Humpback whales in the open waters of Bass Strait 📷 Ken Flannigan (iNaturalist, CC-BY-NC)
Pelagic fauna may gather together (aggregate) for different reasons. Many pelagic fishes school as a means of protection against predators. Aggregation may also occur because many individuals are attracted to a particularly favourable habitat. For example, Orange Roughy (Hoplostethus atlanticus) aggregate around seamounts, pinnacles and canyons as they seek out habitats with particular hydrologic profiles. Southern Right Whales migrate from the Antarctic to Victorian waters and gather together in preferred nursery areas along the western Victorian coast to give birth and nurse calves.
Seabirds, predatory fish and marine mammals all congregate in areas where there is a reliable food supply, such as the upwellings. The Bonney coast of western Victoria is one of only 13 known areas of frequent aggregation for Blue Whales (Balaenoptera musculus).
Plankton and nekton
The water column in Victorian marine waters is a habitat for drifting plankton and for active-swimming animals including fish (also known as nekton). This habitat is obviously present throughout the Victorian marine environment, but it is considerably different along different environmental gradients.
The temperature of marine and coastal waters fluctuate seasonally. There are greater temperature ranges in bays and estuaries, as the smaller bodies of water are more quickly heated and cooled. Ocean temperature is influenced by currents and upwellings. The east of the state is influenced by the warmer East Australia Current, whereas the Otway and Central Victorian Bioregions are influenced by the temperate South Australia Current and Northern Bass Strait waters. The Flinders Bioregion is under the influence of the South Australia Current, East Australia Current, Northern Bass Strait and cold subantarctic surface waters.
Other important environmental gradients include greater wave action in shallower waters and greater turbidity closer to shore. In bays and estuaries, the habitat is highly influenced by nutrient runoff, suspended matter and freshwater inputs from rivers and other drainages.
Plankton are a major source of food for benthic (bottom-dwelling) invertebrates and nekton. They also play a key role in the carbon, nitrogen and other nutrient cycling in marine systems. The photosynthesising fraction of plankton, phytoplankton, is highly productive. In Port Phillip Bay it is responsible for at least two-thirds of primary production.
Diatoms and dinoflagellates are the dominant type of phytoplankton. The abundance of phytoplankton communities in Victoria varies depending on the season, with peaks during summer and lowest concentrations during winter. This is probably because temperatures are higher in the summer and more light tends to be available, while factors limit abundance during colder months.
Phytoplankton abundance is also limited by the availability of nutrients, particularly nitrogen. Concentrations of phytoplankton are often highest in estuaries and river mouths, where there is greater input of nutrient from terrestrial runoff.
Phytoplankton are preyed on by zooplankton (small, floating animals). Zooplankton include a wide variety of organisms, including amoeboids, crustaceans, jellyfish, invertebrate larvae and fish larvae. Crustaceans, particularly copepods and cladocerans, make up a large proportion of the zooplankton in Port Phillip. The composition of the zooplankton, similar to that of phytoplankton, shows some influence of seasonality.
In Victoria, active-swimming pelagic organisms, nekton, are predominantly fishes and cephalopods, but also include marine mammals, penguins and crustaceans (e.g. krill). Nekton play an important role in the trophic pathways of Victorian marine ecosystems. They facilitate the transfer of energy from plankton, through the food web, to higher order organisms; for example plankton are a food source for small nektonic species which in turn are prey for other predators.
High abundances of nekton are often associated areas of high phytoplankton productivity, such as upwellings. In Victoria, nektonic fishes and cephalopods support significant recreational and commercial fisheries. Marine mammals also have high scientific, social, historical and tourism value.
Between November and April, along the Bonney Coast in the Otway bioregion, an oceanographic process called ocean upwelling occurs. Seasonal prevailing winds drive warm, nutrient-depleted surface water away from the coast. This draws deeper, colder water to the surface to replace it. The deep water is nutrient-rich, and as it reaches sunlit surface waters it promotes high productivity of phytoplankton. These phytoplankton are the base of a highly productive food chain that sustains a high biomass of zooplankton and nekton, including krill and commercially important pelagic fishes such as tuna, sardines, and anchovies.
The krill and small fishes are an important food source for pelagic sharks, seabirds, and fur seals during the summer breeding season and support the significant population of blue whales along the Bonney coast.