DYNAMIC ESTUARINE PLUMES AND FRONTS - IMPORTANCE TO SMALL FISH AND PLANKTON IN COASTAL WATERS OF NSW, AUSTRALIA

In 1990, low density estuarine plumes in the vicinity of Botany Bay, A ustralia, extended up to 11 km across a narrow continental shelf (ca 2 5 km) on ebb tides. The shape and seaward extent of plumes varied acco rding to a combination of state of the tide, freshwater input and the direction and intensity of coastal currents. Offshore plumes dissipate d on the flood tide and fronts reformed at the entrance of Botany Bay. Major differences in the abundance and composition of ichthyoplankton and other zooplankton were found over a 400-800 m stretch of water en compassing waters of the plume, front and ocean on seven occasions. Fo r example, highest abundances of the fishes Gobiidae, Sillaginidae, Ge rreidae and Sparidae as well as barnacle larvae and fish eggs were fou nd in plumes. Cross-shelf distribution patterns of zooplankton, theref ore, are influenced by plumes. Distinct assemblages of plankters accum ulated in fronts, e.g. fishes of the Mugilidae and Gonorynchidae and o ther zooplankters (e.g. Jaxea sp.). Accumulation in fronts was variabl e and may relate to variable convergence according to the tide. We arg ue that plumes provide a significant cue to larvae in coastal waters t hat an estuary is nearby. Moreover, although many larvae may be retain ed in the turbid waters of plumes associated with riverine input, larv ae are potentially exported in surface waters on ebb tides.