Sargassum bed, Sydney Harbour
I get quite excited when I return to snorkelling in Parsley Bay, Sydney Harbour in spring. Yes, I admit, I am not too keen on entering the water in winter months even in balmy Sydney ( I made it to July one year!).
So what is it that gives me a thrill—well two things, water clarity and a brown algae of the genus Sargassum.
Sargassum is a remarkable algae in that it undergoes a strong seasonal growth pattern. By the time we enter the waters in spring it is tall and in places towers above the bed. There has been a prolific growth spurt in winter months. Productivity is quite remarkable. I enjoy diving down to swim through its ‘’branches” and ponder just what has been going on during those colder months when I have been absent. But by November the stalks have collapsed and I am left with admiring other species of algae such as Ecklonia and of course the diverse array of fish species of the Harbour.
It appears that Sargassum species are characterised by a seasonal growth pattern. Two papers that I found online indicate that no matter what the temperature range, it was the seasonality that controlled growth such that there is in the words of one paper from India a significant negative correlation with temperature ‘’indicating it is a causal factor for seasonality than the other environmental conditions’( Rao et al. Indian Journal of Marine Sciences,2002, 31, 26-32). In this study, sea temperature in cooler months was in the low 20 degrees C rising to 30 in summer. They noticed a maximum number of primary shoots and larger size classes during the winter season. Another paper on a different species of Sargassum growing on reef flats of the Red Sea showed that the dynamics of the species are directly related to the seasonal extremes in environmental conditions ( Ateweberhan et al., Marine Ecology Progress Series, 2005, 292, 159-171). They review other studies and highlight that while most tropical Indo-Pacific species of Sargassum reach peak abundance and reproduce during the cooler months of the year, some other species grow and reproduce during periods of higher temperatures. In the Red Sea seawater temperatures at 2.5m depth ranged from 27.7 C in winter to 33.4C in summer.
Sydney Harbour does not have the extremes of temperature recorded in these studies. In winter it descends to 17 to 18 degrees for several months and rarely exceeds 23 degrees in summer. So actual temperature does not seem to be a factor but the seasonal change. There may be other factors and I hope there are those at Sydney Institute of Marine Science who are as fascinated as me with the patterns of change are taking systematic observations. For me it is fascinating to encounter each year the seasonal switches of biotic growth similar to what you see on land.
– words by Prof Bruce Thom