A regional hindcast model simulating ecosystem dynamics, inorganic carbon chemistry, and ocean acidification in the Gulf of Alaska Biogeosciences

The coastal ecosystem of the Gulf of Alaska (GOA) is especially vulnerable to the effects of ocean acidification and climate change. Detection of these long-term trends requires a good understanding of the system's natural state. The GOA is a highly dynamic system that exhibits large inorganic carbon variability on subseasonal to interannual timescales. This variability is poorly understood due to the lack of observations in this expansive and remote region. We developed a new model setup for the GOA that couples the three-dimensional Regional Oceanic Model System (ROMS) and the Carbon, Ocean Biogeochemistry and Lower Trophic (COBALT) ecosystem model. To improve our conceptual understanding of the system, we conducted a hindcast simulation from 1980 to 2013. The model was explicitly forced with temporally and spatially varying coastal freshwater discharges from a high-resolution terrestrial hydrological model, thereby affecting salinity, alkalinity, dissolved inorganic carbon, and nutrient concentrations. This represents a substantial improvement over previous GOA modeling attempts. Here, we evaluate the model on seasonal to interannual timescales using the best available inorganic carbon observations. The model was particularly successful in reproducing observed aragonite oversaturation and undersaturation of near-bottom water in May and September, respectively. The largest deficiency in the model is its inability to adequately simulate springtime surface inorganic carbon chemistry, as it overestimates surface dissolved inorganic carbon, which translates into an underestimation of the surface aragonite saturation state at this time. We also use the model to describe the seasonal cycle and drivers of inorganic carbon parameters along the Seward Line transect in undersampled months. Model output suggests that the majority of the near-bottom water along the Seward Line is seasonally undersaturated with respect to aragonite between June and January, as a result of upwelling and remineralization. Such an extensive period of reoccurring aragonite undersaturation may be harmful to ocean acidification-sensitive organisms. Furthermore, the influence of freshwater not only decreases the aragonite saturation state in coastal surface waters in summer and fall, but it simultaneously decreases the surface partial pressure of carbon dioxide (pCO(2)), thereby decoupling the aragonite saturation state from pCO(2). The full seasonal cycle and geographic extent of the GOA region is under-sampled, and our model results give new and important insights for months of the year and areas that lack in situ inorganic carbon observations.

阿拉斯加湾（Gulf of Alaska, GOA）的沿岸生态系统尤其易受海洋酸化与气候变化的影响。要精准识别这类长期变化趋势，需先充分掌握该系统的自然本底状态。阿拉斯加湾是高度动态的海域，在亚季节至年际的时间尺度上展现出显著的无机碳变异性。但由于该海域广袤偏远、观测数据匮乏，学界对这类变异性的认知仍相当有限。我们针对阿拉斯加湾开发了全新的模型架构，将三维区域海洋模型系统（Regional Oceanic Model System, ROMS）与碳、海洋生物地球化学及低营养级（Carbon, Ocean Biogeochemistry and Lower Trophic, COBALT）生态系统模型进行耦合。为深化对该系统的概念性认知，我们开展了1980年至2013年的后报模拟。该模型显式地将高分辨率陆地水文模型输出的时空变化沿岸淡水径流作为强迫场，进而对盐度、碱度、溶解无机碳及营养盐浓度产生影响。相较此前的阿拉斯加湾建模尝试，本研究的模型实现了显著改进。本研究利用现有最优的无机碳观测数据，在季节至年际时间尺度上对模型进行评估。模型在复现5月近底水体文石过饱和状态与9月近底水体文石不饱和状态方面表现尤为出色。模型存在的最大缺陷在于无法准确模拟春季表层无机碳化学特征：其高估了表层溶解无机碳浓度，进而导致同期表层文石饱和状态被低估。我们还利用该模型解析了塞沃德断面（Seward Line transect）在采样不足月份的无机碳参数季节循环及其驱动机制。模型结果显示，受上升流与有机质再矿化作用影响，6月至1月期间塞沃德断面沿线多数近底水体处于文石不饱和状态。这种持续周期较长的文石不饱和现象可能对海洋酸化敏感生物造成危害。此外，淡水输入不仅会降低夏秋季节沿岸表层水体的文石饱和状态，同时还会降低表层二氧化碳分压（pCO₂），进而打破文石饱和状态与pCO₂之间的天然耦合关系。阿拉斯加湾区域完整的季节循环与地理分布仍存在采样不足的问题，本研究的模型结果为那些缺乏原位无机碳观测数据的月份与区域提供了全新且重要的科学认知。