Dynamic equilibrium of sediment carbon content in an estuarine tidal flat: characterization and mechanisms

Abstract

In a long-term and a short-term study of 2 estuarine intertidal flats (sandy and muddy), sediment carbon contents were measured every 2 wk over 2.5 yr and daily for a 2 wk period, respectively. During the long-term study, current velocity was measured twice (for periods of 8 wk and 6 wk). During the short-term study, sediment traps were also deployed. In both tidal flats, sediment carbon content fluctuated but had no long-term trend, indicating dynamic equilibrium. Amplitude, period and mechanisms of dynamic equilibrium differed between the 2 tidal flats. In the sand flat, the carbon content of the surficial sediment (0 to 1 cm) fluctuated, ranging from 0.03 to 0.17%. It is likely that physical transport of particulate carbon (e.g. by deposition) increases sediment carbon content under calm conditions, while carbon is washed out of the sediment through sand resuspension during spring tides. These processes are probably responsible for the dynamic equilibrium in the sand flat. In the mud flat, the carbon content of the surficial sediment varied from 0.5 to 2.4%, showing sharp peaks after summer floods, and gradual increases in winter followed by a decrease in spring to a lower limit of 0.5 to 1.0%. In the mud flat, the consolidated sediment layer probably governs the dynamic equilibrium of sediment carbon content: it prevents a long-term increase in carbon content by withstanding the transport of deposited particulate carbon into the sediment layer and destabilizing it on the sediment surface; it also maintains the lower limit of sediment carbon content by compaction of organic matter. Sediment organic content is crucial for benthos occupation; thus, our results suggest that the dynamics of sediment quality in tidal flats should be taken into account when assessing or managing benthic communities of tidal flats.

Hydrodynamic‐Driven Changes in the Source and Composition of Sedimentary Organic Matter via Grain Size Distribution in Shallow Lakes

“…Average current velocities at the 14 locations ranged from 0.74 to 3.74 cm/s (Figure S2 in Supporting Information ), likely being influenced by lake geomorphology and meteorological conditions (e.g., heavy rainfall) during the sampling period. Current velocities of the lake are calmer than those in estuarine or marine ecosystems (mean of 20–30 cm/s; Sakamaki & Nishimura, 2006, 2007), but they are similar to other lake environments (<10 cm/s; Bhowmik et al., 1991; Hua et al., 2013; Qin, 2008). A significant and positive correlation between TOC and the <63 μm fraction ( y = 16.518ln( x ) + 36.289; R 2 = 0.754, p < 0.05) in opposition to the negative correlation for the >63 μm fraction ( y = −16.52ln( x ) + 63.711; R 2 = 0.754, p < 0.05) indicated the changing sediment characteristics and the increased TOC content that can be expected in muddy sediments (Figure 5b).…”

“…Here, we propose a new concept, current velocity frequency ( F ), based on the random characteristics of the current velocity. This parameter, as the driver for sediment modifications, uses the cumulative frequency of reaching a certain velocity instead of the absolute value of the velocity (Sakamaki & Nishimura, 2006, 2007). The relationships between F and TOC were examined in the lake since the average current velocity failed to predict the sediment TOC.…”

“…This phenomenon was absent in muddy sediments likely because of the different characteristics of muddy sediments, in terms of cohesiveness. The sandy sediments were non‐cohesive (Sakamaki & Nishimura, 2006, 2007). In muddy sediments, fine organic matter (<63 μm) cannot easily be converted into a sedimentary layer with poor permeability and consolidation by advection (Figure 7).…”

“…The settling method of current velocity was described in our previous observations of coastal ecosystems (Sakamaki & Nishimura, 2006, 2007). Briefly, the two‐dimensional velocity meter powered by electromagnetic force (Alec Electronics, compact‐EM) was set at 5 cm above the sediment surface at every study site (S1–S14) to measure current velocity (Figure 1c).…”

“…Hydrodynamic conditions driving the substantial changes in sedimentary TOC and SOM sources were determined from the correlation of sediment characteristics to the current velocity. First, for the weekly sampling intervals during the study period, the frequency at which the observed current velocity V exceeded a certain level was calculated as F ( V > v ), where v is 1 ≤ v ≤ 10 cm/s at 0.5 cm/s intervals (Sakamaki & Nishimura, 2006, 2007). The maximal range of current velocity was generally comparable to other lake environments around the world (approximately <10 cm/s; Bhowmik et al., 1991; Hua et al., 2013; Qin, 2008).…”