What are Macroalgae?
Macroalgae (seaweeds’) are an ancient class of large multicellular plants that resemble vascular plants but lack the complex array of tissues used for reproduction and water transport. They are important elements of shallow coastal waterways and are found in red (Rhodophyta), green (Chlorophyta) and brown (Phaeophyta) divisions. The colours of macroalgae are due to different pigments that the algae use to convert the sunlight into chemical energy via photosynthesis. While all algae have chlorophyll pigments, red algae also have red and blue pigments called phycobillins and brown algae also have orange pigments called carotenoids which result in their multi-hued appearance1. Macroalgae typically grow attached to hard substrates such as rocks, shells and coral skeletons.
Photo 1. Pictures of macroalgae in their natural environment; a) Green algae – Chlorodesmis fastigata, b & c) Brown algae Macrocystis sp. and unknown, respectively, and d) Red Algae – Galaxaura marginata.
Significance of Macroalgae
Macroalgal beds are valued for their intrinsic biodiversity in Australia2. Macroalgae can also provide food and refuge for juvenile fish, crabs and other species, particularly in areas where other habitat is lacking. Particular green and red macroalgae even produce calcium carbonate which plays an important role in building coral reefs1. Red coralline algae have been found to grow to depths of up to 268 m, which is 100 m deeper than which sunlight usually penetrates and the deepest of any photosynthetic organism1.
Humans have also discovered the wonders of macroalgae and subsequently they are widely used commercially for food (e.g. sushi), some yield agar used widely in microbiological culture media, and some yield alginate used in products as diverse as ice cream, instant puddings, dental impression material, wound dressings and pharmaceuticals34.
Macroalgae thrive in waters that receive nutrient pollution and the presence of certain types of macroalgae often indicates nutrient enriched waters. Macroalgal blooms can have direct and indirect impacts on the natural environment. They can deprive seagrass areas of light, causing their eventual decline. Decomposing mats of the macroalgae can also deplete the water column of dissolved oxygen, driving oxygen levels to deleteriously low (“hypoxic”) or zero (“anoxic”) levels. Massive fish kills are possible under these conditions, even if the anoxic event lasts only a few hours. Decaying macroalgae can accumulate on shorelines and be an odorous nuisance to local residents and to the coastal tourism industry.
The strong relationships that have been found between macroalgae and the quality of water that they live in has resulted in much research into using them as indicators of water quality.
Macroalgae as environmental indicators?
Some macroalgal species have a large capacity for nitrogen assimilation and storage over short time intervals5. Such plants can rapidly assimilate event-driven nutrient pulses that can occur in oligotrophic waters, and can retain a signature of the event in their tissues. As such, macroalgal tissues can be used to detect and integrate pulsed nitrogen inputs to coastal waterways that might be missed by routine water quality monitoring programmes5. Macroalgal tissue indicators include:
- the amount of certain amino acids within the algae (e.g. citrulline)6;
- the concentration of pigments (e.g. nitrogen-rich phycobillins)7; and
- the concentrations of nitrogen (N) in dried plant tissues896.
Macroalgae have also been used to fingerprint nutrient sources by using the ratio of nitrogen stable isotopes 15N to 14N (δ15N signatures) in the algae’s tissue. Different sources of nitrogen often have unique δ15N signatures (i.e. δ15N of plant fertilisers is very different from δ15N of sewage) and this is reflected in macroalgae10.
Other macroalgal indicators include:
- the frequency of (macro-)algal blooms; and
- changes in macroalgal bed area (km2) and species composition. These are suggested indicators for State of the Environment reporting (e.g. Indicators 2.1 and 3.1 in the Estuaries and the Sea volume)2.
What causes Macroalgal tissue chemistry, bed areas and species composition to change?
Concentrations of pigments can act as good indicators of nutrient status of the surrounding water quality. Some algae (particularly red macroalgae) that grow in nutrient depleted waters will store nutrients as pigments for times when nutrients are not readily available, much like animals store fat7. It is akin to a litmus test, the deeper the colour of certain macroalgae, the more nutrient-rich they are.
Tissue N and Citrulline content
Tissue N generally reflects external N concentrations896. The amino acid citrulline has an N-rich structure and may be involved in N storage in macroalgae9. The concentration of citrulline increases with pulsed inputs of N to a coastal waterway6.
