Depression of Primary Productivity by Humic Matter in Lake and Reservoir Waters of the Boreal Forest Zone

Abstract

Analysis of water from backwater and main-stem regions of 3 reservoirs and a natural lake belonging to major river systems in northern Manitoba (Canada) showed that primary productivity was inversely related to total dissolved (< 0.45 .mu.m) organic C and nondialyzable dissolved Fe, organic C, N and H-bonded polymeric hydroxyl groups. The chemical data represent allochthonous humic-FeOOH complexes with strong, covalent metal-ligand bonds. Humic matter depressed primary productivity by making Fe unavailable to phytoplankton, but not by attenuating light, lowering the pH or sequestering phosphate (although the dissolved P was mostly in the form of humic-Fe P complexes). Relatively stagnant backwater bays, where humic mater accumulates, were less productive than the more rapidly flushed, less humic main-stem waters. The reservoirs, which were formed 6 mo, 3 yr and 14 yr prior to the time of sampling, revealed short-term anomalies caused by impoundment. Three distinct stages of geochemical and trophic development are represented, the sequence of stages generalized as follows for reservoirs of this region; soon after impoundment, rapid leaching of nutrients from submerged land causes a brief rise in productivity and aberrantly high levels of dissolved low molecular weight P and N in the backwaters; subsequently, a pulse of soil humic matter released more gradually into the backwaters depresses productivity by fixation of minor elements such as Fe, even if the dissolved P levels are high: a steady state is restored when this anomalous accumulation of humic matter is removed from the water column by sedimentation or flushing, allowing productivity to return to higher levels. Even under unperturbed natural conditions humic matter restricts the productivity of surface waters in this geographic region.