Seagrass Research at Grassy Point, Bellarine Peninsula

Pete

For the past two years, Peter Macreadie, a PhD student at the University of Melbourne has been carrying out a study focused on the importance of seagrass to fish, and the potential impacts of seagrass habitat loss and fragmentation on fish species. The following webpage provides information about the experiments completed and their outcomes, and the benefits derived from this study. This site is dedicated to the residents and commercial fishermen of Grassy Point, Bellarine Peninsula, Port Phillip Bay for their cooperation and support.

Seagrass Habitats

What is Seagrass?

Seagrasses are highly specialised marine flowering plants that are adapted to soft sediments of shallow, well-lit coastal waters.

Seagrass blades are supported by the natural buoyancy of water, remaining flexible when exposed to waves and currents.

Seagrasses have extensive root systems that are used to firmly hold the plants in place, and to extract nutrients and minerals from the sediment.

Seagrasses manufacture their own food via the process of photosynthesis in the chloroplasts located in their leaves.

Most Seagrasses have separate sexes and produce flowers and seeds.


Natural seagrass		Artificial seagrass

Seagrass habitats have undergone significant decline in recent times, and despite increased efforts to protect seagrasses, this decline is predicted to continue throughout the 21st century. This is of major concern given the important biological and physical role played by seagrass. Seagrass provides structure to otherwise featureless sandy bottoms thereby stabilising sediments, and provide critical habitats to a variety of mammals, fish and invertebrates. It is therefore no surprise that seagrass loss has been linked to loss of biodiversity, collapse of fisheries and increased sedimentation.

Habitat Fragmentation

The central dogma of habitat fragmentation is that larger, more continuous areas of habitat, are reduced to smaller, more isolated patches. It often, but not always results in habitat loss and an increase in the total number of patches. Seagrass can fragment naturally (e.g. by waves, currents, bottom feeding animals, changes in water temperature) or through human mediated mechanisms (e.g. boat propellers, fishing, nutrient loading). Understanding the effects this has on fauna is important for conservation. To date, the majority of "fragmentation" studies have increased our knowledge of habitat patchiness and its effects on animal distribution patterns but have done little to improve our understanding of fragmentation per se. This is because most studies do not actually perform or observe fragmentation, but instead study a post-fragmented or patchy state. A greater understanding of the effects of seagrass fragmentation may be obtained through studies using artificial seagrass. Artificial seagrass has been shown to be a good mimic of natural seagrass with similar densities of fish and invertebrates found on natural and artificial seagrass. The primary objective of this study was to gain a better understanding of how fish respond to fragmentation and loss of seagrass.


Continous patch of artificial seagrass		Fragmented patch of artificial seagrass

Experiments/ Outcomes

Experiment 1 - Fish Responses to seagrass fragmentation

This experiment was a large-scale simulation of seagrass fragmentation, using artificial seagrass, to assess the response of fish to habitat fragmentation. Artificial seagrass units (ASUs) were used to simulate continous (non-fragmented) and patchy (fragmented) plots of seagrass. Through time, surveys of the marine fauna occupying the plots of seagrass were performed.The results showed that fragmentation had no effect on overall fish abundance. In an attempt to explain this phenomenon, it was hypothesised that positive edge effects (more fish located at the edges compared to the centre) were counteracting the potential negative effects of habitat loss. This hypothesis was explored in experiment 2...

Grassy Point, Bellarine Peninsula - Artifical seagrass units (ASUs). Photograph courtesy of DSE.

Experiment 2 - Edge effects in seagrass

The aim of this experiment was to determine whether positive edge effects can counteract the negative effects of habitat loss. Using the continuous and patchy plots of artificial seagrass from the previous experiment, samples were taken from the inner and outer areas of each seagrass plot. Fish were found to be significantly more abundant at seagrass patch edges. Experiments 3 & 4 were carried out to determine the factors responsible for causing positive edge effects.


Continous (non-fragmented) seagrass plot		Patchy (fragmented) seagrass plot

Experiment 3 - Spatial measurement of food variability

The aim of this experiment was to determine whether plankton (the main source of food for many seagrass fish) abundance differs across seagrass patches, and whether this difference is related to current strength. We hypothesised that fish distribute themselves in seagrass relative to the supply of plankton.

Plankton Tube Traps (PTTs) were used to take plankton samples across seagrass patches and plaster blocks were used to measure current strength. Analysis of the plankton volume and loss of plaster showed that plankton supply is related to current strength, and that plankton is in higher abundance at patch edges than patch interiors. This finding supported the theory that fish remain at patch edges to have greater access to their food source.

Plankton Tube Traps (PTTs)

Experiment 4 - Importance of food in determining edge effects

In the previous experiment, plankton was found to be more abundant at seagrass edges. The aim of experiment 4 was to demonstrate that food supply was driving positive edge effects in seagrass fish. If greater food availability at patch edges is the reason for seagrass fish being more abundant at patch edges, then the deficit in interior fish densities will be less, or even reversed, in patches supplemented with food at patch interiors.

