Geochemistry of organic carbon in surface sediments of a summer hypoxic region in the coastal waters of northern Shandong Peninsula

Abstract

The geochemistry of sedimentary organic matter (SOM) in coastal areas is complex due to its multiple sources and intricate hydrological features. In this study, the biogenic element concentrations and stable carbon (δ 13 C) and nitrogen (δ 15 N) isotopic compositions in the coastal surface sediments of northern Shandong Peninsula, along with some parameters related to water quality, were analyzed to investigate the temporal-spatial variations in SOM and the processes that control its distribution. The results revealed that marine autogenous organic matter is a major contributor to SOM, accounting for 75.4 ± 3.3%, 60.8 ± 6.6% and 67.4 ± 10.3% in August and November 2015 and March 2016, respectively. In summer, TOC and TN concentrations were significantly higher than those in autumn and spring. The relatively high abundances of SOM were found in the offshore areas in summer and spring, which was contrary to those in autumn. Riverine discharge, nutrients, primary productivity and dissolved oxygen (DO) dynamics could all influence the composition and contents of SOM in different seasons. In summer, high primary productivity and hypoxia condition led to high SOM values. In comparison, SOM contents were relatively low due to sufficient DO in bottom water in autumn and spring. Dissolved nutrients in seawater could affect the accumulation of autogenous organic matter by impacting upon primary production. In summer, nitrate in surface water had the most obvious effects on autogenous organic carbon (AOC) and may be the principal factor of limiting the growth of phytoplankton. In autumn, nitrate as well as dissolved silicate had more effects on AOC storage. However, phosphate had the most obvious influence on AOC storage in spring.

Spatial and Seasonal Variations of Sedimentary Organic Matter in a Subtropical Bay: Implication for Human Interventions

“…The differences in TOC and TN between spring and summer suggested that labile OM decomposed during the investigation period [55]. The seasonal variation pattern of TOC and TN in the surface sediment of ZJB was different from that in the coastal area of northern Shandong Peninsula (a temperate coastal area), where TOC and TN concentrations were significantly higher in summer than in spring [14]. The relatively high temperature in ZJB (a subtropical bay) is more conducive to the decomposition of OM and reduces the accumulation of OM [56].…”

“…Surface sediment samples were collected using a stainless steel grab sampler. The sediment surface layer (0-2 cm) was collected at each sampling site [14,23,29]. After collection, the samples were homogenized and placed into sterile polyethylene bags, sealed, and kept on ice in a cooler during transport.…”

“…Diagenetic effects on the δ 13 C signature of OM have been shown to be small [78,79]. Therefore, δ 13 C is a perfect tool for distinguishing different OM sources [14,66], and it has been widely used to distinguish between terrestrial and marine sources of OM [14,19,66]. C 4 plants are uncommon in the area surrounding ZJB, where the dominant cultivated plant is rice (a C 3 plant) and the natural ecosystem is a subtropical forest.…”

“…We believe that the distribution pattern of δ 13 C in this study was influenced by human activities. Relatively high δ 13 C values have also been seen in the coastal area of northern Shandong Peninsula [14]. The channel and the coastal bay had high concentrations of nutrients as a result of human activities, such as agriculture, mariculture, and industry in or around ZJB [41,58,82].…”

“…This study found that mariculture activities were a significant source of SOC, and also significantly influenced the distribution and preservation of SOC. However, most prior studies investigating the influence of anthropogenic activities and seasonal environmental variations on SOM have been undertaken in temperate zones [9,14,15,23], and much less is known about subtropical bays. Besides, to our knowledge, the influence of seasonal variations in rainfall on SOM has rarely been documented in previous studies.…”