BENTHIC AND PLANKTONIC ALGAL COMMUNITIES IN A HIGH ARCTIC LAKE: PIGMENT STRUCTURE AND CONTRASTING RESPONSES TO NUTRIENT ENRICHMENT 1 Sylvia Bonilla 2 Facultad de Ciencias, Universidad de la Repu ´blica, 11400-Montevideo, Uruguay and Centre d’E ´ tudes Nordiques & De´partement de Biologie, Universite ´ Laval, Que´bec QC G1K 7P4, Canada Vale´rie Villeneuve and Warwick F. Vincent Centre d’E ´ tudes Nordiques & De ´partement de Biologie, Universite ´ Laval, Que´bec QC G1K 7P4, Canada We investigated the fine pigment structure and composition of phytoplankton and benthic cyano- bacterial mats in Ward Hunt Lake at the northern limit of High Arctic Canada and the responses of these two communities to in situ nutrient enrich- ment. The HPLC analyses showed that more than 98% of the total pigment stocks occurred in the benthos. The phytoplankton contained Chrysophy- ceae, low concentrations of other protists and Cyanobacteria (notably picocyanobacteria), and the accessory pigments chl c 2 , fucoxanthin, di- adinoxanthin, violaxanthin, and zeaxanthin. The benthic community contained the accessory pig- ments chl b, chl c 2 , and a set of carotenoids domi- nated by glycosidic xanthophylls, characteristic of filamentous cyanobacteria. The black surface layer of the mats was rich in the UV-screening com- pounds scytonemin, red scytonemin-like, and my- cosporine-like amino acids, and the blue-green basal stratum contained high concentrations of light-harvesting pigments. In a first bioassay of the benthic mats, there was no significant photosynthet- ic or growth response to inorganic carbon or full nutrient enrichment over 15 days. This bioassay was repeated with increased replication and HPLC analysis in a subsequent season, and the results confirmed the lack of significant response to added nutrients. In contrast, the phytoplankton in samples from the overlying water column responded strong- ly to enrichment, and chl a biomass increased by a factor of 19.2 over 2 weeks. These results under- score the divergent ecophysiology of benthic versus planktonic communities in extreme latitudes and show that cold lake ecosystems can be dominated by benthic phototrophs that are nutrient sufficient de- spite their ultraoligotrophic overlying waters. Key index words: arctic lakes; bioassays; cyanobac- teria; HPLC; mats; nutrients; photosynthesis; phytoplankton; pigments Abbreviations: DIC, dissolved inorganic carbon; ELA, Experimental Lakes Area; MAAs, myco- sporine-like amino acids; SRP, soluble reactive phosphorus Polar and alpine lakes are typically thought of as low-resource ecosystems in which the photosynthetic communities are severely constrained by nutrient sup- ply (Douglas et al. 1994, Ve´zina and Vincent 1997). The classic work on Char Lake in the Canadian High Arctic drew attention to its ultraoligotrophic charac- teristics, with phytoplankton populations limited by the meager inputs of phosphorus from its polar desert watershed (Rigler 1978). In nearby Meretta Lake, an increase in nutrient loading from anthropogenic sources resulted in a sharp increase in phytoplankton biomass, confirming the nutrient responsiveness of arctic lakes and their susceptibility to eutrophication (Schindler et al. 1974a, Douglas and Smol 2000). Sim- ilarly, in antarctic and alpine lakes, nutrient enrich- ment bioassays have confirmed the strong limitation of their phytoplankton communities by phosphorus and/ or nitrogen availability (Vincent 2000, Dore and Priscu 2001, Goldman et al. 2001). Shallow lake ecosystems contain two separate al- gal communities: phytoplankton in the water column and the phytobenthos attached to bottom substrata. The latter attached communities can be a major source of organic carbon for planktonic as well as bent- hic food webs (Sand-Jensen and Borum 1991, Zimba 1995) and can play a leading role in the production dynamics of the overall ecosystem (Burkholder and Wetzel 1989). The responses to environmental varia- tion have the potential to differ greatly between the plankton and the benthos (Sand-Jensen and Borum 1991, Vinebrooke and Leavitt 1999). Although nutri- ents may severely limit planktonic production, benthic phototrophs may be subject to quantitatively or even qualitatively different controls. Many arctic and 1 Received 22 September 2004. Accepted 7 September 2005. 2 Author for correspondence: e-mail sbon@fcien.edu.uy. 1120 J. Phycol. 41, 1120–1130 (2005) r 2005 Phycological Society of America DOI: 10.1111/j.1529-8817.2005.00154.x