Shifting Seasons of the Sea

Autumn’s Arrived, or at least it feels like it anyway

If you have been observing recent seasonal changes on the land that signal the end of the summer and of autumn’s arrival, you might be, like me, surprised that it feels a little early. Leaves of sycamore are turning that beautiful bronze colour, falling and scattering on the ground, flocks of migratory birds are gathering to begin their long journey south to warmer grounds and swathes of rowan berries are beginning to turn crimson. With still a week and a half of August left, this feels a bit premature, especially as there’s barely been a chance to pick a blackberry before they’ve gone over and turned into mush!

But shifting seasons is a reality and directly linked to climate change, and it goes much deeper than what we observe on our daily walks or drive to the shops. Changes in the timing of natural events such as the warming of the soil in spring or songbird migration can have harmful effects on ecosystems. Different species respond to different environmental cues, therefore resulting in species that rely on one another becoming out of sync. So for me it might mean an earlier and smaller blackberry crumble, but this shift is a much greater issue for plants and animals.

Sea-sonal Changes

Similar to that of the land, the marine biome experiences seasonal changes that are a critical driving force for natural events. Sea temperature, sunlight and ocean currents all play a triggering role. Plants grow and animals spawn, reflecting what’s happening on land, only sea temperatures lag about a month behind that of the atmosphere. The migration of leatherback turtles, which come from tropical waters to the Irish Sea in the summer and autumn months to feed on jellyfish and the growth of seasonal seaweeds, which provide important habitats for marine wildlife; these are just two examples of seasonal occurrences in the waters surrounding Wales.

Plankton shifting polewards

Minke whale’s are the smallest of the baleen whales, and feed on plankton using their mouths as filter feeders. Photo Credit: Eleanor Stone

But perhaps one of the most important events is the blooming of plankton. Acquiring its name from the Greek work ‘planktos’, meaning wanderer, plankton forms the basis of many marine food chains. Plankton blooms when water temperature, salinity and nutrient availability in the correct amounts are all favourable, and these conditions vary for each species. Phytoplankton are microscopic plants, and zooplankton are microscopic animals which feed on phytoplankton. From jellyfish to baleen whales, a great number of marine animals depend on plankton for food.

Lion’s mane jellyfish (Cyanea capillata) feed on zooplankton along with fish and other jellyfish. Photo Credit: Alexander Mustard

Yet, a recent study on zooplankton published in the journal Nature shows that this microscopic but vital fodder is shifting pole wards in response to climate change and an increase in ocean temperature. In fact, it’s found to have moved a whopping 602 Km on average since pre-industrial times. Apart from being greatly concerning for the marine species that rely on zooplankton, this study demonstrates that marine ecosystems have now entered the “Anthropocene”.

Photographs of plankton taken through a microscope. The far left is a zooplankton species, with the right hand two being phytoplankton. Photo credit: Allan Rowat

Why only talk about one study? Being at the bottom of the food chain within the world’s largest ecosystem, and the basis for all marine life that lies above it, the movement of plankton is highly indicative of what’s happening to other marine species too. Therefore, this study demonstrates the shifting of plankton is just the tip of the iceberg.

So the next time you see a berry ripen on a tree early, or you’re wondering why where did the last frost of the winter go, take a moment to ponder the sea, and what the seasons are doing beneath the surface of the ocean.

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