Growth and production inLaminaria solidungula: relation to continuous underwater light levels in the Alaskan High Arctic

Abstract

The growth relations of the kelpLaminaria solidungula were examined in the summers of 1984 and from 1986 to 1989 in relation to continuous year-round measurements of photosynthetically active radiation (PAR) at several sites in the Stefansson Sound Boulder Patch, Alaskan Beaufort Sea. PAR showed large seasonal differences, ranging from a maximum of 5µmol m⁻² s⁻¹ during the ice-covered months to 200µmol m⁻² s⁻¹ during the open-water season. In most years, the light received during the 9 mo ice-covered period represented < 10% of the total PAR reaching the plants. Annual variations in PAR ranged from 45 to over 89 mol m⁻² yr⁻¹ over the 4 yr period among sites. In 1988, annual quantum budgets forL. solidungula varied from 45 to 50 mol m⁻² yr⁻¹, close to that reported in the previous year, and near the annual minimum light requirement reported in other studies for the lower limit ofLaminaria spp. However, the duration of time that the plants were exposed to saturating levels of PAR in 1988 was considerably less than in other years. This was correlated with significant reductions in thallus tissue-density and carbon content during the summer open-water period in 1988. Percentage of dry to wet weight (tissue density) dropped from about 16 to 10%; carbon content from 35 to 28%. The drop in both indices indicated that 1988 summer open-water (ice-free) PAR was insufficient for maintaining maximum photosynthetic carbon fixation. The decreased storage of carbohydrate reserves, which are used for tissue expansion during the dark ice-covered period, resulted in significantly reduced linear growth in all plants the following year (1989). These results provide evidence that in low-light environments, plant production is more a function of exposure to saturating levels of PAR than to the total amount of photons received over the course of a growing season. Under these conditions, under-ice photosynthetic production at very low light levels becomes increasingly important forL. solidungula if it is to meet the metabolic demands imposed by winter growth and respiration.