The ratio of 15N to 14N in macroalgal tissues can be used to determine the major sources of nitrogen to coastal waterways. This is because sewage-derived N (~10 per mil) has a different isotopic signature than fertiliser-derived nutrient (~0 per mil)10.
Algal bed area and species composition
Algal bed areas and their species composition change in response to many known pressures including:
- water quality – especially light, nutrient and turbidity levels2 and water temperature;
- salinity levels – macroalgal species richness usually decreases with salinity11;
- marine pest incursions2;
- fishing pressure2;
- sea level rise and other aspects of global climate change2.
Coastal waterways where indicators are most relevant
Macroalgal indicators are highly relevant as indicators of water quality in areas that have highly variable conditions leading to erratic changes in water quality, or alternatively receive pulsed nutrients from e.g. intermittently discharged sewage. The greatest concern for management authorities is that they are not adequately capturing what is actually happening in the environment with their current monitoring programs. This is particularly the case in tropical to sub-tropical regions where highly variable rainfall makes capturing changes in water quality largely unpredictable.
Considerations for measurement and interpretation
- Algal pigments are typically measured using spectrophotometric techniques12.
- Amino acids are measured using gas chromatography techniques6.
- Tissue nitrogen and stable isotope analysis is best measured using mass spectrometry techniques10.
- Guidelines for monitoring and reporting on algal bed area and species composition can be found in the Estuaries and the Sea Volume of Environmental Indicators for National State of the Environment Reporting2.
Existing information and data
Sporadic and site specific data sets exist in universities and state governments.
Simon Costanzo, National Research Center for Environmental Toxicology, University of Queensland
Chris Roelfsema, Marine Botany, University of Queensland
- Raven PH, Evert RF, Eichhorn SE (1986) Biology of Plants. Worth Publishers Inc., New York.
- Ward, T., Butler, E. and Hill, B. 1998. Environmental Indicators for National State of the Environment Reporting, Estuaries and the Sea, Commonwealth of Australia, pp. 81. www.ea.gov.au/soe/coasts/estuaries-ind.html
- Cribb, A. B. (1996). Seaweeds of Queensland: A naturalists guide. Kingswood Press, Queensland.
- Woelkerling, W.J. 1990. An Introduction In K.M. Cole and R.G. Sheath (Editors), Biology of the Red Algae. Press Syndicate of the University of Cambridge, Cambridge.
- Lapointe, B.E. 1985. Strategies for pulsed nutrient supply to Gracilaria cultures in the Florida Keys: interactions between concentration and frequency of nutrient pulses. J. Exp. Mar. Biol. Ecol. 93, 211-222.
- Costanzo, S.D., O’Donohue, M., and Dennison, W.C. 2000. Gracilaria edulis (Rhodophyta) as a biological indicator of pulsed nutrients in oligotrophic waters. J. Phycol. 36, 680-685.
- Jones, A.B., Dennison, W.C., Stewart, G.R. 1996. Macroalgal responses to nitrogen source and availability: Amino acid metabolic profiling as a bioindicator using Gracilaria edulis (Rhodophyta). Journal of Phycology 32, 757-766.
- Lyngby, J.E. 1990. Monitoring of nutrient availability and limitation using the marine macroalga Ceramium rubrum (Huds.) C. Ag. Aquatic Botany 38, 153-161.
- Horrocks, J.L., Stewart, G.R., and Dennison, W.C. 1995. Tissue nutrient content of Gracilaria spp. (Rhodophyta) and water quality along an estuarine gradient. Marine and Freshwater Research 46, 975-983.
- Costanzo, S.D., O’Donohue, M.J., Dennison, W.C., Loneragan, N.R. and M. Thomas. 2001. A new approach for detecting and mapping sewage impacts. Marine Pollution Bulletin 42(2), 149-156.
- Laverly, P.S. and McComb, A.J. 1991. Macroalgal-sediment nutrient interactions and their importance to macroalgal nutrition in a eutrophic estuary. Estuar. Coast. Shelf Sci. 32, 281-295.
- Parsons TR, Maita Y, Lalli CM (1984) A manual of Chemical and Biological Methods for Seawater Analysis. Pergamon Press, Toronto.