Diatom of the Month February 2019 - Diatoms (and biomonitoring) in dwindling rivers

Andrea Burfeid Castellanos¹ explains to us why we should care about temporary rivers and how diatoms can better assess its ecological status



Intermittent rivers and ephemeral streams (also known as IRES) are a growing concern worldwide. The myriad of ecosystem services and high biodiversity levels they support are severely jeopardized due to the synergistic effects of climate change (i.e., droughts) and increased human usages (Datry et al., 2016). These water bodies are characterized by their temporary or chronic reduction of flow (Stubbington et al., 2017), which can occur naturally or be caused by anthropogenic influences. Most of the studied temporary streams can be found in the Mediterranean climate, where dry summers and torrential downpours in autumn produce substantial variations in water volume (Kalogeropoulos and Chalkias, 2013). These changes also affect water quality, since a reduced flow produces an increase in nutrient and particle concentrations (Becker, 2014).




Climate change predictions posit that Mediterranean water bodies will be affected harshly by increasing droughts (Fig. 1) and that temperate regions could be affected in the future. Although this problem is known, the European Water Framework Directive (European Commission, 2000, p. 60) (WFD) does not cover these types of rivers. The Directive should arguably extend their remit to IRES because these water bodies are often permanently used for agricultural and industry purposes. To support better management of IRES, the LIFE TRivers project was set up to create a software to identify Mediterranean IRES (Gallart et al., 2017) and compare the effectiveness of “normal flow” bioindicators (using macroinvertebrates, fishes, macrophytes and diatoms). Here we will focus on diatoms, since their short reproduction cycle makes them the best indicators of eutrophication (Hering et al., 2006). Their increased taxon richness in temporary rivers also makes them good indicators of connectivity disruption (Novais et al., 2014).




Figure 1. Projections of the change in minimum river flow in Europe. Relative change of minimum river flow in a 20 years recurrence interval, modeling the future period on 1961-1990 (SRES A18) (Source: modified from European Environment Agency, JRC, European Commission, 2012)

Our research group sampled diatoms bimonthly in 24 Iberian rivers and streams following standard European guidelines for (perennial) rivers (CEN, 2014). These rivers were characterized by their flow characteristics (Fig. 2) and water regimes (dividing water bodies into perennial rivers, intermittently dry streams or intermittent pools). We identified 408 diatom taxa (Burfeid Castellanos, 2018) and categorized them by their life forms (Rimet and Bouchez, 2012), ecological guilds (Passy, 2007) and intrinsic characteristics (Round et al., 1990; Van Dam et al., 1994). These categories group diatom species by attachment mode to the substratum, intra- and interspecific species interactions, and resource (nutrients, light) utilization among others. We also calculated index values (index of poluo-sensitivity and biological diatom index, hereafter IPS and IBD, respectively) to compare their suitability in IRES in the case of connectivity loss.


Figure 2. Comparison of aquatic states of Mediterranean streams. The Tossa Stream is depicted during arheic, eurheic and oligorheic conditions, while the Pineda stream is shown during hyporheic conditions.

The results show that, even though diatom species were similar over time at each sampling site, distinct taxa, life forms, and ecological guilds were found in different aquatic states (Fig. 3). Mostly subaerial and epiphytic diatom species were found in hyporheic (saturated substrate) stream samples. In Arheic (pool) samples, however, diatoms tended to be related to lentic conditions in their taxonomy and functional characteristics. Taxa and life forms expected in lotic were found in systems with Oligorheic (reduced flow) and Eurheic (normal flow) states.




Figure 3. Diatom genera and life forms found in different aquatic states (for more information see text above and Rimet and Bouchez, 2012).


We found that diatom indices performed adequately only in permanently connected states in perennial rivers (Burfeid Castellanos et al., 2017a). Depending on the lack of perennial water flow, trophic indices, such as IPS and IBD, cannot reflect the shift towards drought-tolerant diatom taxa which are characterized by different appraisals of sensitivity and tolerance in each index (Burfeid Castellanos et al., 2017b). We concede that in arheic and hyporheic conditions sampling was inconsistent with the diatoms colonizing the sites. Therefore, we need a homogenous sampling methodology and a specialized bioindication index for temporary rivers to obtain more robust findings. We are currently working on a functional characterization of diatom communities of these temporary rivers to create a bioindication tool for these water bodies in the Iberian Peninsula*.



*Recommendation: read about the analogous Balearic Island Index in reconnected temporary rivers (DiatMIB) (Delgado et al., 2012). This index studies not only diatoms, but also chlorophyll a concentration.
This summary is based on the results of the project LIFE TRivers (LIFE13 ENV/ES000341) run by the freshwater ecology, hydrology and management lab of the University of Barcelona and coordinated by Núria Cid, whom I thank profusely. It was part of my doctoral thesis. I also thank Michael Kloster, Xavier Benito and Luca Marazzi for their constructive comments.

¹ Department of Polar Biological Oceanography, Alfred-Wegener-Institute, Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany


If you have comments about this post, you can drop a message below, email Andrea and visit her web or connect with her Twitter.




Bibliography
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