Temporal and spatial patterns in the distribution and abundance of pico, nano and microphytoplankton in an upland lake

Abstract

SUMMARY. 1. An investigation of the seasonal and depth distribution of populations of autotrophic picoplankton (0.2–2 μm), nanophytoplankton (>2<20 μm) and larger microalgal plankton (>20μm) was carried out over 21/2 years, 1988–90, in Llyn Padarn, a mesotrophic upland lake in North Wales.2. Cell numbers of picophytoplankton ranged from <102 to >106 cells cm−3. Maximum numbers of nanoplankton were c. 104 cells cm−3 and the greatest abundance of microalgal plankton, diatoms, reached 12 × 103 cells cm−3.3. Three types of picoalgae were distinguished: coccoid to oval Synechococcus—Synechocystis, the rod‐shaped Synchecococcus capitatus Bailey‐Watts & Komárek and Chlorella minutissima Fott & Nováková, with maximum numbers of 1.2 × 106, 37.8 × 103 and 44.1 × 103 cells cm−3, respectively.4. Picophytoplankton exhibited periods of exponential growth: the first in spring, and the second in August—September with an intervening population minimum in early to midsummer. Specific rates of population increase for picophytoplankton were low, with minimum apparent generation times of 3.8 days in summer 1989.5. Nanophytoplankton included seven species of phytoflagellates and two non‐motile species. These algae were present for about 10 months in each year exhibiting a fluctuation in population density of 102−‐103 cells cm−3.6. There were spring and autumn maxima in chlorophyll a concentrations in the lake water corresponding to the growth of planktonic diatoms. Maximum total biomass concentration was 35 mg m−3 chlorophyll a, whereas pico, nano and microphytoplankton had individual maxima of 7.7, 8.4 and 31.0 mg m−3 chlorophyll a, respectively. Picophytoplankton often contributed > 60% of the total algal chlorophyll a in the epilimnion.7. The growth patterns and seasonal periodicities of the three size‐categories of planktonic algae in Llyn Padarn were distinct. Picophytoplankton persist throughout much of the year with periods of very low abundance, < 100 cells cm−3, occurring in winter and midsummer. Thus for much of the year, there was a large inoculum of these cells in the lake to initiate growth leading to the population maxima in spring and late summer. Nanoplankton populations, a diverse assemblage, fluctuated in numbers over the period February–November; no population decline in midsummer comparable to picophytoplankton was observed. The larger microphyloplankton exhibited classical seasonal periodicity, namely diatom growth in spring and late summer–autumn with growth of large‐celled chlorophytes in the intervening summer period.