Growth and production of planktonic protozoa in Lake Michigan: In situ versus in vitro comparisons and importance to food web dynamics

Abstract

Growth of an entire planktonic protozoan community in Lake Michigan (nanoflagellates, microflagellatcs, and ciliates) was measured on 10 dates in 1988-l 989 in fractionated lake water (<3, ~8, < 30, and < 153 pm) incubated in bottles (in vitro), while in June and July 1989, in vitro growth rate estimates were compared with in situ growth rates derived from the mitotic index. Comparisons of in situ vs. in vitro growth rates for the taxa assayed were similar, indicating that in vitro experiments provide reasonable estimates of protozoan growth, and these rates were similar to those measured from other oligotrophic/mesotrophic habitats. However, containment artifacts in some bottle experiments influenced community growth as some taxa experienced mortality in bottles, thus underscoring the importance of ccnsusing the entire community. Protozoa in Lake Michigan contribute significantly to both heterotrophic and phototrophic carbon pools; phototrophic llagcllates represented 24% of primary production, while heterotrophic nanoflagellates and ciliates collectively constituted 40% of bacterial production. The fate of high bacterial production can be accounted for through grazing by both heterotrophic nanoflagellates and ciliates, indicating the trophic importance of protozoa in Lake Michigan.

Have we overlooked the importance of small phytoplankton in productive waters?

“…Vertical watercolumn sampling (June-November 1991, n = 9) indicated that both phytoplankton chlorophyll and carbon were relatively constant with depth but increased dramatically on the bottom of the lake. This was due to a meroplanktonic algal maximum existing in a flocculent lake bottom layer -0.05 m thick (see Carrick et al 1993a). …”

“…Ranges for total P are -100-500 p,g liter-l while total N values range from 3,000 to > 11,000 pg liter-l (Schelske et al 1992). Concentrations of NO,--N + NO,--N and P043--P are low (<2 pg liter-'), and ammonium (NH,+-N) concentrations are generally <5 pg liter-l (St. John's River Water Management District unpubl.…”

“…Water temperatures are high, commonly >3O"C and rarely <16"C (avg 25 rt 5°C). Secchi depths are generally restricted to the upper 25 cm of the water column, and the midday 1% light level usually occurred at a depth of 0.8 m (Schelske et al 1992). …”

“…Phytoplankton abundance was measured microscopically by using a stratified enumeration technique (Carrick et al 1993a). Owing to the range in cell size and abundance of the phytoplankton, picoplankton (>0.2 and <2 pm in cell size), and cells in the nano-(>2-and <20-pm) to micro-(>20-and <200-pm) plankton size range were measured separately.…”

“…Subsamples (-1.0 ml) were filtered onto prestained (irgalan black) 0.2-pm pore-sizl= Nuclepore filters and mounted between a microscope slide: and coverslip with immersion oil. Slides were stored at --20°C and counted within 1 week to minimize the fading of fluorescence (Carrick and Fahnenstiel 1989). Biomass was estimated by counting a total of 500-2,000 individuals from two duplicate slides (<5% counting error assuming F*oisson statistics) by using a Nikon Optiphot research microscope (1,000X) equipped for Chl a fluorescence (blue light 450-490 nm excitation and >515 nm emission), and by determination of phycobilin proteins (green light 530-560 run excitation and >580 nm emission).…”