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 appearance . 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.
Macroalgal beds are valued for their intrinsic biodiversity in Australia . 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 reefs . 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 organism .
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 pharmaceuticals [3,4].
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.
Some macroalgal species have a large capacity for nitrogen assimilation and storage over short time intervals . 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 programmes . Macroalgal tissue indicators include:
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 macroalgae .
Other macroalgal indicators include:
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 fat . It is akin to a litmus test, the deeper the colour of certain macroalgae, the more nutrient-rich they are.
Tissue N generally reflects external N concentrations [5,6,7]. The amino acid citrulline has an N-rich structure and may be involved in N storage in macroalgae . The concentration of citrulline increases with pulsed inputs of N to a coastal waterway .
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) .
Algal bed areas and their species composition change in response to many known pressures including:
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.
Sporadic and site specific data sets exist in universities and state governments.