Biogeochemistry of sediments in the Aegean Sea

Biochemical composition of sedimentary organic matter (OM), vertical fluxes and bacterial distribution were studied at 15 stations (95-2270 m depth) in the Aegean Sea during spring and summer. Downward fluxes of labile OM were significantly higher in the northern than in the southern part and were higher in summer than in spring. Primary inputs of OM were not related to sedimentary OM concentrations, which had highest values in summer. Sedimentary chlorophyll-a concentrations were similar in the northern and southern parts. Carbohydrates, the main component of sedimentary OM, were about 1.2 times higher in the southern part than in the northern, without significant temporal changes. Total proteins were higher in summer and about double in the northern part. Sedimentary proteins appeared more dependent upon the downward flux of phytopigment than of proteins. Sedimentary OM was characterised by a relatively large fraction of soluble compounds and showed better quality in the northern part. The lack of a depth-related pattern in sedimentary OM and the similar concentrations in the two areas suggest that differences in sedimentary OM quality in the Aegean basin are dependent on system productivity; the bulk of sedimentary OM is largely conservative. Sedimentary bacterial density was about double in the northern part and higher in spring than in summer, but bacterial size was about three times higher in summer, resulting in a larger bacterial biomass in summer. Bacterial density was coupled with total and protein fluxes, indicating a rapid bacterial response to pelagic production. Bacterial biomass was significantly correlated with sedimentary protein and phytopigment concentrations, indicating a clear response to accumulation of labile OM in the sediments. In all cases bacteria accounted for <5% of the organic C and N pools. The efficiency of benthic bacteria in exploiting protein pools, estimated as amounts of protein available per unit bacterial biomass, indicates a constant ratio of about 70 µg proteins/µg C. This suggests a similar bacterial efficiency all over the area studied, unaffected by different trophic conditions.本研究于春夏季在爱琴海15个站位（水深95~2270米）开展，针对沉积有机质（Sedimentary Organic Matter, OM）的生物地球化学组成、垂直通量以及细菌分布特征展开系统调查。北部海域活性有机质（labile OM）的向下沉降通量显著高于南部，且夏季通量高于春季。有机质的初始输入与沉积有机质浓度并无显著关联，而沉积有机质浓度在夏季达到峰值。南北海域的沉积叶绿素a（chlorophyll-a）浓度基本一致。作为沉积有机质的主要组成成分，碳水化合物（carbohydrates）在南部海域的含量约为北部的1.2倍，且未出现显著的季节变化。总蛋白质（total proteins）含量在夏季更高，北部海域的总蛋白质浓度约为南部的两倍。相较于蛋白质通量，沉积蛋白质的丰度更依赖于植物色素（phytopigment）的向下沉降通量。沉积有机质以较高比例的可溶性化合物（soluble compounds）为特征，且北部海域的沉积有机质品质更佳。沉积有机质未呈现与水深相关的分布格局，且南北海域浓度相近，这表明爱琴海盆地沉积有机质的品质差异受控于系统生产力；大部分沉积有机质整体上具有较强的保守性。北部海域的沉积细菌密度约为南部的两倍，且春季的细菌密度高于夏季；但夏季的细菌个体尺寸约为春季的三倍，最终夏季的细菌生物量更高。细菌密度与总通量及蛋白质通量呈耦合关系，表明细菌对浮游生产（pelagic production）具有快速响应能力。细菌生物量与沉积蛋白质及植物色素浓度显著相关，说明细菌对沉积物中活性有机质的积累具有明确的响应。在所有站位中，细菌占有机碳和氮库（organic C and N pools）的比例均低于5%。以单位细菌生物量对应的可利用蛋白质量估算的底栖细菌（benthic bacteria）利用蛋白质库的效率，显示其比值稳定维持在约70 μg蛋白质/μg有机碳。这表明在所研究的整个海域内，细菌的利用效率基本一致，不受不同营养条件的影响。