COMPARATIVE PHOTOSYNTHETIC PHYSIOLOGY OF SHALLOW AND DEEP WATER 

 POPULATIONS OF LAMINARIA IN THE GULF OF MAINE 



Ian R. Davison, Janet E. Kuebler, Robert S. Steneck, 



and Robert L. Vadas 

 Department of Botany and Plant Pathology 

 and Center for Marine Studies 

 University of Maine, Orono, ME 04469 



INTRODUCTION 



Previous research indicated the occurrence of a deep water 

 population of Laminaria at Ammen rock and Ammen rock pinnacle in 

 the central Gulf of Maine (Vadas and Steneck, 1988) . The Ammen 

 Laminaria closely resembles Laminaria diqitata , which occurs 

 through the North Atlantic including the coast of Maine. Ammen 

 plants differ from coastal L. diqitata by having narrower and less 

 digitate blades, by possessing muscilage ducts in the stipe 

 and holdfast and by the frequent occurrence of whorls of haptera 

 in the holdfast. Furthermore, the Ammen Laminaria population 

 occurs at considerable depth (25-35 m) , whereas L. diqitata 

 typically has maximum abundance in shallow water. By themselves 

 these differences are not sufficient to justify placing the Ammen 

 Laminaria in a separate taxon from L. diqitata . L. diqitata is 

 known to exhibit considerable phenotypic plasticity (Sundene, 

 1964) and the taxonomic significance of mucilage ducts is open to 

 question (Chapman, 197 3) . However, recent preliminary studies 

 indicate that large differences occur between chloroplast DNA 

 sequences in Ammen Laminaria and coastal L. diqitata (Fain, Vadas 

 and Steneck, unpublished) , suggesting that Ammen Laminaria may be 

 a distinct species. 



Thus, it is currently an open question as to whether or not 

 the Ammen Laminaria is a new species, or if it is a deep water 

 ecotype of L. diqitata . Irrespective of the taxon to which the 

 Ammen Laminaria population is eventually assigned, a key question 

 is what, if any, adaptations have evolved for life in a deep water 

 environment characterized by low light levels. A priori one would 

 expect this population to have evolved adaptations to low 

 irradiance because of the small size and extreme isolation of the 

 population (Vadas and Steneck, 1988) , which is continuously 

 exposed to low light levels. In contrast, most coastal Laminaria 

 populations are contiguous from shallow to deep water and hence 

 experience a gradient of irradiance. 



The objectives of the present study were to determine if the 

 Ammen Laminaria population does possess specific adaptation to low 

 light levels and to determine their nature. The experimental 

 design involved a comparative study of the Ammen population with a 



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