Seaweed impacts on seagrasses can be partially predicted from basic impact attributes.

<p><i>Plot 3A: Key meta-analytical results schematized</i> (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0028595#pone-0028595-g001" target="_blank">Fig. 1</a>–<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0028595#pone-0028595-g002" target="_blank">2</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0028595#pone-0028595-t001" target="_blank">Table 1</a>). Impact depends on <u>seaweed abundance</u> (low <i>vs.</i> high, cf. y-axis), <u>seaweed attachment</u> (unattached <i>vs.</i> epiphytic <i>vs.</i> rooted, cf. long x-axis) and <u>seagrass size</u> (large <i>vs.</i> small, cf. short x-axis). The impact mechanisms associated with seaweed abundance and seagrass size are simple; the more of the stressor (seaweed) and less of the impacted organism (seagrass) the larger the impact. The mechanisms that cause different effects between attachment types are less obvious; we suggest that oxygen and light reduction and sulphide production cause large negative impact of unattached and epiphytic seaweeds, whereas allelochemicals cause smaller impacts of rooted seaweeds (listed in bullets). Our analysis addressed impact attributes in isolation. Future tests should use factorial designs to identify interactions between attributes. <i>Plot 3B: Figure legend</i>. Standardized seagrass = three green leaves connected with rhizomes; leaves can be large or small. Standardized seaweed = brown frond; can be sparse or abundant (1 <i>vs.</i> 3 fronds), positioned vertical (attached <i>vs.</i> rooted) or horizontal (unattached), and with (rooted) or without (unattached, attached) inter-connecting rhizome. <i>Plot 3C: Non-impacted controls.</i> The impact treatments shown in plot 3A should always be compared to non-impacted seagrass controls, here to ‘large and small seagrass without seaweed stress’.