Biota play an important role in the transfer of nutrients between marine and terrestrial biomes and especially so in the nutrient limited ecosystems of the polar regions. Birds are well-known vectors of nutrients between sea and land (Lindeboom 1984; Graham et al. 2018), while salmon and seals also bring marine-derived nutrients into freshwater and terrestrial ecosystems (Ben-David 1998; Butler 1999). However, while features such as the well-known ‘bird cliffs’ of the high Arctic strikingly illustrate the effect of bird guano enrichment (Eurola and Hakala 1977; Zwolicki et al. 2020), and marine vertebrate fertilization has been described as a major driver of the development of terrestrial biodiversity in Antarctic coastal regions (Bokhorst et al. 2019), remarkably little remains known in detail of how this source of nutrients, or others, enter into or flow through terrestrial food webs or may re-enter and impact the adjacent coastal marine environment. In the current era of multi-faceted environmental and climate changes, which often occur at accelerated rates in the polar regions, the concept of linkages between the major marine and terrestrial biomes is often raised, encouraged by general reductions in sea ice extent (especially coastal), increased glacial melt and runoff from land to sea carrying sediment and nutrients and locally leading to marine freshening, and changes in oceanographic circulations (e.g., Bischof et al. 2019; Siegert et al. 2019; Chown et al. 2022; Pedersen et al. 2022).

This Special Issue proposal was first conceived by the issue editors in late 2019, at the Polish Evolutionary Conference organized by The Committee of Environmental and Evolutionary Biology, Polish Academy of Sciences (https://pec2019.ug.edu.pl/) at the University of Gdansk, at which PC was supported by the Mary & Clifford Corbridge Trust, Cambridge, UK. It was formally proposed to and accepted by the Journal in early 2020. Little were we to have expected the events that subsequently followed and the lasting impact they would have on the academic community, with a particular emphasis on those parts relying on work in the polar regions. First, the covid pandemic in large part shut down many aspects of polar research for approaching two years (Liggett et al. 2024). Plainly covid impacts extended far beyond the practicalities facing those of us privileged to work in the polar regions, with wide-ranging implications ranging from individual health to societal function, including the untimely loss of key individuals in our research communities, personal, and family lives. Of particular relevance to polar and climate change-related research, hard on the heels of covid came the illegal Russian invasion of Ukraine, immediately driving a wedge through both Antarctic and Arctic research communities, in effect dividing these into non-communicating halves (Chuffart et al. 2022; Lopez-Blanco et al. 2024). This new and increasingly hostile and protectionist world order seems unlikely to change on anything other than a generational timescale.

It was in this new and far more challenging environment that the development of this Special Issue set to work. We are grateful for the patience and support of the Journal and especially its Chief Editor, Dieter Piepenburg, over what now amounts to the last three years, far longer than our original intention. We set out to address a particular facet of these linkages which has to date received fragmented attention-that of the importance and consequences of biotically mediated nutrient flows between biomes and, in particular, with those involving the terrestrial biome. This Special Issue, through a combination of primary research papers and reviews, provides a collation and synthesis of the current state of knowledge of nutrient transfer processes and their impacts in different polar ecosystems and groups of organisms.

