Zooplankton as indicators of changing dominance of water masses in Storm Bay, Tasmania: comparing the 1970s to now

Zooplankton are useful indicators of environmental change. Their physiology is strongly coupled to temperature, they exhibit short life cycles and they are generally safe from the pressures associated with commercial fishing.... [ view full abstract ]

Zooplankton are useful indicators of environmental change. Their physiology is strongly coupled to temperature, they exhibit short life cycles and they are generally safe from the pressures associated with commercial fishing. In Storm Bay, Tasmania, salps, copepods and krill are abundant in the zooplankton, with seasonal dominance of species driven by temperature and salinity. Storm Bay is a region of dynamic oceanography that is influenced by (i) warm, low nutrient waters from the East Australian Current (EAC) in the summer, (ii) cooler, nutrient-rich subantarctic waters in the winter, (iii) the Leeuwin (Zeehan) Current flowing along the west coast and (iv) flows from the Derwent Estuary. The relative influence of these water masses is changing; for example, the EAC is now extending further along the east and south coasts of Tasmania and persists for longer, bringing warm, nutrient-poor water into Storm Bay for extended periods. We’ve examined zooplankton distribution on a monthly basis in Storm Bay since 2009 and have observed changes that relate to the persistence of the water masses. A study of the zooplankton in Storm Bay in the early 1970s showed that the oceanic influence from the EAC was strongest in January and February, while subantarctic species were common from August to October. Our study of Storm Bay has shown that the zooplankton biodiversity of Storm Bay has changed to reflect a stronger EAC influence and possibly a weakening of the subantarctic current. Subantarctic indicator species, such as Neocalanus tonsus and Racovitzanus sp., are rarely observed, while species associated with subtropical waters are often dominant and/or persist for several months (Jan-May). These include the copepod Temora turbinata, the salps Salpa fusiformis and Thalia democratica and cyclopoid copepods in the genera Sapphirina and Corycaeus. We will discuss these changes in biodiversity and relate them to changes in oceanography since the 1970’s, in particular the dominance of the EAC and the Leeuwin Current. Changes in the biodiversity may have important implications for energy transfer up the food chain.