Phosphorus and ammonia excretion by planktonic protists

Excretion by grazers is an important source of the nutrients orthophosphate and ammonia to producers of organic particles (phytoplankton and bacteria). Excretion may be dominated by the smallest of particle consumers, planktonic protists, because metabolic rates per unit weight are usually inversely related to size. However, protist taxa are a heterogenous assemblage, ranging over 3 orders of magnitude in size, and include forms which are mixotrophic, profiting simultaneously from photosynthesis and ingestion of particulate matter. Excretion rates from laboratory studies of protists, ciliates and flagellates, both heterotrophic and mixotrophic, were analysed to determine relationships with both cell size and growth rate. Smaller protists, such as flagellates, showed higher weight-specific excretion rates than larger protists like ciliates. Equations relating maximal excretion rate (excr) to size in terms of dry weight (DW) were, for orthophosphate, log excr (mu g P cell(-1) h(-1)) = -2.101 + 0.570 log size (mg DW cell(-1)), r = 0.847, n = 12 and for ammonia, log excr (mu g N cell(-1) h(-1)) = -1.388 + 0.622 log mg DW cell(-1), r = 0.899, n = 15. Mixotrophic protists showed maximum excretion rates similar to like-sized heterotrophic protists. For phosphorus, the maximum weight-specific excretion rates of protists were higher than predicted by extrapolation of most size-excretion rate relationships established for metazoan zooplankton. Maximum excretion rates for ammonia approached the average of reported relationships for metazoan-based equations. Rapidly growing protozoa appear to excrete orthophosphate (measured as soluble reactive phosphorus) relative to ammonia well in excess of the Redfield atom ratio of 16:1, i.e., excretory N:P ratios of 2:1-8:1. For orthophosphate, excretion rate may be predictable based on cell size and growth rate. In contrast, ammonia excretion appeared weakly related to growth rate in five out of ten species, and independent of growth rate in the remaining species, probably due to variability of the form of nitrogenous excretion. Consequently, planktonic protists may be much more important in the regeneration of orthophosphate than ammonia. However, the apparent magnitude and simplicity of phosphorous excretion may be an artefact of considering the dominant excretory compound, soluble reactive phosphorus, as entirely orthophosphate. (C) Elsevier Science B.V. All rights reserved.