Probably, the most recognized route of nutrient transport between ecosystems or biomes, and also the best represented in terms of contributions to this Special Issue, is that vectored by colony-nesting seabirds from sea to land. Even though this field was initiated as long ago as the early to mid-twentieth century (Summerhayes and Elton 1923; Gilham 1956), this subject has still been insufficiently explored in terms of the consequences of living in the vicinity of seabird colonies for different biota in the polar regions. Though studied across the globe, more frequently in easier to access and more hospitable regions than the Arctic or Antarctic (Mulder et al. 2011), it is the latter which respond the most sharply to any external nutrient subsidy. Furthermore, the distribution of seabird colonies is skewed toward the polar regions, with approximately 50% of the global population of breeding seabirds and chicks concentrated at higher latitudes (Otero et al. 2018). The contribution of guano in providing the majority of macro- and micronutrients to otherwise extremely poor polar soils (Bokhorst et al. 2019; Zwolicki et al. 2020), is well known. Here, Szymański et al. delve more deeply into the topic, showing that different ornithogenic phosphorus forms promote the growth of different vascular plant species. Together with nutrients, seabirds can also deliver contaminants and trace elements, such as heavy metals to the land, and Soares et al. show that species holding the highest trophic positions are the most effective contributors due to greater accumulation of these elements in their tissues. The availability of nutrients influences community composition of primary producers (Ellis 2005) and their morphological traits (Wojciechowska et al. 2015). However, some important physiological properties, such as the concentration of carbon-based secondary compounds produced by lichens investigated by Bokhorst et al., even if changing along a penguin-influenced nutrient gradient, may not be directly linked solely to nitrogen availability. Changing microenvironmental conditions associated with enriched soil and consequential vegetation development inherently affect the terrestrial invertebrate fauna, which include among the most abundant species on land, and play a central role in organic matter decomposition (Kampichler and Bruckner 2009). Here, the review of Zmudczyńska-Skarbek et al. confirms that their abundance and species richness generally increase, and community composition changes, in association with vertebrate fertilization. However, they also highlight that terrestrial invertebrate responses differ between the Arctic and the Antarctic, suggesting this is primarily a consequence of the strong geomorphological differences between the locations typically occupied by bird aggregations in the two regions (cliffs inaccessible to most predators for the flying seabirds of the Arctic, vs. largely flatter coastal ground occupied by flightless penguins in the terrestrial-predator-free Antarctic). On the other hand, van der Vegt and Bokhorst highlight the great complexity of invertebrate responses to allochthonous nutrient subsidies, presumably in turn linked to the bird diet, while Segura et al. in one of the very few studies to date of influences on soil prokaryotic communities, demonstrate that there are clear limits to the spatial extent of fertilization. In a further subtlety, Angerbjörn et al. demonstrate that marine-derived nutrients may also be used temporarily when terrestrial resources are scarce and be transferred further inland by larger predators, such as the Arctic fox. Bliska et al. and Culler et al. demonstrate that, locally, small lakes can also act as a nutrient source for surrounding terrestrial food webs through the dispersal of great numbers of emerging insects, resulting in enhanced terrestrial population sizes, although this link is sensitive to drought conditions, with consequences for higher trophic levels.

Far less attention has been given to the potential importance of the return of marine-derived nutrients, via the land, to the adjacent coastal marine ecosystem (Zmudczyńska-Skarbek & Balazy 2017). In this Issue, Finne et al. demonstrate a clear ornithogenic signal linking seabird colonies, through terrestrial streams, to the intertidal biota. In addition, there are indications that bird-derived nutrients create hotspots for algal growth on intertidal mudflats (see Souguires et al.). These animal transfers of materials between biomes are not limited to nutrients but also include redistribution of limiting minerals between different compartments of the marine environment (see Gilbert et al.) and, as noted above, the transfer of pollutants, such as mercury (see Soares et al.). Furthermore, in the Arctic, rivers transport vast amounts of dissolved organic matter from terrestrial ecosystems to the sea (e.g., Rawlins et al. 2021), but there is little or no evidence for animals transferring comparable amounts of terrestrial-derived nutrients or minerals to coastal zones in the polar regions as clearly takes place from ocean to land.

Being aware that terrestrial cyanobacterial and green algal assemblages clearly respond to ornithogenic fertilization (Pietryka et al. 2016), a continued gap in knowledge is that of the role of snow and ice algal communities in the fixing of nutrients into microbial food webs and their subsequent passage into adjacent terrestrial, freshwater, and marine ecosystems. The formation of these sometimes extensive primary producer communities is underpinned by aeolian, geologically and biologically derived deposition of nutrients on glacier and snow surfaces, while, in turn, they support the development of complex and diverse microbial and even micro-invertebrate communities. While attention has been given to their contribution to decreasing albedo and increasing ice surface melt, particularly on the Greenland Ice Sheet in the Arctic, it is only recently that the magnitude of their biological production rates and, hence, potential contribution to surrounding ecosystems has been emphasized (Davey et al. 2019; Gray et al. 2020).

As with snow algal communities, a further key knowledge gap relates to diversity and function in microbial ecosystems, even though these provide the foundation of all ecosystem processes. In this regard, recent and ongoing developments in various fields of molecular biological research, particularly relating to omics and the use and interpretation of environmental DNA, stand to provide new revelations in understanding of the routes by which nutrients pass through and between different elements of polar ecosystems.

Taken together, the 12 papers in this special issue confirm the important role animals play in shaping polar communities and ecosystems in general through their displacement of nutrients. There remain many fundamental questions to explore, in particular with regard to species-specific responses at the receiving end, temporal variability of nutrient transfers, and the variation created by the animal vectors (e.g., Lisi and Schindler 2011). With the ongoing changes in environmental conditions and stressors for animals resulting from climate and other environmental changes, large changes can be anticipated in their populations and distributions, with potentially very influential consequences for polar terrestrial ecosystems and biodiversity hotspots. Finally, we thank all of the contributing authors for their interest in contributing to this Special Issue and their patience with the extended process it involved, as well as all reviewers for their constructive inputs.