Marine macrophytes and ‐algae have undergone major changes in abundance and species composition over the last decades, primarily due to eutrophication. However, comparable studies conducted in the mid 20th century are rare, but potentially valuable for enabling insight into changes in the benthic communities from the early onset of the eutrophication of the Baltic Sea. In the present study, the submerged phytobenthic community in the exposed southern archipelago of the Åland Islands was examined in 2018 and compared with surveys conducted in 1956 and 1993, respectively. The aim was to analyze long‐term changes in the phytobenthic community in relation to the general large‐scale anthropogenic drivers since the 1950s. Between 1956 and 1993, a decrease in the total number of species/taxa, an increase of filamentous algae coverage and a decrease in the depth range of Fucus vesiculosus was observed. These changes in the phytobenthic community continued between 1993 and 2018, suggesting no changes in the previously described negative trends. Between 1956 and 2018, a general shift in the distribution of phytobenthic functional groups, (grouped according to morphology and type of algae; green, brown and red) occurred, with increased coverage of filamentous brown and green algae, and decline in red algae coverage. The depth range of F. vesiculosus also decreased by >50% between 1956 and 2018. The results support findings that the eutrophication of the northern Baltic Sea is still at a high level, which slows down or prevents the recovery of offshore phytobenthic communities, despite the progress seen in other areas. Thus, the likely main drivers behind the changes are the direct and indirect effects of eutrophication in combination with warmer water, i.e. an effect of climate change.

中文翻译：

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Åland群岛南部，波罗的海北部的植物底栖动物的长期变化

在过去的几十年中，主要由于富营养化，海洋大型植物和藻类的丰度和物种组成发生了重大变化。但是，在20世纪中叶进行的可比研究很少见，但对于从波罗的海富营养化初期开始就了解底栖生物群落的变化具有潜在价值。在本研究中，2018年检查了奥兰群岛裸露的南部群岛上的水下植物底栖动物群落，并将其分别与1956年和1993年进行的调查进行了比较。目的是分析自1950年代以来与一般大规模人为驱动因素有关的植物底栖动物群落的长期变化。在1956年至1993年之间，物种/分类单元总数的减少，观察到了墨角藻。植物底栖动物群落的这些变化在1993年至2018年期间持续存在，表明先前描述的负面趋势没有变化。在1956年至2018年之间，发生了植物底栖动物功能组分布的总体变化（根据藻类的形态和类型分组;绿色，棕色和红色），丝状棕色和绿色藻类的覆盖率增加，而红色藻类覆盖率下降。F.vesiculosus的深度范围在1956年至2018年之间也减少了50％以上。研究结果表明，波罗的海北部北部的富营养化水平仍然很高，尽管在其他地区已有进展，但其减缓或阻止了近海底栖动物群落的恢复。因此，变化背后的主要驱动力可能是富营养化与温暖的水相结合的直接和间接影响，即气候变化的影响。