Highly efficient silica sink in monomictic Lake Biwa in Japan

Abstract

-In order to clarify the mechanisms underlying high efficiency of the silica sink in monomictic Lake Biwa in Japan, vertical flux of biogenic silica (BSi) was measured using sediment traps over a period of 15 months. The sediment traps were deployed at depths of 30 and 70 m. On a global scale, BSi fluxes in Lake Biwa were very high, ranging from 20 to 1087 mg Si.m In the winter period, when nutrients are supplied from the hypolimnion to the epilimnion, the distribution of photosynthetically active diatoms was almost homogeneous in all layers, including the aphotic layer. At this time, the diatoms assimilated dissolved silica (DSi) in a wider layer containing a part of aphotic layer in order to produce rigid frustules, which accumulated rapidly in bottom sediments as DSi concentration in the water column decreased. Thus, size of the silica sink in Lake Biwa is enhanced during the winter holomictic mixing period through interaction between physical (thermocline disruption: transfer of diatoms to deep layers by vertical convection), chemical (nutrient supply from deep layers) and biological (dominance of active diatoms in all layers) processes.

Estimation of long-term external nutrient loading from watersheds to Lake Biwa by a combined rainfall-runoff model and loading-discharge curve approach

“…SiO 2 loading from the South ranged from 2,500 to 8,000 tons yr -1 . The estimation for the SiO 2 sink represented by Goto et al (2013) showed that 20,000 ton SiO 2 yr -1 was retained in Lake Biwa, which accounts for 80% of the annual inflow. Figure 5b shows the estimated seasonal change in nutrient loading from rivers and groundwater.…”