Factors regulating phytoplankton production and standing crop in the world's freshwaters

A regression analysis of global data for freshwater phytoplankton production, chlorophyll, and various nutrient parameters revealed the following: A high proportion of the variance in both annual phytoplankton production and mean annual chlorophyll could be explained by annual phosphorus input (loading), once a simple correction for water renewal time was applied. Good relationships were also found between phosphorus loading and mean total phosphorus concentration, and between total phosphorus concentration and chlorophyll. The slope of the regression of total phosphorus on phosphorus loading for stratified lakes was not significantly different from that for unstratified lakes, suggesting that the effect of stratification on phosphorus concentration is insignificant compared to external sources of the element. Nutrient input, which was unavailable in previous analyses, appears to be an important factor in controlling freshwater production. There is some evidence for a correlation between latitude and nutrient input, and it is possible that this may explain the good correlation between latitude and production observed by earlier investigators. In an earlier synthesis of IBP freshwater primary production results, Brylinsky and Mann (1973) and Brylinsky (in press) concluded that variables related to energy availability had a greater influence on phytoplankton production than those related to nutrient availability. They found that 57% of the variability in global primary production could be explained by latitude alone. Brylinsky and Mann, however, recognized the scarcity of good data for nutrient input to lakes in the IBP data set. They suggested (p. 12) that some of the influence attributed to energy might be due to its driving of circulation systems, so that nutrient availability might be affected directly. Recently, more data on nutrients, including input and output dynamics, have accumulated, chiefly as the result of investigations of the eutrophication of lakes. These data, plus our own experimental studies in lakes, suggested that nutrients played a preeminent role in controlling phytoplankton production and standing crop. Of all the variables which may control aquatic production and standing crop. only nutrients are amenable to control by humans. It therefore seemed worthwhile to repeat Brylinsky and Mann’s analysis, including this new information (Schindler and Fee 1974). The following analysis is highly biased toward glacial lakes in north temperate and subarctic regions, where most limnological activity has been concentrated. More complete data are sorely needed for water bodies in tropical and arctic regions, in the southern hemisphere, and in unglaciated areas before a definitive global analysis of production and factors affecting it can be made. Increases in the annual input of nu