Dark water is poor evidence of an empty sea. During the Arctic polar night, a coastline can appear almost lifeless while animals continue to feed below the surface. In summer, the same place offers a different selection of visible life. Each visit reveals part of an annual cycle, and neither season can stand for the whole.
Seasonal biodiversity includes changes in the organisms present in a place, their abundance, and the activities that connect them. Migration changes which species are present. Breeding changes the number and needs of young animals. Less visible changes determine whether those animals encounter suitable food. A list of species can remain reassuringly familiar even as the conditions supporting them become less dependable.
Arctic organisms cope with extreme seasonal conditions, sometimes through remarkable adjustments within their own bodies. Their survival also depends on processes they cannot control. An animal’s response to winter or spring does not guarantee that its food supply will change on a compatible schedule. Protecting seasonal biodiversity requires attention to when feeding relationships work, as well as which species are present.
Winter life beyond human vision
An ecosystem study at 79 degrees north, published by Jørgen Berge and colleagues in 2015, found substantial activity and interactions across most feeding levels during the polar night. In some of the habitats studied, biodiversity and the occurrence of juvenile stages were higher in winter than in the sunlit, more productive season. A summer survey alone would have missed this part of the community’s annual cycle. [1]
Zooplankton, the drifting animals of the water column, can respond to light too faint for people to appreciate easily. In 2016, Kim Last and colleagues reported that their vertical migrations shifted from a 24-hour solar rhythm to the roughly 24.8-hour lunar day when the moon rose above the winter horizon. Around full moons, animals sank from surface waters and accumulated at approximately 50 metres. The evidence came from environments ranging from fjords to the open sea. [2]
The researchers suggested that moonlight could increase exposure to predators, making a deeper position advantageous. Such movement changes when and where prey encounter their predators during a season often treated as inactive. What looks like uniform darkness to a human observer can contain differences that affect an animal’s chance of being eaten. [2]
For biodiversity assessment, this creates a sampling problem. Surveys repeated in the same accessible summer weeks can produce a consistent record while repeatedly overlooking winter life. A reliable annual account needs observations from difficult seasons as well as those when organisms are easiest to see.
The limits of individual flexibility
A different adjustment occurs within the reindeer eye. Karl-Arne Stokkan and colleagues showed in 2013 that the tapetum lucidum, a reflective layer behind the retina, had a golden reflection in summer and a deep blue reflection in winter. The change involves this internal layer, rather than the iris. [3]
The blue winter reflection was associated with greater retinal sensitivity. Changes in the spacing of collagen fibres altered the wavelengths reflected, and the authors proposed that increased scattering helps the eye capture scarce light. They described a trade-off between greater sensitivity and reduced visual sharpness. The adjustment suits particular conditions and carries a cost. [3]
Such flexibility helps a resident species function through very different seasons. It is tempting to take it as reassurance that Arctic animals will accommodate environmental change. Yet an adjustment in one trait establishes only a particular capacity under the conditions studied. Whether food remains accessible is a separate question. An animal can be well equipped for seasonal darkness while still depending on changes elsewhere in its habitat.
Food production beneath the ice
Sea ice can conceal a major burst of food production. In findings published in 2012, Kevin Arrigo and colleagues reported a large phytoplankton bloom beneath consolidated ice in the Chukchi Sea, far from the ice edge. These microscopic photosynthetic organisms were growing beneath a cover that had been expected to restrict substantial production. [4]
Thinner ice and surface melt ponds allowed more light into the water. The authors estimated that satellite-based calculations could underestimate annual production by as much as tenfold in the nutrient-rich shelf waters concerned. This was a regional assessment, rather than a tenfold correction for the Arctic Ocean as a whole. [4]
The finding matters for the calendar of a food web, the network of feeding relationships within a community. Observing the date when open water appears may miss production that began beneath the ice. A survey starting only after breakup risks recording a later stage of the event while treating it as the beginning.
The consequences for consumers require separate investigation. A hidden bloom reveals production that observers had missed; whether animals benefit depends partly on their ability to feed during it. Measuring growth at the base of a food web cannot establish how successfully that food supports the organisms above it. Researchers need to follow the feeding relationship itself.
Food supply and the needs of young birds
Phenology is the study of when recurring biological events take place. A seasonal mismatch occurs when the period of greatest food availability fails to overlap adequately with the period when consumers need it. Such a mismatch can develop if linked organisms adjust their schedules at different rates.
