).…”

“…Intermediate LIMECO categories were also characterised by the highest guild diversity confirming the hypothesis that thick biofilms host strong resource gradients that can be exploited by different guilds in different ways (Stevenson et al, 1996). Once more, the functional analysis of the diatom community provided important and interesting results, which improved the information provided by the taxonomic composition analysis (Falasco et al, 2020;2021a).…”

Functional Responses of Phytoplankton Assemblages to Watershed Land Use and Environmental Gradients

“…In this study, FRic of phytoplankton communities decreased with increasing land-use intensity. Similar patterns of functional changes of community in response to environmental disturbance have been reported, for instance, a reduction in functional diversity of aquatic insects in southeastern Brazil (Castro et al, 2018), and of fishes in Malaysia (Wilkinson et al, 2018), and of benthic diatoms in Italy (Falasco et al, 2021). Potentially higher functional diversity represented much higher levels of productivity and reliability (Dìaz and Cabido, 2001;Vallina et al, 2017).…”

Are protected areas effective in preserving alpine stream morphology and biodiversity? An experience in the first Italian National Park

“…In protected areas, water use for human purposes (such as recreational or energetical) is generally minimal, and catchment integrity is mostly maintained, so as spatial and temporal river connectivity and habitat heterogeneity. All these features significantly contribute to the maintenance of a high functional richness of biological communities, as demonstrated by recent research Falasco, Bona, Risso, & Piano, 2021;B-Béres, Stenger-Kovács, Török, & Török-Krasznai, 2021). It has been recognized that high functional richness increases community resilience against extreme events, which is expected to significantly increase under a global climate change scenario (Mason, Mouillot, Lee, & Wilson, 2005;Doretto et al, 2020;B-Béres et al, 2021).…”