Limnol. Oceanogr., 44(3), 1999, 597–607

Photosynthesis vs. irradiance relationships were determined for phytoplankton communities from seven lakes in the Canadian high Arctic, including ultraoligotrophic Char Lake, nutrient-enriched Meretta Lake, and two meromictic lakes. The derived photosynthetic parameters were low for all samples, with a mean (6SD) light-saturated photosynthetic rate (P ) of 0.46 (60.28) g C g21 chlorophyll a (Chl a) h21 and a mean a (light-limitation parameter) B m of 1.23 (60.56) g C g21 Chl a m2 mol21. The saturation irradiance (Ek) ranged from 50 to 196 mmol quanta m22 s21 and was positively correlated with mean irradiance for the water column. Quantum yields for photosynthesis in the Arctic lake phytoplankton were also low (mostly ,10 mmol C mol21 quanta). An intersystem comparison of a and P values with literature data for algae from other cold environments showed that the photosynthetic B m parameters for phytoplankton in Arctic and Antarctic lakes are threeto sixfold lower than for marine algae, ice algae, and cultures over the same low-temperature range. This may be the result of more severe nutrient stress in high-latitude lakes relative to polar marine environments and to the persistence of nonactive pigments in cold freshwaters. Photosynthetic carbon uptake by phytoplankton is regulated by the dynamic interplay between cellular physiology and the supply of resources. Phytoplankton can optimize growth rate by adjusting their photosynthetic apparatus to the prevailing conditions of light (Falkowski 1980; Prézelin 1981) and nutrients (Geider et al. 1993). However, although the combined effects of light plus nutrient stress have been investigated under laboratory conditions (Kolber et al. 1988), less is known about natural phytoplankton in this regard (Cullen et al. 1992). Temperature is a basic property of the environment that directly affects photosynthetic processes as well as the ability of algae to adjust their photosynthetic apparatus to changes in resource availability. Oligotrophic lakes in the high-latitude regions are characterized by low nutrients and cold water and are therefore likely to present an extreme set of adverse conditions for phytoplankton photosynthesis and growth. The objective of the present study was to measure light absorption and photosynthetic performance of the phytoplankton in a broad range of Arctic lakes and to evaluate these measurements in the context of other low-temperature systems. Seven lakes were selected in the Canadian high Arctic as extreme types of pelagic systems with persistent low temperatures, low-nutrient concentrations, and, for some 1 Present address: National Environmental Research Institute, Department of Marine Ecology and Microbiology, Frederiksborgvej 399, DK-4000 Roskilde, Denmark. 2 Corresponding author.