Spatial Heterogeneity of Summer Phytoplankton and Water Chemistry in a Large Volcanic Spring-fed Lake in Northern Iceland

Abstract

Myvatn, Iceland, is the only lake in the world where diatomite is dredged commercially. Although we anticipated an enormous production ofplanktonic diatoms at the peak of the summer production cycle in mid-July, upon analysis of our samples, we realized that less than 5% of the phytoplankton standing crop was composed of diatoms. Benthic, rather than planktonic diatom species, appeared to be the key primary producers in the lake. The phytoplankton assemblage was dominated by Chlorophyta (green algae), Cyanophyta (blue-green algae), and Chrysophyta (yellow-green algae). The relative absence of planktonic diatoms in this shallow lake (mean depth less than 2.5 m) was associated with the presence of dense stands of macrophytes, which appeared to limit local turbulence. The spatial heterogeneity in phytoplankton abundance and species composition was correlated with the spatial distribution of nutrients in this volcanic spring-fed lake. Phytoplankton abundance was highest where warm spring waters from the north basin flowed into the eastern channel and mixed with the cold spring nutrient-rich waters coming from the southeastern corner of the lake. Intermediate nutrient levels were found in the center of the lake where relatively high phytoplankton standing crops of chrysophytes (Chrysochromulina and Dinobryon), cyanophytes (Anabaena and Gloeocystis), and colonial chlorophytes (Sphaeorocystis, Oocystis, and Pediastrum) developed during the annual phytoplankton production peak. During this midsummer peak in primary production, nitrogen, rather than phosphorus appeared to be the limiting nutrient.