Spatio‐temporal cladoceran (Branchiopoda) responses to climate change and UV radiation in subarctic ecotonal lakes

Abstract

Aim To understand modern and past aquatic community responses to climate‐induced shifts in productivity and ultraviolet radiation (UV) exposure. Location Tree line ecotone from north boreal forest to subarctic tundra in northeastern Finnish Lapland. Taxon Cladocera (Crustacea: Branchiopoda). Methods Thirty‐one small and shallow lakes were examined for summer epilimnetic communities (SEC) and surface sediment fossil integrative communities (FIC) of Cladocera for species distribution and their environmental correlations. A 700‐year down‐core sediment sequence from a tree line lake (Námmájávri) was analysed for FICs and cladoceran‐inferred UV absorbance (ABSUV, indicative of melanin pigmentation) for evidence of long‐term community and photoprotective responses and compared with records of palaeotemperature, solar intensity, and composite sediment biogeochemistry by variance partitioning analysis. Results The SECs were primarily correlated with specific UV absorbance (indicative of UV exposure) and total phosphorus and FICs by mean July air temperature and total nitrogen. The Námmájávri FICs showed subtle changes with a directional shift between the 19th and 21st centuries and were mostly explained by solar intensity. ABSUV exhibited increases during the 18th and 20th centuries, being related to variation in sediment biogeochemistry, which was indicative of changes in auto‐ versus allochthonous production. Main conclusions The ecotonal distribution of cladocerans is sensitive to temperature, nutrients, and allochthonous carbon, which is closely linked with UV exposure. The long‐term community shifts and photoprotection have been governed by solar intensity and biogeochemical shifts through lake water optics, attributable to direct UV impact or climate‐mediated intensification in photodegradation of allochthonous carbon. Estimations of the dual effects and mechanisms of increasing temperatures and UV on subarctic lakes and their biota remain challenging as their individual impacts on key species were partly contradictory.

Proxy reconstruction of ultraviolet-B irradiance at the Earth’s surface, and its relationship with solar activity and ozone thickness

“…Empirical evidence to test this hypothesis is currently lacking, however. Surface UV-B proxy reconstructions based on the abundance of photoprotective pigments in fossil cladocera (water flea) carapaces in arctic and subarctic lakes (Nevalainen et al, 2015(Nevalainen et al, , 2016(Nevalainen et al, , 2018) demonstrated a positive correlation between surface UV-B and solar activity over the last millennium, which is the opposite pattern to that predicted by Rozema et al (2002).…”

“…However, UV-B proxies based on aquatic organisms such as cladocerans are influenced by water transparency as well as ambient UV-B, and so relate at least in part to local climatic and vegetation conditions and anthropogenic land use changes (Nevalainen et al, 2015(Nevalainen et al, , 2018.…”

Cladoceran (Crustacea) Niches, Sex, and Sun Bathing—A Long-Term Record of Tundra Lake (Lapland) Functioning and Paleo-Optics

“…However, the latter ABS UV peaks were likely related to a more transparent water column during the LIA, related to constrained organic matter flux from the catchment and reduced in-lake production driven by longer ice-cover period. A similar pattern of increased aquatic UV exposure during the LIA has occurred in lakes of northern and eastern Finland and in the Alps [30,53]. The pre MCA period at the middle of the first millennium of the Common Era has been characterized to be hydroclimatically dry in Fennoscandia.…”

“…Although solar forcing interacted partly in cladoceran assemblage succession, SSN reconstruction explaining 12% of the species variation (Figure 3), it did not have substantial association with the current ABS UV record (Figure 4). In an adjacent lake, which differed from Loažžejávri by its location in the mountain birch woodland at a lower altitude, millennial variation in ABS UV was in clear connection with solar intensity [13,53]. In Loažžejávri, highest carapace ABS UV was recorded in the early core (~300 and 500 C.E.)…”

“…In arctic and subarctic lakes, long-term changes in UV exposure have been mostly governed directly, or indirectly through photodegradation of OM, by solar radiation intensity [51][52][53]. Although solar forcing interacted partly in cladoceran assemblage succession, SSN reconstruction explaining 12% of the species variation (Figure 3), it did not have substantial association with the current ABS UV record (Figure 4).…”