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

Abstract

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 (DO14C) released by U. rigida. Plants cultured under a full DO14C load showed a significant enhancement of growth. The DO14C disappeared from the culture medium during the first 4 d of culture as a linear function of external DO 14C concentration. This was coupled to a linear increase of radioactive particulate organic carbon (PO14C) in aboveground tissues, while a substantial part of this PO14C was allocated in belowground tissues. Overall, the PO14C incorporated in Z. noltii plants represented ca. 20 to 25% of the DO14C which had disappeared. Therefore, a net transfer of DO14C 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.