Primary Productivity of Phytoplankton

Abstract

Although appreciable quantities of organic matter synthesized by terrestrial plants within the drainage basin can be transported to freshwater ecosystems in either dissolved or particulate forms (allochthonous primary productivity), much of the organic matter of lakes is produced within the lake by phytoplanktonic algae, by littoral macrophytic vegetation, and by sessile algae (autochthonous primary productivity). In situ rates of photosynthesis by phytoplankton are the subject of this exercise; those of other plant forms will be treated separately in Exercise 22.

It is a distinct advantage to measure rates of metabolism directly in situ since extrapolation of laboratory results to natural conditions is usually difficult. When the rates of synthesis of organic matter and changes in primary production can be measured over time, efforts can be directed to the experimental evaluation of causal mechanisms regulating the synthesis, utilization, and loss of the organic matter.

The complex biochemical reactions of photosynthesis can be summarized by the general redox reaction:

Cyanobacteria, prochlorophytes, eukaryotic algae, and higher plants use light energy to oxidize water to molecular oxygen, hydrogen ions, and electrons. The light reaction occurs in photosystem II located in the thylakoid membranes. Mitochondrial (“dark”) respiration occurs both in the light as well as in darkness although at different rates, during which ATP and reductants are formed. Respiration associated with cell synthesis increases with temperature (Q₁₀ ca. 1.7–2.0). Photorespiration, a light-dependent oxidation of ribulose bispho-sphate produces glycolate that is excreted, oxidized, or used to synthesize amino acids, is an energy dissipating process that reduces the light-saturated rate of photosynthesis. Photo-respiration increases with higher ratios of dissolved oxygen concentration to carbon dioxide concentration.

Techniques for measuring rates of photosynthesis are based on the stoichiometry of this reaction, e.g., rates of oxygen production, rates of utilization of CO₂ or water, or changes in the concentration of organic matter. The objective is to modify the natural community as little as possible during assays of in situ rates of photosynthesis. Variations in the metabolic state of the phytoplankton can be large; measurements of primary productivity may reflect the rates of certain species rather accurately, but for other species rates are estimated poorly. Nonetheless, with reasonable precautions and guarded interpretations, the methods provide useful estimates of in situ rates of phytoplanktonic photosynthesis.

Algae suspended in water are circulated within the epilimnion of stratified lakes and exposed to bright light at the surface and then moved in the circulation to low light habitats. In situ incubation is static and can suppress turbulent circulation of water containing algae for several hours during the incubation. Dynamic incubation methods were developed to move incubating samples within the mixed layer to derive an integral of primary production within the zone (e.g., Gervais et al., 1997). In general, however, for short-term incubations, agreement is relatively close between the results from static and dynamic incubations.

Although in situ incubation is most desirable, it is not always practical when studying large lakes where simultaneous measurements at distant stations are necessary. Therefore, samples of natural phytoplankton can be incubated under controlled light and temperature conditions in laboratory incubators that simulate underwater conditions. From the intensity and spectral distribution of light, temperature, and phytoplankton in relation to depth, the relationship between light and photosynthesis can be used to estimate primary production in the natural environment (cf., Strickland, 1960; Saunders et al., 1962; Fee, 1969, 1971, 1973; Parsons et al., 1984; Walsby, 1997a).

Details of the integration of phytoplankton photosynthesis through time and depth in a water column can be calculated by numerical analysis based on the photosynthesis irradiance curve of the phytoplankton, the vertical dis-

tribution of the community, and details of the underwater light field (Walsby, 1997b). Variations in the light field are calculated from continuous recordings of surface irradiance and measurements of vertical light attenuation, with corrections for losses by reflection at the water surface that depend on the sun's elevation and wave action. Effects of changes in phytoplankton distribution, light attenuation, photoinhibition, and water temperatures can be modeled for reasonable estimates of primary productivity within the euphotic depth integrated over 24 hours.

Alternatively, measurements of in situ photosynthesis can be estimated indirectly in the natural environment from changes in environmental parameters affected by photosynthesis, such as changes in CO₂ and oxygen concentrations, pH, or specific conductance. The resultant evaluation is a measure of community metabolism, and a number of critical assumptions must be made when assessing which components of the overall biological communities are causing the observed changes in environmental parameters over short periods of time. This approach will be undertaken in Exercise 24.

Finally, autotrophic productivity of lake ecosystems that are sufficiently large to stratify thermally can be estimated indirectly by measuring long-term changes in biomass, reductions in certain nutrients, or hypolimnetic oxygen deficits or accumulations of CO₂. This systems approach will be treated separately in Exercise 29.