Spatial variability of biogeochemistry in shallow coastal benthic communities of Potter Cove (Antarctica) and the impact of a melting glacier

Measurements of biogeochemical fluxes at the sediment–water interface are essential to investigate organic matter mineralization processes but are rarely performed in shallow coastal areas of the Antarctic. We investigated biogeochemical fluxes across the sediment–water interface in Potter Cove (King George Island/Isla 25 de Mayo) at water depths between 6–9 m. Total fluxes of oxygen and inorganic nutrients were quantified in situ. Diffusive oxygen fluxes were also quantified in situ, while diffusive inorganic nutrient fluxes were calculated from pore water profiles. Biogenic sediment compounds (concentration of pigments, total organic and inorganic carbon and total nitrogen), and benthic prokaryotic, meio-, and macrofauna density and biomass were determined along with abiotic parameters (sediment granulometry and porosity). The measurements were performed at three locations in Potter Cove, which differ in terms of sedimentary influence due to glacial melt. In this study, we aim to assess secondary effects of glacial melting such as ice scouring and particle release on the benthic community and the biogeochemical cycles they mediate. Furthermore, we discuss small-scale spatial variability of biogeochemical fluxes in shallow water depth and the required food supply to cover the carbon demand of Potter Cove’s shallow benthic communities. We found enhanced mineralization in soft sediments at one location intermediately affected by glacial melt-related effects, while a reduced mineralization was observed at a location influenced by glacial melting. The benthic macrofauna assemblage constituted the major benthic carbon stock (>87% of total benthic biomass) and was responsible for most benthic organic matter mineralization. However, biomass of the dominant Antarctic bivalve Laternula elliptica, which contributed 39–69% to the total macrofauna biomass, increased with enhanced glacial melt-related influence. This is contrary to the pattern observed for the remaining macrofauna. Our results further indicated that pelagic primary production is able to fully supply Potter Cove’s benthic carbon demand. Therefore, Potter Cove seems to be an autotrophic ecosystem in the summer season.

沉积物-水界面（sediment–water interface）的生物地球化学通量（biogeochemical fluxes）测量是探究有机质矿化过程（organic matter mineralization）的核心手段，但南极浅海沿岸区域的此类研究极为匮乏。本研究于乔治王岛（又称25 de Mayo岛）波特湾水深6~9米的海域，开展了沉积物-水界面生物地球化学通量的原位（in situ）调查：定量测定了氧气与无机营养盐的总原位通量，同时原位测定了扩散性氧气通量，而扩散性无机营养盐通量则通过孔隙水剖面（pore water profiles）计算得到；此外还测定了生源沉积物组分（色素浓度、总有机碳、总无机碳及总氮含量）、底栖原核生物、小型底栖生物与大型底栖生物的密度与生物量，以及沉积物粒度、孔隙度等非生物参数。本次测量共设置3处采样点位，分别位于波特湾内受冰川融水沉积影响程度各异的区域。本研究旨在评估冰川融水引发的次生效应——包括冰蚀与颗粒物释放——对底栖群落及其介导的生物地球化学循环的影响，同时探讨浅水环境下生物地球化学通量的小尺度空间变异性，以及支撑波特湾浅水底栖群落碳需求所需的食物供给规模。研究结果显示，在一处受冰川融水相关影响中等的区域，软沉积物中的有机质矿化作用显著增强；而在另一处受冰川融水影响较强的区域，矿化作用则出现明显减弱。大型底栖生物群落构成了底栖碳库的主体（占底栖总生物量的87%以上），并主导了绝大多数底栖有机质矿化过程。其中，优势南极双壳类软体动物椭圆拉文蛤（Laternula elliptica）的生物量占大型底栖生物总生物量的39%~69%，其生物量随冰川融水相关影响的增强而升高，这一变化趋势与其余大型底栖生物的响应完全相反。本研究进一步证实，浮游植物初级生产可完全满足波特湾底栖群落的碳需求，由此可见，波特湾在夏季属于典型的自养生态系统。