Search

The limiting effects of stressors like desiccation, light and salinity on seagrass growth and distribution are well-studied. However, little is known about their interactive effects, and whether such effects might differ among populations that are adapted to different local conditions. In two laboratory experiments we tested (a) if growth and development of intertidal, temperate Zostera noltii is affected by emergence time (experiment 1 and 2), and (b) how this is affected by an additional, second stressor, namely shading (experiment 1) or high salinity (25, 30 and 35, experiment 2). In addition, we tested (c) whether the effects of emergence time and salinity varied between three different European seagrass populations (Saint-Jacut/France, Oosterschelde/The Netherlands, and Sylt/Germany), which are likely adapted to different salinity levels (experiment 2). In both experiments, emergence of 8 h per tidal cycle (of 12 h) had a negative effect on seagrass relative growth rate (RGR), and aboveground biomass. Emergence furthermore reduced either rhizome length (experiment 1) or belowground biomass (experiment 2). Shading (experiment 1) resulted in lower RGR and a two-fold higher aboveground/belowground ratio. We found no interactive effects of emergence and shading stress. Salinity (experiment 2) did not affect seagrass growth or morphology of any of the three populations. The three tested populations differed greatly in morphology but showed no differential response to emergence or salinity level (experiment 2). Our results indicate that emergence time and shading show an additive negative effect (no synergistic or antagonistic effect), making the plants still vulnerable to such combination, a combination that may occur as a consequence of self-shading during emergence or resulting from algal cover. Emergence time likely determines the upper limit of Z. noltii and such shading will likely lower the upper limit. Shading resulted in higher aboveground/belowground ratios as is a general response in seagrass. Z. noltii of different populations originating from salinity 30 and 35 seem tolerant to variations in salinity within the tested range. Our results indicate that the three tested populations show morphotypic rather than ecotypic variation, at least regarding the salinity and emergence, as there were no interactive effects with origin. For restoration, this implies that the salinity regime of the donor and receptor site of Z. noltii is of no concern within the salinity range 25–35.

This research was designed to evaluate the extent to which power functions can predict performance on a task when performance context has been altered. Since power functions reliably describe performance improvements during practice, an assumption implicit in some theories of skill acquisition and transfer is that transfer performance will continue to improve as an extrapolation of the practice power function. In the training phase of Experiment 1, 120 participants practiced solving simple problems from the six-times table. In the transfer phase, these same problems were presented again, intermixed with problems from one of the six conditions differing in various respects to the target problems. With the exception of two of these six conditions, performance on the target problems was slower than was predicted by training phase power function extrapolations. Where the nature of the task was altered, this disruption only occurred in the initial stages of transfer, with performance returning quickly to predicted levels. Where an increased scope of knowledge was required to perform the task, the disruption was more prolonged. Experiment 2 demonstrated that a spacing effect explanation of the results of Experiment 1 was not valid. Results were interpreted as reflecting the effect a change in the conceptual context of a task has on transfer performance. These findings have implications for theories of skill acquisition and transfer that assume transfer performance of established skills will continue to improve according to an extrapolation of the practice power function regardless of the conceptual context of the task.