Biogeochemical and photobiological responses of subarctic lakes to UV radiation.

Solar ultraviolet radiation (UV) induces photodegradation of optically and functionally important organic compounds in lakes and may negatively impact aquatic biota. We disentangled UV impacts on dissolved organic matter (DOM) transformation, and algal and zoobenthic micro-organisms in two shallow subarctic lakes in NW Finnish Lapland; in a high-UV + low-DOM (tundra, Iso-Jehkas) and a low-UV + high-DOM (mountain birch woodland, Mukkavaara) system. In addition to site and seasonal comparisons, in situ experiments with three treatments (DARK, photosynthetically active radiation [PAR], UV + PAR) were set up floating on the lakes for four weeks during midsummer. Lake water and experimental lake water were analyzed for basic limnology, optical properties (dissolved organic carbon [DOC], specific UV absorbance [SUVA], colored DOM [CDOM], and DOM compounds) as well as for photosynthetic and photoprotective pigments in algae and microzoobenthos. DOC concentrations remained largely unchanged after the exposure period in seasonal and experimental samples in both lakes yet the biochemical composition of the carbon pools was distinctly altered. CDOM and SUVA decreased seasonally and under UV exposure in the experiments, and terrestrial DOM compounds decreased in the experiments, suggesting UV induced photodegradation of large molecular size DOM of terrestrial origin. Higher seasonal and experimental (UV + PAR vs. PAR) proportional CDOM degradation occurred in Iso-Jehkas (32%, 29%) than in Mukkavaara (19%, 9%). Accordingly, the high-UV + low-DOM lake was more sensitive to photodegradation despite originally low CDOM relative to the low-UV + high-DOM system where DOM biodegradation likely prevailed. Experimental results showed elevated algal carotenoid/chlorophyll ratios and microzoobenthic melanin under UV exposure indicating photoinhibition and photoprotective pigmentation. UV has a significant impact on aquatic food webs of subarctic lakes altering the biogeochemical composition of organic matter and organisms through mechanisms of photodegradation, photoinhibition and photoprotection.

Recent changes in chironomid communities and hypolimnetic oxygen conditions relate to organic carbon in subarctic ecotonal lakes.

A key question in aquatic elemental cycling is related to the influence of bottom water oxygen conditions in regulating the burial and release of carbon under climate warming. In this study, we used head capsules of Chironomidae larvae to assess community and diversity change between the past (estimated as Pre-Industrial Period) and present and to reconstruct changes in hypolimnetic oxygen conditions from 30 subarctic ecotonal lakes (northeastern Lapland) using the top-bottom paleolimnological approach applying surface sediment (topmost 0-2 cm) and reference (4-5 cm) samples. Subsequently, we tested the findings against dissolved organic carbon (DOC) concentration of the sites. We found that the benthic communities were statistically dissimilar between the past and the present with largest changes occurring in the more transparent oligo-mesohumic lakes. However, murky polyhumic lakes displayed uniformly a decrease in diversity. The chironomid-inferred oxygen values showed a general decrease toward the present with largest shifts in low-DOC lakes, whereas no significant changes were found in the hypolimnetic oxygen conditions of high-DOC lakes, which were often located in wetland areas. These finding suggest that lakes associated with constant organic carbon inputs are more resilient toward climate-induced reductions in hypolimnetic oxygen.

Temperature controls organic carbon sequestration in a subarctic lake.

Widespread ecological reorganizations and increases in organic carbon (OC) in lakes across the Northern Hemisphere have raised concerns about the impact of the ongoing climate warming on aquatic ecosystems and carbon cycling. We employed diverse biogeochemical techniques on a high-resolution sediment record from a subarctic lake in northern Finland (70°N) to examine the direction, magnitude and mechanism of change in aquatic carbon pools prior to and under the anthropogenic warming. Coupled variation in the elemental and isotopic composition of the sediment and a proxy-based summer air temperature reconstruction tracked changes in aquatic production, depicting a decline during a cool climate interval between ~1700-1900 C.E. and a subsequent increase over the 20th century. OC accumulation rates displayed similar coeval variation with temperature, mirroring both changes in aquatic production and terrestrial carbon export. Increase in sediment organic content over the 20th century together with high inferred aquatic UV exposure imply that the 20th century increase in OC accumulation is primarily connected to elevated lake production rather than terrestrial inputs. The changes in the supply of autochthonous energy sources were further reflected higher up the benthic food web, as evidenced by biotic stable isotopic fingerprints.