Year

2011

Degree Name

Master of Science - Research

Department

School of Biological Sciences

Abstract

Little is known about larval processes that occur in the temperate estuarine waters of Australia. Meroplankton, a term used to describe organisms that spend usually their larval stages of their life-cycle in a planktonic form, are common inhabitants of the estuarine system. In the saltmarsh and mangroves that fringe most estuaries in temperate Australia, some meroplankton groups have developed endogenous timing to environmental (exogenous) factors. These environmental factors are the result of periodic cycles that arise from the forces of the sun and moon acting on the earth.

Specifically, meroplankton time larval release and supply to diel (light/dark), tidal (flood/ebb), tidal amplitude and lunar cycles. Coinciding the timing of larval release and supply to these cycles is thought to increase their likelihood of survival, by reducing times when meroplankton are exposed to predation and inhospitable physical factors. Few studies have examined the life histories of estuarine intertidal fauna in Australia and this study attempts to add to this limited body of knowledge.

The current study aimed to examine the role each environmental cycle has on the abundance of larval meroplankton and their spatial and temporal patterns during these cycles. This was tested by collecting larvae in the lower saltmarsh and mangrove habitats at two estuaries during times when key phases of the environmental cycles occurred. Multifactorial designs and analyses were used to test diel, tidal, tidal-amplitude and lunar factors and that may drive spatial and temporal patterns.

The light/dark and tidal cycle interaction was the main driver for larval release and supply for crab larvae, which usually accounted for the majority of the meroplankton in the samples. Specifically, larval crabs occurred at great densities during the nocturnal ebb tide. Gastropod and polychaete larvae, which were also substantially large in most samples, appeared to show no conclusive release or supply patterns to environmental cycles, although at times were greater in densities on specific diel and tidal phases. During the lunar and tidal phases, great meroplankton abundances occurred during the ebb tides compared with flood tides, and these results were significant during new-moon ebb tides. However, significant month-to-month variation during the ebb tides could have just as likely explained differences between meroplankton abundances.

This study builds on the few studies in Australia that have examined larval release and supply. For organisms with a planktonic life history, the dispersal of their larvae largely influences adult population community structure. Further, the periods during which these processes occur increase the productivity of the estuary through predator-prey interactions. As a result, the ecological value of the saltmarsh and mangrove habitats is being understood and recognised. Further studies are needed to gain a greater insight into larval meroplankton life histories and the processes that influence them, until then it remains difficult to make generalisations about larval release and supply within and between temperate estuaries in Australia.

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