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Model assessment of present-day Phaeocystis colony blooms in the Southern Bight of the North Sea (SBNS) by comparison with a reconstructed pristine situation

机译：通过与重建的原始情况进行比较，评估北海南部南部（SBNS）现今Phaeocystis菌落繁殖的模型

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摘要

We performed simulations with the existing MIRO&CO-3D ecological model to assess the present-day magnitude and geographical extent of undesirable Phaeocystis colony blooms in the Southern Bight of the North Sea (SBNS) receiving anthropogenic nutrient inputs from large European rivers that mix with the inflowing Atlantic waters. The criterion of 4·10~6 Phaeocystis colonial cells L~(-1) of Lancelot et al. (2009) is used to scale the presence of undesirable bloom. These simulations are compared with a reconstructed pristine SBNS ecosystem making use of nutrient inputs calculated with the Seneque/Riverstrahler model of the river system when all human activities on the watershed have been erased. Interannual variability is considered by performing model runs for two contrasted meteorological years: wet (2001) and dry (2005). Results show a large excess of nitrogen (N) and phosphorus (P) delivery to the SBNS, i.e., 12 and 5 times the pristine situation respectively. In contrast, the total silicon (Si) input is decreased with respect to natural conditions due to freshwater diatom growth in some eutrophied rivers. Qualitatively, rivers deliver nutrients in large Si excess for pristine condition but N excess for both 2001 and 2005, when scaled to the N, P and Si requirement of coastal diatoms. Responding to the river nutrient inputs, either natural or of anthropogenic origin, phytoplankton blooms are simulated in the vicinity of the river mouths, especially in the eastern SBNS receiving 78-98% of the direct river inputs. In this area nutrients cumulate along a SW-NE gradient and the bloom is forming a wide ribbon parallel to the coast. A short time-delay is simulated between the western and eastern SBNS due to light limitation imposed by the amount of suspended particles carried by the Thames River. A spring diatom-Phaeocystis succession is simulated for both present-day and pristine condition. Diatoms dominate the bulk of the pristine phytoplankton community; Phaeocystis colonies develop in the whole domain, especially in the Thames and Scheldt river plume though their biomass remains low and never exceed the threshold of 4·10~6 cells L~(-1). In contrast, present-day Phaeocystis colonies start growing when diatoms attain their maximum in April, co-occur with and supplement them in May and June, reaching colonial cells densities ＞4·10~6 cells L~(-1) in many locations of the SBNS. Overall, Phaeocystis colonies are favored by wet meteorological conditions that enhance river N delivery. The geographical extent of undesirable Phaeocystis blooms thus varies between dry and wet years and affects 72% of the modeled SBNS domain distributed in three areas: the eastern band (90%), the Thames plume (6%) and a small offshore area (4%) possibly connected to the eastern band.
机译：我们使用现有的MIRO＆CO-3D生态模型进行了模拟，以评估北海南部海岸线（SBNS）上不受欢迎的Phaeocystis菌落繁殖的现今规模和地理范围，这些人接受了来自欧洲大河的人为营养输入并与入流混合大西洋水域。 Lancelot等人的4·10〜6个囊藻菌落细胞L〜（-1）的判据。 （2009年）用于衡量不良水华的存在。将这些模拟与重建的原始SBNS生态系统进行了比较，该生态系统使用了河流系统的Seneque / Riverstrahler模型计算出的养分输入（当流域上的所有人类活动都被消除时）。通过对两个相反的气象年进行模型运行来考虑年际变化：湿（2001）和干（2005）。结果表明，向SBNS输送的氮（N）和磷（P）大大过量，分别是原始情况的12倍和5倍。相反，由于某些富营养化河流中淡水硅藻的生长，相对于自然条件而言，总硅（Si）输入量减少了。从质的角度来看，河流的养分含量高出原始状态所需的大量硅，而2001年和2005年则按沿海硅藻的氮，磷和硅需求量定为氮。响应自然或人为来源的河流养分输入，在河口附近模拟了浮游植物的开花，特别是在SBNS东部，接受了直接河流输入的78-98％。在该区域，养分沿西南偏北梯度积聚，水华形成一条平行于海岸的宽条带。由于泰晤士河携带的悬浮颗粒数量所限，光线受到限制，因此在SBNS西部和东部之间模拟了短时延。模拟了春季和原始状况下的春季硅藻-囊藻演替。硅藻主导着原始浮游植物群落的大部分。尽管囊藻菌落的生物量仍然很低并且从未超过4·10〜6细胞L〜（-1）的阈值，但在整个域中都发育着囊藻菌落，特别是在泰晤士河和Scheldt河羽流中。相反，当硅藻在4月达到最大数量时，当今的囊藻菌落开始生长，在5月和6月同时存在并补充它们，许多地方的菌落密度达到> 4·10〜6细胞L〜（-1）。 SBNS。总体而言，Phaeocystis菌落受到湿润的气象条件的青睐，这些条件增强了河流N的输送。因此，不想要的Phaeocystis盛开的地理范围在干燥和潮湿的年份之间有所不同，并且影响了分布在三个区域的SBNS建模区域的72％：东部带（90％），泰晤士河羽流（6％）和小的海上区域（4 ％）可能与东部频段有关。

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