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Miniature marine grazers harvest the sun’s bounty

Hyperia macrocephala, a species of zooplankton, in action. Photo Uwe Kils at ecoscope.com.

Hyperia macrocephala, a species of zooplankton, in action. Photo Uwe Kils at ecoscope.com.

Mary Gardner

These past few weeks, the waves drop and surfers are becalmed. As the days lengthen, the spring flush of phytoplankton, the free-floating mass of single-cell life, makes the water a murky green. Visibility closes in for me and other snorkelers. All we can do is wait.

The phytoplankton may seem like a flush of grass in a paddock but it’s more like a blizzard of edible particles. Nutrients in a current or upwelling meet sunshine. Living cells use solar energy to mix ingredients and voila! – they become abundant. They’ve accomplished ‘primary production’. This attracts tiny grazers, zooplankton, who are certainly not like miniature cows. But they are animals.

These crowds of animals are a cosmopolitan mix doing ‘secondary production’. Some are strict vegetarians and others not. Some stay small, living rapid lifecycles. Best known as the copepods, forming one of the world’s largest biomasses of animals.

But others will outgrow this size and take up new roles. They are youths wandering before settling down and becoming adults. Corals, barnacles, clams, those clumps of tube worms or oysters on rocks.

Still others are the young of big free-swimming adults. The fish that live in a defined homeland. The fish that roam farther reefs and open waters. The krill that school, the squid that grow nonstop all their single year-long life.

Also in this plankton mob are the small versions of gelatinous animals. Salps, jellyfish of all sorts, comb jellies and arrow worms. There are 2,000 known species, with origins some 500 million years ago. Neither ghosts nor magical beings, their transparent bodies are 95 per cent water. Most animals are 60–70 per cent water.

Up to this point, the story is sometimes said to be a battlefield with winners and losers. Or a feeding orgy dominated by first one hungry animal and then the other. But defying these easy descriptions, other creatures swim in, scoop up everyone and collapse all metaphors. Yes, the very large filter-feeding sharks and whales.

An overlooked reality is that these whales digest and then excrete a flocculent poo. This mass of particles floats and fertilises another round of phytoplankton, supporting another round of zooplankton. Of course, the next round depends on the good life of the previous generation. Enough of them must grow up and reproduce. Some need weeks or months. Others need years or decades. Many marine animals are better breeders when they are older, larger, fatter and perhaps even wiser. Long time frames envelop short ones. Cycles turn within cycles.

There are twists within these cycles. For instance, most reef fish change sex over their lifetime. Consider blue gropers, the western ones who can live for 70 years. They all start life as females. They need 20 years to mature and at least 35 years before any become males.

From this murky knowledge appear counter-intuitive insights. Larger numbers of baleen whales raise, not lower, marine productivity. Many populations of fish rely on their elders. Beds of filter feeders such as oysters and clams require minimum sizes and long-term peaceful tenancies.

But region after region reports the numbers of these marine animals are fractions of what were known 30, 50, 100 or more years ago. So many populations are now in single-digit percentages of their former numbers. They may not be extinct but what if they are no longer successfully reproducing? What is lost when their great numbers are so diminished? What are the names for their ineffable contributions to the sea and to us humans?

Where fishing is prohibited, trawling banned and nets forbidden, there is respite for marine life and a new chance for future sea people. What’s been degraded over several generations often needs decades to reorganise. Pass the message around. Tell the politicians and schoolkids, too.  Patience is a virtue. What’s gone may not ever reappear, but something else as wise may develop. All we can do is wait.


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