SPATIAL HETEROGENEITY OF SUMMER PHYTOPLANKTON AND WATER CHEMISTRY IN A LARGE VOLCANIC SPRING-FED LAKE IN NORTHERN ICELAND

Myvatn, Iceland, is the only lake in the world where diatomite is dred ged commercially. Although we anticipated an enormous production of pl anktonic diatoms at the peak of the summer production cycle in mid-Jul y, upon analysis of our samples, we realized that less than 5% of the phytoplankton standing crop was composed of diatoms. Benthic, rather t han planktonic diatom species, appeared to be the key primary producer s in the lake. The phytoplankton assemblage was dominated by Chlorophy ta (green algae), Cyanophyta (blue-green algae), and Chrysophyta (yell ow-green algae). The relative absence of planktonic diatoms in this sh allow lake (mean depth less than 2.5 m) was associated with the presen ce of dense stands of macrophytes, which appeared to limit local turbu lence. The spatial heterogeneity in phytoplankton abundance and specie s composition was correlated with the spatial distribution of nutrient s in this volcanic spring-fed lake. Phytoplankton abundance was highes t where warm spring waters from the north basin flowed into the easter n channel and mixed with the cold spring nutrient-rich waters coming f rom the southeastern comer of the lake. Intermediate nutrient levels w ere found in the center of the lake where relatively high phytoplankto n standing crops of chrysophytes (Chrysochromulina and Dinobryon), cya nophytes (Anabaena and Gloeocystis), and colonial chlorophytes (Sphaeo rocystis, Oocystis, and Pediastrum) developed during the annual phytop lankton production peak. During this mid-summer peak in primary produc tion, nitrogen, rather than phosphorus appeared to be the ''limiting'' nutrient.