A large British study by Stephen Thackeray and colleagues, published in 2016, examined more than 10,000 phenological datasets covering 812 terrestrial and aquatic species and other taxonomic groups. Responses varied widely, but climate sensitivity also differed systematically across feeding levels. Secondary consumers, which feed on other consumers, generally showed less sensitivity than organisms producing food at the base of the food web and those feeding directly on them. The study gives a reason to measure how closely seasonal events coincide. [5]
Arctic shorebirds show how local the problem can be. Eunbi Kwon and colleagues examined six species and their invertebrate prey at ten sites across the North American Arctic, using field data from 2010 to 2012. Their 2019 paper found greater variation in mismatch among sites than among bird species at the same site. Snowmelt and its relationship with temperature helped explain the differences. [6]
The team assessed both the separation between peak food supply and estimated peak chick demand and the overlap of supply and demand through time. [6] To see why that matters, imagine food peaking on the same day at two sites. A brief spike and a longer period of high availability could share a peak date while offering chicks very different feeding opportunities. Duration belongs alongside timing in any assessment of food supply.
The findings suggested possible consequences for regional bird populations without establishing mismatch as the sole cause of their declines. They also caution against assuming that an earlier spring has the same effect everywhere: local combinations of snow, temperature, and prey availability matter. [6]
Conservation monitoring should capture that variation. A bird count and a prey survey, taken separately, do not necessarily establish whether chicks can feed adequately during their most demanding days. Following food availability through the breeding period makes the relationship visible. It also makes a reserve’s performance easier to assess: the presence of breeding birds alone cannot show whether conditions allow their young to develop successfully.
When ice blocks winter forage
Winter can damage those relationships without removing the food itself. Rain falling on snow may refreeze around vegetation, making forage inaccessible to grazing animals. In a 2013 Svalbard study, Brage Hansen and colleagues found that extreme weather, especially heavy rain on snow, synchronised population fluctuations among resident vertebrate herbivores. Arctic fox populations showed delayed correlations with the herbivores, extending the pattern to a secondary consumer. [7]
Measuring plant cover would miss the obstacle created by ice. An animal may occupy apparently suitable habitat while being unable to reach its food. Resource abundance and resource access can give very different accounts of the same winter. [7]
The event also shows why annual average temperature cannot fully describe biological conditions. A short sequence of rain and freezing can have consequences out of proportion to its duration. Seasonal monitoring needs to retain the history of such events and investigate their later effects. Otherwise, a population change detected in one season may be explained only by the weather prevailing when the count was made.
The shorebird and grazing examples suggest a shared test of habitat quality. Resources must remain accessible during the periods when organisms need them. An inventory records what is present; seasonal observations can establish whether it can be used.
Conservation through the whole year
The studies support conservation that follows organisms through their annual cycles and measures the conditions under which interactions succeed. They also expose a difficulty: an observer can inadvertently alter the behaviour being studied.
In 2020, Berge and colleagues demonstrated that a ship’s normal working lights affected Arctic fish and zooplankton during the polar night, with responses detected at least 200 metres below the surface. Responses differed among the locations. Illumination could bias sampling and acoustic surveys: a change in the echo recorded by an instrument could partly reflect animals turning, rather than a rapid change in their numbers or depth. [8]
These results support testing survey methods against darker conditions where feasible and assessing unnecessary illumination near sensitive waters. The practical response would depend on local conditions and safe working requirements. Darkness itself deserves consideration as a habitat condition that human activity can disturb. [8]
Long-term monitoring could pair observations of organisms with measurements of the conditions that determine their access to resources. A shorebird study could follow prey availability through chick development and record snowmelt at the same site. A winter grazing study could record ice barriers as well as vegetation. Each programme would need methods adapted to its location and a record long enough to separate unusual years from persistent change.
Community participation could extend that record through repeated, standardised observations. Schools and local groups could follow specified events at fixed sites, using protocols developed with researchers. Reliable records of ordinary years would be especially valuable, because a spectacular anomaly is difficult to interpret without knowing the usual range of conditions.
An earlier bloom or a longer ice-free period may appear encouraging in isolation. The ecological consequences become clearer when organisms affected by such changes are followed through the rest of the year. An adequate assessment of biodiversity needs to establish whether the community still provides the opportunities required for feeding and reproduction.
The Arctic’s seasonal adaptations reveal considerable flexibility, but survival also depends on events outside an animal’s own body and often outside the season in which its success is measured. Protecting seasonal biodiversity means preserving the conditions under which those dependencies can continue to work. A familiar bird returning to a familiar shore is the beginning of the evidence. What matters next is whether its young can find enough food to grow.
By: Timur Tekfi
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