Since ‘crossing the line’ we have sampled eight stations in the southern hemisphere to find out what is going on in the Southern Gyre.
So what is a Gyre? It is a swirling vortex, which in the ocean is created by wind or currents. There are two main gyres in the Atlantic Ocean, which are created by currents; the northern Gyre which circulates clockwise and is created by the North Equatorial and North Atlantic currents and the southern Gyre which swirls anti-clockwise, created by the South Equatorial and Antarctic Circumpolar currents.
Each of the Gyres has quite distinct biological, chemical and physical properties. The southern Gyre is more productive than the northern Gyre as it occupies a larger area and on average fixes 2.86Gt carbon per year as opposed to 1.12Gt carbon per year for the northern gyre. The phytoplankton species Synechococcus and Prochlorococcus occur in the Northern and Southern Gyre where they dominate the phytoplankton community; Prochlorococcus is generally more abundant in both, but the ratio between them can be altered in each of the gyres depending on the amount of grazing, light and nutrient limitation that they encounter.
We first entered the northern Gyre on 10th October at about 38°N and stayed within the Gyre until approximately 19°N on 17th October. The mixed layer in this Gyre was at 40 to 60 meters and the phytoplankton biomass resided at between 100 and 125m. At each individual station we sample however, the northern Gyre seems more productive than the southern Gyre and as a result of this we saw an abundance of squid, dolphins, brown booby, frigate birds etc., in the northern Gyre. By contrast, in the southern Gyre, the mixed layer and phytoplankton biomass have deepened to 90 and 130 meters respectively. The water seems more oligotrophic and as a result, there is less phytoplankton biomass. To date, we have only seen small squid chasing minnow like flying fish.