Satellite imagery of the top of the South has revealed huge plumes of sediment caused by heavy rain and flooding last weekend.
Last weekend’s deluge devastated roads, homes and walking tracks – but the sea suffered in the flooding, too, as illustrated in satellite imagery captured before and after the storm.
The CawthronEye images, using NASA satellites, taken before and after the deluge, show a visible change in colour in shallow ocean bays, and plumes of sediment stretching sometimes several kilometres out to sea.
Marine biophysicist Ben Knight, from Cawthron Institute, said the plumes were linked to river management.
“Historically, rivers would have spread over floodplains, but now with things like stop-banks, it’s going out into the sea rather than building land.”
He said research into sedimentation had shown an increase in the amount of sediment going into the sea over time.
“Since Europeans arrived [in New Zealand] it’s increased markedly, those deposits,” he said.
“I’m not arguing against stop-banks, they’re a crucial part of how we build towns, but it’s a bit of a trade-off.”
He said the effect of sediment on the ocean could be quite significant, and in shallower bays like the Tasman the effects could linger thanks to turbulence which kicked partially-settled sediment back up into the water-column.
He said benthic microalgae (algae the lives on or near the sea-floor) usually worked to bind sediment together and help it settle, but when sediment lingered in the water column it could block light the algae needed to survive, decreasing its ability to bind the sediment, which in turn made it more likely for lightweight sediment to linger.
He said the type of sediment had different effects on the sea, depending on where in the catchment it came from.
“There’s a really interesting thing in the Nelson catchment, up in the Hackett … there’s quite barren land up there, because the soil is quite toxic. It’s naturally high in heavy metals, just a natural mineral belt. That can have quite an impact if sediment enters the bay.”
Sediment in the bays is more than an aesthetic problem, Knight said, and could have long-lasting effects on the make-up of the ocean floor and the animals that live there.
“There has been some suspicion that sediment events played a part in why baby scallops couldn’t survive in the bay. That’s a real shame, a big loss for the region,” he said.
He said while it was likely the collapse of the scallop beds around the top of the south were caused by many contributing factors, there was “potential” that sedimentation played some part.
He said events like the recent flooding and sedimentation were a chance to highlight the interconnected health of waterways.
“Effects on land have an impact on the sea,” he said.
“Riparian planting and those sorts of thing on the banks of rivers have been proven to reduce sedimentation, so there are things that we can do – but it’s a long-term investment.”
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