Dissolved carbon, greenhouse gases, and δ 13 C dynamics in four estuaries across a land use gradient

Abstract

Land use is a well known driver of nutrient dynamics in estuaries, however less is known about how land use may influence the coastal carbon (C) cycle. Here, we report dissolved carbon concentrations, δ 13 C isotopic signatures, and water–air CO 2 and CH 4 fluxes for four Australian estuaries with contrasting catchment land use ranging from 9 to 72% natural. Water samples were collected during dry and wet hydrologic extremes. Dissolved organic carbon (DOC) concentrations were highest and total dissolved nitrogen and dissolved phosphorous were lowest in the least impacted estuary (Pine Creek). The DOC δ 13 C signature was enriched for the estuary with 91% urban-agricultural land use (Coffs Creek), implying a shift in the source of DOC along the land use gradient. Low DOC and high nutrient availability decreased CO 2 fluxes to the atmosphere in the impacted systems during both wet and dry conditions, and increased CH 4 concentrations and fluxes during the wet conditions. Variability of CO 2 and CH 4 were influenced by multiple drivers including hydrologic forcing, dissolved nutrients, as well as shifts in the nature of allochthonous versus autochthonous sources within each estuary.