Phytoplankton abundance and Late Paleozoic extinctions: A reply

Abstract

Factors affecting nutrient supply and utilization changed in the Mid Paleozoic to result in reduced phytoplankton populations. Among these were the rise of vascular plants which for the first time retained such nutrients in a large terrestrial biomass, both as living plants and detrital accumulations. Prior phytoplankton populations had been adapted to a higher rate of influx. Modern aquatic vascular plants and filamentous algae are successful competitors for limited nitrogen abundance, and their growth is inhibiting to phytoplankton. Abundant bacteria, suggested by other data to have characterized the Permian, also would have competed for dissolved phosphorus. Although Mid Paleozoic extinctions were in part balanced by newly evolved introductions, the nature of the particular deletions and replacements also is consistent with modern communities of low net production, the highest extinction rates affecting sessile suspensions feeders and the higher trophic levels, and lowest rates characterizing detritus feeders and active predators. Offshore communities of lowest productivity underwent rapid changes. Invertebrates evolved new methods of feeding or turned to new kinds of food, such as the organic aggregates produced from dissolved matter by bacterial action or adsorption on bubbles and mineral particles, in response to reduced phytoplankton productivity. The direction of evolution supports the evidence of low productivity in the Late Paleozoic presented by the fossil record. Phytoplankton and invertebrate abundance did not immediately follow continental emergence as the reduced numbers had involved extensive extinctions. The annual exponential growth of phytoplankton resulting in the spring “blooms” results from a pre-existing, although reduced, wintering population or the germination of encysted individuals. Renewal of entire communities by evolution of new taxa is less predictable and far slower even on a geologic scale.