Springtime – so we’re in for warming waters! As our air temperatures begin to climb, so will the ocean temperatures, but ocean temperatures move much less and lag behind. In Sydney, our average water surface temperature range is just 5.2 degrees, from a minimum of 18.5 in September to a maximum of 23.7 in February. Contrast this with an average air temperature range of 10 degrees, from 13 degrees in July to 23 degrees in January. Of course these are averages, which mask the range of highs and lows, and the extremes on land are much greater.
As divers we know it’s often warmer on the surface than deeper, and the averages in the paragraph above are surface temperatures from satellite data (http://www.seatemperature.org/australia-pacific/australia/sydney.htm). Right now, for example (October), the sea surface temperature in Sydney is showing as 21 degrees, but my computer on my dive at Bare Island yesterday recorded a minimum of 14 degrees! Maybe my computer is a bit inaccurate, but it certainly felt like 14 through my 7 mm semi-dry, thermal top and hood.
Warming waters aren’t all good either. In Sydney, warmer weather allows phytoplankton to thrive, creating an algal bloom which reduces visibility. Whether it’s a pro or con, warmer waters also allow tropical species to settle in Sydney, from damsels and surgeonfish to tropical urchins and nudibranchs. In summer, there is more to see but it’s harder to see it!
Any discussion of ocean temperatures is incomplete without reference to global warming. Whilst Australian sea temperatures have warmed overall by around half a degree, the south-east is a hot-spot thanks to increasing strength and warmth of the East Australian Current, reaching 2 degrees above average in March this year. Marine “heat waves” like this can wreak havoc on marine life, which have much less tolerance to temperature fluctuations than terrestrial organisms.
So whilst we’ve always had an influx of tropicals in spring and summer in Sydney, many believe this phenomenon is increasing. “Tropicalisation” of our marine communities can benefit one species at the expense of others, for example the arrival of herbivorous surgeonfish in Sydney may impact on our kelp beds. If weedy seadragons are in fact declining in Sydney, is it related to warming waters and / or declines in kelp densities? Only time and science will tell.