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Title: A meta-analysis of seaweed impacts on seagrasses: generalities and knowledge gaps
Authors: Thomsen, Mads S.
Wernberg, Thomas
Tuya, F. 
Vanderklift, Mat A.
Holmer, Marianne
McGlathery, Karen J.
Arenas, Francisco
Kotta, Jonne
Silliman, Brian R.
Engelen, Aschwin H.
UNESCO Clasification: 241705 Biología marina
Keywords: Eelgrass Zostera-Marina
Macroalga Chaetomorpha-Linum
Nutrient Availability
Thalassia-Testudinum, et al
Issue Date: 2012
Journal: PLoS ONE 
Abstract: Seagrasses are important habitat-formers and ecosystem engineers that are under threat from bloom-forming seaweeds. These seaweeds have been suggested to outcompete the seagrasses, particularly when facilitated by eutrophication, causing regime shifts where green meadows and clear waters are replaced with unstable sediments, turbid waters, hypoxia, and poor habitat conditions for fishes and invertebrates. Understanding the situations under which seaweeds impact seagrasses on local patch scales can help proactive management and prevent losses at greater scales. Here, we provide a quantitative review of available published manipulative experiments (all conducted at the patch scale), to test which attributes of seaweeds and seagrasses (e.g., their abundances, sizes, morphology, taxonomy, attachment type, or origin) influence impacts. Weighted and unweighted meta-analyses (Hedges d metric) of 59 experiments showed generally high variability in attribute-impact relationships. Our main significant findings were that (a) abundant seaweeds had stronger negative impacts on seagrasses than sparse seaweeds, (b) unattached and epiphytic seaweeds had stronger impacts than ‘rooted’ seaweeds, and (c) small seagrass species were more susceptible than larger species. Findings (a) and (c) were rather intuitive. It was more surprising that ‘rooted’ seaweeds had comparatively small impacts, particularly given that this category included the infamous invasive Caulerpa species. This result may reflect that seaweed biomass and/or shading and metabolic by-products like anoxia and sulphides could be lower for rooted seaweeds. In conclusion, our results represent simple and robust first-order generalities about seaweed impacts on seagrasses. This review also documented a limited number of primary studies. We therefore identified major knowledge gaps that need to be addressed before general predictive models on seaweed-seagrass interactions can be build, in order to effectively protect seagrass habitats from detrimental competition from seaweeds.
ISSN: 1932-6203
DOI: 10.1371/journal.pone.0028595
Source: PLoS ONE [ISSN 1932-6203], v. 7(1): e28595
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