Spencer Gulf is a reverse estuary with two different marine environments- the sea and the open ocean. The northern Spencer Gulf is defined as a wind generated short waves sea environment. Summer has prevailing strong southerly winds. A minimum of fresh water enters the northern section of the gulf with salinity rising up to 45% as it reaches Point Lowly. The water temperature ranges from 11ºC to 24ºC.
The deepest point is at Point Lowly reaching of 23 m at mean depth. The tides generate a strong southern current. The effect of narrowing of the gulf, in combination with the tides, creates two individual strong currents at Backy Point and Point Lowly. The profile of the gulf basin resembles a letter V with one arm substantially lower than the other, which is more like the letter L, as the deeper parts come closer to the Eyre Peninsula coast. The sea floor gradually rises to very shallow waters on the eastern shore.
The area has two low tides and two high tides per lunar cycle with regular dodge tides. A dodge tide means no or very little tidal movement for a period for up to two days. This unique phenomenon is observed only in Northern Spencer Gulf, Gulf St Vincent, Torres Strait and Gulf of California.
(Source: Santos Port Bonython study, Adelaide, 1981. Natural History of Eyre Peninsula, Adelaide, 1985.)
The northern Spencer Gulf has one of the most diverse temperate water marine environments in Australia. The combination of high salinity and high water temperature combined with diverse underwater habitats creates an environment that serves as a breeding ground and also attracts oceanic visitors.
The bottom of the northern Spencer Gulf is formed by a combination of sand with sea grass meadows encountering rocky reefs and mangroves. Marine species in the area include resident dolphins, sharks and stingrays, occasional migrating whales and seals, snapper, kingfish, whiting, salmon, boar fish, puffer and goby fish, cuttlefish, squid and octopus, crabs and many species of anemones, corals, mussels and nudibranchs. The two prevalent species of sea grass in the area are Posidonia australis and Posidonia sinnosa.
The mangroves of the Northern Spencer Gulf are the closest growing nearest the South Pole of any species in the world. The line of mangroves between Whyalla and Cowell is considered to be a breeding ground for many shark species including the White Pointer shark. Fitzgerald Bay has become a basis for predatory finfish aquaculture industry growing yellowtail kingfish, which are native to the gulf.
(Source: Natural History of Eyre Peninsula, Adelaide, 1985. Paula Brown, The Fish Habitat Handbook, Adelaide 2001)