66. Estuarine Drift Algae

Organised by Mads Thomsen (Edith Cowan U) to be held at Edith Cowan University, Perth. First meeting 30 November - 5 December 2009

PARTICIPANTS INCLUDE Mads Thomsen (leader) - Edith Cowan U - coastal ecology, meta-analysis Thomas Wernberg - Edith Cowan U - algal ecology, meta-analysis Mat Vanderklift - CSIRO Marine and Atmospheric Research, Perth - estuarine ecology and seagrass biology Marianne Holmer - Syddansk U (Denmark) - biogeochemical impacts of estuarine drift algae PARTICIPATING BUT NOT ATTENDING THE MEETING Brian Silliman - U Florida (USA) - community ecology of coastal systems

GOAL

The proposed working group will compile all estuarine manipulative drift algal ‘impact’ studies and extract and compare standardized effects sizes, i.e. mean and dispersal values for control plots (without drift algae) against treatment plots with experimentally added drift algae. In addition, all relevant biogeographical and ecological data (e.g. study-region, depth, nutrient levels, temperature, algal density, taxonomic data, etc) will be tabulated in numerical form and analysed. Based on this review and meta-analysis we aim to develop a model that will predict impacts based on drift algal species identity and density, the impacted habitat, and temperature and nutrient levels. Such a model – founded on experimentally derived data - will potentially have great pure and applied scientific applicability.

From a pure science perspective our work will demonstrate how data-extraction and meta-analysis of experimental ecological data can be coupled with predictive modelling. But this model will also have practical importance by quantifying environmental thresholds that, if exceeded, may cause irreversible negative impacts to special-valued ecosystems (reefs, seagrass beds). Such a quantitative model can aid managers to establish monitoring programs, to set target values (e.g. for acceptable levels of drift algae) and to identify habitats that are particular susceptible to drift algal impacts. Finally, our quantitative review will identify key research gaps and be used as a platform to suggest the productive future research agendas.

BACKGROUND

Drift algae are a natural and vital component of estuarine ecosystems in the same way as seagrass beds, oyster reefs and salt marshes. Drift algae can be important primary producers that filter out land-derived nutrients and provide structurally complex habitat and food for invertebrates and fishes. In addition, tumbling drift algae increase habitat-connectivity, by ‘transporting’ invertebrates and juvenile fishes between patches of suitable habitats (e.g. between seagrass patches).

However, drift algae can also be an unwanted nuisance if they accumulate in high densities; drift algae can smother seagrasses, compete for light and nutrients, filter out propagules before they reach a suitable substratum, and they can cause low oxygen levels in sediments and the water column due to respiratory processes and tissue decay. Such hypoxic or even anoxic conditions may lead to local extinctions of seagrasses, reef-formers (e.g. oysters, mussels), mobile invertebrates and fishes; indirectly cause sediment destabilization and ultimately large-scale regime shifts from clear water systems characterized by slow-growing habitat-formers to turbid systems composed of fast-growing ephemeral drift algae. Indeed, drift algal mass accumulations and associated anoxia and negative ecosystem effects are becoming increasingly common due to nutrient pollution (eutrophication). In addition, large masses often accumulate on popular beaches causing aesthetic problems that are expensive to mitigate.

Many experimental studies have documented negative ecosystem effects associated with drift algal accumulations. Specifically, it has been shown that the magnitude of effects typically depends on drift algal density and species identity, the affected habitat type, temperature, and nutrient levels. However, most studies have only focused on how a single environmental determinant impact a specific estuarine habitat and the literature of environmental drift algal drivers and impacts is highly scattered.

Last Updated June 2009