Living on the edge: the importance of estuaries

In this section you’ll learn about why estuary ecosystems are so important for humans and biodiversity alike. Hi, I’m Dr. Christopher Henderson and I’m a coastal and marine ecologist at the University of the Sunshine Coast. Estuaries provide humans with a large number of ecosystem services that we covered. They aid in shipping through the construction of ports, coastal protection, where they buffer the impacts of the ocean to the land. And finally, they are home to numerous fish and invertebrate species that are of recreational and commercial fishing importance. Estuaries are positioned at the transition between terrestrial and marine environments with the impacts that occur throughout both on the land and in the sea, having an effect in the estuary.

Therefore, it is the effects of terrestrial shoreline modification, habitat fragmentation, and the centralization of impact from modified environments generally, that combined in the lower parts of estuaries and cause significant changes to the structure and functioning of these regions. One key example on land is extensive urbanised areas in the building of ports and cities which can result in a high volume of pollutants, nutrients and sediment, which runoff into nearby waterways. These all drain into the lower parts of estuaries and result in negative outcomes for the composition of fish. In addition, estuarine fringe habitats are often reduced in their extent, quality and conductivity by coastal development, destructive fishing and practices such as dredging.

Estuaries are made up of a series of different habitats that are intertwined with one another, making these complex seascapes a true hotspot for biodiversity. Some of the key habitats that make up estuaries include mangrove forests, salt marsh, seagrass meadows, Rocky and oyster reefs, and even bare sandy areas, that these are all important for some spaces that use estuaries. Now depending on where you are in the world, it may change the habitats that you most commonly see. Here in subtropical Australia, we often see that our estuaries are dominated by mangrove forests. However, if you are on the Atlantic or Gulf Coast of the United States of America, you might find extensive salt marsh habitats in those positions.

How these different habitats are intertwined together in a broad mosaic can shape the composition of animal assemblages, the distribution of ecological functions and the structure of the food web as a whole across these really important seascapes. Estuaries support a suite of habitats that provide foraging and resting sites for adult fishes and nursery habitats for juveniles of many species that move offshore as adults. Fish migrate among estuarine habitats with tidal, diel and seasonal changes that govern either their accessibility or suitability as fish habitat, or they may move with ontogenetic changes in resource requirements.

Thus, it is the spatial properties of estuarine habitats for example, their size, isolation, position and configuration, which can shape both the level of larval recruitment and the frequency of visitation by juvenile and adult fishers. It is widely appreciated that variation in habitat extent, isolation and connectivity can modify the composition of animal assemblages in estuaries. For example, fish diversity and abundance is often greatest in structurally complex ecosystems that provide habitats for fish, particularly when those patches are both large and located close either salt marshes, mangroves, seagrass meadows, oyster reefs, or rocky reefs. So why do we care so much about fishing estuaries? Well, one of the main reasons is that we like to eat them.

And as I’ve previously said, many of the species that live in estuaries are of recreational and commercial importance to our society. However, the second reason is that they play a really important role in the ecosystem. Each of these spaces perform a series of important ecological functions that are crucial to maintaining the health of the ecosystem as a whole. Animals perform a range of important ecological functions, for example, herbivory, predation and scavenging, which shape the condition of habitats structure of food webs, and underpin the capacity of ecosystems to resist or recover from disturbance.

It is widely accepted that ecological functions are tightly linked to biodiversity, which one may consider to be a great for somewhere like estuaries, and positive correlations between animal diversity in the delivery of an ecological function have been reported in many different ecosystems globally, particularly in those high diversity systems such as tropical rainforests or coral reefs. However, what we do find in estuaries is something a little different. In research completed here at USC on the Sunshine Coast in eastern Australia. We have found that in many cases ecological functions might only be performed or dominated by a single species. The role of species domination in functional ecology. So that’s the delivery of functions in an ecosystem is gaining traction.

And it’s been suggested that a relatively small pools of species can underpin the provision of several ecological functions. For example, herbivory in predation on the land and in the sea. Species that dominate key ecological functions are likely to be relatively common, and could therefore be important targets for refined spatial management, as their protection might help to maintain the function of interest across a landscape or a seascape. So what does this mean for how we manage an estuary? Do we manage for biodiversity? Do we manage for the abundance of a single spaces that may be important to an ecological function? Well, I think it needs to be somewhere in the middle.

And this is where the concepts of functional redundancy and functional complementarity come in. functional redundancy is when multiple species can coexist by using the same resources and performing an identical ecological function. Whereas functional complementarity is when species access similar resources in different contexts, and can therefore perform a similar ecological function but at a distinct location or time. functional redundancy and complementarity are important components of animal assemblages, which help to maintain the structure and functioning and the resilience of those ecosystems to environmental change should be a combination of these two concepts that we are managing for.

This is where in a marine context, fisheries management and ecosystem protection approaches could be combined to better maintain the biodiversity and ecological functioning the estuaries required to provide humans with ecosystem services that were covered. In the next activity, you will read a paper about the drivers of fish abundance, diversity and ecological functions that they provide, leading to a discussion on how you would best like to see management of these biodiversity and ecological function hotspots change as we move into the future.