Ocean temperature roller coaster and its local legacy
Although most of NGT’s work is in terrestrial and wetland ecology, you may have noticed I have a keen interest in the coastal and marine environments, through the activities I run with various groups.
Over the summer, I spent many hours at the beach with my family. I noticed the water temperature to be warmer than usual, particularly at Robe where it’s known to be icy cold even on the hottest days. A recent email from the Great Southern Reef Foundation prompted me to investigate sea surface temperature data.
This Sea Surface Temperature (SST) Anomaly map above shows the difference between the recorded temperature for the week ending 26th January 2025 and the long-term average. Zero, shown as white, shows areas where the SST for the week ending 26th January 2025 is the same as the long-term average, while the areas in dark red (around Western Australia) are 4°C above the long-term average.
As a side issue let’s hope this anomaly doesn’t stick around for long off Western Australia. as it could lead to a coral bleaching event.
Back to focusing on southeastern South Australia and South West Victoria, particularly Robe, where I felt the difference. The temperature for the week ending 26th January was recorded as 3°C above the long-term average. This got me thinking about last year and we had a bit of a discussion in NGT’s Mt Gambier office. Bryan recalled that the water at Robe in January 2024 was colder than usual; so cold that his feet and ankles hurt! Looking at the SST anomaly map below, you can see why. The recorded temperature for the week ending 7th January 2024 was 1-2°C colder than the long-term average. Taking into consideration that the average SST is 16-18°C, this takes the temperature down to about 14°C. That’s pretty chilly for swimming!
Interestingly, during January of 2024 we also had a strong Bonney Upwelling, a natural phenomenon, where the wind conditions move surface water away from the land, and oceanic deep water moves up the continental shelf to the surface. This brings with it nutrient-rich water, low water temperatures, and slightly lower salinity. As this event was so strong, more nutrients were brought to the surface than usual and an extensive phytoplankton bloom was spotted off the coast in January 2024. At the same time, a fish die-off was observed by locals in the Port MacDonnell area, with many different species washing up on beaches.
Early media reports of the die-off quoted SA’s Department of Primary Industries and Regions as saying “early indications based on water testing are that low salinity caused by recent high rainfall is the most likely reason for the fish mortality event”. We had quite a lot of rain leading up to the event, but Eight Mile Creek and Deep Creek, which flow into the ocean where the dead fish were found, do not have flow gauges, so we can’t say for sure that the rain increased their flows significantly. The Glenelg River, further east, had a small pulse of around 500 ML/day but this is pretty minor in the scheme of things, so that doesn’t shed any more light. Nevertheless, it may be that the combined impacts of the rain and upwelling did lower salinity.
Since this event, local recreational Pipi fishers in Discovery Bay, in South West Victoria, have not been able to find any Pipis. One Nelson resident said that he had not seen anything like this in his lifetime. My initial thoughts were that the Pipis too suffered from the intense upwelling event, and a decrease in water temperatures and salinity. In terms of salinity, some research suggests that elevated salinity has an impact on early life stages and juveniles while another study (on a different species – Donax ohesulus) showed 100% mortality at extreme lowering of salinity, like you’d get during riverine flooding, rather than what probably occurred during the events of January 2024. In terms of temperature, adult Pipi’s are more susceptible to higher temperatures although lower temperatures slow their burrowing rates. Another study looking at the life cycle of Pipis in captivity noted deaths after the water temperature had suddenly decreased by up to 5°C. These temperature shocks are unlikely in the wild so it seems that the larvae are most susceptible to changes in salinity and temperature. Based on this, there is potential that recruitment was impacted in the summers of both January 2023 (when it was cooler) and January 2024 (when it was warmer). Pipi’s do live for four of five or five years but maybe these consecutive events have meant that the adults haven’t been replaced by the next cohort, and this could explain their recent scarcity in Discovery Bay.
But there is some good news! During my recent summer at the beach, my kids have spent some time digging big holes in the sand, bringing me the treasures they find. Of note was a 10 mm Pipi. And, a recent beach walk between Piccaninnie Ponds and the Glenelg Estuary also uncovered some similar-sized Pipis. They grow to between 51mm and 54mm in their first year so these were obviously new recruits. So it’s good to see some new recruitment, and these guys still have some growing to do!
It’s difficult for many people to visualise what’s coming in terms of climate change, without seeing the effects firsthand. In this respect, the temperature roller coaster happening in our local oceans is something worth watching. With big swings in temperatures on both sides of the long-term average in the past two years and the impacts that have been observed on local species, it’s important to keep track of what’s going on. On that note, if you have any local observations you’d like to share with us, we’d love to hear about them. Please email Jess at .
