Attributes of seaweeds and seagrasses that may influence seaweed impact on seagrass.

We had a priori expectations about the direction of impact for the first seven attributes (above the dotted line). These directional hypotheses are based on simple rules; we expect a large impact when there is (a) more of a stressor (the seaweed) in either space or time, or (b) less of the impacted organism (the seagrass). Summary of tests-results are shown in the table (significant values in bold, see also Fig. 1, 2 and Appendix S3; NT = not tested because data were inadequate).1Impact of seaweeds on seagrasses may also be modified by habitat attributes, including the resource levels (e.g., nutrients, light, O2, space), abiotic conditions (e.g., temperature, salinity, desiccation, sedimentation, substrate conditions, day-length) and resident animals living in and around the seagrass habitat [21].2The categorical test based on experiments that explicitly tested for abundance effect was significant, but the correlation conducted across all experiments was not significant.3We assume that abundant seagrasses have more resources to withstand stress. Alternatively, abundant seagrass may suffer from intra-specific competition resulting in abundant seagrass being more susceptible to stress (i.e. the opposite expectation may be equally valid).4We assume that invaders have superior impact (seaweeds) and resistance (seagrass), e.g., as novel weapons [44].5Poor ‘condition/health’ of the seaweed results in decomposition and production of anoxia, sulphide and ammonia. Unattached mats often decompose when lower layers are shaded by higher layers.6For seagrasses, integration is a continuous attribute that encompasses below ground storage products and ability to translocate products between ramets.7Dri = Drift/unattached, Epi = epiphytic to seagrass leaves, Roo = rooted in sediment with rhizoids and rhizomes.8A few seagrasses can attach to rocks, but no studies have quantified seaweed impacts on attached seagrass.9Adapted from Littler and Littler (1980); She = sheets, Coa = Coarsely branches, Fil = filaments, Coe = coenocytic.10Ulv = Ulva, Gra = Gracilaria, Lau = Laurencia, Cau = Caulerpa.11Amp = Amphibolis, Tha = Thalassia, Cym = Cymodocea, Zos = Zostera, Had = Halodule, Hap = Halophila.