icolls - Sustainable Tourism CRC

ECOLOGY, THREATS AND MANAGEMENT OPTIONS FOR SMALL ESTUARIES AND ICOLLS
Chapter 2
MARINE SUBSIDIES SUPPORT ICOLL FOOD WEBS: THE
DEVELOPMENT OF A MARINE PULSE CONCEPT
ICOLLS are abundant and important ecosystems along the Australian coastline. In many regions along the coast,
ICOLLs provide highly sought-after sites for tourism and recreation and the sustainability of ICOLLs in light of
visitor activities has been afforded very little attention. In this component of the project, we aimed to gain a
better understanding of ICOLL ecology and the sustainability of these systems in response to increasing coastal
development, including tourism and recreational demands. Specifically, we used stable isotope signatures of
carbon and nitrogen to examine food web structure and function in small ICOLLs in northern New South Wales,
Australia, with a view to gaining a better understanding of the processes that sustain these systems.
Community composition across the study systems was highly variable, in space and time, reflecting the
strong influence of entrance status on the composition and abundance of taxa. During periods of closure,
ICOLLS support a wide variety of freshwater taxa, which subsequently are lost when connectance with the
ocean is reinstated.
Despite considerable variability in species composition and community structure, all of the sampled ICOLLS
appeared to rely on autochthonous algal production as a major carbon source. During periods of opening, marine
subsidies, in the form of macroalgae and incoming taxa appeared to be important.
Marine subsidies play a strong structuring role in influencing food webs of ICOLLS.
Implications of Catchment Modifications on ICOLL Food Web Structure
and Function
Given their variable physical and chemical characteristics, it comes as no surprise that ICOLLs are biologically
diverse, with species representative of freshwater, estuarine and marine habitats occurring concurrently in many
systems. Similarly, the natural variability in ICOLL opening and closing events may be expected to ensure that
food web structure in these systems is spatially and temporally dynamic. Furthermore, whilst there have been
some studies investigating the ecology and diets of commercially important fish species in ICOLLs, very little is
known of the productivity and trophic dynamics of these ecosystems.
Conceptual Approach to Carbon Flow in ICOLL Food Webs
There is little data on which we can base our understanding of ICOLL food webs. However, given that these
systems are fundamentally distinct from large permanently open estuaries (Pollard 1994a) and are often
characterised by conditions more typical of freshwater creeks, lakes and lagoons, conceptual and theoretical
models derived for freshwater systems might be useful in enhancing our understanding of carbon flow through
these coastal systems.
To date, there have been no models developed to describe the carbon dynamics that support food webs in
ICOLLs. Whilst some models of carbon flow have been derived from research conducted in permanently open
estuaries (e.g. Heymans & Baird 1995; Howarth, Schneider & Swaney 1996; Otero, Culp, Noakes & Hodson
2000), the documented differences between these systems and ICOLLs suggest that their applicability may be
limited. In this study, we decided to build on existing knowledge from riverine ecosystems to describe patterns
and processes in ICOLLs. This application of riverine models to ICOLL ecosystems (and subsequent
development of a novel ICOLL carbon flow model) represents a completely original approach.
Three primary conceptual models have been developed to describe the flow of energy (carbon) in riverine
ecosystems (Figure 1). The River Continuum Concept (RCC) assumes that the relative importance of
allochthonous (terrestrial) carbon sources to stream food webs will diminish progressively from small headwater
streams to large rivers (Vannote, Minshall, Cummins, Seddell & Cushing 1980; Minshall, Cummins, Peterson,
Cushing, Bruns, Seddell & Vannote 1985). The Flood Pulse Concept (FPC) suggests that in large meandering
rivers with extensive floodplains, floods provide consumers with significant pulses of organic matter derived
from the floodplain (Junk, Bayley & Sparks 1989). The Riverine Productivity Model (RPM) assumes that
within-stream production can be a significant source of carbon to consumers in medium to large systems (Thorp
& Delong 1994). Significantly, all of these carbon flow models have been developed to characterise energy flow
14