Effect of shading by Ulva rigida canopies on growth and carbon balance of the seagrass Zostera noltii

The effects of macroalgae blooms on seagrasses were assessed by shading Zostera noltii Hornem. with Ulva rigida C. Agardh mats under laboratory and field conditions. In the laboratory, where there was no direct contact between U. rigida and Z. noltii, leaf, rhizome, and root elongation rates, as well as gross production, declined as a function of U. rigida layers, causing a mobilization of non-structural carbohydrates in both above- and belowground tissues to meet carbon demands. However, when shading was performed in the field, where direct contact exists between Z. noltii and U. rigida, Z. noltii responses were not proportional to the number of Ulva layers. Elongation rates and gross production were reduced by U. rigida shading, with the lowest values under 2 Ulva layers, while there were no significant differences between controls and 4 U. rigida layers. This suggests another Ulva effect occurs besides shading. To test the likely effect of dissolved organic carbon (DOC) derived from U. rigida, Z. noltii plants were cultured under light limitation with radioactive dissolved organic carbon ((DOC)-C-14) released by U. rigida. Plants cultured under a full (DOC)-C-14 load showed a significant enhancement of growth. The (DOC)-C-14 disappeared from the culture medium during the first 4 d of culture as a linear function of external (DOC)-C-14 concentration. This was coupled to a linear increase of radioactive particulate organic carbon ((POC)-C-14) in aboveground tissues, while a substantial part of this (POC)-C-14 was allocated in belowground tissues. Overall, the (POC)-C-14 incorporated in Z. noltii plants represented ca. 20 to 25% of the (DOC)-C-14 which had disappeared. Therefore, a net transfer of (DOC)-C-14 from U. rigida to Z. noltii has been documented. Other additional possibilities, such as a light quality effect or other kind of signals (i.e. growth factors), are discussed.