What are the effects of macroalgal blooms on the structure and functioning of marine ecosystems? A systematic review protocol

Background: Anthropogenic activities are believed to have caused an increase in the magnitude, frequency, and extent of macroalgal blooms in marine and estuarine environments. These blooms may contribute to declines in seagrasses and non-blooming macroalgal beds, increasing hypoxia, and reductions in the diversity of benthic invertebrates. However, they may also provide other marine organisms with food and habitat, increase secondary production, and reduce eutrophication. The objective of this systematic review will be to quantify the positive and negative impacts of anthropogenically induced macroalgal blooms in order to determine their effects on ecosystem structure and functioning, and to identify factors that cause their effects to vary. Methods: We will search a number of online databases to gather empirical evidence from the literature on the impacts of macroalgal blooms on: (1) species richness and other univariate measures of biodiversity; (2) productivity and abundance of algae, plants, and animals; and (3) biogeochemical cycling and other flows of energy and materials, including trophic interactions and cross-ecosystem subsidies. Data from relevant studies will be extracted and used in a random effects meta-analysis in order to estimate the average effect of macroalgal blooms on each response of interest. Where possible, sub-group analyses will be conducted in order to evaluate how the effects of macroalgal blooms vary according to: (1) which part of the ecosystem is being studied (e.g. which habitat type, taxonomic group, or trophic level); (2) the size of blooms; (3) the region in which blooms occurred; (4) background levels of ecosystem productivity; (5) physical and chemical conditions; (6) aspects of study design and quality (e.g. lab vs. field, experimental vs. observational, degree of replication); and (7) whether the blooms are believed to be anthropogenically induced or not. Background Coastal marine ecosystems are under increasing pressure from multiple drivers of human-induced environmental change, including resource extraction, habitat modification and destruction, species introductions, ocean acidification, and inputs of pollutants and nutrients [1]. One of the consequent changes experienced by these ecosystems is an increase in "species outbreaks", or large, rapid, and temporary increases in the abundance of a species. These outbreaks can, in turn, cause changes in the structure and function of the broader ecological community, modify the physical and chemical environment, and affect human health, ecosystem services, and the economy [2-5]. * Correspondence: [email protected] Lead reviewer, University College Dublin, Dublin, Ireland Full list of author information is available at the end of the article © 2012 Lyons et al.; licensee BioMed Central L Commons Attribution License (http://creativec reproduction in any medium, provided the or Large blooms of macroalgae provide a striking example of species outbreaks. These blooms are normally formed by opportunistic, ephemeral algae, which are a natural component of shallow-water marine communities [6]. However, humans are believed to have increased the magnitude, extent, frequency, and duration of their proliferation by increasing nutrient loads in coastal waters and inducing trophic cascades that release seaweeds from herbivory [5,7]. The effects of macroalgal blooms are varied, with social, economic and ecological dimensions. Blooming macroalgae interfere with recreation and aesthetic enjoyment of coastal areas, while toxins released from decomposing algae pose a threat to the health of humans and domestic animals [8,9]. As a result, macroalgal blooms have the potential to reduce income from tourism and necessitate costly algal removal programs. The ecological effects of td. This is an Open Access article distributed under the terms of the Creative ommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and iginal work is properly cited. Lyons et al. Environmental Evidence 2012, 1:7 Page 2 of 6 http://www.environmentalevidencejournal.org/content/1/1/7 macroalgal blooms are generally seen to be negative. Macroalgal outbreaks have been linked to declines in the abundance of seagrasses and non-blooming macroalgae [10,11]. Decaying blooms can contribute to hypoxia and increased free sulphide in sediments and surrounding waters, thereby reducing the abundance and diversity benthic invertebrates [12-15]. However, the effects of macroalgal blooms are not uniformly negative. Algal accumulations can increase habitat complexity, provide tolerant species with food or shelter, and enhance both dispersal and secondary production [16-19]. The negative effects of macroalgal blooms on seagrasses may not occur unless herbivory is intense [20]. Macroalgal blooms may even help to reduce the eutrophication that causes them. They transfer nutrients from eutrophic waters to the sediments and other macroalgae, thereby reducing nutrient uptake by phytoplankton positive feedbacks to further eutrophication [21,22]. Given the varied positive and negative ecological effects of macroalgal blooms, quantifying their overall effects on ecosystem structure and functioning, and identifying the factors that determine how and why these effects vary are an important challenge. Meeting this objective would enable us to obtain more complete estimates of their effects on ecosystem services and the economy. Finally, it would provide relevant authorities with evidence-based knowledge that will facilitate decisions about how to best manage macroalgal blooms and the anthropogenic drivers that cause them. Objective of the review This review will synthesize the evidence for effects of ephemeral macroalgal blooms on the organization and dynamics of marine and estuarine ecosystems. The focus will be on quantifying effects that may influence the services that these ecosystems provide, and the economic activity that they generate. This review will have a global geographic scope. However, it will not address the wellstudied effects of macroalgae on coral reefs, which have been extensively reviewed [e.g. 23-25], synthesized, and evaluated in a recent meta-analysis [26]. The primary research question is: What are the effects of macroalgal blooms on the structure and functioning of marine ecosystems? Ecosystem structure and ecosystem functioning are complex concepts, with varying definitions. In quantifying the effects of macroalgal blooms on ecosystem structure, we will focus on how the diversity and evenness of the biological communities within the ecosystem are affected, as well as the abundance of other primary producers (e.g. seagrasses, other algae), fish, invertebrates, and birds. In quantifying the effects on ecosystem functioning, we will focus on functions such as: the productivity of other primary producers; secondary productivity; biogeochemical cycling of carbon, nitrogen, phosphorus, oxygen, and sulphur; and other flows of energy or material (e.g. subsidies to other ecosystems, trophic interactions). In assessing the evidence for these effects, we intend to address seven secondary questions: 1. Do different components of the ecosystem (e.g. particular trophic levels, functional groups or taxonomic groups) respond differently to macroalgal outbreaks? 2. How do the effects of outbreaks vary with their magnitude, duration and spatial extent, or the duration and extent of the studies examining them? 3. How do the effects differ in different regions (i.e. "large marine ecosystems")? 4. How do the effects vary between habitat types (e.g. pelagic vs. benthos, subtidal vs. intertidal, estuary vs. ocean) 5. Do the effects differ in areas of differing productivity? 6. Do the effects vary with environmental variables, such as temperature, water flow, and nutrient concentrations? 7. Do blooms attributed to anthropogenic causes have different effects from those that are naturally occurring?