Phytoplankton community structure under ice in shallow North Dakota lakes

Winter phytoplankton dynamics under ice in temperate lake systems are not well-understood. The general model of freshwater phytoplankton seasonal distribution suggests that phytoplankton survival is minimal under winter conditions. A review of the primary literature indicates that winter phytoplankton dynamics are oversimplified in this model. The winter phytoplankton communities are often highly diverse and may produce a significant portion of the overall biomass in temperate systems. However, long-term studies of winter phytoplankton community structure have not been performed. For 3 consecutive winters (1994-1997), phytoplankton samples were collected under ice at 4 sites on 3 lakes in Arrowwood National Wildlife Refuge (ANWR), near Pingree, North Dakota. Three separate studies evolved from the ANWR collections: (1) phytoplankton community structure, (2) phytoplankton blooms associated with elevated oxygen levels, and (3) diversity of coccoid green algae. In the first study, 85 taxa were identified and enumerated. The winter phytoplankton community under the ice was dominated by winter annuals such as Synura uvella and Peridinium aciculiferum and eurythermal organisms such as cryptomonads, euglenoids, and coccoid green algae. In the second study, I observed a dramatic increase in oxygen concentration that was associated with a phytoplankton bloom during late February and early March in both 1996 and 1997. In 1996, the bloom was composed of the dinoflagellate, Peridinium aciculiferum, and several species of cryptomonads. However, in 1997, a similar bloom of P. aciculiferum was followed by a bloom of several species of euglenoids. In the third study, HPLC- and PCR-based methods were used to examine the diversity of autosporic coccoid green algae isolated from ANWR. Six 18S rDNA genotypes of coccoid trebouxiophytes, 3 genotypes of coccoid chlorophytes, and 1 genotype of coccoid eustigmatophytes were detected in winter isolates from the Arrowwood refuge lakes. The results of these studies clearly indicate that a diverse phytoplankton community thrives despite the thick ice cover and very cold conditions.