The interior of each seagrass plot was supplemented with a continual, slow release supply of food (live artemia) using a dripper system. After a set period of time, interior and edge samples were taken and species richness and abundance was recorded. The results showed that fish responded to the food by moving from the edges to the interiors of plots. This demonstrated that food supply is driving positive edge effects in seagrass fish.


Live Artemia Dispenser (LAD) dripper system		Using drop nets to take interior and edge samples

Experiment 5 - Influence of habitat growth and habitat loss on fish assemblages

Experiment 5 compared the influence of habitat growth and habitat loss on fish assemblages. To do this, artificial seagrass was used to simulate habitat growth and habitat loss. When patches reached the same size via each of these mechanisms, the fish assemblages were compared and monitored through time. This experiment provided an understanding of how different processes determine community structure and showed that fish will crowd into remaining habitat following habitat loss; a phenomenon reported as the "crowding effect" in terrestrial research.

frag

Schematic diagram illustrating the process of habitat loss and habitat growth.

Common Species observed within the seagrass:

Common name - Species name

Bridled leatherjacket – Acanthaluteres spilomelanurus
Cobbler - Gymnapistes marmoratus
Crested Weedfish – Cristiceps australis
Decorator crab - Naxia sp
Dumpling squid - Euprymna tasmanica
Eastern Australian Salmon - Arripis trutta
Flathead - Platycephalus sp
Greenback flounder – Rhombosolea tapirina
Hardyhead - Kestratherina esox
King George Whiting - Sillaginodes punctata
Knobby Seahorse - Hippocampus breviceps
Little Weed Whiting - Neoodax balteatus
Port Phillip pipefish - Vanacampus phillipi
Pygmy squid - Idiosepius notoides
Siphon fish - Siphaemia cephalotes
Smooth Toadfish - Tetractenos glaber
Southern pygmy leatherjacket - Brachaluteres jacksonianus
Spotted pipefish - Stigmatopora argus
Weedfish - Heteroclinus sp
Wide-body pipefish - Stigmatopora nigra

Benefits derived from this study

Seagrasses are a conspicuous element of Australian marine environments, and are crucial in the conservation and maintenance of biodiversity. Because seagrasses occur mostly in shallow water, at the confluence of land and sea, they are in a prime position for early identification, monitoring and prediction of detrimental impacts attributable to anthropogenic activities, including global warming, pollution, urbanization, sedimentation and fishing effects. There have been significant declines of seagrass habitat in recent years and predictions point toward continued loss. These changes to seagrass have been linked with increased coastal erosion, severe loss of biodiversity, and collapse of fisheries. Unfortunately, however, the limited understanding of processes important in the functioning of seagrass habitat in Australia reduces our ability to manage these systems sustainably.

This project focused on the importance of seagrass to fish and assessed the potential impacts of seagrass habitat loss and fragmentation on fish species. Seagrasses in Australia are important for fish because they act as a nursery habitat for juvenile fish through their provision of food, shelter and protection from predation. Seagrasses are therefore thought to play an important role in the sustainability of recreational and commercial fisheries. With an accelerating trend of seagrass loss and fragmentation of seagrass habitat, it is important to understand what processes are important in determining abundances and species richness of fish associated with seagrasses.

This study contributes to Research Priority 1 of Australia's National Research Priorities; 'to develop an Environmentally Sustainable Australia'. Specifically, this study increased our understanding of how processes determining faunal associations with seagrass interact with the arrangement of seagrass landscapes and has applications in the sustainable use of Australia's biodiversity, as well as responding to climate change and variability. By providing important information about fish movements, this study improves our ability to predict changes to the viability of local fisheries as the arrangement and value of seagrass landscapes change. It also addresses a number of gaps in our current knowledge of the effects of seagrass habitat loss and may be used to assist the management of Australia's increasingly fragmented coastal landscapes.

Acknowledgments

Melbourne University
Prof Mick Keough
The Keough Lab

Department of Primary Industries (DPI) Queenscliff
Dr Greg Jenkins
Dr Jeremy Hindell

CSIRO Marine and Atmospheric Research (CMAR)
Dr David Smith

Griffith University
Prof. Rod Connolly

Victorian Marine Science Consortium (VMSC)
Liz McGrath
Rod Watson

A huge thank you to Roytal Enterprises for the construction of the artificial seagrass

Field assistance:
Fiona Warry
Tim Smith
Rod Watson
Allyson O’Brien
Hannah Murphy
Bec Loughman
Gerry Ryan
Katie Baker
Jo Brown
Meg Wright
Jessica Smith
Bec Bray

GIS assistance:
David Ball

This page was created by Jessica Smith and Peter Macreadie

Financial support provided by: