ENHANCEMENT OF NET PRIMARY PRODUCTIVITY BY HERBIVORE GRAZING IN AQUATIC LABORATORY MICROCOSMS1

The role of herbivore grazing intensity as a factor affecting net primary productivity was investigated by introducing varying biomasses of a starved herbivore (Notropis spilopterzcs) into replicate autotrophic microcosms, After 20 clays, the enhancement of net primary productivity in the experimental microcosms was directly related to herbivore biomass up to a certain density and inversely related above this. The relationship approximates the first derivative of a sigmoid population growth model. Enhancement of primary production under these experimental conditions appears to be due to reductions of standing crop and increased turnover rates of producer populations. The results suggest that these responses were independent of increased nutrient regeneration rates brought about by grazing. The distinction between productivity (a rate function) and standing crop (a static mcasurcment) has often been made. Yet, the notion that reduction in producer standing crop corresponds to a reduction in food material for a consumer population is taken as axiomatic in many investigations of production-consumption relationships. Several observations of changes in productivity of producer populations immediately following rapid removal of consumers suggest that measurements of standing crop are not necessarily indicative of the amount of food material available for consumption by a consumer population. For example, Paine and Vadas ( 1969) found that removal of sea urchins ( StrongyZocentrotus spp, ) resulted in significant alterations in benthic algal species composition and increases in primary productivity. Also, the high mortality of limpets on the Cornish shorts subsequent to detergent spraying following the Terre!/ Canyon oil spill in 1967 resulted in enhanced primary productivity by attached green algae (Smith 1968). But thcrc has been little direct invcstigation of the dynamic effects of hcrbivorc 1 This research was supported by a research fellowship and grant-in-aid from the SUNY Research Foundation JAC/UAC 365-570. ’ Prcscnt address: Battelle Columbus Laborntories, 505 King Avenue, Columbus, Ohio 43201. consumption on rates of primary productivity. Hargrave ( 1970) varied numbers of the herbivorous amphipod Hyalda axteca in sediment cores. The productivity of the sediment microflora was stimulated with increased amphipod numbers within the range of natural density. Above this lcvcl, with increased numbers of amphipods, algal production declined. The results suggest that increased grazing, within the range of natural grazing intensities, scrvcs to increase the primary productivity of benthic microflora. One explanation for the cnhancemcnt of net primary productivity by low level grazing is that herbivores might increase nutrient cycling rates in nutrient-limited situations. Nutrient rcgcncration by aquatic consumers has been discussed by Johanncs ( 1968) and is undoubtedly important for producer populations in situations of nutricnt limitation. However, several reports suggest that the cnhanccmcnt of net primary productivity by grazing might bc due to other factors. Welch ( 1968) found that nutrient reserves in sand recdgrass bccamc deplctcd in rcsponsc to grazing and deduced that the plants wcrc utilizing thcsc rcscrves to satisfy a newfound growth potential in the aftermath of standing crop rcductions due to grazing. Examples such as this from tcrrcstrial habitats may or may LIMNOLOGY AND OCEANOGRAPIIY 31 JANUARY 1973, V. 18( 1)