Carbon and nitrogen stable isotope ratios are increasingly used to study long-term change in food web structure and nutrient cycling. We retrospectively analyse elemental composition (C, N and P) and stable isotopes (δ¹³C, δ¹⁵N) in archived monitoring samples of two important taxa from the bottom of the food web; the filamentous ephemeral macroalgae Cladophora spp. and the blue mussel Mytilus edulis trossulus from three contrasting regions in the Baltic Sea (coastal Bothnian Sea and Baltic Proper, open sea central Baltic). The aim is to statistically link the observed spatial and interannual (8–24 years’ time-series) variability in elemental and isotope baselines with their biomass trends and to the oceanographic monitoring data reflecting the ongoing environmental changes (i.e., eutrophication and climate) in this system. We find clear differences in isotope baselines between the two major Baltic Sea basins. However, the temporal variation in Mytilus δ¹³C was similar among regions and, at the open sea station, mussels and algae δ¹³C also correlated over time, likely reflecting a global (Suess) effect, whereas δ¹⁵N of both taxa varied with local and regional dissolved nitrogen concentrations in water. δ¹⁵N in source amino acids allowed detection of diazotrophic N in Mytilus, which was masked in bulk δ¹⁵N. Finally, Cladophora N:P reflected regional nutrient levels in the water while P%, which differed for both taxa, was linked to food quality for Mytilus. This study highlights the potential of a multi-taxa and multi-stable isotope approach to understand nutrient dynamics and monitor long-term environmental changes.

碳和氮稳定同位素比率越来越多地用于研究食物网结构和营养循环的长期变化。我们回顾性分析了来自食物网底部的两个重要分类群的存档监测样本中的元素组成（C、N 和 P）和稳定同位素（δ ¹³ C、δ ¹⁵ N）；丝状短暂的大型藻类Cladophora spp。和蓝贻贝Mytilus edulis trossulus来自波罗的海的三个对比鲜明的地区（波的尼亚海沿岸和波罗的海地区，波罗的海中部的公海）。目的是将观测到的元素和同位素基线的空间和年际（8-24 年时间序列）变异与其生物量趋势和海洋学监测数据联系起来，这些数据反映了当前的环境变化（即富营养化和气候）。这个系统。我们发现两个主要波罗的海盆地之间的同位素基线存在明显差异。然而，贻贝δ ¹³ C的时间变化在区域之间相似，在公海站，贻贝和藻类 δ ¹³ C 也随时间相关，可能反映了全球（苏斯）效应，而 δ ¹⁵两种分类群的 N 随水中的局部和区域溶解氮浓度而变化。源氨基酸中的δ ¹⁵ N 允许检测贻贝中的固氮 N ，它被大量 δ ¹⁵ N掩盖。最后，枝藻N:P 反映了水中的区域营养水平，而两种分类群不同的 P% 与Mytilus 的食品质量。这项研究强调了多分类群和多稳定同位素方法在了解营养动态和监测长期环境变化方面的潜力。