Controls on dissolved organic matter (DOM) degradation in a headwater stream: the influence of photochemical and hydrological conditions in determining light-limitation or substrate-limitation of photo-degradation

摘要

We investigated how absorption of sunlight by chromophoric dissolved organicmatter (CDOM) controls the degradation and export of DOM from Imnavait Creek,a beaded stream in the Alaskan Arctic. We measured concentrations ofdissolved organic carbon (DOC), as well as concentrations and characteristicsof CDOM and fluorescent dissolved organic matter (FDOM), during ice-freeperiods of 2011–2012 in the pools of Imnavait Creek and in soil watersdraining to the creek. Spatial and temporal patterns in CDOM and FDOM inImnavait Creek were analyzed in conjunction with measures of DOM degradationby sunlight and bacteria and assessments of hydrologic residence times andin situ UV exposure. CDOM was the dominant light attenuating constituent inthe UV and visible portion of the solar spectrum, with high attenuationcoefficients ranging from 86 ± 12 m?1 at 305 nm to3 ± 1 m?1 in the photosynthetically active region (PAR). Highrates of light absorption and thus light attenuation by CDOM contributed tothermal stratification in the majority of pools in Imnavait Creek underlow-flow conditions. In turn, thermal stratification increased the residencetime of water and DOM, and resulted in a separation of water massesdistinguished by contrasting UV exposure (i.e., UV attenuation by CDOM withdepth resulted in bottom waters receiving less UV than surface waters). Whenthe pools in Imnavait Creek were stratified, DOM in the pool bottom waterclosely resembled soil water DOM in character, while the concentration andcharacter of DOM in surface water was reproduced by experimentalphoto-degradation of bottom water. These results, in combination with watercolumn rates of DOM degradation by sunlight and bacteria, suggest thatphoto-degradation is the dominant process controlling DOM fate and export inImnavait Creek. A conceptual model is presented showing how CDOM amount andlability interact with incident UV light and water residence time todetermine whether photo-degradation is "light-limited" or"substrate-limited". We suggest that degradation of DOM in CDOM-richstreams or ponds similar to Imnavait is typically light-limited under mostflow conditions. Thus, export of DOM from this stream will be less underconditions that increase the light available for DOM photo-degradation (i.e.,low flows, sunny days).