Multiple tracers demonstrate distinct sources of dissolved organic matter to lakes of the Mackenzie Delta, western Canadian Arctic

Abstract

Lakes of the Mackenzie Delta occur across a gradient that contains three clear end members: those that remain connected to river-water channels throughout the summer; those that receive only brief inputs of river water during an annual spring flood but contain dense macrophyte stands; and those that experience significant permafrost thaw along their margins. We measured dissolved organic carbon (DOC) concentration, dissolved organic matter (DOM) absorption and fluorescence, and stable isotopes of DOM, DOM precursor materials, and bacteria to elucidate the importance of river water, macrophytes, and thermokarst as DOM sources to Mackenzie Delta lakes. Despite standing stocks of macrophyte C that are sevenfold to 12-fold greater than those of total DOC, stable isotopes indicated that autochthonous sources contributed less than 15% to overall DOM in macrophyte-rich lakes. Instead, fluorescence and absorption indicated that the moderate summertime increase in DOC concentration in macrophyte-rich lakes was the result of infrequent flushing, while bacterial d 13 C indicated rapid bacterial removal of autochthonous DOC from the water column. In thermokarst lakes, summertime increases in DOC concentration were substantial, and stable isotopes indicated that much of this increase came from C released as a result of thermokarst-related processes. Our results indicate that these distinct sources of DOM to neighboring arctic Delta lakes may drive between-lake differences in C cycling and energy flow. Rapidly assimilated macrophyte DOM should be an important contributor to microbial food webs in our study lakes. In contrast, the accumulation of thermokarst-origin DOM allows for a significant role in physico-chemistry but indicates a lesser contribution of this DOM to higher trophic levels.

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

“…The elevational gradient among delta lakes creates an exceptionally diverse aquatic environment (Squires et al 2009;Lesack and Marsh 2010), and generates gradients in carbon (Spears and Lesack 2006;Tank et al 2011), nutrients (Lesack et al 1991), primary productivity Squires and Lesack 2003a), and bacterial abundance (BA) and production (Spears and Lesack 2006). Due to the complexity of the lake-rich delta landscape, and the regular inputs of nutrients and sediments carried in flood and river water, the Mackenzie Delta is a hotspot of productivity and biodiversity in the circumpolar Arctic supporting large populations of birds, fish, and mammals and serves as a feeding and breeding ground for migratory birds (Squires et al 2009).…”

“…Heterotrophic grazers subsequently consume bacteria, which shunts otherwise unavailable carbon, organic matter, and energy back into the aquatic food web via the microbial loop (Azam et al 1983). This is a critical process that mobilizes otherwise unavailable DOM molecules in high-latitude aquatic environments (Cole 1999;Cory et al 2014) via bacteria, contributing to overall food web production in Mackenzie Delta lakes (Spears and Lesack 2006;Tank et al 2011), the Mackenzie River (Vallières et al 2008), and the nearshore Beaufort Sea (Garneau et al 2008;Bell et al 2016). Because DOM is an important component of carbon budgets in aquatic ecosystems, an understanding of how photodegradation alters DOM bioavailability is critical to understand carbon fluxes through the landscape.…”

“…3). They are continually flushed with river water, which has high sediment loads and a DOM pool that is primarily terrigenous in origin (Tank et al 2011). High sediment levels limit light penetration in CON lakes (Gareis et al 2010), Fig.…”

“…The DOM pool in TK lakes tends to increase throughout the open-water season due to leaching from thawed permafrost and evaporative concentration. Unaltered TK DOM is a poor bacterial substrate (Tank et al 2011). The TK bacterial community was collected from Lake 520.…”

“…This creates ideal conditions for abundant MAC growth, which may reach 350 g m −2 (dry weight; ; Table 1). MAC-derived DOM generally supports much higher levels of BP and C cycling than terrestrially-derived DOM (Tank et al 2011). The MAC bacterial community was collected from Lake 56.…”