MEPS 265:85-96 (2003)  -  doi:10.3354/meps265085

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 DO¹⁴C 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.

KEY WORDS: Eutrophication · Heterotrophic DOC uptake · Dissolved organic carbon · Seagrass · Non-structural carbohydrates · Light reduction