Photodegraded dissolved organic matter from peak freshet river discharge as a substrate for bacterial production in a lake-rich great Arctic delta

Jolie A.L Gareis,Lance F.W Lesack

doi:10.1139/as-2017-0055

Abstract

Lake-rich Arctic river deltas are recharged with terrigenous dissolved organic matter (DOM) during the yearly peak water period corresponding with the solstice (24 h day−1 solar irradiance). Bacteria-free DOM collected during peak Mackenzie River discharge was exposed to sunlight for up to 14 days in June 2010. As solar exposure increased, carbon and lignin concentrations declined (10% and 42%, respectively, after 14 days), as did DOM absorptivity (62% after 14 days), aromaticity, and molecular weight. Photochemical changes were on par with those normally observed in Mackenzie Delta lakes over the entire open-water season. When irradiated freshet DOM was provided as a substrate, no significant differences were observed in community-level metabolism among five bacterial communities from representative delta habitats. However, bacterial abundance was significantly greater when nonirradiated (0 day) rather than irradiated DOM (7 or 14 days) was provided, while cell-specific metabolic measures revealed that per-cell bacterial production and growth efficiency were significantly greater when communities were provided irradiated substrate. This complex response to rapid DOM photodegradation may result from the production of inhibitory reactive oxygen species (ROS), along with shifts in bacterial community composition to species that are better able to tolerate ROS, or metabolize the labile photodegraded DOM.

Highlights

The Mackenzie River Delta, located in northwest Canada where the Mackenzie River empties into the Beaufort Sea basin of the Arctic Ocean, is both lake-rich and ecologically diverse
The initial floodwater sample taken from the Mackenzie River on 19 May (MACK) was strongly terrigenous in character (DOC:DON = 37), as were values from the RIV sampling site throughout June (Figure 5a)
Photodegradation under ambient solar conditions on the large, lake-rich, and seasonallyflooded Mackenzie Delta floodplain may lead to substantial changes in dissolved organic matter (DOM) quality that are on par with those normally seen over the course of the entire open-water season in delta lakes

Summary

Introduction

The Mackenzie River Delta, located in northwest Canada where the Mackenzie River empties into the Beaufort Sea basin of the Arctic Ocean, is both lake-rich and ecologically diverse. High levels of productivity are sustained by the annual spring flood, when discharge in the north-flowing Mackenzie peaks due to melting snow in the large subarctic watershed, and water levels peak in the delta due to restricted flows as channels become clogged with fractured ice cover and rubble. As water levels exceed channel bank heights, sediment-, nutrient-, and organic-rich floodwater (Gareis and Lesack 2017) spreads out over the delta floodplain in a thin layer covering an average of 11,000 km (Emmerton et al 2007). The timing of the annual flood corresponds with the Arctic summer solstice, when solar exposure is continuous and there is high potential for dissolved organic matter (DOM) photodegradation. Direct sampling of floodwater is difficult, since unstable ice cover and moving debris in river channels means that the only safe method of accessing sampling sites is by helicopter. The spring freshet is a relatively understudied part of the annual hydrograph, and at present Mackenzie River freshet

Methods

Results

Discussion

Conclusion