Dr Maria Schreider

Trophic relay is an ecological model that involves the movement of biomass and energy from vegetation, such as saltmarshes, within estuaries to the open sea via a series of predator-prey relationships. Any potential for trophic relay is therefore affected by water movements within an estuary and by the ability of a predator to "switch" prey in response to fluctuating abundances of those prey. Saltmarsh-dwelling grapsid crabs, which feed on saltmarsh-derived detritus and microphytobenthos, release zoeae into ebbing tides that inundate saltmarshes during spring-tide cycles within tidally-dominated estuaries, such as Brisbane Water Estuary, therefore providing an opportunity to examine whether prey-switching and/or trophic relay may occur in fish that feed on those zoeae (such as the highly abundant estuarine ambassid, Ambassis jacksoniensis). This model was examined by sampling A. jacksoniensis near saltmarshes in a large, temperate south-eastern Australian estuary during flood and ebb tides on days of saltmarsh inundation and non-inundation over four spring-tide events in 2012. Stomach fullnesses of A. jacksoniensis were generally highest during ebb tides on days of saltmarsh inundation, implying that feeding was most marked at these times. Caridean decapods dominated diets during flood tides and on days of no saltmarsh inundation, while crab zoeae dominated diets during ebb tides and on days of inundation, suggesting that, when saltmarsh-derived zoeae became abundant, A. jacksoniensis switched to feeding on those prey. Three potential zooplankton prey (calanoid copepods, caridean decapods and crab zoeae) did not differ calorimetrically, indicating that switching of prey by A. jacksoniensis is not directly related to their preying on energetically greater prey, but reflects opportunistic feeding on more abundant and/or less elusive prey. As A. jacksoniensis is able to switch prey from estuarine caridean decapods to saltmarsh-derived crab zoeae, this very abundant ambassid would be well-placed to promote any trophic relay, via further water movements or other predator-prey relationships, to the adjacent marine environment.

Saltmarsh-dwelling grapsid crabs release free-swimming larvae (i.e. zoeae) into ebbing tides during spring-tide cycles that inundate saltmarshes, where initial inundation is a cue for larval release on subsequent inundations. In a saltmarsh environment, crab zoeae are the main food for fish (including the glassfish, Ambassis jacksoniensis), which 'fast' at other times. This saltmarsh-feeding model was tested by obtaining glassfish from near saltmarshes in a reasonably unmodified tributary of a large temperate estuary on flood and ebb tides during the night in two spring-tide events in austral autumn of 2009. Glassfish fed only on ebbing tides, with stomachs being similarly full on both spring-tide events. Thalassinid larvae (including Trypaea australiensis) dominated the dietary volumes, especially on the night before saltmarsh inundation, presumably being released during inundation of intertidal mud and sand habitats. Although glassfish progressively 'switched' to feeding on greater volumes of crab zoeae (presumably released after inundation of a saltmarsh) over both spring-tide cycles, such zoeal contributions never exceeded those of thalassinid larvae. The above differences highlight that, although ebb tides trigger feeding by glassfish, this ambassid focuses on different prey in a reasonably unmodified environment. The ability of glassfish to switch prey, and thus accommodate environmental differences, helps explain their high abundance in estuaries of this region.

© 2015 Elsevier Ltd. The shift from seagrass to macroalgae, particularly the "boom and bust" cycle of the ephemeral Chlorophyta in estuarine environments has occurred worldwide, most likely as a result of anthropogenic nutrient loading. Contrary to popular view, these algal blooms are not harmful to humans, although they are perceived as unsightly and negatively affect aquatic leisure activities. Macroalgal blooms mostly occur in spring and summer when rainfall prior to and during this time is low and temperatures are high. Water quality data have been collected before and during an algal bloom at Avoca lagoon on the Central Coast of NSW, Australia, to identify changes in water quality associated with a bloom. Water dissolved oxygen declined significantly whereas turbidity and available phosphate increased significantly during the bloom. The changes could be detected only using intensive spatial and temporal replication. Identifying water quality changes during a bloom is a crucial first step for understanding the conditions under which extensive growth of algae occur and has important implications for prediction and management of the blooms in coastal lagoons.