Drainage‐induced browning causes both loss and change of benthic biodiversity in headwater streams

Abstract

Browning of freshwaters is known to modify food-web structure in lakes via changes to light and nutrient regime. Its effects on biodiversity, particularly in streams and benthic habitats therein, remain largely unknown. We explored the effects of browning on benthic invertebrate biodiversity across a wide gradient of dissolved organic carbon (DOC) concentrations in streams impacted by peatland use. Invertebrate diversity decreased consistently along the gradient whereas community composition exhibited an abrupt change at 12-13 mg DOC L À1 . A few tolerant taxa benefited from browning whereas more vulnerable species, particularly algal scrapers, were greatly reduced. These results show that forestry-related land use enhances browning which not only reduces benthic biodiversity but also causes biodiversity change by homogenizing species assemblages at the high-DOC end of the gradient.

Water quality shapes the community structure of zoobenthos in rivers but only has a minor effect on the fatty acid composition of zoobenthos and salmonids

“…These rivers are dark in water color (ranging 110–190 mg L −1 Pt), which is typical for rivers in the study region (Rouvinen, 2010). Increased concentration of DOC can reduce the abundance and biodiversity of zoobenthos communities (Arzel et al, 2020; Brüsecke et al, 2022) but some taxa, such as mayfly Baetis and Chironomidae might even benefit from it (Bellamy et al, 2019; Robbins et al, 2020). The concentration of DOC (especially humic matter) also decreases the pH (Oliver et al, 1983), which has been found to drive the taxonomical diversity of zoobenthos communities, together with nutrient availability (Baker et al, 2022; Heino et al, 2003).…”

“…These rivers are dark in water color (ranging 110-190 mg L À1 Pt), which is typical for rivers in the study region (Rouvinen, 2010). Increased concentration of DOC can reduce the abundance and biodiversity of zoobenthos communities (Arzel et al, 2020;Brüsecke et al, 2022) but some taxa, such as mayfly T A B L E 4 Mean percentage (%) and content (c, μg mg DW À1 ) of essential fatty acids (EFAs), total fatty acid (FA) content (c, μg mg DW À1…”

Brownification shapes the food web of aquatic invertebrates: a review

“…Brownification was also found to decrease macroinvertebrate abundance (Brothers et al, 2014;Arzel et al, 2020;Turunen & Aroviita, 2024), species richness (Brüsecke et al, 2023;Strandberg et al, 2023a), density (Estlander et al, 2010) and habitat use (Horppila et al,2018), while some taxa, such as Chironomidae and Corixidae, became more abundant along the increasing water colour (Wissel et al, 2003;Feuchtmayr et al, 2019). Macroinvertebrates, however, are much less studied than zooplanktons (Figure 2).…”

“…Second, the effects of brownification can be species-specific. Within macroinvertebrates, some shredders, such as Asellus aquaticus, were found to respond positively to increasing water colour, while other shredders, such as Ephemerella aroni, responded negatively in Finnish rivers (Brüsecke et al, 2023). Within zooplankton, increasing water colour had positive effects on cladoceran biomass and recruitment, but no effects on copepods (Ekvall & Hansson, 2012;Koizumi et al, 2023).…”

“…Even under Cladocera, some species, such as Bosmina longirostris and Thermocyclops oithonoides, positively responded to increasing water colour and were tolerant to brownification, while many species responded negatively to brownification (Lehtovaara et al, 2014). Such different responses of taxa or even species to brownification could have been an explanation for the different responses of invertebrate communities found in browning waters (Wissel et al, 2003;Williamson et al, 2020;Gohil, 2022;Brüsecke et al, 2023;Koizumi et al, 2023;Adamczuk, 2021).…”