Mapping the fish fry feeding prism in a saltmarsh-estuary ecotone

摘要

It is a common understanding that coastal wetland plays an important role in marine fisheries recruitment. However, mechanistic detail of the processes that link the two ecosystems is not well defined. This study aims to describe and map ecological landscape features of a saltmarsh-estuary ecotone at the entrance to Mambo Creek in Salamander Bay, at Port Stephens, on the southeast coast of Australia. The creek drains from 175 ha of estuarine-freshwater area which forms part of the Tomaree Wetland complex. Based on field surveyed data and GIS analysis, a dynamical system is depicted of a tidal salt-wedge that results from the interaction of the wetland and estuary water. The creek entrance lies inside the extensive Posidonia seagrass beds of Salamander Bay. It is constricted by a shallow sandbar at the entrance and is fringed by mangroves that stabilize the creek channel. Analysis of the ebb-tide outflow from the wetland shows it contains micro-particles (10–100 µm diameter) at densities >20,000 particles ml⁻¹, high levels of H2S, low pH, lows-alinity and low dissolved oxygen and is thus adverse for most fish fry. In the vicinity of the entrance channel to the creek: i) the outflow from the wetland forms a tidal salt wedge and disperses across the water surface where it meets denser saline estuary water; ii) the micro-particles sink out of the surface layer at the tip of the salt wedge in a sharp, readily observable boundary, due to flocculation effects. At this boundary or ecotone, the continuous presence of a “feeding prism” which contains very large numbers of at least three species of marine fish fry <20 mm total length, particularly in warmer months, was observed. The fry remain only within this creek area and feed on the ad-libitum supply of outwelling wetland micro-food particles while swimming in near oceanic condition water below the salt wedge; iii) the seston material varies with sea--son; in warmer months it is principally a mixture of purple and green bacteria coated micro-particles and in cooler months detritus predominates; iv) the location of the halocline is dynamic and is related to the morphology of the creek and tide; and, v) the creek entrance also contains a deeper area in which small numbers of large apparently gravid fish were regularly observed. The fish fry aggregations are mapped in relation to the halocline in space and time during summer and winter seasons and linked to tidal sea-level and the morphology of the creek. The authors suggest the “feeding prism” ecotone is an important link between wetland process functions and marine fish recruitment for at least three commercially and recreationally important coastal fish species and perhaps others. It is this ecotone that needs to be considered as a focus of management measures and environmental impact assessment associated with adjoining developments and landscape modifications that may affect coastal processes in the vicinity of the creek. For example, the sand-dune system surrounding Mambo Wetlands provides a freshwater aquifer reservoir that is recharged by precipitation and discharges over time into the low-lying saltmarsh.