Tracing the effects of eutrophication on molluscan communities in sediment cores: outbreaks of an opportunistic species coincide with reduced bioturbation and high frequency of hypoxia in the Adriatic Sea

Estimating the effects and timing of anthropogenic impacts on the composition of macrobenthic communities is challenging because early 20th century surveys are sparse and the corresponding intervals in sedimentary sequences are mixed by bioturbation. Here, to assess the effects of eutrophication on macrobenthic communities in the northern Adriatic Sea, we account for mixing with dating of the bivalve Corbula gibba at two stations with high sediment accumulation (Po prodelta) and one station with moderate accumulation (Isonzo prodelta). We find that, first, pervasively bioturbated muds typical of highstand conditions deposited in the early 20th century were replaced by muds with relicts of flood layers and high content of total organic carbon (TOC) deposited in the late 20th century at the Po prodelta. The 20th century shelly muds at the Isonzo prodelta are amalgamated but also show an upward increase in TOC. Second, dating of C. gibba shells shows that the shift from the early to the late 20th century is characterized by a decrease in stratigraphic disorder and by an increase in temporal resolution of death assemblages from ~25-50 years to ~10-20 years in both regions. This shift reflects a decline in the depth of the fully-mixed layer from more than 20 cm to few centimeters. Third, the increase in abundance of the opportunistic species C. gibba and the loss of formerly abundant, hypoxia-sensitive species coincided with the decline in bioturbation, higher preservation of organic matter, and higher frequency of seasonal hypoxia in both regions. This depositional and ecosystem regime shift occurred in ~1950 AD. Therefore, the effects of enhanced food supply on macrobenthic communities were overwhelmed by oxygen depletion even when hypoxic conditions are limited to few weeks per year in the northern Adriatic Sea. Preservation of trends in molluscan abundance and flood events in sedimentary sequences was enhanced by eutrophication that reduced bioturbational mixing.

评估人为活动对大型底栖生物群落（macrobenthic communities）组成的影响及其发生时间颇具挑战，原因在于20世纪早期的调查资料稀缺，且沉积序列（sedimentary sequences）中的对应层段受到生物扰动（bioturbation）的混合作用。本研究为评估亚得里亚海北部富营养化（eutrophication）对大型底栖生物群落的影响，选取了两个沉积堆积速率较高的站位（波河三角洲前缘，Po prodelta）与一个堆积速率中等的站位（伊松佐河三角洲前缘，Isonzo prodelta），通过对双壳类（bivalve）Corbula gibba进行测年来校正生物扰动混合效应。研究结果显示：其一，在波河三角洲前缘站位，20世纪早期沉积的、代表高水位体系域特征的普遍受生物扰动的泥质沉积物，被20世纪晚期沉积的、含有洪水层残余且总有机碳（total organic carbon, TOC）含量较高的泥质沉积物所取代；伊松佐河三角洲前缘站位的20世纪贝壳泥质沉积物发生了混合叠加，但同样呈现出总有机碳含量向上递增的趋势。其二，对C. gibba壳体的测年结果表明，两个区域均出现了从20世纪早期至晚期的转变：地层紊乱（stratigraphic disorder）程度降低，死亡组合（death assemblages）的时间分辨率从约25~50年提升至约10~20年，这一转变反映出完全混合层的深度从20厘米以上缩减至数厘米。其三，两个区域均出现机会种（opportunistic species）C. gibba的丰度上升、原本占优势的低氧敏感（hypoxia-sensitive）物种消失的现象，这与生物扰动作用减弱、有机质保存程度提升以及季节性低氧（hypoxia）发生频率增加同步发生。这一沉积与生态系统状态转换（regime shift）发生于公元1950年左右。因此，即便亚得里亚海北部的低氧条件仅局限于每年数周，食物供给增加对大型底栖生物群落的影响仍被氧含量降低所抵消。而富营养化通过减弱生物扰动混合作用，提升了沉积序列中软体动物丰度变化趋势与洪水事件记录的保存程度。